Multicellular Parasites

M U L T I C E L L U L A R P A R A S I T E S 

A number of species of parasitic worms can inhabit the human 

intestinal tract and cause disease. While not microorganisms, 

they are studied and diagnosed using microscopic and immunologic 

techniques familiar to microbiologists. Some invade tissue 

and others do not, so that their presence in the body is often 

called an infestation rather than an infection. These worms, or 

helminths, are divided into two groups, the roundworms and 

the flatworms, and the flatworms are further divided into the 

tapeworms and the flukes. Tapeworms are ribbon-shaped and 

segmented, while flukes are generally shorter and leaf-shaped. 

Tapeworms differ from the other parasitic helminths in that they 

lack a digestive tract; they absorb nutrients through their skin. In 

most instances the helminths themselves are difficult to find and 

examine, so that they are identified by the microscopic appearance 

of their eggs, or ova, which are often present in the host’s 

feces in large numbers. The ova of each helminth species are distinctive 

and can be identified on the basis of size, shape, thickness 

of the outer covering, embryonation, and other features. 

Some helminths have complex life cycles, involving one or more 

intermediate hosts where early stages of development occur, 

and a definitive host where the sexually mature forms occur. 

Generally, helminths have male and female forms, but most of 

the flatworms are hermaphroditic, meaning that both male and 

female sex organs reside in the same worm. 

Diseases Due to Intestinal 

Roundworms 

Some notable examples of diseases due to roundworms (nematodes) 

are discussed in this section. Pinworm disease, whipworm 

disease, and ascariasis are acquired by ingesting ova. Hookworm 

disease and strongyloidiasis are contracted by skin penetration 

by worm larvae, and trichinosis by eating undercooked meat 

containing worm larvae. 

Pinworm Disease (Enterobiasis) 

Pinworm disease, enterobiasis, is caused by the pinworm, Enterobius 

vermicularis. It is the most common helminth disease seen 

by American physicians, and pinworms infest more than 200 

million people worldwide. 

Symptoms 

About one-third of individuals infested with E. vermicularis are 

asymptomatic. The most common symptom in adults is anal 

itching, sometimes accompanied by vaginal irritation. Sleeplessness, 

nightmares, nervousness, irritability, anal pain, and weight 

loss are common in affected children. 

Causative Agent. 

Enterobius vermicularis is a tiny roundworm; the females average 

about 10 mm in length, and the males, 2 to 3 mm. The 

female has a sharply pointed tail end, which inspired its popular 

name pinworm. The worms can often be detected in the anal 

area by using a flashlight about 2 hours after retiring, or early in 

the morning. Ova can be collected from the anal area by pressing 

transparent tape against the anus. The pinworm ova are 

elongated and have a distinctive appearance, flat on one side and 

containing a larval worm (figure 24.26a). 

Pathogenesis 

The worms live mainly in the upper part of the large intestine. 

They may, however, migrate throughout the intestinal and 

female genital tracts during their 1- to 2-month life span, feeding 

on bacteria and other intestinal material. Tiny ulcerations 

and inflammation occur where the worms attach to the intestinal 

epithelium. The male worms die soon after mating. The 

mature females containing 5,000 to 15,000 ova migrate out of 

the anus, usually while their host is sleeping, deposit their eggs 

in a sticky matrix, and die. Figure 24.26b shows the enormous 

numbers of ova present in a single worm. The sensation caused 

by worms exiting the anus leads to scratching, bleeding, and 

bacterial infection. If the anal area is not kept clean, eggs 

deposited there hatch, and the larval worms crawl through the 

anus, enter the intestine, and perpetuate the infestation. 

Epidemiology 

Scratching deposits E. vermicularis ova on the fingers and under 

the fingernails. Because of the enormous numbers of ova, clothing,

(a) 

(b) 

Ova 

Body 

100 µm 

0.5 mm 

Figure 24.26 Pinworm, Enterobius vermicularis (a) Microscopic view of 

the ova, showing flattened side and developing worm larva. (b) Body of the adult 

worm surrounded by thousands of its ova. 

bedding, various surfaces, and even the air in the household 

become contaminated. Inhaled and ingested ova are swallowed 

and hatch in the small intestine, and the larval worms reach 

maturity in 2 to 6 weeks. Ova can survive for about a week in 

cool moist conditions, but they quickly die if conditions are hot 

and dry. Enterobiasis occurs worldwide and, unlike most other 

helminth diseases, is more common in temperate climates and 

does not spare upper socioeconomic groups. Humans are the 

only host for E. vermicularis. 

Prevention and Treatment 

Hygienic habits and good ventilation minimize the risk of 

acquiring E. vermicularis. Treatment is easy because effective 

medications are available, but preventing reinfestation is difficult. 

All members of a household must be treated simultaneously, 

even those without symptoms. Clothing and bedding 

should be washed in hot water, and household surfaces thor- 

(a) 

(b) 

20 µm 

Figure 24.27 Whipworm, Trichuris trichiura (a) The slender anterior 

portion is embedded in the mucous membrane, while the thicker portion protrudes into 

the intestinal lumen. (b) Ovum. Notice the long oval shape, thick wall, and plugs at 

each end. 

oughly cleaned. Ideally, heat in the household should be turned 

up and all members take a vacation for a week. 

Whipworm Disease (Trichuriasis) 

Whipworm disease, trichuriasis, affects an estimated 500 to 800 

million people in warm and wet regions of the world where people 

defecate on the ground or use human feces for fertilizer. 

Symptoms 

Most infestations, even with as many as 100 worms, are asymptomatic. 

Very heavy infestations can cause anemia from blood 

loss, weight loss, abdominal pain, diarrhea, and protrusion of 

the bowel through the rectum from forcefully straining to expel 

the worms. 

Causative Agent 

The causative agent of trichuriasis is the whipworm, Trichuris 

trichiura. The worms are 3 to 5 cm long, with a slender anterior 

portion and a short, thicker posterior portion, hence the 

name whipworm (figure 24.27a). The unmistakable ova can be 

found by microscopic examination of the feces (figure 24.27b).

Pathogenesis 

Trichuriasis is contracted by ingesting the mature T. trichiura ova. 

The ova hatch in the small intestine, and the emerging larvae burrow 

into pockets in the intestinal epithelium. Here they develop 

for about a week, then return to the bowel lumen and migrate to 

the large intestine. They become sexually mature and start producing 

eggs in 1 to 2 months. The slender anterior region of the 

worm is inserted into the intestinal mucosa, with the thicker portion 

protruding into the bowel lumen. The intestinal epithelium 

is damaged by the penetrating worm, causing blood loss and an 

inflammatory response. Sometimes people infested with T. trichiuria 

develop hives because of allergy to worm antigens. 

Epidemiology 

Infection is by the fecal-oral route. Each mature female worm produces 

about 5,000 ova per day. These ova are eliminated with the 

feces and can contaminate hands and foods, but they are not immediately 

infectious. The ova require about 3 weeks in a warm, moist, 

and shady place before they are capable of parasitizing a new host. 

The worms can live 3 to 8 years, so that repeated ingestions of eggs 

can cause a large buildup of the numbers of the worms in a person’s 

intestine. Also, because of the long life of T. trichiura, the 

worms are frequently found in people who traveled to endemic 

areas years ago. Humans are the only reservoir for T. trichiura. 


Trichuriasis is prevented by proper disposal of human feces and 

other sanitary measures. Several medications provide safe and 

effective treatment of the disease. 

Ascariasis 

Ascariasis is a helminthic disease caused by Ascaris lumbricoides, 

the largest and most prevalent human roundworm. Worldwide, 

an estimated 1 billion people are infested. 

Symptoms 

Most cases of ascariasis are asymptomatic. Many individuals 

infested with the worms, however, develop one or more bouts of 

fever, trouble breathing, coughing, and wheezing. Some infested 

people have the frightening experience of vomiting up a large 

worm, or passing one in their feces; others develop abdominal 

pain when the worms obstruct the intestine or gallbladder. 


Ascaris lumbricoides (see figure 24.28) is a roundworm prevalent 

worldwide in areas where human feces contaminate the 

soil. Female worms are enormous, ranging from 20 to 45 cm 

long and 3 to 6 mm wide, while males are somewhat smaller. 

The ova are nearly spherical, with a thick wall and irregular 

outer surface (see figure 24.28). The life cycle of A. lumbricoides 

is complex. The ova released by mature females are eliminated 

with the host’s feces. The tiny worm larva inside the ova 

must develop in soil or other location outside the host’s body 

for at least 1 1 / 2 weeks before they can survive and mature in a 

new host. After being swallowed by the new host, the ova hatch 

in the duodenum. At this point, the tiny larval worms do some- 

thing totally unexpected. Instead of staying in the intestine, 

they burrow through the intestinal epithelium, enter the circulatory 

system, and are carried throughout the body. Those that 

reach the lungs push through the capillary walls into the alveoli. 

Here they undergo further development, reach 1 to 2 mm 

in length, and shed their covering twice to get ready for the rest 

of their journey. After about 10 days, they move out of the alveoli 

into the air passages of the lungs, and they are coughed up 

and swallowed (figure 24.28). The developmental changes 

that take place in the lungs make them resistant to stomach 

juices, and so they pass safely through the stomach into the 

small intestine. They mature to adulthood after about 2 months 

in the upper small intestine, feeding on intestinal contents. 

They live about 1 year. 

Pathogenesis 

Large masses of worms can obstruct the intestine, while wandering 

worms can obstruct or perforate various organs. Except 

for the larval worms that reach the lung, circulating larvae lodge 

in capillaries throughout the body and die, causing an inflammatory 

and immune response and consequent aches, pains, and 

fever. The larval penetration into the lungs causes microscopic 

bleeding, which if widespread, can predispose one to bacterial 

pneumonia; lesser amounts of damage provoke inflammatory 

and immune responses, coughing, wheezing, and pain. 

Roundworms of some species of animals can penetrate 

human tissues but cannot complete their life cycle. Instead, their 

larvae wander through the tissues and eventually die, causing an 

intense inflammatory reaction. Preschool children are especially 

susceptible to this condition, called visceral larva migrans, 

because of their tendency to put rocks and dirt into their 

mouths. If the material is contaminated with certain roundworm 

ova, the eggs hatch, and the larvae migrate through body 

tissues. One example is Toxocara canis, an intestinal roundworm 

of dogs that resembles A. lumbricoides. This helminth can cause 

damage to various organs of a child, even blindness when its larvae 

lodge in the eyes. 

Epidemiology 

Humans are the only natural hosts of A. lumbricoides. A closely 

similar helmith of pigs can sometimes infect people. The 

helminth female produces more than 200,000 ova daily. These 

ova are remarkably resistant to heat, cold, drying, and disinfectants. 

Moreover, the ova remain infectious for up to 10 years, 

contaminating soil, dust, and food. In many areas of the world, 

human feces is used as fertilizer for food crops, and children 

defecate outdoors causing extensive soil contamination. Young 

children commonly place soiled objects into their mouth, and 

they are prone to eat soil if they are anemic or iron deficient. 

Children may have 50 or more worms, the record being well 

over 1,000. 


Prevention of ascariasis depends on hygienic practices and sanitary 

disposal of human feces. Effective and safe medications are 

available for treating the disease.

Bronchiole 

Alveolus 

(c) Larvae in the lung 

leave the capillaries 

and enter alveoli 

and bronchioles 

(d) Larvae from lung 

enter esophagus 

and then gastrointestinal 

tract 

Figure 24.28 (a–f) Life Cycle of Ascaris 

lumbricoides, the Largest Roundworm Infesting 

the Human Intestine Curiously, larvae hatching in the 

intestine migrate through the lungs and back to the 

intestine before maturing to adulthood.The ova are nearly 

spherical, thick-walled, with a ruffled border. 

(f) Eggs passed 

in feces 

Hookworm Disease 

Hookworms infest about 900 million people in warm, wet areas 

around the world where people defecate on the ground and do 

not wear shoes. Hookworm disease was once common in the 

southeastern United States. Asymptomatic individuals are still 

seen in most states, but they have contracted their hookworms 

in other countries. 

Symptoms 

The vast majority of hookworm infestations are asymptomatic. 

An itchy foot rash commonly occurs at the site of skin penetration 

by the worms. Cough, fever, and shortness of breath can 

occur, and later, diarrhea, nausea, and vomiting. The most 

important symptoms of hookworm disease are mental and physical 

retardation in children. 

Causative Agents 

Necator americanus and Ancylostoma duodenale account for 

most cases of human hookworm disease. The worms are about 

10 mm long and live in the small intestine, attaching by means 

of small hooks or plates located about their mouth. The hookworm 

life cycle is shown in figure 24.29. The female worm 

(e) Larvae mature 

to adulthood 

(a) Worm eggs from 

contaminated 

soil are ingested 

(b) Ingested eggs 

hatch; larvae 

penetrate intestinal 

capillaries and 

are carried to lung 

6 cm 

FPO 

100 µm 

releases her eggs, and they are discharged with the feces, contaminating 

soil in places that lack toilets. The eggs hatch in the 

soil, releasing tiny larvae that develop in the soil and become 

infectious. These larvae generally penetrate the skin of a person’s 

feet, wiggle their way into the bloodstream, and are carried 

to the heart and lungs. In the lungs, as in ascariasis, they 

push through the capillary walls into the alveoli, migrate into 

the bronchial tubes, and then enter the gastrointestinal system 

when lower respiratory tract secretions are coughed up and 

swallowed. Once in the small intestine, the larvae attach and 

mature to complete their life cycle. 

Pathogenesis 

Hookworms feed by sucking the blood of the host. More than 

1,000 worms may be found in a single person, and the constant 

loss of blood frequently produces anemia. Children who acquire 

hookworms may be undernourished already, and the anemia 

resulting from hookworm disease may cause weakness, fatigue, 

and physical and mental retardation. Initial attachment of the 

worms can cause nausea and vomiting. Migration of the larvae 

through the lungs causes microscopic bleeding and inflammatory 

and immune responses that result in coughing, wheezing, 

and shortness of breath. The itchy rash, called ground itch, that

(c) Larvae 

mature in the 

small intestine 

and produce ova 

Large 

intestine 

(d) Hookworm ova 

from feces 

hatch in soil; 

larvae develop 

into an 

infectious 

form 

50 µm 

Bronchioles Alveoli 

(b) Larvae in the 

lung leave the 

capillaries, 

enter the 

alveoli and 

bronchioles 

(a) Larvae penetrate 

skin and enter 

capillaries 

Figure 24.29 (a–d) Life Cycle of a Hookworm Infection begins with penetration of the 

skin by the worm larvae.The micrograph shows the thin-walled ovum containing the developing 

worm embryo. 

often develops between the toes at the site of larval penetration, 

is probably an allergic response to repeated exposures. 

The larvae of dog and cat hookworms can contaminate 

soil and penetrate the skin, especially of people such as plumbers 

that sometimes have to crawl in dirt under buildings. The larvae 

wander just under the skin surface, causing itchy red tracts 

(figure 24.30) that advance as much as 1 to 2 cm per day for a 

month or more, before dying. This condition is called cutaneous 

larva migrans. 

Epidemiology 

Hookworm disease occurs around the world in temperate and 

tropical climates where the soil is moist, human feces is 

deposited on the ground, and people are barefooted. Each 

female worm can produce 25 million ova in her lifetime of 2 to 

10 years. 


Prevention of hookworm disease depends on sanitary 

disposal of human feces and wearing shoes. Highly 

effective medications are available to eliminate the 

infestation. 

Strongyloidiasis 

Strongyloidiasis, the disease caused by Strongyloides 

stercoralis, occurs in a spotty distribution in warm, wet 

areas of the world, with an estimated 400,000 cases in 

the southeastern United States and Puerto Rico. This 

disease differs from most other helminthic diseases in 

that it perpetuates itself indefinitely in the absence of 

reexposure to the causative agent. 

Symptoms 

Most infections are asymptomatic, but abdominal 

pain, diarrhea, recurrent rashes on the buttocks and 

lower back, and respiratory symptoms can occur with 

heavy infestations. 


The adult Strongyloides stercoralis (figure 24.31), 

sometimes called a threadworm, is only about 2 mm 

long. Its life cycle is similar to that of hookworms with 

two important exceptions: (1) the worms can multiply 

sexually in the soil; and (2) ova produced by intestinal 

worms hatch before they are discharged in the feces. 

These intestinal larvae can mature to an infectious 

form, penetrate the intestine or the anal skin, and 

thereby perpetuate the infection indefinitely. 

Pathogenesis 

Most individuals with strongyloidiasis are asymptomatic 

because their immune system limits tissue invasion. 

Heavy infestations, however, can damage the 

lungs and intestine, sometimes leading to invasion by 

bacterial pathogens. Recurrent rashes result from 

hypersensitivity to the larvae at the sites of penetration. 

In people who are debilitated, alcoholic, or immunosuppressed, 

massive numbers of the worms arise, penetrating 

all parts of the body, including the brain. These massive 

infections are rapidly fatal unless treated. 

Epidemiology 

Strongyloidiasis occurs in much the same distribution as hookworm, 

but favoring tropical areas. Since the organisms can multiply 

in soil, contaminated areas may remain infectious even 

when there are no new deposits of human feces. Human infections 

can last for a lifetime, and they can be transmitted to other 

people through close physical contact. Many American soldiers 

contracted the disease during World War II while imprisoned in 

Southeast Asia. 


Prevention of strongyloidiasis is the same as for hookworm disease. 

Effective medications are available but require a longer

Figure 24.30 Cutaneous Larva Migrans, an Occupational Hazard of 

Plumbers and Others Who Crawl Under Buildings Where Dog or Cat Feces 

Have been Deposited Larval hookworms from the feces penetrate and wander 

under the skin, causing serpentine tracts of inflammation before dying. 

100 µm 

Figure 24.31 Strongyloides stercoralis Larva in Feces The adult worm is 

only about 2 mm long, and has a life cycle similar to hookworms. Unlike other 

intestinal roundworms, however, S. stercoralis can multiply in the soil and in the body 

of a single host. 

period of treatment than for hookworm, and even so, relapses 

may occur. 

Trichinosis (Trichinellosis) 

Trichinosis, also known as trichinellosis, differs from all the roundworm 

diseases discussed previously in that it is contracted by eating 

inadequately cooked meat. Since pork is a leading source of 

infection, the incidence of the disease has fallen markedly in the 

United States over the years due largely to federal regulations on 

how pigs raised commercially are fed. Nevertheless, the potential 

for large outbreaks of the disease still exists because the meat of a 

number species of animals can be infectious. 

Symptoms 

Trichinosis is characterized by abdominal pain and diarrhea, followed 

in about a week by fever, muscle pain, swelling around 

the eyes, and sometimes a rash. Occasional cases are fatal 

because of damage to the heart or brain. The disease occurs 

worldwide except for Australia and some Pacific Islands. 


The cause of trichinosis is usually Trichinella spiralis, a 1 to 4 mm 

long roundworm that lives in the small intestine of meat-eating 

animals, especially rats, pigs, bears, dogs, and humans. Its life 

cycle is shown in figure 24.32. The female worm discharges her 

living young into the lymph and blood vessels of the host’s 

intestine without an intervening egg stage, and those larvae are 

carried to all parts of the body. Most of the larvae are killed by 

body defenses, but some survive in the muscles of the host 

where they become encased with scar tissue. The worms then 

stay alive for months or years within the muscle. If the flesh of 

an infested pig or other carnivorous animal is eaten by humans 

or other animals, the digestive juices of the new host release the 

larvae, permitting them to burrow into the new host’s intestinal 

lining. The larvae then mature, and the females begin producing 

larvae in the new host to complete the life cycle of the 

worm. Each female adult Trichinella may live 4 months or more 

and produce 1,500 young. 

Pathogenesis 

The penetration of larval worms into the host’s tissues is responsible 

for the symptoms of trichinosis. Abdominal pain and diarrhea 

begin within the first week after eating meat containing T. 

spiralis, when the worms mature and begin discharging their 

larvae. Fever, muscle pain, rash, and facial swelling result from 

inflammatory and immune reactions to larvae lodged in various 

tissues. Only larvae that penetrate skeletal muscle survive. 

Epidemiology 

In trichinosis, the same animal generally serves as both the 

definitive and the intermediate host, human beings one hopes 

being the exception. Almost all warm-blooded carnivores can 

be hosts for T. spiralis. Many cases of the disease have been contracted 

from the meat of wild animals, including bear, wild boar, 

walrus, and cougar. 


Prevention of trichinosis depends on thorough cooking of meat 

so that all parts reach at least 170°F (77°C). Pigs raised commercially 

must by law be fed only cooked garbage to ensure that 

they do not receive meat scraps containing viable Trichinella larvae. 

Government inspection of meat does not detect Trichinella 

infestation. Larvae in pork are generally killed at 5°F (:15°C) or 

lower for 3 weeks if the meat is less than 15 cm thick. Wild 

game, however, often harbors Trichinella species that are not 

killed by freezing. Medication is available that can probably prevent 

the disease in persons who have eaten meat shown to contain 

Trichinella, but there is no effective treatment. 

MICROCHECK 

Parasitic worms are divided into the roundworms 

(nematodes) and the flatworms. Flatworms are further 

divided into tapeworms (cestodes) and flukes

(b) Larvae deposited 

by adult worms into 

blood or lymph vessels 

(a) Infested meat 

is ingested 

(trematodes). Roundworm diseases include pinworm, 

whipworm, and ascariasis contracted by ingesting worm 

ova; hookworm disease and strongyloidiasis, contracted 

through skin penetration by the worm larvae; and 

trichinosis, acquired by ingesting inadequately cooked 

meat containing encysted worm larvae. 

■ Why might asymptomatic strongyloidiasis pose a serious 

threat to an individual who develops 

immunodeficiency? 

■ Would you be surprised to see new cases of ascariasis 

turning up 3 years after all the inhabitants of a 

Southeast Asian village began using outhouses to 

deposit their feces? Why or why not? 

Diseases Due to Intestinal Flatworms 

This section gives some examples of diseases due to tapeworms, 

cestodes, and an example of a disease caused by a very large 

intestinal fluke, or trematode. ■ cestodes, p. 317 ■ trematodes, p. 318 

(c) Larvae lodge 

in muscle 

and encyst 

150 µm 

Figure 24.32 (a–c) Life cycle of 

Trichinella spiralis, the Cause of 

Trichinosis Most cases of the disease 

result from eating inadequately cooked 

pork or bear meat. 

Tapeworm Disease 

Tapeworms can exceed 10 m (more than 30 ft.) in length and 

have a ribbonlike appearance. Their structure is quite distinctive in 

that there is no digestive tract. The head end, called a scolex, is 

only 1 to 2 mm in diameter and is merely an attachment device to 

anchor the organism so that it is not eliminated by peristalsis. The 

neck region continuously produces segments called proglottids 

that become more mature the further they are from the scolex. 

The proglottids are little more than hermaphroditic egg factories; 

depending on the species, ova are discharged from an opening in 

the proglottid, or the proglottid separates from the rest of the 

worm and releases its ova by breaking apart. People become 

infested with tapeworms by eating inadequately cooked red meat 

or fish. Three important tapeworms are the beef tapeworm, the 

pork tapeworm, and the fish tapeworm. Beef, pork, and fish refer 

to the sources of human infection, not where the adult worms 

exist; human beings are the definitive host for all three. 

Symptoms 

Most tapeworm victims have few or no symptoms. An occasional 

individual will develop vomiting and abdominal pain.

Rarely, weakness, unsteadiness, mental abnormalities, tumorlike 

swellings, blindness, or epilepsy occur. 

Causative Agents 

The tapeworms Taenia saginata or beef tapeworm, Taenia 

solium or pork tapeworm, and Diphyllobothrium latum or fish 

tapeworm can be distinguished by the structure of their scolices, 

their proglottids, and their ova. Taenia saginata and T. solium 

each have a single intermediate host, cattle and pigs, respectively, 

while D. latum has two or more. 

Cattle ingest T. saginata ova when they feed on pasture 

contaminated with feces from people who carry the adult worm. 

The ova hatch in the cattle intestines, releasing larvae that burrow 

through the intestinal wall and lodge in the flesh of the animals. 

Here each of the parasites develops into a cysticercus, a 

cystic structure containing an immature 

scolex. When the inadequately cooked beef is 

eaten by a person, the cysticercus is released 

from the meat by digestive enzymes, matures, 

attaches to the intestinal wall, and soon develops 

into an egg-producing adult tapeworm. 

The life cycle for the pork tapeworm, Taenia 

solium, is similar except pigs rather than cattle 

are the intermediate host. Cysticerci in inadequately 

cooked meat represent the only 

source of adult worms in people. 

The intermediate hosts of D. latum are 

water fleas and fish, whereas the definitive 

host, the mature egg-producing form, occurs 

in humans; bears, dogs, and other animals can 

be definitive hosts for the same or similar 

tapeworms. When ova of D. latum passed 

with a person’s feces enter fresh water, they 

soon release ciliated larvae, which are 

devoured by water fleas (figure 24.33). The 

larvae lose their cilia, claw their way into the 

water flea’s body using tiny hooks, absorb 

nutrients from the flea’s blood, and increase 

in size. These larvae can develop further only 

if the water flea is eaten by a fish. In a fish’s 

muscle, the organism continues developing 

and becomes infectious for humans. Usually, 

however, it gets passed from one fish to 

another as bigger fishes eat smaller ones, 

before being eaten by the human host. In the 

human host, the flesh of the fish is digested 

away by stomach juices, releasing the wormlike 

larva. It attaches to the small intestine, 

rapidly matures, and begins producing ova. 

Pathogenesis 

Symptoms of tapeworm disease can arise from 

the presence of adult worms in the intestine. 

Nausea, vomiting, and abdominal pain sometimes 

arise because the worms partly obstruct 

the intestine. Also, large fish tapeworms can 

absorb almost 100% of vitamin B 12 from 

(h) Degenerating 

worm segments 

containing 

many eggs 

(g) Adult worm 

develops in 

human small 

intestine 

intestinal juices, depriving the host of this essential vitamin and 

causing anemia, weakness, unsteadiness, and other manifestations 

of nervous system malfunction. 

The intermediate developmental forms of tapeworms can 

also infect humans and cause symptoms. This occurs in the case 

of D. latum when infected water fleas are accidentally ingested, as 

might happen with drinking lake water. Infection can also result 

from the practice of applying raw meat or fish to open wounds in 

the mistaken belief that the meat will aid recovery. If larvae are 

present in the flesh, they penetrate the tissues via the wound. 

Pork tapeworms are the most dangerous of the three 

types, because its eggs may hatch before being discharged in the 

persons’s feces or ova contaminating fingers or foods can be 

ingested and hatch. Larvae emerging from the ova do the same 

as they would do in a pig. Although humans are a poor excuse 

Swimming larva 

(a) Ovum in 

fresh water 

(f) Infested fish 

eaten by 

human 

(d) Small fish eaten 

by large fish 

(c) Water flea eaten 

by small fish 

(b) Larva 

eaten by 

water flea 

Figure 24.33 (a–h) Life Cycle of the Fish Tapeworm, 

Diphylobothrium latum Notice that this species requires two or more 

intermediate hosts. Distinctive features of the ovum are its large size, oval 

shape, and operculum, a trapdoor through which the larva exits the 

ovum. Infestation is a risk for those who are fond of eating uncooked fish. 

20 µm 

Knob 

(e) Larvae in 

muscle of 

fish 

Operculum

for an intermediate host, the larvae penetrate the intestinal wall 

by means of tiny hooklets. Then they enter the bloodstream, 

from which they invade tissues throughout the body, including 

the eyes, brain, heart, and muscles. In the tissues they develop 

into cysticerci, 0.5 to 5 or more cm in diameter. The condition 

resulting from cysticerci in the tissues is called cysticercosis and 

can mimic a brain tumor or cause epilepsy (figure 24.34). 

Epidemiology 

Tapeworm disease occurs wherever people eat raw meat or fish. 

The beef tapeworm is the most common in the United States, 

reported from eight states in one survey. The fish tapeworm 

mainly occurs in the north central part of the United States, 

where formerly outhouses were constructed so as to discharge 

directly into the lakes. The pork tapeworm is rare in the United 

States, but it may occur in immigrants from parts of Mexico, 

India, the Philippines, and other parts of the world where eating 

raw pork is traditional. 


Control of tapeworms depends on adequate cooking of meat 

until it is no longer pink in the center. Fish should also be thor- 

Hooklets 

Adult worm 

200 µm 

Scolex 

Proglottid 

Sucker 

Cysticerci 

(d) Cysticercus develops 

into mature tapeworm 

in small intestine 

(e) Ova and ova-filled 

proglottids passed 

in human feces 

oughly cooked, or if eaten raw should be deep frozen for a week 

before eating. Proper disposal of human feces and good 

hygienic habits are especially important for controlling pork 

tapeworm disease. Effective medicines are available both for ridding 

the body of adult worms and for treating cysticercosis. 

Giant Intestinal Fluke Disease (Fasciolopsiasis) 

Fasciolopsiasis, disease caused by Fasciolopsis buski, may be 

acquired in a number of countries of Southern Asia, including 

parts of China, India, Myanmar (Burma), Thailand, Laos, Cambodia, 

Vietnam, Malaysia, Indonesia, and the Philippines. An 

estimated 10 million people are infested worldwide. 

Symptoms 

Symptoms of fasciolopsiasis are diarrhea, abdominal pain, generalized 

weakness, and swelling of the face and other parts of the 

body. 


Fasciolopsis buski, also known as the giant intestinal fluke, can be as 

large as 7.5 cm long and 2.0 cm wide. This hermaphroditic 

helminth lives in the upper and middle small intestine; attachment 

(c) Inadequately cooked 

pork containing 

cysticerci ingested 

by human 

Figure 24.34 (a–e) Life Cycle of the Pork Tapeworm, Taenia solium, Acquired by Eating 

Inadequately Cooked Pork If worm ova hatch in the intestine, larval forms called cysticerci can develop throughout 

the person’s tissues. A cysticercus in the brain can cause epilepsy. 

(f) Ova ingested 

by human leads 

to cysticercosis 

(b) Cysticercus develops 

in flesh of pig 

(a) Pig ingests embryonated 

ovum from human feces

and movement is aided by two suckers. Unlike tapeworms, flukes 

like F. buski have a digestive system. Ova are passed in the feces of 

the definitive hosts, humans and pigs, and the enclosed embryos 

mature in warm fresh water in about 6 weeks (figure 24.35). The 

ova then hatch, releasing ciliated larvae, which penetrate the flesh 

of certain snail species that serve as intermediate hosts. In the snail, 

the organisms develop and multiply asexually, eventually exiting 

the snail in the form of motile long-tailed larvae called cercariae. 

These cercariae attach to the outer covering of water plants such 

as water chestnuts, where they form cysts infectious for pigs and 

human beings. People generally become infected when they peel 

off the outer portion of water plants with their teeth in order to 

eat the edible inner portion of the plant. The worms exit from 

their cysts in the duodenum, attach, and after several months 

begin producing ova. 

Pathogenesis 

Inflammation occurs at the site of attachment of the helminths, 

followed by ulceration and bleeding. Heavy infestations can interfere 

with nutrition or even obstruct the intestinal tract. Severe 

symptoms also arise from toxicity and allergy to substances 

released by the worms and absorbed by the host’s circulation. 

(e) Asexual multiplication 

and differentiation into 

cercariae occur over a 

number of weeks 

(g) Cercariae attach to 

water plant and encyst 

(f) Cercariae exit 

the snail 

(d) Miracidium 

burrows into 

flesh of snail 

(c) Miracidium 

emerges after 

3 to 6 weeks 

Epidemiology 

Fasciolopsiasis occurs in warm climates where people raise pigs 

and eat raw freshwater plants, and where a suitable snail intermediate 

host exists. Feces from infested humans or pigs must 

reach the ponds inhabited by the snails and edible water plants. 

Each adult worm produces more than 20,000 ova per day and 

lives about 1 year. 


Fasciolopsiasis can be prevented by killing encysted F. buski on 

water plants by dipping them in boiling water for a few seconds 

before peeling and eating. Rinsing the edible portion thoroughly 

in clean water after peeling is probably also effective. 

The disease is effectively treated with praziquantel. Table 24.17 

compares some features of these parasitic helminths. 

MICROCHECK 

Tapeworms are ribbonlike creatures, consisting largely 

of a string of hermaphroditic egg-producing segments. 

The pork tapeworm, Taenia solium, is the most 

dangerous because its ova can hatch in the human 

intestine, the larvae entering brain and other tissues, 

(h) Cyst is ingested by human or pig; 

larva excysts in the duodenum 

and becomes adult worm 

Oral sucker 

Ventral sucker 

Figure 24.35 (a–h) Life Cycle of the “Giant Intestinal Fluke,” Fasciolopsis buski The infectious larva of 

this worm encysts on certain edible water plants, including a kind of water chesnut. People become infested when they 

peel the edible portion with their teeth. 

(a) Adult worm 

in duodenum 

(b) Ova in feces reach 

warm fresh water, 

embryo develops

TABLE 24.17 Notable Helminthic Diseases of Human Beings 

Disease and Causative Agent How Acquired Disease Characteristics Prevention and Control 

Enterobiasis (pinworm), Hand to mouth; inhalation Itching of anal region, restless- Handwashing, daily change of 

Enterobius vermicularis ness, nervousness, irritability, 

poor sleep 

underclothing and bed sheets 

Trichuriasis, (whipworm) Ingestion of eggs of parasite Abdominal pain, bloody stools, Sanitary disposal of human 

Trichuris trichiura along with contaminated food 

or water 

diarrhea, and weight loss feces; frequent handwashing 

Ascariasis, Ascaris lumbricoides Ingestion of eggs of parasite or Abdominal pain, live worms Sanitary disposal of human 

water along with contaminated food vomited or passed in stool feces 

Hookworm disease, Larvae penetrate bare feet Anemia, weakness, fatigue, Sanitary disposal of human 

Necator americanus and physical and mental retardation feces; wearing shoes 

Ancylostoma duodenale in children 

Strongyloidiasis, Larvae penetrate bare feet Skin rash at site of penetration, Sanitary disposal of human 

Strongyloides stercoralis cough, abdominal pains, weight 

loss 

feces; wearing shoes 

Trichinosis, Trichinella spiralis Eating raw or undercooked Fever, swelling of upper eyelids, Adequate cooking of meat 

meat, usually pork muscle soreness 

Tapeworm disease, Eating raw or undercooked Abdominal pain, anemia, Thorough cooking or deep 

Taenia solium, T. saginata, 

Diphyllobothrium latum 

meat or fish cysticercosis freezing of meat and fish 

Fasciolopsiasis, Eating raw water plants with Diarrhea, abdominal pain, Brief treatment of plants in 

Fasciolopsis buski encysted larvae weakness, body swelling boiling water 

resulting in cysticercosis. Tapeworm disease is usually 

acquired by eating inadequately cooked beef, pork, or 

fish. Fasciolopsiasis is caused by the giant intestinal 

fluke, a relatively short, nonsegmented hermaphoditic 

flatworm usually acquired when a person uses their 

teeth to peel certain edible aquatic plants having 

encysted fluke larvae on their surface. 

■ Describe the anatomy of a tapeworm. 

■ How might you explain the development of epilepsy in 

a Mexican man fond of eating traditional dishes made 

from raw pork? 

F U T U R E C H A L L E N G E S 

Defeating Diarrhea 

Development of better preventive and treatment techniques 

for alimentary tract diseases has an urgency 

arising from massive food and beverage production 

and distribution methods. For example, in 1994, an 

estimated 224,000 people became ill because a tanker truck used to 

transport ice cream mix had previously carried liquid eggs. One 

day’s production from a ground beef factory can yield hundreds of 

thousands of pounds of hamburgers, which are soon sent to many 

parts of this or other countries. The challenge is to better educate 

the producers and transporters, to develop guidelines to help them 

avoid contamination, and to utilize newly approved methods such 

as meat irradiation. 

Other challenges include the following: 

■ Exploit the power of molecular biology techniques to 

produce effective vaccines against rotavirus, hepatitis C 

virus, and bacterial pathogens. There is special need for 

bacterial vaccines that can be administered by mouth and 

evoke long-lasting mucosal immunity. 

■ Find better ways of preventing and reversing resistance to 

antibacterial medications. In the early 1980s, 0.6% of 

Salmonella Typhimurium isolates were resistant to 

ampicillin, chloramphenicol, tetracycline, streptomycin, 

and sulfa drugs. Now, in some areas, over 30% of isolates 

of this common salmonellosis cause are resistant to all 

these medications. 

■ Explore further the influence of global warming on 

alimentary tract diseases. A study of Peruvian children by 

Johns Hopkins University scientists found an 8% increase 

in clinic visits for diarrhea with each 1°C increase in 

temperature from the normal. 

■ Explore new prevention and treatment options. Scientists 

at the University of Florida have developed a genetically 

engineered Streptococcus mutans that does not produce 

lactic acid but readily displaces wild strains of the dental 

decay causing bacterium. Researchers at the University of 

Alberta have custom-designed a molecule that binds 

circulating Shiga toxin, potentially preventing hemolytic 

uremic syndrome. Others work on finding an effective 

therapy for cryptosporidiosis.

STD Caused by 

Multicellular Parasites 

Two common STDs are caused by lice or mites, multicellular 

skin parasites that are distributed worldwide and infect only 

human beings. Nonsexual transmission can sometimes occur in 

both instances. Besides the discomfort they cause, their chief 

importance is to raise the possibility that other, unrecognized, 

STDs might be present. Fortunately, neither of these parasites 

are vectors of microbial diseases. 

“Crabs” (Pubic Lice, Pediculosis Pubis) 

Pediculosis pubis results from infestation by a species of louse. 

Lice are wingless insects dependent for nutrition on sucking 

blood from their host. 

Symptoms 

The principal symptoms are itching in the pubic area, and psychological 

stress from seeing the tiny lice and their eggs, called 

nits, both visible to the unaided eyes. Sometimes, tiny spots of

lood are seen on the underclothing. In heavy infestations, 

these lice may cling to hair of parts of the body other than the 

pubic area, sometimes even the eyebrows and eyelashes. 


Pediculosis pubis is caused by Phthirus pubis, a slow-moving 

insect resembling a microscopic crab (figure 25.22). The adult 

form is approximately 2 mm in diameter; smaller juvenile forms 

feed along with the adults. The females lay about six eggs daily, 

attached tightly to body hairs. Hatching occurs in about 1 week, 

and after several developmental stages, adulthood is reached in 

about 2 weeks. Their life span is approximately 1 month. 

Pathogenesis 

Crab lice have piercing mouthparts with which they penetrate 

the skin to obtain a blood meal. They usually feed for several 

hours twice daily, leaving a tiny puncture site that can ooze a little 

blood. Symptoms are due to an allergic reaction to the feeding 

lice, and they do not appear for about a week after the 

infestation begins. If the host becomes reinfested at a later date, 

symptoms begin almost immediately. The intense itching leads 

to scratching, which can in turn lead to secondary bacterial 

infection. 

Epidemiology 

Phthirus pubis is slow moving and prefers to cling to the host’s 

hair using its clawlike hooks. Since it can live only about 24 hours 

away from a human host, transmission generally occurs during 

close physical contact, mainly but not exclusively during sexual 

intercourse. Fomites such as a towel or a theater seat can occasionally 

transmit the organisms. 


Condoms do not prevent transmission. Several insecticides are 

approved for treating pediculosis pubis, but they must be used 

exactly as directed to avoid possible serious toxicity. Bedding 

1 mm 

Figure 25.22 Crab Louse, Phthirus pubis, is Usually Sexually 

Transmitted It is not known to be a vector of infectious agents. 

and clothing is washed in hot water, dry cleaned, or kept from 

human contact for 3 days. Any sexual partners over the prior 

month are examined and treated if needed. All are reexamined 

after 1 week and retreated if necessary. 

“Seven-Year Itch” (Scabies) 

Scabies, also known as the “seven-year itch,” results from infestation 

with a mite, a tiny member of the arachnids, the class of 

eight-legged creatures that includes spiders and scorpions. Other 

mites, such as the house mouse mite, can transmit microbial diseases, 

and the dust mite causes allergic asthma. The scabies mite 

is restricted to human beings, and it does not transmit microbial 

diseases. Scabies occurs worldwide predominantly associated 

with poverty and crowding, but anyone can contract the disease. 

Symptoms 

As with pediculosis pubis, the dominant symptom of scabies is 

intense itching. Besides the pubic area, the space between the 

fingers, the wrists, and the area under the breasts are often 

involved. Rarely, the entire body below the neck is infested, 

causing a scaly rash. 


The causative agent of scabies is Sarcoptes scabiei. The adult 

female (figure 25.23a) is only 330 to 450 µm long; the male is 

smaller. The organisms live on the surface of the human skin. 

The females make burrows into the outer layers of epidermis 

(figure 25.23b), forming tunnels where they lay several eggs 

daily over a life span of 1 to 2 months. The tunnels are visible 

to the unaided eyes as short, dark-colored wavy lines, sometimes 

with a vesicle overlying the end containing the mite. The mites 

can be scraped from the tunnels with a scalpel and identified 

microscopically, the only way accurately to diagnose scabies. Sixlegged 

larvae hatch from the eggs and mature on the skin surface 

to the eight-legged form in about 2 weeks. 

Pathogenesis 

Usually it takes 2 to 6 weeks after contracting scabies for symptoms 

to appear. If following a cure scabies is contracted again, 

however, symptoms appear in 1 to 4 days. Thus as with pediculosis 

pubis, the host’s allergic response to the mites and their 

feces is largely responsible for symptoms of the disease. In the 

usual individual with scabies, the number of mites is less than 

100 and the disease goes away by itself in a matter of months. 

In individuals with AIDS and other immunodeficiencies, however, 

millions of the organisms can be present, spreading over 

much of the body and causing a severe rash with thickening and 

peeling of the skin. Opportunistic bacterial pathogens, introduced 

by scratching, are a serious threat to patients with scabies. 

Epidemiology 

Scabies is usually transmitted by close contact with a person who 

has the disease. In adults, transmission often occurs as a result 

of sexual contact; in children, this is rarely so. Adults with the 

disease should be checked for other possible STDs. Nonsexual 

transmission of scabies occurs readily among children, especially 

those less than five years old.

(a) 

TABLE 25.17 Pediculosis Pubis and Scabies 

0.25 mm 


Scabies is prevented by avoiding contact with those who have 

scabies, their bedding, and their clothing. Sexual and other close 

contacts within the month prior to diagnosis need to be examined 

and treated. Bedding and clothing is washed in hot water, 

dry cleaned, or removed from human contact for 3 days. Insecticides 

suitable for use on the skin are effective against S. scabiei, 

as is a sulfur-containing ointment, and ivermectin, an antiparasitic 

medication that paralyzes the organism. These medications 

must be used exactly as prescribed in order to minimize the risk 

of serious side effects. 

The main characteristics of pediculosis pubis and scabies 

are presented in table 25.17. 

MICROCHECK 

The main importance of pubic lice and scabies is to 

raise the possibility of other sexually transmitted 

diseases. The possibility of secondary infection 

Skin surface 

Epidermis 

Figure 25.23 Seven-Year Itch Mite, Sarcoptes scabiei, Commonly Sexually Transmitted 

(a) Microscopic appearance in scrapings of a scabies burrow. (b)The female mite burrows into the outer layer of skin to 

lay her eggs, causing an intensely itchy rash. Sarcoptes scabiei is not known to be a vector of infectious diseases. 

introduced by scratching the involved area is another 

important consideration. 

■ Explain why it takes 2 to 6 weeks for symptoms to 

appear after first contracting scabies, but only a few 

days after contracting the disease again. 

■ What further testing should be considered once pubic 

lice or scabies is diagnosed in an adult? 

■ What evidence can you present that the immune system 

plays a role in controlling scabies? 

■ Why will condoms not prevent the spread of crab lice? 

F U T U R E C H A L L E N G E S 

Getting Control of Sexually Transmitted Diseases 

Few problems are as complicated as getting control of sexually 

transmitted diseases because of the psychological, 

cultural, religious, and economic factors that are 

involved, which vary from one population to another. 

Pediculosis Pubis (“Crabs”) Scabies (“Seven-Year Itch”) 

Symptoms Intense itching, visible lice and eggs Intense itching 

Incubation period Usually about 1 week Usually about 1 month (2 to 6 weeks) 

Causative agent Phthirus pubis, a louse Sarcoptes scabiei, a mite 

Pathogenesis Skin penetration by a blood-sucking insect; Burrowing into the epidermis by an arachnid; 

allergic reaction to it allergic reaction to it 

Epidemiology Transmitted by sexual intercourse and other 

close physical contact; sometimes by fomites 

Same as for pediculosis pubis 

Prevention and treatment Avoidance of persons with the disease, their Same as for pediculosis pubis. 

clothing and bedding; treatment of contacts; Additional treatment options include ivermectin 

insecticide medications applied to the skin and a sulfur ointment 

(b)

Multicellular Parasites 

A number of species of roundworms can infect the blood and 

lymphatic vessels of humans. Mostly, they are contracted in 

tropical countries, transmitted by biting insects. The adult 

female worms live in the lymphatic vessels and deposit their offspring, 

tiny microfilaria, directly into the skin, lymphatics, or 

bloodstream, where they can be identified in biopsies or blood 

smears to make the diagnosis. The disorder these roundworms 

produce is termed filariasis, and it may be asymptomatic or 

produce dramatic symptoms. Two examples are elephantiasis, 

marked swelling of a body part due to lymphatic obstruction, 

caused by Wuchereria bancrofti and Brugia sp., and river blindness, 

caused by Onchocerca volvulus. 

Completely different kinds of parasitic worms live in the 

blood vessels that carry venous blood from the intestines to the 

liver, or in the veins of the bladder. They are responsible for the 

disease schistosomiasis. 

Schistosomiasis 

Schistosomiasis, endemic in countries of Africa, Asia, the 

Caribbean, and South America, involves more than 200 million 

people worldwide and causes over 500,000 deaths annually. 

About 400,000 individuals with the disease now live in the 

United States, having emigrated from places where schistosomiasis 

is endemic. Schistosomiasis is caused not by roundworms, 

but by flukes. Flukes are short, bilaterally symmetrical worms 

that usually attach by one or more sucking discs. 

Symptoms 

Itching skin may occur at the time of exposure to fresh water containing 

the worm larvae. The itching subsides, and weeks later, a

generalized acute illness occurs, with fever, hives, cough, abdominal, 

joint and muscle pain, and diarrhea. Some people die during 

this stage, but usually the symptoms subside and infected individuals 

are free of symptoms for a number of years. Then a chronic, 

slowly progressing illness appears, with weakness, accumulation of 

fluid in the abdominal cavity, and sometimes vomiting blood. 


Most cases of schistosomiasis are caused by three species of 

flukes in the genus Schistosoma. Other genera of flukes that 

infect humans are hermaphroditic, meaning that each worm has 

both male and female reproductive organs, but Schistosoma 

species have male and female worms. Schisto-soma means “splitbody,” 

referring to a deep groove running along the male’s 

body in which he clasps his female partner. Schistosoma mansoni, 

the only species established in the Americas, and the most common 

cause of schistosomiasis worldwide, is 10 to 20 mm long 

Figure 28.13 Life Cycle of 

Schistosoma mansoni (a) Eggs (ova) 

from feces reach fresh water. (b) First larval 

form (miracidium) hatches from ovum and 

(c) infects snail. (d) Asexual reproduction in 

body of snail and transformation into 

another larval form (cercaria). (e) Cercariae 

break out of snail and penetrate human 

skin.Tail left behind, larva enters capillary, is 

carried by bloodstream to intestinal veins, 

and develops into mature fluke. (f) Ova 

deposited, break into the intestine or are 

swept to the liver by the blood. 

(f) Ova that fail 

to rupture 

into the 

intestine 

are swept 

to the 

liver, 

cause 

scarring 

Blood vessel 

(e) Larvae from 

skin capillaries 

mature in 

intestinal vein and 

produce ova 

Eggs 

and lives in the small veins of the human intestine. Because it 

lives in blood vessels, it is called a blood fluke. The life cycle of 

S. mansoni is shown in figure 28.13. The slender female worm 

deposits ova that rupture through the tiny intestinal veins and 

wall of the intestine to enter the lumen, ultimately to be eliminated 

with the feces. The ova hatch in fresh water, releasing ciliated 

larvae called miracidia which can live up to 6 hours. When 

a miracidium encounters a certain species of snail, it penetrates 

the snail’s body and multiplies asexually. Over about 6 weeks, 

thousands of elongated fork-tailed larvae called cercariae 

develop and leave their snail intermediate host to enter the 

water. When they encounter a human being wading in the 

water, they burrow through his or her skin, leaving their tails 

behind them. These larvae proceed to enter the circulatory system, 

are carried by the bloodstream through the heart and 

lungs, and eventually reach veins of the intestine, where they 

mature, mate, and begin ova production. 

(a) Ovum 

(d) Cercaria 

from snail 

penetrates 

skin 

(c) Snail 

(b) Miracidium

Pathogenesis 

Many of the cercariae die upon entering the skin, causing an 

inflammatory and immune response that becomes more and 

more intense with each exposure to the parasites. Each skin penetration 

by the schistosomes causes an itchy skin rash that gradually 

subsides. Weeks later, when the mature worms begin to 

deposit their ova, a generalized illness occurs, probably due to 

circulating schistosomal antigens reacting with antibodies. 

Although some people die from the reaction, usually it subsides 

within several months. Unfortunately, the worms continue to 

deposit hundreds of ova per day over a lifetime that can exceed 

a quarter century. Perpetuation of the species depends on ova 

staying close to the intestine, where an intense inflammatory 

response liquefies the tissue and allows them to rupture into the 

intestinal lumen. The spine on the ovum probably helps hold 

them in place, but many are swept away by the bloodstream to 

the liver. The same inflammatory response to the ova occurs in 

the liver, causing gradual destruction of liver cells and their 

replacement with scar tissue. The end result is malnutrition and 

a buildup of pressure in the abdominal veins and connecting 

veins in the esophagus. Fluid accumulates in the abdominal cavity 

(figure 28.14), and hemorrhage occurs if the engorged 

esophageal veins rupture. 

Swimmer’s itch is a schistosomal disease that is common 

across the United States. The disease is characterized by an itchy 

rash caused by cercariae of schistosomes of wild birds and other 

animals. The cercariae penetrate the skin of swimmers and then 

die. These schistosomes are otherwise harmless, since they are 

unable to complete their life cycle in humans. 

Epidemiology 

Transmission occurs in northern and eastern South America, 

parts of the Caribbean, much of the African continent, parts of 

the Middle and Far East, Philippines, Southeast Asia, China, 

and Japan. The distribution depends on abundant fresh water, 

Figure 28.14 Child with Schistosomiasis Notice the distended abdomen, 

probably due to inflammatory enlargement of the liver and spleen. Later in the 

development of the disease the abdomen will fill with fluid, the result of scarring that 

obstructs the flow of blood throught the liver. 

extensive contamination of water by human feces, and a suitable 

snail intermediate host. Farmers and people who fish or 

wade in fresh water are at high risk for the disease. Large irrigation 

projects to enhance agriculture have extended the 

range of the disease. Schistosomiasis cannot be contracted in 

continental United States because there is no appropriate snail 

intermediate host. 


Preventing schistosomiasis depends on avoiding skin exposure 

to fresh water contaminated with the miracidia, preventing 

untreated human feces from entering fresh water, treatment of 

cases of the disease, and measures, both chemical and biological, 

to control the snail intermediate hosts. Promising synthetic 

peptide and DNA vaccines are under development. Effective 

medications are available that kill the parasites in disease victims. 

The main features of schistosomiasis are presented in 

table 28.9. 

TABLE 28.9 Schistosomiasis 

Symptoms Acutely: fever, hives, cough, abdominal, joint, and 

muscle pain.These symptoms subside and after years 

are replaced by generalized weakness and swelling 

of the abdomen 

Incubation period Usually 2 to 6 weeks 

Causative agents Most cases are caused by three species of Schistosoma, 

the most prevalent being S. mansoni.The male 

schistosome has a longitudinal slit in its body in which 

he clasps the female. Perpetuation of the life cycle 

depends on rupture of their eggs into the lumen of the 

intestine from the tiny intestinal veins where the 

adults live.The eggs hatch in fresh water, releasing a 

free-swimming miracidium that penetrates into the 

flesh of the snail intermediate host. In the snail, the 

organism multiplies and differentiates into forked-tail 

cercariae that leave the snail and can infect humans 

by penetrating their skin 

Pathogenesis Inflammatory and immune response to skin 

penetration by the cercariae causes intense itching. 

Allergic reaction to released schistosomal antigens as 

the worms mature and begin depositing ova causes 

the acute illness. Chronic malnutrition and swelling of 

the abdomen result from liver damage caused by 

ova that fail to penetrate into the intestinal lumen, 

and are carried to the liver by the bloodstream 

Epidemiology Distribution of the disease is favored by large 

bodies of shallow fresh water, extensive contamination 

of the water with human feces, and a suitable snail 

intermediate host. Farmers, people who fish, and 

children who wade in fresh water at high risk 

Prevention and Prevention depends on proper disposal of human 

treatment feces, identifying and treating infected patients, and 

measures to control the snail intermediate host. 

Praziquantel is effective treatment

MICROCHECK 

The health and economic burden of multicellular 

parasite infections challenges that of malaria. The 

complex life cycle of Schistosoma mansoni provides a 

number of possibilities for attacking schistosomiasis. 

■ Trace the life cycle of Schistosoma mansoni. 

■ How long would it take S. mansoni to become extinct 

if snail-eating fish eliminated the intermediate host?

Multicellular Parasites

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