Peste des petits ruminants (PPR) in Ethiopia: analysis of a national serological survey

Abstract

Background: Peste des petits ruminants (PPR) is a contagious viral disease of small ruminants in Africa and Asia. In 1999, probably the largest survey on PPR ever conducted in Africa was initiated in Ethiopia where 13 651 serum samples from 7 out of the 11 regions were collected and analyzed by competitive enzyme-linked immunosorbent assay (cELISA). The objective of this paper is to present the results of this survey and discuss their practical implications for PPR-endemic regions.

Methods: We explored the spatial distribution of PPR in Ethiopia and we investigated risk factors for positive serological status. Intracluster correlation coefficients (rho), were calculated for 43 wereda (administrative units).

Results: Seroprevalence was very heterogeneous across regions and even more across wereda, with prevalence estimates ranging from 0% to 52.5%. Two groups of weredas could be distinguished on the basis of the estimated rho: a group with very low rho (rho < 0.12) and a group with very high rho (rho > 0.37).

Conclusion: The results indicate that PPRV circulation has been very heterogeneous, the values for the rho may reflect the endemic or epidemic presence of the virus or the various degrees of mixing of animals in the different areas and production systems. Age appears as a risk factor for seropositive status, the linear effect seeming to confirm in the field that PPRV is highly immunogenic. Our estimates of intracluster correlation may prove useful in the design of serosurveys in other countries where PPR is of importance.

Figure 1

Seroprevalence of PPR across wereda in Ethiopia. Administrative map of Ethiopia indicating the regions and weredas boundaries. For each wereda seroprevalence of PPR was calculated by dividing the number of positive valid samples by the number of individual sampled in the wereda. As the colour gets browner higher is the seroprevalence found in the area. In grey, wereda for which no data was available.

Figure 2

Distribution of the correlation coefficient (ρ) across wereda. Histogram showing the values of the intra-cluster correlation coefficient calculated for 43 weredas for which information about the kebelle of origin of the samples was available as indicated in the Methods section. Two groups can be distinguished: one including almost 80% of the weredas with low values of ρ (ρ < 0.12) and the other with 9.3% of the weredas showing a strong intracluster correlation (ρ > 0.37).

Figure 3

Distribution of wereda ρ values across Ethiopia in 1999. Geographical distribution of the values for the intra-cluster correlation coefficient (ρ) by wereda. The red is more intense in the weredas with a higher value for ρ.