Molecular epidemiology of african animal trypanosomiasis in western Kenya

Abstract

Studies on the epidemiology of African bovine trypanosomiasis have concentrated on the temporal aspects with the longitudinal studies rarely considering the spatial dimension of disease prevalence. The problem is amplified by the low sensitivity of the parasitological diagnostic methods used in these surveys. This study aimed at combining highly sensitive molecular diagnostic methods, and Geographical information system (GIS) for spatial analysis, to provide accurate information and a better understanding on the epidemiology and control of bovine trypanosomiasis in Teso and Suba districts, Western Kenya. In addition, diagnostic capacities of three PCR based methods already developed namely; species specific primers, single, and nested PCR's based on primers amplifying the Internal Transcribed Spacers (ITS) region of rDNA was evaluated and compared. Blood samples obtained from forty four animals sampled from Teso and fifty nine from Suba was collected together with the geographical coordinates of every sampling site. In total 103 samples were screened for trypanosomes using the three peR based diagnostic assays to compare and evaluate the diagnostic capacities of each of the methods. Using ArcView and ArcGis Geographical information systems software, the distribution of all cases detected positive for trypanosomes was analysed. Results showed higher trypanosome prevalence in Suba district (40.67%) compared to 29.44% in Teso with T. vivax infections being the most prevalent species in both districts. T. brucei infections were relatively higher in proportion in Suba district (11.7%) compared to Teso (2.27%), a significant finding since parts of Suba are known to be sleeping sickness foci. Nested PCR detected the highest number of trypanosome infections (28.1%), the single ITS based PCR detecting 26.2% while the species specific primers detected 10.7% of the samples as positive for trypanosomes. Cohen kappa statistic was used to determine the agreements between the three tests. The results showed highest agreement (0.6) to be between the two ITS based tests, and the lowest (0.2) between the nested PCR test and the species specific primers. To determine the spatial pattern of the trypanosome cases detected, an average nearest neighbour analysis was used and gave a Z - Score of 0.9 and - 1 for Suba and Teso districts respectively, indicating a random pattern of the disease distribution. An overlay with the tsetse maps showed cases lying outside the tsetse infested areas, mostly being cases of T. vivax infections, which are known to be transmitted both biologically by tsetse and mechanically through other biting flies. These findings suggest a need to design control strategies that target not just the biological vector tsetse, but also possible mechanical transmitters as iatrogenic methods that could help explain the high prevalence of T. vivax.