Roundworms and hookworms
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Infection by nematodes (roundworms), Ascaris lumbricoides and
Trichuris trichiura, or hookworms, Necator americanus and Ancylostoma
duodenale, is prevalent in developing countries.
Adult Ascaris are found in the intestine; the female worm produces
up to 2000 eggs per day. The eggs survive in the soil where they
mature into infective forms that can be ingested. After hatching in
the small intestine, the organism undergoes a migratory cycle
through the liver and lungs, where it is coughed up and swallowed
developing into the adult worm in the intestine. Transmission is
aided by poor sanitation or when food crops are manured with
human faeces. In warm, moist climates, the eggs can survive for
many years in the soil. Hookworm eggs hatch into an infective
larva that is able to burrow through intact skin to cause
These parasites cause disease by competing for nutrients; thus, the
severity of symptoms is proportional to the number of worms
present (parasite load). Hookworms take blood, leading to irondeficiency
anaemia that can be severe. Heavily infected children
have poor growth and lowered school performance, which is
attributed to micronutrient deficiency, especially with Trichuris
Infection is usually asymptomatic, but heavy Ascaris infection may
lead to intestinal obstruction and heavy Trichuris infection to a
The diagnosis is made by examining up to three stool samples for
the presence of the characteristic eggs.
Intestinal nematodes are treated with albendazole or mebendazole.
Improved sanitation is required to control the spread of infection.
Humans are the only host of Enterobius vermicularis. Living in the
large intestine, the females migrate to the anus where they lay their
eggs on the perianal skin. Symptoms are few; thread-like worms
may be found in the faeces or patients may complain of perianal
itching, often worse at night. Scratching allows contamination of
the fingers with larvae-containing eggs which, when placed in the
mouth, initiate a new cycle of infection. Occasionally, Enterobius
can be found in the appendix.
Diagnosis is made by sending an adhesive-tape swab to the laboratory
where D-shaped eggs are seen. Treatment is with mebendazole
or piperazine. It is often necessary to treat the whole family,
repeating treatment after 2 weeks, and hygiene should also be
Strongyloides stercoralis larvae are passed in the stools, where they
either undergo a free-living cycle in the soil or differentiate into
infective larvae that invade another host via intact skin. Inside the
human host, they can initiate another development cycle. As a
consequence, infection with Strongyloides can be prolonged.
Resistance to Strongyloides depends on efficient cell-mediated
immunity. Individuals who are infected with human T-cell leukaemia
virus 1 (HTLV-1) or are taking steroids are especially susceptible
Migrating larvae leave a red, itchy track, which fades after about
48 h. If the patient is given immunosuppressive therapy, uncontrolled
multiplication of the Strongyloides may occur, which is
characterized by fever, shock and the signs of septicaemia and
Stool culture from multiple specimens may reveal infective larvae.
Alternatively, samples of jejunal fluid are examined for the presence
of larvae. A sensitive enzyme immunoassay (EIA) technique
for serum is available.
Ivermectin is the optimal drug for treatment, with the imidazoles
(e.g. albendazole) as alternatives. Relapse occurs in up to 20% of
patients. The hyperinfection syndrome is often accompanied by
Gram-negative septicaemia, which requires urgent treatment.
The risk of infection can be reduced by wearing appropriate footwear
to prevent larvae penetrating the skin.
Two Taenia spp.
infect humans: the pork worm, T. solium, and
the beef worm, T. saginata. Infection is acquired by eating meat
from the intermediate hosts that contains the tissue stages of the
Pathogenesis and Clinical features
Tapeworms compete for nutrients and infections are usually
Taenia solium can use humans as an intermediate as well as the
definitive host. When an individual ingests T. solium eggs, the eggs
hatch and disseminate, forming multiple cyst-like lesions in the
muscles, skin and brain. These 'measly' lesions, similar in appearance
to infected pork meat, are known as cysticercosis. Inflammatory
responses to parasitic antigens that leak from cysts in the
brain may lead to epileptic seizures.
This is made by finding characteristic eggs in the patient's stool.
Cysticercosis is diagnosed by a specific EIA and confirmed by
demonstrating the presence of multiple tissue cysts by X-ray, CT
Treatment and Prevention
Treatment is with praziquantel. Specialist advice should be sought
for the management of Taenia infections in the central nervous
system as severe inflammatory reactions can occur.
Humans are the definitive host of this rare tapeworm, acquiring
infection from undercooked freshwater fish. The parasite competes
for nutrients and causes deficiency of vitamin B12. The Diagnosis
is made by detecting the characteristic eggs in faeces and
treatment is with praziquantel.
Humans are the only host of this small tapeworm. Infection is
usually asymptomatic and diagnosis is made by detecting the characteristic
eggs in the faeces. Treatment is with praziquantel.