Selected US Ticks and Tick-borne Pathogens - tabpi

The epidemiology of tick-borne disease is 

changing: ticks are moving into new areas and 

are being linked to new diseases. VETERINARY 

FORUM talks with leading experts to update you 

on the problem. 

The tick population in the United States is exploding. Driven by socioeconomic 

factors, changing climate and an increase in the number and 

distribution of wildlife hosts, ticks have successfully spread across the 

country, changing the landscape of zoonotic diseases. 

Two ticks are particularly troublesome in the United States: Amblyomma americanum 

(the Lone Star tick) and Ixodes scapularis (the black-legged or deer tick). 

They have spread across nearly half of the country, overlapping in many states, 

and have been implicated in transmitting nearly a dozen infectious disease 

pathogens to both humans and animals. 

“Based on clinical evidence and the literature, there appears that at least two major 

tick species have expanded their ranges and even their density within those 

ranges over the past 15 to 20 years,” explains Michael W. Dryden, DVM, PhD, who is 

professor of veterinary parasitology at the Kansas State University College of Veterinary 

Medicine. “Those tick species are Ixodes scapularis in the eastern half of the United States, 

which is our major Lyme disease and human anaplasmosis vector, and Amblyomma americanum, 

which is a big ehrlichiosis vector.” 

B y M a r i e R o s e n t h a l , M S • E x e c u t i v e E d i t o r 

May 2007 | Veterinary Forum 45

Massive 

increase in the 

deer population 

over the past 

100 years 

Suburbanization 

bringing people, 

wildlife and ticks 

together 

I. scapularis has been reported from 

Minnesota to Miami and from Bangor, 

Maine, to Corpus Christi, Texas, and A. 

americanum has been found from Corpus 

Christi to southern Michigan and from 

southern Florida to Maine, according to 

Dryden. 

Katherine M. Kocan, PhD, agrees, 

adding that the rise in the role of A. 

americanum has been one of the most 

fascinating aspects of her field of study. 

“Historically, the Lone Star tick, or A. 

americanum, was considered insignificant 

as a vector of disease, but now it is considered 

the number one vector. It used 

to be if someone got a Lone Star tick on 

them, I’d say don’t worry, you don’t 

have any risk of disease,” says Kocan, 

who is regents professor and Walter R. 

Sitlington endowed chair of food animal 

research in the department of pathobiology 

at the Center for Veterinary 

Health Sciences at Oklahoma State University. 

”But in the past 15 years or so, 

the Lone Star tick has become our number 

one tick vector in the United States. 

I think it is one of the most interesting 

and dramatic things going on in the 

study of tick-borne diseases in the 

United States.” 

© Wolfgang Rücki, Svetlana Larina, Michael West, Mark/Shutterstock 

Source: CDC 

Dispersal of 

juvenile ticks by 

migratory birds 

VETERNARY FORUM 

Increase in 

outdoor 

recreational 

activities 

And then there were? 

Ticks are the most common transmitters 

of vector-borne diseases in the 

United States. They wait in vegetation for 

a host and then attach themselves using 

a hypostome to pierce the skin. During a 

blood meal, they transmit a variety of 

pathogens, including bacteria, spirochetes 

and rickettsiae. 

Most of the pathogens linked to A. 

americanum cause emerging diseases, 

Kocan explains, and the number associated 

with the tick has risen from zero to 

seven: Ehrlichia chaffeensis, which causes 

human monocytic ehrlichiosis; E. ewingii, 

which causes canine and human granulocytic 

ehrlichiosis; Francisella tularensis, 

the cause of tularemia; Rickettsia amblyommii, 

the cause of a rickettsial spotted 

fever that has not yet been named; non- 

46 Veterinary Forum | May 2007

pathogenic Theileria cervi in deer; and Borrelia 

lonestari, which has been associated 

with southern tick–associated rash illness 

(STARI), a newly described disease in humans. 

Another newly described disease 

that is associated with the Lone Star tick 

but does not yet have a name has been 

found in a goat in the United 

States and is caused by 

Ehrlichia ruminantium, which 

appears to be closely related 

to the pathogen 

that causes the African 

cattle disease known 

as heartwater. 

There are two other major tick vectors 

in the United States that are especially 

important for animal diseases: the brown 

dog tick and the American dog tick. Rhipicephalus 

sanguineus (the brown dog tick) 

transmits pathogens that cause the following 

canine diseases: Anaplasma platus, 

which causes a milder form of 

ehrlichiosis; Babesia canis, 

which causes babesiosis; 

and Ehrlichia canis, which 

causes canine monocytic 

ehrlichiosis. Outside 

the United 

States, the tick has 

Just because you did not diagnose this disease 

in your practice yesterday doesn’t mean you 

won’t diagnose it tomorrow. 

— Michael W. Dryden 

“In Africa, Ehrlichia ruminantium causes 

the cattle disease heartwater. It was reported 

in the Caribbean islands and 

Puerto Rico some years ago, and there 

is a big push to eradicate the tick from 

the Caribbean and Puerto Rico to reduce 

the risk of introducing heartwater 

into the United States. Heartwater is a 

major disease in cattle in Africa,” Kocan 

explains. “Just recently, a closely related 

Erhlichia organism was described 

in a goat in the Northeast.” 

I. scapularis transmits Anaplasma phagocytophilum, 

which causes human granulocytic 

anaplasmosis; Babesia microti, the 

cause of rodent babesiosis as well as human 

infection; B. odocoilei, which appears 

to be host-specific for deer (cervid 

babesiosis) and Borrelia burgdorferi, the 

cause of Lyme disease. These diseases 

occur in a variety of animals and humans 

(see charts on pages 50 and 52). 

been implicated in the transmission of 

Hepatozoon canis, which causes canine hepatozoonosis, 

and B. gibsoni, which causes 

babesiosis. The brown dog tick has also 

been implicated in the spread of Rickettsia 

rickettsii, the cause of Rocky Mountain 

spotted fever (RMSF) in both humans 

and dogs. 

Another vector of RMSF is D. variabilis 

or the American dog tick, which also 

transmits Anaplasma marginale, the cause 

of anaplasmosis in cattle; Cytauxzoon felis, 

the cause of cytauxzoonosis in domestic 

and wild cats; Ehrlichia canis, the cause of 

canine ehrlichiosis; and E. chaffeensis, 

which causes ehrlichiosis in humans and 

dogs. 

Although it may seem as if ticks are 

being infected with more organisms, 

which is why they are transmitting more 

diseases, both Dryden and Kocan say 

that probably isn’t the case. Instead, re- 


searchers and epidemiologists have the technical means to 

find pathogens they couldn’t find before. “Ticks are not changing 

their ability to transmit disease,” explains Dryden, “but we 

are recognizing more diseases because we are looking for 

them.” 

Adds Kocan: “Molecular technologies are fabulous. We 

can do things that we could never do before. For instance, 

you can collect ticks and, using polymerase chain reaction 

[PCR], you can test for half a dozen parasites at one time. So 

ticks may have carried these pathogens all along, but we 

just haven’t recognized them.” 

Because of tick spread, increased tick density and recognition 

of more diseases from tick vectors, veterinarians 

might find it useful to include vector-borne diseases that are 

not common in their area as part of the diagnostic differentials 

if the animal’s clinical signs and presentation match the 

disease definition. Dryden adds: “What I tell practitioners 

when I get in front of an audience is very simple: ‘Whatever 

you knew about ticks 5 years ago, it is different today and 

whatever you know about ticks today will be different 5 

years from now.’ 

“And that is because these ticks continue to move and 

expand and increase in importance. Just because you did 

not diagnose this disease in your practice yesterday doesn’t 

mean you won’t diagnose it tomorrow. The situation is not 

static. This is a dramatic, dynamic situation we have today in 

the eastern half of the United States.” 

Hard ticks begin their life cycle as larvae, which hatch 

from eggs and immediately begin seeking hosts, often rodents. 

After the larvae successfully feed, they fall off the 

host and live in the soil and decaying vegetation where they 

then molt into nymphs, which seek their blood meal from a 

small vertebrate. If a nymph fails to find a blood meal, it 

dies. If it succeeds, it falls off the host and lives in the soil to 

molt into an adult. The adult seeks a larger host for its blood 

meal, and the preferred host for I. scapularis and A. americanum 

is the white-tailed deer, where the adult feeds and mates 

over the winter. The adult female tick is capable of laying 

several thousand eggs before she dies, according to Kocan. 

“The thing that drives the tick populations is deer. If we 

didn’t have deer, we wouldn’t have half the tick populations 

that we have,” Kocan says. 

When the United States was young, white-tailed deer 

flourished in this country, but they were over-hunted, explains 

Dryden. Through the 1800s, they were a major source 

of food in this country — even Army units supplied themselves 

by hunting deer. “The estimates indicate there were 

only between 200,000 and 300,000 white-tailed deer left in 

the early 1900s. Then it became illegal to hunt deer in many 

states,” Dryden says, as many state gaming departments 

tried to repopulate the species. 

(text continues on page 54) 


Tick Species 

Selected US Ticks and Tick-borne Pathogens 

Pathogen(s) and Host(s) 

Scientific Name Common Name Scientific Name Disease/ Main Host(s) 

Common Name 

Amblyomma americanum Lone Star tick Borrelia lonestari Southern tick-associated Humans; deer 

rash illness (STARI) 

Ehrlichia chaffeensis Human monocytic Humans, deer 

ehrlichiosis (HME) and dogs 

Ehrlichia ewingii Canine granulocytic Dogs, deer and 

ehrlichiosis in the humans 

United States 

Newly described; closely No disease name Goats 

related to Ehrlichia 

ruminantium, which causes 

the ruminant disease 

heartwater primarily 

in Africa. 

Francisella tularensis Tularemia Humans; rabbits, 

cats, other 

mammals 

Rickettsia amblyommii Spotted fever rickettsiae Humans and 

group; no disease name other mammals 

Theileria cervi Nonpathogenic White-tailed deer 

Amblyomma maculatum Gulf Coast tick Ehrlichia ruminantium Experimental vector Cattle 

of heartwater disease 

in cattle 

Hepatozoon americanum Canine hepatozoonosis Dogs and coyotes 

in the United States 

Rickettsia parkeri No disease name Humans (?) 

Boophilus annulatus Cattle fever tick Anaplasma marginale Bovine anaplasmosis Cattle 

Babesia bigemina Bovine babesiosis or Cattle 

piroplasmosis 

Boophilus microplus Tropical cattle tick Anaplasma marginale Bovine anaplasmosis Cattle 

Babesia bigemina Bovine babesiosis or Cattle 

piroplasmosis 

Dermacentor albipictus Winter tick Anaplasma marginale Bovine anaplasmosis Cattle 

Dermacentor andersoni Rocky Mountain Anaplasma marginale Bovine anaplasmosis Cattle 

wood tick 

Rickettsia rickettsii Rocky Mountain Humans and dogs; 

spotted fever (RMSF) rodent reservoir 

hosts 

(table continues on page 52) 

Credit: Katherine Kocan, Susan Little and Mason Reichard, Center for Veterinary Health Sciences, Oklahoma State University 


Selected US Ticks and Tick-borne Pathogens (continued) 

Tick Species 

Pathogen(s) and Host(s) 

Scientific Name Common Name Scientific Name Disease/ Main Host(s) 

Common Name 

Dermacentor nitens Tropical horse tick Babesia equi Equine piroplasmosis Horses 

Dermacentor variabilis American dog tick Anaplasma marginale Bovine anaplasmosis Cattle 

Cytauxzoon felis Feline cytauxzoonosis Wild and domestic 

cats 

Ehrlichia canis Canine monocytic Dogs 

ehrlichiosis (CME) 

Ehrlichia chaffeensis Human monocytic Humans, dogs, 

ehrlichiosis (HME) 

and deer 

Rickettsia rickettsii Rocky Mountain Humans and dogs; 


hosts 

Francisella tularensis Tularemia Humans; rabbits, 

cats, other 

mammals 

Ixodes pacificus Western Borrelia Lyme disease Humans, dogs, 

Black-legged tick burgdorferi reptiles, rodents, 

birds 

Ixodes scapularis Black-legged tick Anaplasma Human granulocytic Humans and 

phagocytophilum anaplasmosis (HGA) many reservoir 

hosts 

Babesia microti Rodent babesiosis Rodents and 

Humans 

Babesia odocoilei Cervid babesiossis Deer 

Borrelia burgdorferi Lyme disease Humans, dogs 

and cattle; rodent 

reservoir hosts 

Rhipicephalus Brown dog tick Anaplasma platys* Milder form of Dogs 

sanguineus 

ehrlichiosis 

*Not confirmed 

**Not confirmed in the United States 

For references, please contact Marie Rosenthal at mrosenthal@vetlearn.com. 

Credit: Katherine Kocan, Susan Little and Mason Reichard, Center for Veterinary Health Sciences, Oklahoma State University 

Babesia canis Canine babesiosis Dogs 

Babesia gibsoni** Canine babesiosis Dogs 

Ehrlichia canis Canine monocytic Dogs 

ehrlichiosis (CME) 

Hepatozoon canis** Canine hepatozoonosis Dogs 

outside of the United States 

Rickettsia rickettsii Rocky Mountain Dogs and humans; 


hosts 


It was one of the most successful 

conservation efforts in history, and today, 

there are more than 30 million deer 

in this country. Humans also have eliminated 

the natural predators of deer, so 

they have flourished. 

“By anyone’s estimates, there are almost 

100 times the deer today than 

there was 100 years ago,” Dryden 

says. “I don’t care where you 

live, whether it is Kansas 

or New Jersey, when you 

were a kid, to see a 

deer was a phenomenon. 

Now they are 

everywhere, and the 

white-tailed deer is 

Other driving forces 

The other major driving forces behind 

the emergence of tick-borne diseases 

are the suburbanization of 

America; climate change; an increased 

incidence in outdoor recreational activities 

and migrating birds, according to 

the Centers for Disease Control and 

Prevention (CDC). 

“You have changing trends in tick 

transmission and diseases because the 

environment and people are changing,” 

Kocan says. As people move from urban 

areas to suburbs and rural neighborhoods, 

they drive animals and their attached 

ticks to outlying areas moving 

them across the map. 

In addition, the trend today is not to 

cut forests to build tract houses but to integrate 

the homes with the natural surroundings. 

Although this is aesthetically 

pleasing, it brings people, wildlife and 

ticks together. “Significant reforestation 

has occurred in the eastern United 

States over the past 100 years 

from massive clear cutting 

that occurred in the 

1700s and 1800s,” Dryden 

says. “We just are 

not doing that to the 

extent that we once 

did.” 

You have changing trends in tick transmission 

and diseases because the environment and 

people are changing. 

— Katherine M. Kocan 

the primary host for adult I. scapularis 

and a major host for all stages of the 

Lone Star tick. Neither of those ticks can 

be maintained in high numbers in an 

ecosystem without white-tailed deer.” 

In addition, people are spending 

more recreational time outdoors, often 

with their dogs, providing another opportunity 

for ticks to attach. “Dogs are the 

ones that go out and run around and 

bring ticks back into the human environment,” 

Kocan says. “I don’t think people 

understand that dogs can be the big connector 

between people and ticks.” 

Migratory birds are also being recognized 

as both a host for various zoonotic 

pathogens and a carrier of ticks, which 

can drop off and lay eggs in new territory. 

According to a recent article in 

Emerging Infectious Diseases, North American 

birds host vector-borne pathogens, 

such as Anaplasma spp and B. burgdorferi, 

that infect Ixodes larvae. Although rodent 

species, such as the white-footed 

mouse, are the most common hosts of 

immature Ixodes ticks, alternative hosts, 


such as migratory birds, play a larger 

role than previously thought and may 

be hosts of epidemiologically important 

vector ticks. 

Finally, climate change is also influencing 

tick populations. “We are not 

experiencing as hard and as cold winters 

that we used to historically for 

whatever reason,” explains Dryden. 

“We need to protect ourselves and our 

dogs and cats. With the winters not being 

as hard as they used to be in many 

areas, we’ve gone to year-round tick 

control for our pets.” 

Although the winters didn’t necessarily 

kill ticks, they stopped attaching 

during the really cold months. Now, as 

soon as there is a sunny day, they are 

out again. 

“We have ticks all year-round,” explains 

Kocan from Oklahoma, “and each 

tick has its own biology and its own seasons. 

So, this time of year, we have the 

Lone Star tick, and by the end of the 

summer, it is I. scapularis, and then later, 

the Dermacentor albipictus comes out. 

“People think they only need tick 

control in the summer, but we have 

something out there all year around. 

Oklahoma has really become a hotspot 

for tick-borne diseases.” 

Although tick-borne diseases and 

ticks are a major problem in the Northeast, 

South and Midwest, it doesn’t 

mean the West is without a tick problem, 

according to Curtis Fritz, DVM, 

MPVM, PhD, DACVPM (epidemiology) 

and Anne Kjemtrup, DVM, MPVM, PhD, 

both of the vector-borne disease section 

of the California Department of 

Health Services. 

“There are changes in tick densities 

and numbers yearly, but we don’t see an 

overall increase or change in location as 

they are seeing in other parts of the 

country,” Kjemtrup says. People moving 

into rural or coastal areas encounter more 

wildlife and ticks, but most transmission 

occurs through recreational activities. 

“We developed differently out here,” 

she says. “People live in high-density 

© iStockImages 

areas, where they don’t end up with 

large backyards. Tick-borne diseases in 

California for the most part are a result 

of recreational activities, where people 

and animals go out into wooded areas.” 

There is a seasonality to tick infestations 

in the West, she adds, and different 

ticks spread different diseases. For 

instance, Ixodes pacificus, instead of I. 

scapularis, is the tick that transmits Lyme 

disease in California. That tick primarily 

feeds on lizards, instead of deer, and 

lizards have a natural substance in their 

bloodstreamthat kills Borrelia organisms. 

“As a result, our adult tick populations 

are infected at a much lower prevalence 

with Borrelia than is seen in the eastern 

United States,” Kjemtrup says. 

Still there are some novel 

pathogens being seen, Fritz adds. For 

Black-legged Tick: Life Cycle 

Eggs 

Nymph 

Nymph 

dormant 

Adults lay 

eggs 

SPRING 

WINTER 

Larvae 

Adults 

instance, Arizona recently reported an 

outbreak of RMSF, which doesn’t normally 

occur in that state, and it probably 

resulted from a different tick 

vector, the brown dog tick. 

Kjemtrup adds that canine babesiosis 

is a problem in California. Three different 

species of canine Babesia have 

been documented in Californian dogs: 

Babesia canis, B. gibsoni, and the recently 

described B. conradae. Epidemiologists 

SUMMER 

FALL 

Nymph 

dormant 

The life cycle of the black-legged tick lasts about 2 years. 

Adults lay about 5,000 eggs at a time. 

Source: CDC 

are reporting B. gibsoni in dogs used in 

dog fighting, and it is related not only to 

tick transmission, but also to the exchange 

of body fluids during the fights. 

“This is something that has been reported 

in the literature recently,” 

Kjemtrup says, adding that “scary dog 

fighting websites have started posting 

treatment recommendations” because 

the owners are concerned about this 

disease. 

There are also more reports of Lyme 

disease in canines, she added. In fact, 

IDEXX Laboratories recently reported 

that Lyme disease has been found in 

canines in 48 states. However, Fritz says 

this doesn’t mean that dogs have a current 

infection, but that some dogs were 

exposed to B. burgdorferi at some point 

and developed antibodies. Nevertheless, 

these reports suggest 

that exposure of 

dogs to B. burgdorferi 

may be more widespread 

than previously 

supposed. 

“The tick populations 

have increased dramatically 

and moved into 

new areas. If there is one 

tick out there, it’s not a 

big deal. If there are 

1,000 ticks out there, it is 

a big deal,” Kocan explains. 

“The average tick 

burden of a moose in 

Canada is about 37,000 

ticks.” 

Most readers think 

about ticks in reference 

to dogs and cats. The average pet 

owner might find one or two ticks and 

pick them off, but ticks are a larger 

problem in the cattle industry, where a 

steer can harbor more than 3,000 ticks 

at once. As the ticks feed, they can lead 

to weight reduction and secondary infections, 

she says. Kocan and her team 

are working on a tick vaccine for A. americanum, 

she adds. There is a tick vaccine 

for the cattle tick, Boophilus, that is being 


used in Cuba, which José de la Fuenté 

developed. He is now at Oklahoma 

State University working with Kocan. 

“We are doing a lot of work to find tickprotective 

antigens and develop tick 

vaccines,” she says. 

Ticks have few natural predators. 

Fire ants destroy them but the cure is 

worse than the problem, and 

Guinea hens eat them but 

they are not a preferred 

food for most other 

birds. So, the best defense 

is a good offense, 

all the experts 

say. Recommend yearround 

tick protection 

where the rodents might move in. You 

want to keep your house from being 

attractive to a tick host, be that a deer 

or a rodent. One of the worst things 

that a person can do is put a deer 

feeding station in their backyard, but 

people do that all the time. There are 

enough deer out there, so we don’t 

need to be doing that. 

“I’m not advocating getting 

rid of the deer, but we 

need to recognize that 

there has been a consequence 

to moving 

so close to nature. 

“If owners understand 

why this has 

Recommend year-round tick protection for 

animals and humans, and advise owners to do 

tick checks after walking in wooded areas. 

for animals and humans, and advise owners 

to do tick checks before returning 

home if they walk or play in wooded areas. 

“If you don’t keep up with that, an 

engorged female can drop off in your 

yard and lay 5,000 eggs. You’ll have a big 

problem in a short period. Tick control is 

a major consideration,” Kocan says. 

In addition, tell owners to make sure 

their home is not attractive to wildlife 

and ticks. “There are a lot of things you 

can do around your house to make sure 

ticks don’t become established,” Kocan 

says. “You can cut down bushes and 

mow your grass and put a gravel area 

around your yard, because they won’t 

cross that. Use an integrated pest management 

approach.” 

Dryden adds: “Try things that reduce 

animal harborage. You want to 

reduce rodent populations. Don’t 

keep a wood pile next to your house 

happened, then they will be far more 

willing to accept the consequences, 

which is year-round, lifelong tick control 

for their pets.” 

vF 

For more information: 

Comstedt P, Bergström S, Olsen B, et al: Migratory 

passerine birds as reservoirs of Lyme 

borreliosis in Europe. Emerg Infect Dis [serial 

on the Internet]. 2007 Jul [date cited]. Available 

from http://www.cdc.gov/ncidod/EID/ 

vol12no07/06-0127.htm 

Kocan AA, Kocan KM: Tick-transmitted protozoan 

diseases of wildlife in North America. 

Bull Soc Vector Ecol 1991;16(1):94–108. 

Raghavan M, Glickman N, Moore G, et al: Prevalence 

of and risk factors for canine tick infestation 

in the United States, 2002–2004. 

Vector-borne Zoonotic Dis 2007;7:65–75. 

Dryden MW, Payne PA: Biology and control of 

ticks infesting dogs and cats in North America. 

Vet Ther 2004;5:139–152. 

Kjemtrup AM, Wainwright K, Miller M, et al. 

Babesia conradae, sp. nov., a small canine Babesia 

identified in California. Vet Parasitol 2006;138: 

103–111.

Selected US Ticks and Tick-borne Pathogens - tabpi

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