Tropical Medicine: A Clinical Text, 8th Edition, Revised and Expanded

Larval Helminth Infections

Larval Helminth Infections Human disease may be caused by infection with the larval stage of intestinal helminthic parasites of animals. In the case of Trichinella spiralis , both adults and larvae are present, but in cysticercosis and hydatid disease, only the larval forms develop in human tissues. It need hardly be noted that only in exceptional circumstances does man become a true intermediate host. Trichinosis As the parasites do not exist outside mammalian carnivore hosts, infection is independent of climatic conditions, and trichinosis occurs in arctic, temperate, and tropical climes. The Parasite Trichinella spiralis is a small, slender nematode. The adult worms measure 1.5 to 3 mm in length. They live in the small intestinal mucosa and submucosa where, during a life span of 4 to 16 weeks, the female produces 1,500 larvae. The larvae, 100 microns in length, are carried by lymph or blood to all parts of the body. Those that reach transversely striated muscle penetrate into the cells where they coil up, and within 17 to 20 days, become encysted. In this situation, they may remain viable and infective for many years. After death of the larvae, cysts often become calcified. The life cycle is completed when a carnivore eats the flesh of an infected animal, and the cysts are digested by gastric juices, releasing infective larvae. Pathology Adult worms do traumatize the small intestinal mucosa, but the principal pathological effects follow the arrival of larvae in muscle cells. They ................. 18086$ CH15 07-15-11 15:40:11 PS larval helminth infections 171 elicit an inflammatory response with edema, swelling, cellular infiltration , eosinophilia, and a rise in serum creatinine phosphokinase. The myositis may persist for 5 to 6 weeks and then gradually subsides with encapsulation of the parasites. Affected muscles include those of the extremities, the diaphragm, intercostal muscles, the tongue, and eyes. Larvae may also be deposited in other tissues, such as the brain and the heart, where although they do not survive, they may cause a transient inflammatory reaction. Clinical Features Most infections are mild and pass unnoticed. More severe infections tend to occur in local outbreaks, such as when a bear is killed in Thailand , and villagers feast on undercooked meat, or following the killing of a bush pig or wart hog in Africa or a pork barbecue in Hong Kong. Heavy infections may be accompanied by nausea, vomiting, abdominal pain, diarrhea, or occasionally constipation. The main symptoms commence 1 week or more after exposure with a high fever, headache, myalgic pains, and severe malaise. Other features may include pain, tenderness and weakness of muscles, periorbital edema, and splinter hemorrhages under the nails. Blurring of vision, paresis, psychotic disturbance , cardiac arrhythmias, coma, and sudden death can occur. After 5 to 6 weeks, symptoms tend to abate gradually, but convalescence may be attended by persistent muscle pain. Diagnosis The diagnosis is suggested by the clinical picture, a history of recent ingestion of undercooked or raw pork, bear or bush pig meat, a high blood eosinophilia, and a raised serum creatinine phosphokinase. The diagnosis is confirmed by ELISA demonstration of specific IgE and IgM antibodies; muscle biopsy is seldom required. Treatment Severe symptoms are best controlled first by prednisone, 40 to 60 mg daily, and then specific therapy with mebendazole, in a dosage of 20 mg per kg bw for 10 to 30 days, to destroy the parasites. Mebendazole ................. 18086$ CH15 07-15-11 15:40:12 PS 172 larval helminth infections FIGURE 24: Trichinella spiralis in a fresh-pressed preparation of muscle. is not advised during the first 3 months of pregnancy; thiabendazole may be used instead. Cysticercosis Man may act as host for the larval stage of Taenia solium as well as for the adult worm. This is most likely to occur where there is widespread contamination of the ground by human feces from individuals infected by the pig tapeworm. The most important endemic regions are India, South Africa, and Latin America. Outbreaks can occur among the most unusual contacts, as when a foreign domestic worker infected an orthodox Jewish Community in New York. Pathology The outer covering of the Taenia solium egg dissolves in the stomach, and the oncosphere is released in the duodenum. Here, it penetrates the mucosa and is carried by the blood stream to practically every part ................. 18086$ CH15 07-15-11 15:40:20 PS larval helminth infections 173...