One of the most common roundworm infections is hookworm. Like ascarids,
hookworms are picked up as a result of unsanitary conditions. Hookworm
eggs are passed in human feces onto the ground where they develop into
infective larvae. When the soil is cool, the worms crawl to the nearest
moist area and extend their bodies into the air. They remain there – waving
their bodies to and fro – until they come into contact with the skin of
a suitable host or until they are driven back down by the heat.
Hookworm is endemic in those tropical and subtropical countries in which
people defecate on the ground and soil moisture is most favorable. Necator
americanus is the prevailing species in the southeastern United States.
The infection is usually contracted by persons walking barefoot over contaminated
soil. In penetrating the skin, the larvae may cause an allergic reaction.
It is from the itchy patch at the site of entry that the early infection
gets its nickname "ground itch." Once larvae have broken through
the skin, they enter the bloodstream and are carried to the lungs. (Unlike
ascarids, however, hookworms do not usually cause pneumonia.) The larvae
migrate from the lungs up the windpipe to be swallowed and carried back
down to the intestine.
Maturation of the worms in the intestine before eggs appear in the stool
is sometimes marked by the onset of diarrhea, particularly in a previously
uninfected person. Other signs and symptoms at this stage include vague
abdominal pain, intestinal cramps, colic, and nausea.
Scientists have learned that persons in good health and on a diet containing
adequate iron can tolerate the presence of these worms in small or moderate
numbers with no ill effects. In chronic infections, if the number of parasites
become great enough, serious anemia can occur as a result of blood loss
from the worms attaching themselves to the intestine and sucking the blood
and tissue juices of the host.
To diagnose hookworm, the doctor will examine stool specimens to look
for and count the number of eggs. If the egg output is large enough –
more than 2,000 eggs per gram of stool – the doctor will assume that the
infection may cause anemia and start treating the patient.
If humans come into contact with larvae of the dog hookworm or the cat
hookworm, or of certain other hookworms that do not infect humans, the
larvae may penetrate the skin. But the larvae are unable to complete their
migratory cycle in humans. Instead, the larvae move just below the skin
producing snake-like markings. This is referred to as a creeping eruption
or cutaneous larva migrans.
Ancylostoma canium, an illness caused by a particular species of
dog hookworm, has been described in Australia. This worm may almost complete
its development in the lower small intestine, but produces a severe inflammatory
reaction in the bowel, causing abdominal pain, diarrhea, and an increase
in peripheral blood eosinophils.
Once diagnosed with hookworm, the patient can be treated effectively with
drugs such as mebendazole. Frequently, the doctor will supplement the
drug therapy with dietary iron.
National Institute of Allergy and Infectious Diseases (NIAID), National
Institutes of Health (NIH) Bethesda, MD 20892, October 1998.