Glossinidae (Tsetse Flies) & Reduviidae (Bugs)
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Tsetse flies in the family Glossinidae vector both animal and human Trypanosomiasis or "Sleeping Sickness." As of 2017 there were 23 species of tsetse flies known, 6 of which are divided into two or more subspecies all in the family Glossinidae and genus Glossina (Service 2008). Most species are found in tropical sub-Saharan Africa (Fig. 1) with only two in Arabia. Separate treatment is given to the disease in Africa under Gambian and Rhodesia forms. For humans the most important vectors of trypanosomiasis are Glossina palpalis, G. tachinoides, G. fuscipes, G. pallidipes and G. morsitans. A separate disease in South America, Chagas, caused by Trypanosoma cruzi, is vectored by Phthiraptera bugs in the family Reduviidae.
The Glossinidae are all potential vectors of different species of trypanosomes that cause Sleeping Sickness in humans and mammals, but only a few are attracted to humans. They resemble stable flies of the genus Stomoxys with their large stout proboscis projected forward (See Heads: Fig. 2). But they are larger and make a shrill buzzing sound when entering enclosures, such as automobiles. Adult flies hold their wings crossed over the abdomen when at rest (Fig. 3), there is a hatchet- or cleaver-shaped cell on the wing (Fig. 4), and the shape of their larvae and pupae are uniquely stout and rounded (Fig. 5)
A distinctive reproductive process allows tsetse flies to hatch their eggs one at a time within the mother's uterus where a single larva completes development (Fig. 6). The larva is fed with special "milk glands" inside the uterus until mature after which it is placed in dry shaded soil near a water source. Pupation occurs in the soil and adults emerge 20-60 days later depending on season and species.
Different species of Protozoan trypanosomes, are transmitted among the Glossina species. They are parasitic in the blood stream and tissues of vertebrates. Matheson (1950) noted that most of the many trypanosome species are dependent on an invertebrate host such as the tsetse flies to complete their life cycle. For humans the most important species vectored by Glossina, are Trypanosoma gambiense, T. rhodesiense and T. brucci. In Africa wild game animals can serve as reservoirs of the trypanosomes for some species, although their importance in perpetuating the disease is probably not great.
Medical Importance and Adult Fly Behavior
Considerable behavioral information has been obtained from Tsetse flies because of their great importance to human health. All species can serve as vectors of Trypanosomiasis, but few are natural vectors because they rarely feed on humans. Also the amount of contact the flies have with infected reservoir hosts determines their threat. In Africa the human population suffers greatly from the disease, with estimates given by the World Health Organization of some 400,00 new cases every year accompanied by about 55,000 deaths. The two most important subspecies involved are Trypanosoma brucci gambiense and T. brucei rhodesiense. Although difficult to distinguish each produces different clinical symptoms and different epidemiologies.
A summary of important characteristics by Service (2008) is given as follows: "Both male and female tsetse-flies bite people, a large variety of domesticated and wild mammals, and sometimes reptiles and birds. No species of tsetse feeds exclusively on one type of host but most show definite host preferences, often associated with host availability. For example, in East Africa Glossina swynnertoni feeds mainly on wild pigs and G. morsitans on wild and domesticated bovids as well as on wild pigs, whereas in West Africa G. morsitans feeds mainly on warthogs. In East Africa G. pallidipes feeds principally on wild bovids, while in West Africa G. palpalis feeds predominantly on reptiles and humans, and in West Africa. Glossina tachinoides feeds on humans and bovids, but in southern Nigeria it feeds predominantly on domestic pigs. Tsetse-flies blood-feed about every 2-3 days, although in cool humid conditions it may be about every 10 days. Feeding is restricted to the daytime and vision, as well as olfactory cues emanating from host breath and urine, are important in host location, dark moving objects being particularly attractive. On pale-skinned people, such as Caucasians, tsetse-flies often bite through dark clothing such as socks, trousers and shorts in preference to settling on the skin. During feeding blood sucked up the proboscis passes to the crop and later to the mid-gut, where digestion proceeds."
Trypanosoma spp. Development Cycle
(Derived from Service 2008)
The male and female tsetse flies, both of which bite, become infected with trypanosomes when drawing blood from an infected person or animal. Trypanosomes then pass through the fly esophagus to the crop and peritrophic tube that lines the middle intestines. After 9-11 days the trypanosomes penetrate the middle section of the peritrophic membrane and pass across into the space between the membrane and migrate to the proventriculus. Then they pass down the food channel to the proboscis and up into the salivary duct to the salivary glands, where they develop into epimastigotes and multiply. Varying times after a blood meal (18-35 days) the flies become infective and the metacyclic trypanomastigotes are passed to a vertebrate host during a blood meal. Sometimes when Trypanosoma species that cause Nagana only in animals are found in tsetse flies they may be mistakenly believed to be a threat to humans. There are also different forms of Sleeping Sickness, the most common being Gambian and Rhodesian. The Gambian form is more chronic and less severe than the Rhodesian form.
Tsetse Fly Range (see Map)
Adult tsetse flies range over a variety of landscapes during wetter seasons, but remain closer to water sources during dry seasons. Their habits are largely synchronized with the availability of animal herds. Service (2008) separated the flies into three main groups depending on their range and types of habitat: (1) Fusca group (Forest flies), (2) Morsitans group (Savanna flies) and (3) Palpalis group (Riverine & forest flies).
Fusca Group.-- There are 13 species of Glossina in this group of very large flies. All except one (G. longipennis) inhabit dry forests of East Africa and equatorial forests of West and West-central Africa. Service (2008) reports that these flies do not often attack humans and no species is a vector of the trypanosomes.
Morsitans Group.-- There are 5 species in this group of medium-sized flies, which usually inhabit savannahs that occur from costal areas or the edges of forests to the drier areas. Glossina morsitans is common in the savannahs of West, Central and East Africa, while G. pallidipesis restricted to East and southern Africa. G. swynnertoni occurs in savannahs of a small larea of East Africa. G. morsitans and G. pallidipes range from wooded areas near the edges of forests and dry thickets, while G. swynnertoni is found primarily in dry thickets. All of these flies are vectors of trypanosomes, the most important being G. morsitans through most of its range.
Palpalis Group.-- There are 9 species and subspecies of small to medium-sized flies included here. They occur all over Africa in vegetation of wetter areas, such as forests, and along rivers, lakes and swamps. Glossina palpalis is common in all these habitats while Glossina fuscipes is found primarily in Central Africa and parts of East Africa. Gossina tachinoides is a riverine species in West and Central Africa and portions of Ethiopia and Sudan. All species are vectors of trypanosomes.
Control of Tsetse Flies
Avoidance of places where tsetse flies occur, as near rivers and certain vegetation, can reduce infection, but it is difficult for people who use such areas for obtaining water, fishing and gathering plant products. Elimination of reservoir animal hosts has been deployed with success, but reliance on them for food minimizes effectiveness. Insecticides continue to be used to kill adult flies, but the inevitable problem of insecticide resistance reduces effectiveness in time. Trapping methods are being deployed because they are less expensive (Green 1994), but the resistance problem eventually reduces effectiveness over time also. Service (2008) noted that genetic control with sterile tsetse male releases has been successful at times, as flies were actually eradicated from the island of Zanzibar with this approach (Gooding & Krafsur 2005). He suggested that the sterile males be used against residual populations of tsetse flies that were controlled with insecticides.
Also of great importance for control is a knowledge of where the adult flies pass most of their time. Both fed and unfed flies pass the night and most of the day resting in dark and humid places. Service (2008) noted that about 23 hours a day are spent resting on vegetation. Favored resting sites of most species are on twigs, branches and trunks of trees and bushes, but they shy away from areas where the temperature exceeds 35 deg. Centigrade. During the night the flies rest on the upper surface of leaves. The height at which adults rest on trees determines where insecticides should to be directed, and this is usually below four meters and even at ground level in some areas.
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Vectors of Trypanosomiasis in the Americas are all in the family Reduviidae, and they cause a disease called Chagas, which is treated separately (See: Chagas Disease ).
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