Hookworm infection remains a major health burden in developing
countries.1 As many as 740 million people in the
world are infected with Ancylostoma duodenale, Necator americanus, or both.2 Infections
with A duodenale occur in focal regions of Africa,
Asia, and South America, whereas N americanus is
the predominant hookworm worldwide, with the greatest number of infections
in North and South America, equatorial Africa, much of Southeast
Asia, and some Pacific islands. It is important to recognize that there
is significant overlap in the geographic pattern of infection and
that mixed infections occur frequently. Although common in southern
states in the early part of the 20th century, today there is little
evidence of hookworm transmission in the United States. Other species
that occasionally cause intestinal disease in humans include Ancylostoma
ceylanicum, found in India and Southeast Asia,3 and
the dog hookworm Ancylostoma caninum, which has
been associated with eosinophilic enteritis in Australia.4 Zoonotic
infection with Ancylostoma braziliense causes cutaneous
Populations at highest risk for significant disease sequelae
include preschool and school-age children, adolescents, and women
of childbearing age, although adults who work in agricultural occupations
are also at risk for high-intensity infection. Vulnerable populations such
as young children and pregnant or lactating women are at greater
risk of anemia due to relatively high iron requirements.6 Unlike Ascaris
lumbricoides and Trichuris trichiura,
the intensity of hookworm infection appears to increase with age,
defining the elderly as another high-risk group for severe disease.7
The hookworm life cycle begins with the excretion of fertilized
eggs within the feces of an infected individual. The eggs hatch
to release first-stage (L1) larvae, which undergo two subsequent
molts to the infective third stage (L3). These L3 hookworm larvae
migrate along moisture and temperature gradients within the soil
until they encounter a permissive host. When larvae contact the
skin, they quickly penetrate the epidermis and dermis, ultimately
invading small blood vessels and entering the venous circulation.
They are then carried passively to the heart and lungs, where they
lodge in the pulmonary capillaries and break through to the alveolar
space. Larvae then migrate up the respiratory tree, are swallowed,
and undergo their final developmental molts to the adult stage when
they reach the small intestine. Once in the proximal small bowel,
the adult worms attach to the mucosal surface and begin to feed
(Fig. 329-1 and eFig. 329.1).
Adult hookworms secrete anticoagulants, platelet inhibitors, and
hemoglobin-degrading proteases that facilitate blood feeding and
digestion of red blood cells.8-10 When the plug of
intestinal mucosa at the site of attachment has been digested, the
worm releases and reattaches at a new site. Male and female worms mate,
and the female releases 10,000 to 30,000 eggs per day into the intestinal
lumen. It takes approximately 6 weeks for eggs to appear in the
feces of an infected individual.
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