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Helminthes
Three groups comprise the helminthes;
these are (1) nematodes (roundworms), (2) cestodes
(tapeworms) and (3) trematodes (flukes).
Nematodes (Roundworms)
Intestinal roundworms acquired by
ingestion
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Enterobius vermicularis (pinworm)
is transmitted by the fecal-oral route (ingestion of ova) and treatment is
mebendazole.
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Ascaris lumbricoides can be a
particular problem with persons who have AIDS. Infection is again by ingestion
of ova and the worm may cause intestinal obstruction. Infective stages are
found where human feces are used as a fertilizer.
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Trichuris trichiura (whipworm) is
again transmitted by ingestion of the ova. Symptoms are asymptomatic unless
the worm burden reaches a certain point at which time there is anemia,
eosinophilia and bloody diarrhea. Transmission occurs where sanitation is poor
and human feces are used for fertilizer.
Intestinal roundworms usually acquired
by passage through skin
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Necator americanus and Ancylostoma
duodenale usually penetrate the skin of the legs and feet ("ground itch")
and are usually found where the soil is shaded, but warm and the air is humid.
Wearing shoes helps prevent exposure. Treatment is mebendazole or iron therapy
to treat anemia. Ancylostoma brziliense is a dog and cat hookworm that
penetrates human (inadvertant host) skin but cannot develop further. People
often get infected after dogs have been brought to the beach.
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Strongyloides stercoralis (strongyloidiasis)
is a particular problem for AIDS patients. Treatment is with thiabendazole.
Tissue Nemathelminths
(Roundworms)
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Trichinella spiralis cause
trichinosis after eating poorly cooked meat which contains encysted larval
forms. No eggs are involved (just larvae) in their life cycle. Freezing
meat for longer than 20 days in a home freezer will kill larvae.
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Toxocara canis and T. cati have a
similar life cycle to A. lumbilcoides in humans. Corticosteroids may be
helpful in treatment.
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wucheretia bancrofti and Brugia
malayi are transmitted by mosquitoes and are not endemic to the US.
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onchocera volvulus are
transmitted by blackflies and cause river blindness and are not endemic to the
U.S.
Cestodes (Tapeworms)
Tapeworms are long (mm to meters) and have
an extensive reproductive system (proglottids). If man is the
definitive host, disease is minor but if man is the intermediate host serious
disease may develop. All tapeworms are hermophrodditic, with male and female
reproductive organs present on each mature proglottid.
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Taenia saginata (beef) contain
sucking discs but no hooklets like the pork tapeworm. Niclosamide
is the drug of choice for treatment.
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Taenia solium (pork) have
hooklets. Niclosamide can be used to treat it.
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Diphyllobothrium latum (fish)
results from eating improperly cooked fresh water fish (can occur in Florida)
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Echinococcus ganulosis (dog tapeworm):
Man is an accidental/inadvertant host.
Trematodes (Flukes and
Schistosomes)
Trematodes are hermaphroditic with both
male and female reproductive organs in a single body, with the
exception of schistosomes. They are broad, leaf-like flat worms. 2 intermediate
hosts are required; snail and then man.
Three schistosomes most frequently
associated with human disease are mansoni, haematobium and japonicum.
They collectively produce the disease schistosomiasis. The female lives in the
ventral groove of the male for life. The avian species is acquired by
wading in fresh water where aquatic birds frequent.
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