Strongyloides stercoralis - threadworms

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Strongyloides stercoralis

Taxonomy, Common Name, Disease

 

* CLASS: SECERNENTEA

* SUBCLASS: RHABDITIA

* ORDER: RHABDITIDA

* SUBORDER: RHABDITINA

* SUPERFAMILY: RHABDITOIDEA

* FAMILY: STRONGYLOIDIDAE

 

Scientific name - Strongyloides stercoralis

 

Common name - threadworm

Hosts

S. stercoralis is the threadworm parasite of man. There are about 38

species of threadworms found in sheep, swine, goat, ox, deer, camel,

rabbit, primates, dogs, and cats. Some species are parasites of birds,

reptiles and amphibians. Worms indistinguishable from S. stercoralis

have been recovered from dogs and cats.

Distribution

The threadworm of man occurs from about 35 degrees north latitude to

30 degrees south latitude. Generally the distribution is limited to

warm moist areas because such climates are favorable to the survival

of the juvenile stages.

Life Cycle

These worms have a heterogenetic life cycle which consists of: a) a

parasitic generation (Homogenic life cycle); and b) a free-living

generation (Heterogonic life cycle).

 

The heterogonic life cycle is advantageous to the parasite as it

affords the opportunity for the parasite to exist and reproduce for

one or more generations in the absence of a host. Also the number of

infective juveniles may be increased by reproduction of the

free-living females and males.

 

Parasitic females anchor themselves with their mouths to the mucosa of

the small intestine or burrow their anterior ends into the submucosa.

Reproduction in the host is by parthenogenetic females which lay

several dozen eggs a day. Eggs are released into the lumen of the gut

or the submucosa where they hatch and juveniles pass into the lumen.

These first-stage juveniles are 300 to 380 um long and are usually

passed with the feces. Juveniles develop either to free-livingadults

or to infective filariform juveniles. Third stage juveniles are the

infective stage. They are 490 to 630 um long. This is a resting stage

which does not develop further until it penetrates through skin or is

ingested. Following skin penetration they are carried by the blood to

the lungs, where they exit into the alveoli, travel up the trachea,

are swallowed, and mature in the small intestine. If ingested,

migration through the lungs is not necessary. The free-living adults

can produce successive generations of free-living adults. Both

free-living and parasitic females can produce juveniles that will

become filariform, infective juveniles and juveniles that will mature

into free-living adults. Autoinfection is also possible if juveniles

have time to molt twice during passage down the digestive tract. In

these cases they penetrate the lower gut mucosa or perianal skin,

migrate in the typical way, and mature. Both free-living and parasitic

females can produce juveniles that will become filariform, infective

juveniles and juveniles that will mature into free-living adults.

Autoinfection is also possible if juveniles have time to molt twice

during passage down the digestive tract. In these cases they penetrate

the lower gut mucosa or perianal skin, migrate in the typical way, and

mature.

Symptoms-Pathogenicity

 

1. Dermatitis is produced by migration of the infective juveniles

through the skin (cutaneous infection).

2. Mild to severe symptom of pneumonia during migration to air-sacs

of lungs. (Cases of reproduction in the air-sacs have been observed

but they are relatively rare).

3. Inflammation of the intestinal mucosa.

4. Diarrhea accompanied by emaciation and exhaustion.

5. In massive infections death may result unless therapeutic

measures are taken.

 

Management

Sanitation involving proper disposal of human wastes. Infected persons

and animals may be treated with appropriate anthelmintics. (Vermox or

Pyrantyl Pamoate; Thiabendazole or Cambendazole).

 

Diagnosis is based on finding juveniles in freshly passed stools, by a

direct smear in cases of heavy infection or following concentration by

Baermann isolation or zinc flotation with centrifugation. Feces which

have been in contact with soil my be contaminated with soil dwelling

rhabditids.

Characteristics

Parthenogenetic females reach a length of 2.0 to 2.5 mm. The buccal

capsule of both sexes is small, and they possess a long, cylindrical

esophagus that lacks a posterior bulb. The vulva is in the posterior

third of the body; the uteri are divergent and contain only a few eggs

at a time. The free-living adults both have a rhabditiform esophagus.

The male is up to 0.9 mm long and 40 to 50 um wide. The male has two

simple spicules and a gubernaculum; its pointed tail is curved

ventrad. The female is stout and has a vulva that is about equatorial;

the uteri generally contain more eggs than do those of the parasitic

female. Parasitic males are about 0.7 mm long and appear identical to

free-living males.

References

 

1. Schmidt, G.S., and L.S. Roberts. 1989. Chapter 25. Order

Rhabditata: Pioneering Parasites. in Foundations of Parasitology.

Times Mirror/Mosby College Publishing. St. Louis. 750 pages.