Within-herd spread of contagious bovine pleuropneumonia in Ethiopian highlands
Introduction
Contagious bovine pleuropneumonia (CBPP) is a major threat for cattle health and production in Africa. A list-A disease in the classification of the World Organisation for Animal Health (OIE) (Lefèvre, 2000), it was reported from 17 countries in 2001 (OIE, 2003). The disease is caused by the small-colony type of Mycoplasma mycoides subspecies mycoides (MmmSC) (Cottew and Yeats, 1978, Nicholas and Bashidurin, 1995). In its acute form, general and respiratory signs are observed: polypnea, coughing, painful breathing (Curasson, 1942, Martel et al., 1985, Provost et al., 1987); a marbled pneumonia and an exudative pleurisy are the most-obvious lesions. Recovered cattle often have necrotic lung tissue, encapsulated in sequestra where mycoplasmas can survive.
Transmission occurs from direct and repeated contacts between sick and healthy animals. The involvement of chronic carriers in the perpetuation of the infection has been suggested by several authors (Mahoney, 1954, Martel et al., 1985, Provost et al., 1987, Egwu et al., 1996) but is still debated (Windsor and Masiga, 1977). Risk factors for its spread include high-density confinement in night housings and use of common grasslands and watering places (Provost et al., 1987). In Africa, between-zone or -country contagion essentially is related to cattle movements caused by trade, transhumance and social troubles (Roeder and Rweyemamu, 1995).
In past years, prevention of CBPP indirectly relied upon internationally funded rinderpest-control programs. Pan-African mass-vaccination campaigns were carried out, during which cattle were immunized against both rinderpest and CBPP. Rinderpest was nearly eradicated from Africa and most countries recently stopped vaccination (and increased rinderpest surveillance) to be officially recognized as free of rinderpest (Yaya et al., 1999). Without further support, most African veterinary services were unable to achieve mass vaccination against CBPP, or to implement specific disease surveillance. This situation is thought to be partly responsible for the reappearance of CBPP in countries where it had been eradicated (or, at least, kept under control) (Masiga et al., 1996, Windsor, 2000b).
Because it is unlikely that pan-African CBPP mass-vaccination campaigns will be funded in the near future, research priorities should focus on improving the potency of the vaccines and looking for alternative control strategies (at the farm, zonal and national levels) (OIE, 1994, Masiga and Domenech, 1995). Economic assessment of these strategies is not possible without good evaluations of the epidemiological processes of the infection (e.g. the dynamics of new cases). Unfortunately, longitudinal data on the within-herd spread of CBPP are rare in general (Bygrave et al., 1968) and absent for mixed crop–livestock systems. These systems are common in Africa (especially in the East African highlands) and characterized by small herds managed by individual farmers (Gryseels and Anderson, 1983, de Leeuw et al., 1995).
In the Ethiopian highlands, cattle are the cornerstone of the agricultural system (draft power, milk, meat, manure, etc.) (Laval and Workalemahu, 2002). Veterinary services recently reported CBPP cases in the province of West Wellega (Laval, 2002). A research program was set up to estimate the epidemiological parameters of infection spread, build simulation models of CBPP dynamics and use them to test different strategies for disease control. Our goals in the present paper were to estimate the within-herd CBPP-spread parameters in newly infected herds, and to assess the effect of different disease management strategies (as actually implemented by the local farmers) on these parameters.
Section snippets
Studied area and agricultural system
We first identified a CBPP-infected area; the western part of Ethiopia was indicated as such by the national veterinary services. A preliminary survey was conducted and the Boji district was selected (Fig. 1). Mixed crop–livestock farming is the dominant agricultural system, providing a subsistence economy at the farm level. Few towns and villages are found in this area; farms are scattered in the countryside—making it difficult to implement mass-vaccination campaigns. Cattle are mostly of the
CBPP serological incidence
Ninety-nine seroconversions were observed during the survey period (Table 1). The statistical analysis of the sero-incidence provided similar results with the different logistic-regression models (Table 2). The time effect was negative and significant (P<0.01 at the most): the sero-incidence risk decreased with time. The estimated herd-effects were not distributed as Gaussian (Fig. 2, left plot), so a confidence interval based on the distribution quantiles was used (rather than tests) to assess
Discussion and conclusions
One of the major difficulties of the study was to identify newly CBPP-infected herds. From our field investigations, including discussions with the local veterinary-services staff and farmers, we believed that CBPP was resurgent in the Boji district. However, the observed clinical CBPP cases were relatively rare in the area. Further difficulties were the insidious nature of the disease and the lack of sensitivity and specificity of the pen-side diagnostic test (Turner and Etheridge, 1963). To
Acknowledgments
We thank the Ministry of Agriculture of Ethiopia, the West Wellega Zonal Agricultural Office (Gimbi) and the Boji Woreda Agricultural Office (Bila, including the staff of Bila veterinary clinic) for their friendly co-operation on this herd monitoring project. We are warmly grateful to the field enumerators and the farmers of Boji district for their help in data collection. This work was funded by the Scientific Direction of CIRAD, through the grant “ATP 60/2000—Modélisation d’une enzootie
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- 1
Present address: International Livestock Research Institute (ILRI), P.O. Box 5689, Addis Ababa, Ethiopia.
- 2
Present address: Ambassade de France, Service de Coopération et d’action Culturelle, BP 834, Antananarivo 101, Madagascar.