Organism Trichinella spiralis (Fig. 9.7), T. nelsoni, T. nativa, T. britovi and T. pseu-dospiralis, nematode worms.
Clinical features The severity of the disease depends upon the dose of larvae that have encysted in the tissues. During the second week of infection, there is headache, insomnia, pain, dyspnoea and pyrexia with
oedema of the orbit and eosinophilia. If the symptoms are sufficiently severe, death can result; otherwise, once the attack is over, the cysts cause no further trouble, gradually die and calcify.
Diagnosis is made by muscle biopsy of the deltoid or thigh muscles where the encysted larvae are found.
Transmission The life cycle is a simple one, encysted larvae in the muscles are eaten by another animal and the liberated larva develops into an adult to produce numerous new larvae, which are then carried to all parts of the body in the circulation. Only the larvae that reach striated muscle survive, the diaphragm, tongue, throat, eye and thorax being the favoured sites.
In the different climatic zones of the world where different groups of animals live off each other, several transmission cycles have evolved. In Africa, the warthog and the bushpig form the vital link in the cycle. Being the favoured prey of lions and leopards, these carnivores, with the hyenas and jackals that finish off the remains, become infected. The general scavenging nature of the warthog and pig inadvertently eating the remains of dead animals allows the cycle to be completed. Humans come in as intruders, a dead end to the cycle when they feast on a recently killed bushpig.
In Europe and Asia, the rat is the reservoir of infection, but by its scavenging nature, the pig acquires infection and when cooked on a spit or otherwise eaten in an improperly cooked way, humans become infected. Sausages made from food scraps or hamburgers contaminated with bits of pork can be potent sources. In the Arctic, the seal and polar bear are involved in the transmission cycle; the latter acquires very high levels of infection. The death of some Arctic explorers has been attributed to killing and eating polar bears infected with trichinosis.
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