Page 482 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 56 Nematodes
471
TABLE 56–2 Medically Important Stages in Life Cycle of Intestinal Nematodes (Roundworms)
Important Stage(s)
Stage(s) in Humans Most
Stage That
Insect
Infects Humans
Associated with Disease
Vector
Outside of Humans
Organism
Enterobius
Eggs
Female worm migrates out anus and lays
None
None
eggs on perianal skin, causing itching
Trichuris
Larvae migrate to lung, causing pneumonia
None
Eggs
Eggs survive in environment
Ascaris
None
Ancylostoma None Eggs Worms in colon may cause rectal prolapse Eggs survive in environment
Worms in colon cause blood loss (anemia)
Filariform larvae
Egg → rhabditiform larvae → filariform
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enter skin
larvae
and Necator
Worms disseminate to various tissues in immu-
Egg → rhabditiform larvae → filariform
Filariform larvae
Strongyloides
None
nocompromised (autoinfection)
larvae; also “free living” cycle in soil
enter skin
Trichinella
None
Larvae encyst in muscle causing myalgia
Larvae in muscle of pig, bear, and other
Larvae in meat
animals
ingested
Larvae in fish
Larvae in submucosa of GI tract
Larvae in muscle of fish
Anisakis
None
ingested
Unlike those of other intestinal nematodes, these
carried to the mouth by fingers after scratching the itch-
ing skin.
adult worms can be found in the stools or near the
Pathogenesis & Clinical Findings eggs are not found in the stools. The small, whitish
anus of diapered children. No serologic tests are
available.
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Perianal pruritus is the most prominent symptom. Pruri-
tus is thought to be an allergic reaction to the proteins of
Treatment
either the adult female or the eggs. Scratching predisposes
to secondary bacterial infection.
pyrantel pamoate. These drugs kill the adult worms in the
Epidemiology
colon but not the eggs, so retreatment in 2 weeks is sug-
gested. Reinfection is very common. Household members
Enterobius is found worldwide and is the most common
should also be treated.
helminth in the United States. Children younger than 12
years of age are the most commonly affected group.
Laboratory Diagnosis
There are no specific means of prevention but
The eggs are recovered from perianal skin by using the Prevention
washing hands when preparing food and washing bed
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Scotch tape technique and can be observed microscopi-
sheets, towels, diapers, and clothing to remove eggs is
helpful.
cally (see Figure 56–4).
TABLE 56–3 Medically Important Stages in Life Cycle of Tissue Nematodes (Roundworms)
Associated with Disease
Outside of Humans
Insect Vector
Stage That Infects Humans
Organism
Larvae
Mosquito
Mosquito ingests microfilariae
Wuchereria
Adult worms in lymphatics
(elephantiasis)
in human blood → larvae
Onchocerca
in eye (blindness)
human skin → larvae
Deer fly
Loa Blackfly Larvae Adult worms in skin; microfilariae Blackfly ingests microfilariae in
Deer fly ingests microfilariae →
Adult worms in tissue (skin,
Larvae
(mango fly)
conjunctivae)
larvae
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Dracunculus
Copepods ingest larvae
Female worms cause skin blister;
None
Larvae in copepods are swallowed
see head of worm
in drinking water
Eggs in dog feces
Toxocara canis
None
→ eggs
Filariform larvae penetrate skin
Ancylostoma
None
Larvae in subcutaneous tissue
Adult worms in dog intestine
→ eggs → larvae
caninum
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