In eastern Africa, nagana, or savannah sleeping sickness, is well known. The symptoms are unmistakable--exhaustion, dribbling, loss of sensitivity to pain, and eventually death. Only in the late nineteenth century (1895) was the cause of the illness found. Trypanosome protists (Trypanosoma brucei) infect the blood, lymph glands, and central nervous system after inoculation by the bite of the tsetse fly (dipteran flies of the genus Glossina). Unusually, both the male and the female tsetse fly are the intermediate hosts that disperse the parasites. When they bite an infected person or animal, the blood they draw contains trypanosomes, which then travel from the fly's stomach to its salivary glands, multiplying and producing the infective forms that the tsetse fly inoculates when it bites another victim. Wild artiodactyls are the reserves of the trypanosomes, which are fatal to domesticated livestock.
Tsetse flies live in the eastern savannahs (Glossina palpalis) and western savannahs (G. morsitans), respectively, and spread true sleeping sickness (Trypanosoma brucei gambiense) or nagana (T. brucei rhodesiense). Fighting the tsetse fly is difficult because, unlike other dipteran flies, they do not have a larval stage that occurs in a well-defined habitat. The embryos are produced one by one in the female's oviducts and feed on the secretions of specialized glands. The gravid female lays a huge, whitish larva, always on damp wet soils, which immediately burrows into the ground and pupates within an hour. After three or four weeks, a new adult emerges. This lifecycle has few points vulnerable to preventive measures.
There are areas of Africa and America where vision is the privilege of the young, who act as guides for the many adults blinded by onchocerciasis, or "river blindness." The vectors of this parasitic disease are female black flies (Simulium), which, like tsetse flies, are blood-sucking dipterans. As in most other dipteran flies, it is only the females that suck blood; the males have less robust mouthparts and suck only nectar. When they bite an infected victim, the female black fly takes in larvae of the nematode that causes the illness, the filarian worm (Onchocerca volvulus). These migrate to the insect's mouthparts and are injected into a new host. The worms development to the adult state terminates with the formation of a small tumor in the subcutaneous tissue that contains the adult filaria, paired or in groups. The eggs laid by the females hatch immediately and liberate mobile larvae that--while they are waiting to infect the intermediate host--settle in the lymphatic tissues and in the conjunctive layer of the eye, causing blindness. Black fly larvae live in shallow, fast-flowing streams, where they capture particles in suspension. As the adult flies are not very mobile, they always occur near rivers with some difference in level, the areas where there are endemic foci of onchocerciasis and where humans suffer most.
Schistosomiasis, also known as bilharziasis, is a third parasitic disease affecting human beings. The infection is caused by a trematode of the genus Schistosoma (=Bilharzia), the adults of which live in the host's abdominal blood vessels, normally in the veins of the intestines and the bladder. The worm's eggs find their way into the bladder or intestines and are expelled with the urine or feces. On entering contact with freshwater, they release a ciliated larvae that swims until it finds an appropriate host, an individual of one of several species of freshwater snail (Bulinus, Physopis, Planorbis), within which it multiplies asexually. A single snail infected by a single larvae can release thousands of fork-tailed larvae (Cercariae) that swim until they come into contact with the skin of the definitive host. They quickly burrow through the skin and enter the blood system. Eleven different species of these worms are known in Africa. (Schistosoma haematobium and S. mansoni are the most frequent in the savannahs.) They are specific parasites of certain artiodactyls, rodents, or humans, and also have their own host species of ramshorn snail (Planorbidae), most of them with geographically restricted distributions. Dozens of different sets of symptoms all lead to the same result--blindness.
Development that does not respect the environment may make the situation worse. The construction of reservoirs and canals often modifies the distribution of the vectors of onchocerciasis and favors the vectors of schistosomiasis. Drastic solutions to eliminate the vectors are not always effective and often have major side effects. Pesticides, for example, affect many other species and may accumulate to dangerous levels. The felling of thickets where tsetse flies rest and the extermination or confinement of wild ungulates both eliminate vital resources. A rational alternative would be to interrupt the parasites' lifecycles by preventing one of the transmission phases. This, however, requires considerable research and a sustainable conception of development.
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|Publication:||Encyclopedia of the Biosphere|
|Date:||Mar 1, 2000|
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