The various types of Hookwarms and how they affect man

Hookworm is found worldwide, especially in tropical areas. The global prevalence of hookworm infection is greater than the past estimate of nearly 500 million, so therefore, its effect to the human health should not be ignored.

 

Trichinella spiralis

 

This causes trichinosis. It occurs worldwide, especially in eatern Europe and West Africa. The Adult female worms are up to 3-4 X 0.6mm; the adult male worms are up to 1.5 X 0.04mm. The incysted larvae (1mm) is enclosed in a fibrous cyst wall. It is localized in the small intestine (adult worms) and straited muscles (larvae). Any mammal (rat, bear, fox) can be infected, but pigs are the most important reservoirs of human disease. The infective stage of the Trichinella spiralis is the Larva. It is transmitted through the alimentary (eating raw or undercooked meat, usually pork, containing larvae encysted in the muscle) route.

 

The larvae excyst and mature into adults within the small intestine of host. Male worms die after fertilization, female worms lay larvae. They are released and distributed via the bloodstream to striated muscles (diaphragm, tongue, m.deltoideus, m.pectoralis, m.intercostalis). Larvae encyst in the muscles within fibrous capsule and can remain viable for several years. Humans are end-stage hosts, because the infected flesh is not consumed by other animals.

 

Clinical manifestations are as follows: initially diarrhea, abdominal pain followed by 1-2 weeks later by fever, muscle pain, periorbital edema and eosinophilia. Death, which is rare is usually due to congestive heart failure or respiratory paralysis.

 

Laboratory diagnosis involves muscle biopsy which would reveal larvae within striated muscle; serologic test (become positive 3 weeks after infection). Treatment therapy includes anti-parasitic drugs such as Thiabendazole, Albendazole, mebendazole, prednisone, depending on what the doctor prescribes. Prevention is done by properly cooking pork and by feeding pigs only with cooked garbage: pork inspection in slaughter houses using a trichinoscope.

 

Ancylostoma duodenale and Necator americanus (hookworms)

 

They cause ancylostomiasis (hook worm infection). Disease occurs primarily in children and construction workers who are exposed to infected soil.

 

Adult worms are 1cm in length; Eggs are translucent, oval with blunt poles, 40-60 micrometer in size. The rhabditiform larva is about 0.25-0.5 micrometer with rhabditiform oesophagus (1/3 body length), pointed tail end; the filariform larvae is about 0.6-0.7 micrometer with cylindrical esophagus (1/4 body length), sharply pointed tail. Its host is the humans; mode of transmission is by penetration of the skin by filariform larva. Its infective stage is the filariform larva.

 

Filariform larvae penetrate the skin, usually of feet or legs after exposure to infected soil. They are carried by the blood to the lungs, migrate into the alveoli and up the bronchi and trachea, and then are swallowed. Larvae develop into adults in the small intestine, attach to the wall with either cutting plates (Necator) or teeth (Ancylostoma). They feed on blood, up to 0.1-0.3ml per worm can be lost per day. Immature eggs pass in the feces about 2 months after infection. The eggs develop into rhabditiform larvae and then into filariform larvae (infective stage) in warm, moist soil.

 

Its clinical manifestations are as follows:

 

1. Invasive stage (the larvae penetrate the skin); dermatitis and itching (‘ground itching’).
2. migration stage: pneumonia with esosinophilia
3. intestinal stage: anemia, diarrhea, abdominal pain, nausea.

 

Laboratory diagnosis is based on the determination of eggs in the stool; blood in the feces is a frequent finding. Mebendazole and levamisole drugs have proven effective in the treatment of ancylostomiasis. Prevention is done by disposing of sewage properly and wearing shoes.

 

Strongyloides stercoralis

 

It causes strongyloidiasis. It occurs primarily in the tropics, especially in southeast Asia. It can be in adult form, egg form, rhabditiform larvae, filariform larvae and free-living female and male forms. It is localized in the small intestine of humans. Its mode of transmission is by penetration of the skin by filariform larva.

 

Life cycle

Strongyloides stercoralis has 2 distinct life cycles, one within the human body and the other free-living in the soil.

 

1. Within the human body:

Filariform larvae penetrate the skin, usually of feet or legs after exposure to infected soil. They migrate to the lungs, enter the alveoli, pass up the bronchi and trachea and then are swallowed. Larvae develop into adults in the small intestine and produce eggs. The eggs form rhabditiform larvae that are passed in the feces and appear in stool within 4 weeks of infection. In the soil, the rhabditiform larvae develop into filariform larvae (infective for man). Some rahbditiform larvae form filariform larvae, which penetrate the intestinal wall directly without leaving the host and migrate to the lungs (autoreinfection).

 

2. Free-living in the soil:

If the larvae are passed in the feces and enter warm, moist soil, the rhabdiform larvae moulth into free-living males and females. female worms lay eggs in the soil. The eggs develop into rhabitiform larvae; which forms the filariform larvae (infective for humans).

 

Clinical manifestation

1. Invasion stage: pruritis (ground itch) at the site of larval penetration of the skin
2. migration stage: penumonia with eosinophilia
3. intestinal stage: diarrhea, abdominal pain.

 

Laboratory diagnosis is based on the determination of the rhabditiformm larvae in the stool. Basically, it is treated with Thiabendazole, but can be prevented by disposing of sewage properly and wearing of shoes regularly.

 

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