Trypanosomiasis
Tsetse are biological vectors of trypanosomes meaning that tsetse, in the process of feeding, acquire and then transmit small, single-celled organisms called trypanosomes from infected vertebrate hosts to uninfected animals. Some tsetse transmitted trypanosome species cause trypanosomiasis, an infectious disease. In humans, tsetse transmitted trypanosomiasis is called sleeping sickness. In animals, tsetse vectored trypanosomiases include nagana, souma, and surra according to the animal infected and the trypanosome species involved, although the usage is not strict and nagana is occasionally used for any form of animal trypanosomiasis.
Trypanosomes are animal parasites, specifically protozoa of the genus Trypanosoma. These organisms are approximately the size of red blood cells. Different species of trypanosomes infect different hosts as can be seen in the table attached to this section. Trypanosomes range widely in their effects on the vertebrate hosts. Some species, such as Trypanosoma theileri, do not seem to cause any health problems except perhaps in animals that are already sick.
Some strains are much more virulent. Tsetse seem to be unaffected by the infection of trypanosomes but it is entirely possible that the parasites alter tsetse behavior or have other effects that improve the chances of transmission and survival. These trypanosomes are highly evolved and have developed a life cycle that requires periods in both the vertebrate and tsetse hosts.
Tsetse transmit trypanosomes in two ways, mechanical and biological transmission.
- Mechanical transmission involves the direct transmission of the same individual trypanosomes taken from an infected host into an uninfected host. The name mechanical reflects the similarity of this mode of transmission to mechanical injection with a syringe. Mechanical transmission requires that tsetse feed on an infected host and acquire trypanosomes in the bloodmeal, and then, within a relatively short period, for tsetse to feed on an uninfected host and regurgitate some of the infected blood from the first bloodmeal into the tissue of the uninfected animal. This type of transmission occurs most frequently when tsetse are interrupted during a bloodmeal and attempt to satiate themselves with another meal. Other flies, such as horse-flies, also can cause mechanical transmission of trypanosomes.
- Biological transmission requires a period of incubation of the trypanosomes within the tsetse host. The term biological is used because trypanosomes must reproduce through several generations inside the tsetse host during the period of incubation, which requires extreme adaptation of the trypanosomes to their tsetse host. In this mode of transmission, trypanosomes reproduce through several generations, changing in morphology at certain periods. This mode of transmission also includes the sexual phase of the trypanosomes. Tsetse are believed to be more likely to become infected by trypanosomes during their first few bloodmeals. Tsetse infected by trypanosomes are thought to remain infected for the remainder of their lives. Because of the adaptations required for biological transmission, trypanosomes transmitted biologically by tsetse cannot be transmitted in this manner by other insects.
The relative importance of these two modes of transmission for the propagation of tsetse-vectored trypanosomiases is not yet well understood. However, since the sexual phase of the trypanosome lifecycle occurs within the tsetse host, biological transmission is a required step in the life cycle of the tsetse vectored trypanosomes.
The cycle of biological transmission of trypanosomiasis involves two phases, one inside the tsetse host and the other inside the vertebrate host. Trypanosomes are not passed between a pregnant tsetse and her offspring so all newly emerged tsetse adults are free of infection. An uninfected fly that feeds on an infected vertebrate animal may acquire trypanosomes in its proboscis or gut. These trypanosomes, depending on the species, may remain in place, move to a different part of the digestive tract, or migrate through the tsetse body into the salivary glands. When an infected tsetse bites a susceptible host, the fly may regurgitate part of a previous bloodmeal that contains trypanosomes, or may inject trypanosomes in its saliva. It is believed the inoculation must contain a minimum of 300 to 450 individual trypanosomes to be successful, and may contain up to 40,000 individuals.
The trypanosomes are injected into vertebrate muscle tissue but make their way, first into the lymphatic system, then into the bloodstream, and eventually into the brain. The disease causes the swelling of the lymph glands, emaciation of the body, and eventually leads to death. Uninfected tsetse may bite the infected animal prior to its death and acquire the disease, thereby closing the transmission cycle.
The tsetse vectored trypanosomiases affect various vertebrate species including humans, antelopes, bovine cattle, camels, horses, sheep, goats, and pigs. These diseases are caused by several different trypanosome species that may also survive in wild animals such as crocodiles and monitor lizards. The diseases have different distributions across the African continent and are therefore transmitted by different species of tsetse. The following table summarizes this information:
| Disease | Species affected | Trypanosoma agents | Distribution | Glossina vectors |
|---|---|---|---|---|
| Sleeping sickness — chronic form | humans | T. brucei gambiense | Western Africa | G. palpalis G. tachinoides G. fuscipes G. morsitans |
| Sleeping sickness — acute form | humans | T. brucei rhodesiense | Eastern Africa | G. morsitans G. swynnertoni G. pallidipes G. fuscipes |
| Nagana — acute form | antelope cattle camels horses |
T. brucei brucei | Africa | G. morsitans G. swynnertoni G. pallidipes G. palpalis G. tachinoides G. fuscipes |
| Nagana — chronic form | cattle camels horses |
T. congolense | Africa | G. palpalis G. morsitans G. austeni G. swynnertoni G. pallidipes G. longipalpis G. tachinoides G. brevipalpis |
| Nagana — acute form | domestic pigs cattle camels horses |
T. simiae | Africa | G. palpalis G. fuscipes G. morsitans G. tachinoides G. longipalpis G. fusca G. tabaniformis G. brevipalpis G. vanhoofi G. austeni |
| Nagana — acute form | cattle camels horses |
T. vivax | Africa | G. morsitans G. palpalis G. tachinoides G. swynnertoni G. pallidipes G. austeni G. vanhoofi G. longipalpis |
| Surra — chronic form | domestic pigs warthog (Phacochoerus aethiopicus) forest hogs (Hylochoerus spp.) |
T. suis | Africa | G. palpalis G. fuscipes G. morsitans G. tachinoides G. longipalpis G. fusca G. tabaniformis G. brevipalpis G. vanhoofi G. austeni |
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