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Taxonomy of Helminths

Medically important Helminths

Medically important Protozoa

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Medically Important Helminths

 

 Diagnosing Medical Parasites: A Public Health Officers Guide to Assisting Laboratory and Medical Officers 

Medically Important Parasitic Diseases - Lifecycles

     WHO Training Manual on Diagnosis of Intestinal Parasites
   Medically important Helminths    
  Medically important Protozoa     


In contrast to the protozoa, helminths (worms and flukes) are multicellular with complex reproductive systems and life cycles involving intermediate hosts for the development of larval stages and a definitive host for the adult form. Adults may be dioecious with separate sexes or hermaphroditic.

Nematodes - the filariases

These are arthropod (insect)-borne infections caused by filarial worms. The classic example is elephantiasis caused by Wuchereria bancrofti. Larvae of W. bancrofti develop in the mosquito and, as in malaria, human infection results from the bite of the insect. When bitten, the larvae (as male and female forms) pass through the lymphatics and mature to thread-like adults, 4-8 cm long, in the lymphatic glands. After mating the females develop eggs and larvae that are released as microfilariae into the peripheral circulation.

The localization of the adult filariae in the lymph glands causes obstructions in the lymphatic drainage. This then results in the grossly disfiguring condition of elephantiasis that typically involves massive swelling of the legs, scrotum and other extremities.

Other arthropod-borne microfilarial infections of man are loaiasis caused by Loa loa. The disease is spread by biting flies and the adult worms migrate along connective tissue, usually reaching the conjunctiva of the eye. Onchocerciasis ("river blindness") is caused by Onchocerca volvulus, a tissue dwelling nematode, the microfilariae of which are predominantly found in the eye and skin.

Soil-Mediated Helminthiases

Soil-transmitted helminthic infections are of two types: the hookworms, which undergo a cycle of development in the soil (the larvae being infective), and a group of nematodes that survive in the soil merely as eggs that have to be ingested in order for the cycle to continue.

Hookworms. The most common hookworms are Ancylostoma duodenale and Necator americanus. Adults attach to the walls of the jejunum and females lay large numbers of eggs that are passed out with the feces. The eggs hatch in the soil and infect man by usually burrowing through the soles of the feet. The larvae then migrate to infect the heart and lungs before passing into the tracheae, pharynx and then the small intestine.

Strongyloides stercoralis. Females live in the mucosal glands of the small intestine. Eggs hatch in these glands and the larvae are passed with the feces into the soil. As with other hookworms, infection results from the larvae burrowing into the skin. The rest of the life cycle is as for A. duodenale and N. americanus.

Ascariasis. Adult worms of Ascaris lumbricoides live in the small intestine where they lay large numbers of eggs that are passed out with the feces. Unlike the hookworms, the eggs are the infectious form in which the larvae develop. When ingested, the eggs hatch in the jejunum, penetrate the mucosa and are carried through the hepatic circulation to the heart and lungs. They again enter the stomach via the tracheae and esophagus before growing to adulthood in the small intestine. Pneumonitis and intestinal obstruction may accompany heavy infestations.

Toxocariasis. The disease results from the accidental infection of man with eggs of the ascarid roundworm of the dog, Toxocara canis, and cat, T. cati. The life cycle is the same as that of Ascaris but the invasive larvae become arrested in various tissues where they are phagocytosed. In the process they induce marked eosinophilia and local tissue reaction commonly involving the liver and eye.

Trichuriasis. Trichuris trichiura ("whipworm") inhabits the cecum where they attach to the mucosa. Eggs from the mature worms are passed with the feces and develop in the soil. When swallowed, the eggs hatch in the small intestine and the developing larvae pass directly to their attachment sites in the large intestine. Heavy infections can cause abdominal pain and chronic bloody diarrhea that may result in rectal prolapse.

Snail-Mediated Helminthiases

This important group of snail-transmitted helminthiases is all caused by trematodes (flukes) that undergo a complicated cycle involving various species of land or aquatic snails. The most significant of these fluke infections is schistosomiasis and over 200 million people are estimated to be infected world-wide. The three common species infecting man, Schistosoma mansoni, S. japonicum and S. haematobium have similar life cycles. Eggs are passed in the urine (S. haematobium) or feces (Schistosoma mansoni and S. japonicum) and hatch in natural waters. Miracidia hatch from the eggs, penetrate suitable snails and develop two generations of sporocysts. The last of these then produces fork-tailed cercariae. These cercariae penetrate the skin when a new host comes into contact with the contaminated water. Once through the skin the cercariae shed their tails and become schistosomulae that then migrate through the tissues to the liver. Here male and female flukes copulate and migrate to either the bladder or rectum where eggs are laid. Schistosomiasis can result in chronic liver, spleen and bladder damage.

Fascioliasis. Fasciola hepatica is found in most herbivores (but primarily sheep) that graze in wet pasturage where the intermediate host, snails of the genus Lymnaea, are found. F. hepatica eggs, shed from the infected primary host, mature into the embryonated form in the environment. These then hatch and release a motile miracidia that seeks out and penetrates the tissue of the intermediate snail host. Cercaria are produced in the snail that when released into the environment can encyst to produce metacercariae. In temperate climates man is often infected by eating wild watercress on which metacercariae have collected. After being ingested, the metacercariae pass through the duodenal wall and penetrate the liver capsule. Following maturation of the young flukes, the adults finally come to lie in the bile ducts or adjacent liver tissue. Here they cause severe damage to the biliary tract and eggs are passed with the bile into the feces to continue the cycle.

Helminth infections acquired through the gastrointestinal tract

Trichinosis. Trichinella spiralis is the cause of trichinosis in man. The nematode circulates between rats and pigs with man becoming infected from eating raw or inadequately cooked pork products. Encysted larvae in the meat excyst (hatch) in the intestine and develop into minute adults in the mucosa. These mature and the females deposit larvae that then migrate through the tissues to reach skeletal muscles in which they encyst. Human infections may be asymptomatic but can include fever, orbital edema, myalgia and eosinophilia. In the extreme, infection can be fatal through myocarditis or encephalitis.

Enterobiasis. Enterobias vermicularis is a small thread-like "pinworm" mainly infecting young children. The female emerges to the perianal region usually at night and lays some 10,000-15,000 eggs and then dies. In the process they cause severe pruritis (itching). The embryonated eggs are infectious on ingestion and hatch in the duodenum. The larvae pass to the cecum where they mature into adults. Because of the pruritis, children often re-infect themselves from eggs under their fingernails. Bedding is also a source of infection and can be a means of spreading the organism in families and institutions such as orphanages and boarding schools.

Taenia solium (pork tapeworm). The adult lives in the small intestine of man that is the definitive host. Segments of the worm pass through the anus and release large numbers of eggs that can survive for long periods outside of the body. When ingested by pigs, the eggs hatch and each releases an oncosphere that migrates through the intestinal wall and blood vessels to reach striated muscle where encystment occurs. When inadequately cooked pig meat is eaten by man, excystment occurs in the small intestine and an adult cestode (worm) develops. If the eggs are released into the upper intestine of man (e.g. through regurgitation) they can invade the host setting up a potentially dangerous larval infection known as cysticercosis in muscle and other sites.

Taenia saginata (beef tapeworm). This also infects man through cattle. The life cycle is similar to T. solium and in both species the adult tapeworm can grow up to 10 meters in length.

Hydatidosis. This is caused by the tapeworm Echinococcus granulosus. The adult worm inhabits the small intestine of dogs from which the eggs of the species are passed. These eggs can be ingested by herbivorous animals and hatch in the duodenum. The embryos enter the circulation where they are carried to various sites to develop into cysts. Dogs become infected when they eat contaminated offal. Humans are infected if they accidentally ingest eggs from infected dogs and the liver is the most common site of infection in which hydatid cysts form. Life Cycle.

Source: University of Leicester

References:
Baker, J.R. and Muller, R (Eds). Advances in Parasitology. Academic Press, London.
Knight, R. (1982). Parasites Diseases in Man. Churchill Livingstone, London.
Peters, W. and Gilles, H.M. (1995) Colour Atlas of Tropical Medicine and Parasitology. Mosby-Wolfe.