Trichinella spiralis. Infection in humans results from the ingestion of uncooked or undercooked pork (occasionally bear meat) containing the encysted larvae of T. spiralis. The larvae are liberated from their cysts by the gastric juices and develop into adult male and female worms in the duodenum and jejunum. After fertilization, the female burrows into the intestinal mucosa, where she deposits several successive batches of larvae. These make their way—via the lymphatics, regional lymph nodes, thoracic duct, and bloodstream—into all parts of the body. The new larvae penetrate all tissues but survive only in striated muscle, where they become encysted and eventually calcify. Animals are infected in the same way as humans, and the cycle can be repeated only if a new host ingests the encysted larvae. Gould has written an authoritative review of this subject.
The early symptoms of the disease, beginning a day or two after the ingestion of pork, are those of a mild gastroenteritis. Later symptoms coincide with invasion of muscle by larvae. The latter begins about the end of the first week and may last for 4 to 6 weeks. Low-grade fever, pain and tenderness of muscles, edema of the conjunctivae and particularly of the eyelids, and fatigue are the usual manifestations. The myopathic aspects of Trichinella infestation are considered fully in Chap. 45.
Particularly heavy infection may be associated with a CNS disorder from larval migration through the nervous system without encystation. Headache, stiff neck, and a mild confusional state are the usual symptoms. Delirium, coma, hemiplegia, and aphasia have also been observed on occasion. The spinal fluid is usually normal but may contain a moderate number of lymphocytes and, rarely, parasites.
An eosinophilic leukocytosis usually appears when the muscles are invaded. Serologic (precipitin) tests become positive early in the third week. The heart is often involved, manifested by tachycardia and electrocardiographic changes; sterile brain embolism may follow the myocarditis. These findings may aid in the diagnosis, which can be confirmed by finding the larvae in a muscle biopsy, using the technique of low-power scan of wet tissue pressed between two glass slides.
Trichinosis is seldom fatal. Most patients recover completely, although myalgia may persist for several months. Once recurrent seizures and focal neurologic deficits appear, they may persist indefinitely. The latter are based on the rare occurrence of trichina encephalitis (the filiform larvae may be seen in cerebral capillaries and in cerebral parenchyma) and emboli from mural thrombi arising in infected heart muscle.
Treatment (See also Chap. 45.) In the treatment of severe trichinosis, albendazole, an antihelminthic agent, and corticosteroids are of value. This drug prevents larval reproduction and is therefore useful in patients known to have ingested trichinous meat. It also interferes with the metabolism of muscle-dwelling larvae. Fever, myalgia, and eosinophilia respond well to the antiinflammatory and immunosuppressant effects of prednisone (40 to 60 mg daily), and a salutary effect has been noted on the cardiac and neurologic complications as well.
Other nematodes, mainly toxocara (the cause of visceral larva migrans), strongyloides, and angiostrongyloides may rarely migrate to the brain, but each is characterized by a systemic illness, which is far more common than the neurologic one. Parasitic meningitis is discussed later.