Rift Valley fever outbreak in livestock in Kenya, 2006-2007

Abstract

We analyzed the extent of livestock involvement in the latest Rift Valley fever (RVF) outbreak in Kenya that started in December 2006 and continued until June 2007. When compared with previous RVF outbreaks in the country, the 2006-07 outbreak was the most extensive in cattle, sheep, goats, and camels affecting thousands of animals in 29 of 69 administrative districts across six of the eight provinces. This contrasted with the distribution of approximately 700 human RVF cases in the country, where over 85% of these cases were located in four districts; Garissa and Ijara districts in Northeastern Province, Baringo district in Rift Valley Province, and Kilifi district in Coast Province. Analysis of livestock and human data suggests that livestock infections occur before virus detection in humans, as supported by clustering of human RVF cases around livestock cases in Baringo district. The highest livestock morbidity and mortality rates were recorded in Garissa and Baringo districts, the same districts that recorded a high number of human cases. The districts that reported RVF in livestock for the first time in 2006/07 included Kitui, Tharaka, Meru South, Meru central, Mwingi, Embu, and Mbeere in Eastern Province, Malindi and Taita taveta in Coast Province, Kirinyaga and Murang'a in Central Province, and Baringo and Samburu in Rift Valley Province, indicating that the disease was occurring in new regions in the country.

Figure 1.

A map of Kenya showing districts that confirmed outbreak of Rift Valley fever in humans and animals in 2006/07.

Figure 2.

Distribution of the livestock cases for acute Rift Valley fever (RVF) virus infection by antiviral IgM antibodies in seven administrative sublocations affected by the outbreak. The highest proportion of acute livestock cases was in Sintaan and Longewan sub-locations.

Figure 3.

(A) Spatial distribution (dot density) of Rift Valley fever (RVF) confirmed cases in humans and livestock herds, overlain on the various soil types of Baringo district. The red circles (o) represent the GPS coordinates of livestock confirmed herd cases distributed randomly in the sub-location (1 dot = 1%). The black cross sign (+) represents the Global Positioning System (GPS) coordinates of confirmed human cases. The gold color represents regions carrying the Solonchak soil type where most of the RVF cases were located. (B). Map of Baringo district showing the 13 administrative divisions. Most of the RVF cases were concentrated in Marigat and Mukutani divisions.