EPIDEMIOLOGY OF BOVINE TRYPANOSOMOSIS IN SELECTED SITES OF THE NEWLY ESTABLISHED SETTLEMENT AREAS EAST WOLLEGA ZONE, WESTERN ETHIOPIA
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2006-06
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Abstract
A study on the epidemiology of bovine trypanosomosis in selected sites of the newly established settlements of Anger I, Anger II and Kenaf areas was conducted in East Wollega Zone, Oromiya Region, Western Ethiopia. The purpose of the study was to determine the prevalence of the disease and associating risk factors and estimate the apparent densities and distributions of tsetse and other biting flies. It was also to generate baseline data that may assist in decision making for planning and implementation of the settlement programmes. Retrospective study and questionnaire survey were conducted to collect background descriptive data and cross-sectional survey to determine the prevalence and vector apparent densities and distributions in late rainy and dry seasons in 2005/06. A total of 576 cattle were sampled and the buffy coat technique was applied to diagnose the trypanosome infection and 96 mono-pyramidal traps to capture the vector fly. Differences between parameters were tested for significance at probability levels of 0.05 or less. Results of the retrospective study and questionnaire survey indicated that bovine trypanosomosis is the major constraint to cattle production in the settlement areas. The curative and preventive doses of trypanocidal drugs are commonly used to control the disease. However, many cattle owners have developed the practices of selling out their work oxen after each crop season as the coping mechanism with the disease risk to replace by new animals and generate additional incomes from sales. The entomological survey showed that only two tsetse species, Glossina morsitans submorsitans and G. tachinoides were found to exist along with other biting flies of the tabanid and muscid groups. The G. m. submorsitans was detected at a mean apparent density (fly/trap/day) of 0.01 and G. tachinoides at 0.35. The overall apparent densities (fly/trap/day) were found to be 0.36 (95% CI = 0.22 – 0.50), 1.26 (95% CI = 0.55 – 1.97) and 41.15 (95% CI = 28.8 – 53.5) for tsetse, tabanids and muscid flies respectively in the study areas. The seasonal apparent densities were 0.40 and 0.33 (p > 0.05), 2.18 and 0.34 (p < 0.05), 76.0 and 6.29 (p < 0.05) for tsetse, tabanids and muscid fly in late rainy and dry seasons in corresponding order. The fly apparent densities in different vegetation types were assessed but the differences were not statistically significant (p > 0.05) for all fly kinds. The sex ratio for tsetse fly was determined and fewer proportion of male flies detected (21/58) and (15/47) during late rainy season and the dry season, respectively. The average age for male tsetse fly sample population
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was estimated from wing fray analysis and observed to be 19 and 16 days for late rainy season and dry season, respectively. The overall prevalence of bovine trypanosomosis was 8.9% (95% CI = 7 – 11%) the seasonal infection rate being 14.9% (95% CI = 11 – 19%) and 2.8% (95% CI = 1 – 5%) with statistically significant difference (p < 0.001) between late rainy and the dry seasons respectively. In all cases the dominant trypanosome species was Trypanosoma congolense 56.8% (95% CI = 43 – 71%) followed by T. vivax 31.4% (95% 21 – 47%). The prevalence of trypanosome infection in newly introduced cattle from tsetse free areas was 12.4% (95% CI = 8 -16%) whilst in native animals 4.3% (95% CI = 2 – 6%) with a statistically significant difference (p < 0.01) between the two groups. The prevalence by sex was 8.4% (95% CI = 4 – 12%) in the female and 9.1% (95% CI = 6 – 12%) in male counterpart. The infection rates in different age groups were nil, 8.0% (95% CI = 5 – 12%) and 10.0% (95% CI = 7 – 13%) in animals of less than 1, 1 – 4 and above 4 year-old, respectively. The differences in infection were not statistically significant (p > 0.05) between sexes and age groups. Analysis of the interaction of the assumed risk factors indicated season and origin to be the risk factors (p < 0.01) for trypanosome infection among the cattle population of the settlement areas. However, sex was not (p > 0.05) while age was signified to be risky which is largely determined by absence or presence of exposure to tsetse challenge. The risk of acquiring infection was lower in the dry season as compared to late rainy season (OR = 0.13, 95% CI = 0.06 – 0.28) and 3.36 times higher (OR = 3.36, 95% CI = 1.64 – 6.88) in the newly introduced than in the native cattle population. The mean PCV values were 27.23% (95% CI = 26.77 – 27.69%) and 20.22% (95% CI = 18.75 – 21.68%) with statistically significant difference (p < 0.001) for the non-parasitaemic and parasitaemic animals respectively. The mean PCV value was negatively correlated (r = -0.35) with the trypanosome prevalence. Bovine trypanosomosis is the major constraint in the newly established settlement areas. Introduction of susceptible cattle population to tsetse infested settlement areas would result in heavy losses especially animals coming from tsetse free areas being the high risk group when the vector challenge is high. Different epidemiological circumstances appeared to exist that would maintain the contact between tsetse and the host animal. Interiorly tsetse-infested wooded borders of many rivers and streams that traverse the areas as well as scrap of forest at the edge of settlements to which G. tachinoides seems well adapted would sustain their constant interactions. On the other hand moderately infested peripheries to which G. m. submorsitans receded that have a link with the major tsetse belt constitute a potential risk to the areas. Furthermore, the frequent
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movement of animals coming from or crossing the tsetse-infested areas to the major market outlet in Uke, Kenaf prefecture is the major source of infected animals. The vast potential land in the settlement areas would only be exploited if the problem of animal trypanosomosis is resolved under the present crop-livestock farming system. Therefore, community based integrated control option should be promoted with strong surveillance and monitoring activities.
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Keywords
Bovine, Origin, Settlement, Season