T. equiperdum is a venerealy transmitted, bilaterally from stallions to mares, and, it is also thought to be transmitted vertically in utero and perorale from the mare to the foal. Since blood phase of T. equiperdum is relatively short and high peaks of parasitaemia are not described, T. equiperdum is most probably not transmitted by mechanical vectors.

T. vivax is biologically transmitted by tsetse flies in their distribution area of Africa, but also mechanically, thus including other areas of Africa, and in Latin America. T. evansi is not able to implement a cycle in tsetse flies, due to the loss of some genetic material of its kinetoplastic DNA. Consequently, both T. vivax and T. evansi are mechanically transmitted by haematophagous flies such as Tabanids and stomoxes. They may also be transmitted iatrogenically, when doing serial injections for example. Vertical transmission is suspected but not often confirmed.

Mechanical vectors are haematophagous dipters such as Tabanids and Stomoxyine flies. Tabanids have more than 4,500 species world-over and 70 genera but the most commonly involved in trypanosome transmission are of the genera Tabanus (horseflies), Chysops (deer flies), Haematopota (clegs) and others such as Atylotus and Ancala. Stomoxyine flies are mostly represented by 18 species of Stomoxys, amongst which the cosmopolite S. calcitrans, and also by the smaller but very abundant flies of the genus Hematobia. Other mechanical vectors may have occasional importance such as flies of the genus Hippobosca, or midges of the genus Simulium, but their impact is not well know.

Trypanosoma evansi is also transmitted by vampire bats in Latin America. Vampires can get the infection by perorale when sucking blood from infected host (horse or cattle), and they develop surra disease, but may recover and exhibit intermittent parasitaemia, including salivary glands invasion by trypanosomes. From that stage, vampire bats can transmit the infection when biting their congeners inside the bat’s colony, or toward hosts. Meanwhile they are acting both as hosts, reservoir and vectors. When infected they can also transmit when biting (through infected saliva) or when being bitten (through infected blood). This phenomenon in which T. evansi behave similarly but multiplies in the vector is called biological tranmission, to be distinguished from « cyclical transmission » implemented by tsetseflies for true African trypanosomes.

As mentioned above, T. evansi can be transmitted perorale, thus not only to vampire bats, but also to carnivores, and it is frequent to observe dogs infections when feeding them with fresh infected meat, or in stray dogs eating around slaughter houses, and in hunting dogs, or even in carnivoresin the wild or in the zoo. Finally T. evansi is also transmitted through wounds, as was observed in some rare human cases in India and Vietnam.

Control of vecors is highly challenging since tabanids and stomoxes are highly prolific oviparous insects, laying 600-4,000 eggs in a life, conversely to tsetse flies, very low prolific larviparous flies, giving a maximum of 10-12 products in a life. Consequently, the insecticide impregnated blue screens used in Africa for the control of tsetse flies can hardly apply to mechanical vectors. However, new technology allowing to develop polyethylen insecticide impregnated blue screens allow, thanks to its very low production cost and highly sustainable activity, allow to use a set of 20-30 blue screens in one farm, in a protocol called « multi-target method ». This method is currently evaluated in sevral countries under a research porgramme called » FlyScreen » ; results are highly promising for the control of mechanical vectors, at least at the scale of a farm.

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