J.M. Reifenberg, P. Solano, D. Cuisance, G. Duvallet
Centre de Coopération Internationale en recherche agronomique pour le développement Département d'Elevage et médecine vétérinaire (CIRAD-EMVT) B.P. 5035, 34032 Montpellier Cedex 1, France e-mail: [email protected]
References
Four trypanosomes that are pathogenic to African livestock, i.e. Trypanosoma brucei, T. congolense, T. simiae, and T. vivax, have been fully characterized through molecular biology techniques (Kukla et al., 1987).
These species are actually composed of different but morphologically similar taxonomic groups. Species belonging to the subgenus Nannomonas (including congolense and simiae), for instance, are among the most virulent cattle parasites. Four different types of T. congolense have been characterized with molecular markers: 'savanna', 'forest', 'kilifi', and 'tsavo' (Majiwa et al., 1985, Majiwa and Webster, 1987, Gibson et al., 1988, Majiwa et al., 1993).
Since 1992, CIRAD, ORSTOM, and the Centre international de recherche-developpement sur l'elevage en zone subhumide (CIRDES), in collaboration with the International Livestock Research Institute (ILRI), and the Tsetse Research Laboratory (TRL), which supply probes, have been developing molecular biology techniques to detect and characterise trypanosomes in mammal hosts, along with their vectors (tsetse flies and other biting insects).
In a recent study conducted in Côte d'Ivoire (Solano et al., 1995), molecular biology techniques were effective for accurate identification of these pathogens in known trypanosomosis disease foci. Ninety percent of all disease cases were found to be caused by the subgenus Nannomonas; this included many mixed infections with savanna- and forest-types of T. congolense that were detected in Glossina longipalpis. The two latter trypanosomes were also found in G. tachinoides in Burkina Faso (Solano et al., 1996). Surveys carried out in other regions of western Africa highlighted parasite-vector specificity between savanna tsetse flies (of the morsitans group) and the savanna-type of T. congolense, and also between gallery forest tsetse flies (of the palpalis group) and the forest-type of T. congolense (McNamara et al., 1989, McNamara and Snow, 1991).
Laboratory studies, using tsetse flies reared by CIRAD-ORSTOM, confirmed some trends concerning these parasite-vector compatibilities noted in the field (Reifenberg et al., 1996, Reifenberg et al., 1997, Reifenberg, 1996).
Our results showed:
- low vectorial competence index of G. tachinoides infected with T. congolense savannah type,- complicated affinity relationships between, on one hand, the savannah type of T. congolense and the morsitans-group tsetse flies, and, on the other hand, the riverine-forest type of T. congolense and the palpalis-group tsetse flies,
- PCR interpretation difficulty in evaluating the mechanical transmission of the disease.
Molecular diagnosis techniques are still not fully controlled and cannot yet be used in large-scale epidemiological studies because of their prohibitive cost; nevertheless they are very useful for small-scale targeted field and laboratory investigations. In the long term, it is essential to understand how pathogenic trypanosomes circulate within disease hot-spots where several parasites and vectors are present, and to determine the most dangerous parasite-vector combinations. This information would be useful for disease protection services to accurately target control campaigns.
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Kukla, B.A., Majiwa, P.A.O., Young, J.R., Moloo, S.K. and Ole-MoiYoi, O.K., 1987. Use of species-specific DNA probes for the identification of trypanosome infection in tsetse flies. Parasitology, 95:1-16.
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Reifenberg, J.M., 1996. Etude des relations parasites-hôtes dans l'épidémiologie moléculaire des trypanosomoses bovines au Burkina Faso. These de doctoral, Université de Montpellier, 6 décembre 1996, CIRAD-EMVT, Montpellier, France.
Reifenberg, J.M., Cuisance, D., Frezil, J.L., Cuny, G. and Duvallet, G., 1997. Comparison of the susceptibility of different Glossina species to simple and mixed infections with Trypanosoma (Nannomonas) congolense Savannah and Riverine-Forest types. Medical and Veterinary Entomology, in press.
Reifenberg, J.M., Cuisance, D., Gidudu, A. and Frezil, J.L., 1996. Evaluation de la compétence vectorielle de Glossina tachinoides (Diptera: Glossinidae) vis-a-vis de Trypanosoma (Nannomonas) congolense Broden 1904: implications épidémiologiques. Parasite, 3: 267-276.
Solano, P., Argiro, L., Reifenberg, J.M., Yao, Y. and Duvallet, G., 1995. Field application of the polymerase chain reaction (PCR) to the detection and characterization of trypanosomes in Glossina longipalpis (Diptera:Glossinidae) in Cote d'lvoire. Molecular Ecology, 4:781-785.
Solano, P., Reifenberg, J.M., Amsler-Delafosse, S., Kabore, I., Cuisance, D. and Duvallet, G., 1996. Trypanosome characterization by polymerase chain reaction in Glossina palpalis and G. tachinoides from Burkina Faso. Medical and Veterinary Entomology, 10: 354-358.
