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Intestinal Nematodes 149

or more, but it is known that a considerable number may through the hostile environment of the stomach and to
die within a year or two after the initial infection of the enter the small intestine, a process that takes about a
host. But there is a difference in the life span of the two week to complete.
hookworms. A. duodenale adults live much shorter lives Upon entering the digestive system the parasites
than N. americanus, as they usually survive for an av- complete their life cycle in the intestine and during this
erage of only about 6 months. But the infection can be time the larvae mature into adult worms. A third hook-
prolonged because there are always inactive or dormant worm of dogs, Ancylostoma caninum, is currently be-
larvae that can be drawn sequentially from areas of tis- coming more familiar as a parasite of humans. It has been
sue storage over a substantial number of years in order found in a number of countries in both tropical and tem-
to replace the dead adult worms. This will generate a perate areas that include North America. It was always
rise in the population of the organisms on a seasonally considered a host-specific canine parasite that very rarely
fluctuating basis. A fluctuation in transmission during invaded humans and, when it did, caused essentially
certain weather conditions such as warm and moist soil asymptomatic infections. It is difficult to rid the body of
conditions may contribute to the prevalence and speed A. caninum once the adults are attached to the intestinal
of transmission of the infection. mucosa (Figure 6-10).
The mature hookworms mate inside the host and In contrast to most intestinal parasites, where the
the females will often lay up to 30,000 eggs per day, which heaviest parasitic loads tend to occur in children, hook-
translates to several million eggs per female during a life- worm prevalence and intensity can be even higher among
time. Most of these eggs tend to pass out through the feces adult males. The explanation for this is that hookworm
of the host. Because it takes 5 to 7 weeks for adult worms infections tend to be occupational so that plantation
to reach maturity and then to mate and produce eggs, workers, coalminers, and certain other groups maintain
there may be few or no eggs detected in the early stages of a high prevalence of infection among themselves by con-
very heavy infection. Therefore, several fecal specimens taminating their work environment. However, in most
may be necessary to detect either eggs or worms follow- endemic areas adult women are more severely affected by
ing the appearance of acute symptoms. Diagnosis may anemia than men, mainly because they may have much
be difficult unless care and diligence in collecting and higher physiological needs for iron because of menstrua-
processing specimens is practiced. Remember, the pos- tion and repeated pregnancies. But an additional factor
sibility of recovering eggs and larvae or mature worms of that contributes to poor health in women is that they cus-
many parasites from known infections does not approach tomarily have access to much poorer food than the men
a 100 percent rate even under the best of circumstances who may hunt and work in the fields providing greater
and when an experienced and diligent laboratory practi- access to food. The larvae of hookworms and those of
tioner is examining the specimens. S. stercoralis are quite similar and may easily be confused

Disease Transmission

Infection by hookworm of the host is by the larvae and
not the eggs from the adult organism. Although A. duode-
nale can be ingested and cause infection, the most com-
mon method of infection is through skin penetration of
the larvae. Walking barefoot through contaminated water
and soil containing fecal matter almost always results in Source: Centers for Disease Control and Prevention (CDC)
contraction of the hookworm larvae. The scarcely visible
larvae are able to penetrate the skin of those who walk
barefoot in contaminated areas and once inside the body
the microfilariae migrate through the vascular system
to the lungs. From there they are coughed up into the
trachea before being swallowed. As the organisms pass FIGURE 6-10 The hookworms, Ancylostoma
down the esophagus they are safely equipped to pass caninum, attached to the intestinal mucosa