[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 8 The Nematodes
205
TABLE 8-11
0.5 mm in width (see Fig. 8-17). Males are typically
smaller, ranging in size from 5 to 10 mm by
0.2 to 0.4 mm. The male is equipped with a
prominent, posterior, umbrella-like structure
that aids in copulation, known as a copulatory
bursa (see Fig. 8-15).
Differences in the makeup of the buccal
capsule distinguish adult N. americanus and A.
duodenale worms. The buccal capsule of N.
americanus (see Fig. 8-14) contains a pair of
cutting plates, whereas that of A. duodenale consists
of actual teeth (see Fig. 8-16).
Laboratory Diagnosis
Hookworm Adults: Typical
Characteristics at a Glance
General Characteristics
Color
Grayish-white to pink
Cuticle
Somewhat thick
Anterior end Conspicuous bend, hook
Characteristic Female Adults Male Adults
Size
9-12 mm long,
0.25-0.50 mm
wide
5-10 mm long,
0.2-0.4 mm
wide
Other features
Prominent
posterior
copulatory
bursa
Buccal Capsule Characteristics
Necator Contains pair of cutting plates
Ancylostoma Contains actual teeth
The primary means for laboratory diagnosis of
hookworm is the recovery of the eggs in stool
samples. Larvae may mature and hatch from the
eggs in stool that has been allowed to sit at room
temperature, without fixative added. Differentiation
of these larvae from those of S. stercoralis
is crucial to ensure proper diagnosis and subsequent
treatment (see later). Recovery and examination
of the buccal capsule is necessary to
determine the specific hookworm organism (i.e.,
whether it is A. duodenale or N. americanus).
Life Cycle Notes
Humans contract hookworm when third-stage
filariform larvae penetrate through the skin,
particularly into areas such as unprotected
feet. Once inside the body, the filariform larvae
migrate to the lymphatics and blood system. The
blood carries the larvae to the lungs, where they
penetrate the capillaries and enter the alveoli.
Migration of the larvae continues into the bronchioles,
where they are coughed up to the
pharynx, subsequently swallowed, and deposited
into the intestine.
Maturation of the larvae into adult hookworms
occurs in the intestine. The resulting
adults live and multiply in the small intestine.
Adult females lay 10,000 to 20,000 eggs/day.
Many of the resulting eggs are passed into the
outside environment via the feces. Within 24 to
48 hours and under appropriate conditions—
warm, moist soil—first-stage rhabditiform larvae
emerge from the eggs. The larvae continue to
develop by molting twice. Third-stage infective
filariform larvae result and are ready to begin a
new cycle.
Epidemiology
It is estimated that almost 25% of the world’s
population is infected with hookworm. The frequency
of hookworm infection is high in warm
areas in which the inhabitants practice poor sanitation
practices, especially with regard to proper
fecal treatment and disposal. Mixed infections
with any combination of hookworm, Trichuris,
and Ascaris are possible because all three organisms
require the same soil conditions to remain
viable. The specific geographic locations for
each species of hookworm are presented later.
Persons at risk for contracting hookworm in
these areas are those who walk barefoot in fecescontaminated
soil.
N. americanus is primarily found in North
and South America. However, because of spread
from international travel, this species is also
known to exist in China, India, and Africa.