Tropical Diseases - Filariasis
The species of mosquitoes that transmit malaria and yellow fever, diseases caused by protozoa and viruses, also transmit filariasis , caused by a parasitic worm—a nematode or round-worm. Filariasis affects 300 million people living in tropical and subtropical areas of the world. The worm invades the subcutaneous tissues and lymph system of the human body, blocking the flow of lymph and producing symptoms of inflammation, edema, abscesses, and, in one form of the disease, blindness. Filariasis is not unknown to Americans; some 15,000 soldiers contracted the disease during World War II fighting in the Pacific Theater, and cases have been reported along the Carolina coast area. But most of the victims of filariasis live in a region extending from Africa through Asia to the islands of New Guinea and Borneo.
Symptoms of filariasis can develop insidiously during an incubation period that may last from three months to a year after infection. There can be brief attacks of a low-grade fever, with chills and sweating, headache, nausea, and muscle pain. The patient also may feel sensitive to bright lights. Signs and symptoms more specifically related to filariasis are the appearance of red, swollen skin areas with tender spots that indicate the spread of the threadlike worms through the lymphatic system. Most likely sites for the first signs of filariasis are the lymph vessels of the legs, with later involvement of the groin and abdomen, producing the swollen lower frontal effect known as elephantiasis . Diagnosis of the disease is confirmed by finding the tiny worms in the lymph; the infecting organism also may be found in blood tests, but only at certain times. The worms of one form of the disease are only 35 to 90 millimeters long in the adult stage, and those of a second type of the disease are only half that size. Larvae, or embryos, of the worms may be only 200 microns (one-fifth of a millimeter) in size.
An oral medication, diethylcarbamazine, is available to kill the larvae in the system; the drug has only limited value in destroying the adult worms. The drug is taken orally for three weeks, but courses may have to be repeated over a period of two years because relapses can occur. Other therapeutic measures include bed rest during periods of fever and inflammation, antibiotics to control secondary infections, and, occasionally, surgery to remove damaged tissues that may interfere with normal working activities following recovery.
The type of filariasis that causes blindness is transmitted by a species of blackfly that introduces or picks up the worm larvae while biting. As in mosquito-transmitted filariasis, the worms work their way through the skin to the lymphatic system but tend to migrate to eye structures. Blackfly filariasis, also called onchocerciasis , occurs most frequently in Africa and from southern Mexico to northern South America. More than one million cases of onchocerciasis have been found in the upper basin of the Volta River of Africa, with thousands of patients already blinded by the infection.
A third variation of filariasis is called loiasis . It is carried from man to monkey or from monkey to man by a biting fly. The larvae develop into adult worms that migrate under the skin and sometimes through the eye. Migration of a worm through the skin causes swelling, irritation, and redness. The disease is treated with drugs to kill the larvae, as well as by antihistamines and, occasionally, surgery to remove the adult worms.
Control of filariasis requires eradication of the flies and mosquitoes that transmit the parasitic worms and perhaps the wild animals that can serve as reservoirs. As in the examples of other tropical diseases, it frequently is easier to separate the humans from the areas infested by the insects than to eradicate the insects.