A cross-sectional epidemiological survey of bovine trypanosomosis and its vectors in the Savelugu and West Mamprusi districts of northern Ghana

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Abstract

The epidemiology of bovine trypanosomosis was investigated in two districts (Savelugu and West Mamprusi) of Northern Ghana with different land use and environmental characteristics. The land use intensity and environmental change was suspected to be higher in the Savelugu District. A cross-sectional entomological survey conducted along the White Volta river and its tributaries confirmed the presence of only Glossina palpalis gambiensis and G. tachinoides. The challenge index as measured by the product of tsetse density and tsetse infection rate was much higher in the West Mamprusi (19.6) than in the Savelugu district (4.7). A total of 1013 cattle (508 in Savelugu and 505 in West Mamprusi) were bled from a random selection of 16 villages in the Savelugu District and 13 villages in the West Mamprusi District. Blood samples were examined for trypanosomes by the buffy coat technique (BCT). Blood samples that were positive in the BCT or negative in the BCT but with packed cell volume (PCV) values below 21 were further tested with a polymerase chain reaction for trypanosomal DNA. Plasma samples of all cattle were serologically tested with an indirect ELISA for trypanosomal antibodies. The parasitological and serological prevalence of bovine trypanosomoses was significantly higher in West Mamprusi (16 and 53%, respectively) than in Savelugu District (8 and 24%, respectively). An evaluation of animal health at the village herd level, using PCV as an index of anaemia, provided various epidemiological scenarios prevalent in the entire study area.

Introduction

The direct and indirect impacts of African Animal Trypanosomosis on agriculture constitute a major constraint to the socio-economic development of tsetse fly (Glossina) infested areas of Africa (Swallow, 1998). Various options are available for the control of animal trypanosomosis; they include the suppression and/or eradication of tsetse populations (Jordan, 1986), the use of trypanotolerant breeds of livestock (D’Ieteren et al., 1998), chemotherapy (Peregrine, 1994) or combinations of these (Holmes, 1997).

Studies conducted in West Africa amply demonstrate that decision making on the control of tsetse-transmitted trypanosomosis, at the regional level, should be founded on an objective assessment of the impact of the disease on production systems (Hendrickx et al., 1999). At the local level, it is important that due cognisance is given to the dynamic nature of trypanosomosis, and in particular to the evolution of the disease with human population growth and agricultural expansion (Bourn et al., 2001). Although the morsitans group of tsetse flies (subgenus Glossina s. str.) is expected to decline appreciably with demographic pressure, the more tenacious palpalis (Nemorhina) group of tsetse flies could continue to persist in isolated habitats (Reid et al., 2000). A study conducted on an agro-pastoral zone of Burkina Faso, showed that changes in the distribution of Glossina palpalis gambiensis Vanderplank and Glossina tachinoides Westwood, over a 15-year period, were partially due to the proximity of cropping to river banks (de La Rocque et al., 2001); the said study did not, however, state the impact of such changes in fly distribution on the epidemiology of trypanosomosis.

This present survey sought to obtain and compare prevalence estimates of bovine trypanosomosis, transmitted by tsetse flies of the palpalis group, in two geographically contiguous areas (Savelugu and West Mamprusi Districts of Northern Ghana) with different land use patterns and to elucidate prevailing epidemiological scenarios in the two districts.

Section snippets

Study area

The two districts are located in the northern part of Ghana at the limits of the Guinea and Sudano-Guinean savanna zones. The riparian vegetation of the main river, which is locally referred to in Ghana as the “White Volta” and its tributaries are infested with G. palpalis gambiensis and G. tachinoides (Draeger, 1983). In the Savelugu District, which has a relatively high human population (60 people/km2), some segments of riparian vegetation have been obliterated by high human activity causing

Tsetse fly species distribution, population density and infection rate

There was a downward gradient in the apparent density of tsetse per site from the northern to the southern limits of the survey area (Fig. 1). Throughout the range of the tsetse survey area, the species found were G. palpalis gambiensis and G. tachinoides. The apparent density varied widely between sites. In the case of G. palpalis gambiensis, the number of tsetse flies caught per trap per day at a given site varied between 0 and 18 with a mean of 2. No G. palpalis gambiensis were found at the

Discussion

The trypanosome infection rate in tsetse flies was approximately the same for both districts but the tsetse density per site in the West Mamprusi District was almost four-fold higher than that of the Savelugu District, hence the trypanosome challenge in the former was much higher. Tsetse blood meal analysis would have provided the true trypanosome challenge that cattle in the areas were exposed to. However, blood meal analysis was considered impractical because of the low tsetse fly counts at

Conclusion

The study underscores the usefulness of cross-sectional studies as a precursor to tsetse and trypanosomosis control interventions. The results of the survey, besides identifying herds and villages where trypanosomosis constitutes a major problem, as far as animal health is concerned, also provided information on the prevalence of trypanosome species and strains as well as their vectors in the Savelugu and West Mamprusi Districts. The combined use of entomological, parasitological and

Acknowledgements

We are very grateful to the Management of CIRDES, Burkina Faso for having given support to this work through the budget of PROCORDEL, funded by the EU. The judgement of the Director of Veterinary Services (Dr. M. Agyen-Frempong) in the approval of this piece of work as a national priority of Ghana is deeply appreciated. Special thanks also go to the staff of the Tsetse and Trypanosomiasis Control Unit, who by hard work and commitment to duty made this research achievable.

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