Trypanosoma vivax: mechanical transmission in cattle by one of the most common African tabanids, Atylotus agrestis

Abstract

The role of mechanical vectors in the transmission of African livestock trypanosomes has always been controversial relative to tsetse flies, their cyclical vectors. An experiment was carried out in Burkina Faso to demonstrate mechanical transmission of Trypanosoma vivax by one of the most common tabanids in Africa: Atylotus agrestis. Eight heifers (crossbred zebu × Baoulé), free of trypanosome infection, were kept in a corral covered by a mosquito net, together with two heifers infected experimentally with a local stock of T. vivax. On average, 324 A. agrestis, freshly captured with Nzi traps, were introduced daily over 20 days. Parasitological, PCR and serological examinations were carried out regularly to assess infections and levels of parasitaemia. Microscopic examination of buffy-coats indicated that five of the eight receiver-heifers were infected on days 8, 13, 32, 41, and 48. PCR results indicated that these five heifers were already infected by day 13. Mechanical transmission of T. vivax by A. agrestis was demonstrated unequivocally, at a high rate (63% in 13–20 days). Conditions of transmission in this experiment are discussed in terms of natural rates of challenge. The importance of tabanids as mechanical vectors of T. vivax should be re-considered, in light of these results. Creation of tsetse free zones in Africa will generally lead to the disappearance of T. congolense, T. brucei, and most often T. vivax as well; however, in areas where T. vivax can be mechanically transmitted, clearance of tsetse may not be sufficient to eradicate livestock trypanosomosis. Index Descriptor and Abbreviations: Trypanosoma vivax (Trypanosomatidae) is a pathogenic trypanosome found in wild and domestic herbivores, principally in cattle ( Bos taurus, Bos indicus, and crossbreeds), in South America and Africa. In Africa it is principally cyclically transmitted by tsetse flies ( Diptera). In America it is only mechanically transmitted by tabanids (Tabanidae). The present paper describes the mechanical transmission of T. vivax in cattle by one of the most common African tabanids: A. agrestis. The main conclusion is that not only tsetse can transmit T. vivax in Africa, consequently the epidemiology of trypanosomosis in cattle is also tabanid dependant, and, the eradication of tsetse flies will not necessarily lead to eradication of T. vivax. ADT, Apparent density of insects per trap per day (mean number of insects caught in one type of trap per 24 h of trapping); PATTEC, Pan African Tsetse and Trypanosomiasis Eradication Campaign; RHD, rate of hematocrit decrease; D, day; ELISA, enzyme linked immunosorbent assay; NS, not significant (in terms of statistics); PCR, polymerase chain reaction