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Epidemiology of Nematodes, Cestodes, and Trematodes 121

NEMATODES LIFE CYCLE of—
Hookworm
The phylum Nematoda is a class of the phylum Nem-
athelminthes and includes the true roundworms as Swallowed Attached to
Pharynx small intestine
well as those called threadworms. Six separate orders of
Nematoda contain species that infect humans and other Trachea
mammals, and it should be noted that a number of spe- Lungs
cies also are capable of infecting plants and other forms Adults in
small intestine
of life, such as mollusks and insects. Of the parasitic Circulation MAN
roundworms that are able to infect humans, the hook-
Penetrates skin
worm is of the most medical importance and also is the
most prevalent form of the roundworm found as a para-
Eggs in feces
site of humans. The hookworm is a parasitic nematode Filariform larva (diagnostic stage)
(infective stage)
worm that lives in the small intestine of its host, which
may be a mammal, including dogs, cats, and humans.
Ancylostoma duodenale is found primarily in northern EXTERNAL ENVIRONMENT
Africa and India, whereas Necator americanus is chiefly Source: Centers for Disease Control and Prevention (CDC)
confined to the so-called New World, which includes Rhabditiform larva
hatches
North and South America.
Until somewhat recently, A. duodenale was found Rhabditiform larva in soil
routinely in southern Europe but efforts toward eradi-
(occasionally in old feces)
cation there have greatly diminished the numbers of
cases of infection by this parasite. By international FIGURE 5-1 Hookworm infection by direct contact
organizational estimates, hookworms are thought with contaminated soil
to infect perhaps as many as 800 million people
worldwide. Species other than A. duodenale and
N. americanus exist but do not affect humans to any ap-
preciable extent. The A. braziliense and A. tubaeforme
species commonly infect cats, but are not known to
infect humans. A. caninum infects mainly dogs but it is
possible for this organism to infect humans, especially
children who have close contact with dogs and their Source: Centers for Disease Control and Prevention (CDC)
excrement (Figure 5-1).
Especially in a number of the developing countries
of the tropics and subtropics, hookworm infections are
considered to be a major cause for both maternal and
child morbidity. Considerable attention has been paid to
endemic regions where hookworm infections are preva-
FIGURE 5-2 Hookworm infection of toes the foot,
lent. Children of these regions are particularly at risk
also known as “ground itch”
because their future development and potential mental
and physical advancement are often stymied. Physical
and intellectual development characterized by cognitive the larvae penetrated the skin (Figure 5-2). Where
retardation, stunting of growth, poor fetal development sanitary facilities are not found in developing countries
leading to long-term health problems in low birth weight of the world, the organism enters the body through the
and premature births are prevalent in mothers who suffer feet, when the victim walks in contaminated soil. Seldom
from hookworm infections. does death ensue even from heavy hookworm infections,
One of the first symptoms to appear following but heavy economic and social costs are borne by the
infection by hookworms is dermatitis of the skin where populations of the areas where hookworm infections