Rift Valley fever in Kenya: history of epizootics and identification of vulnerable districts

Abstract

Since Kenya first reported Rift Valley fever (RVF)-like disease in livestock in 1912, the country has reported the most frequent epizootics of RVF disease. To determine the pattern of disease spread across the country after its introduction in 1912, and to identify regions vulnerable to the periodic epizootics, annual livestock disease records at the Department of Veterinary Services from 1910 to 2007 were analysed in order to document the number and location of RVF-infected livestock herds. A total of 38/69 (55%) administrative districts in the country had reported RVF epizootics by the end of 2007. During the 1912-1950 period, the disease was confined to a district in Rift Valley province that is prone to flooding and where livestock were raised in proximity with wildlife. Between 1951 and 2007, 11 national RVF epizootics were recorded with an average inter-epizootic period of 3·6 years (range 1-7 years); in addition, all epizootics occurred in years when the average annual rainfall increased by more than 50% in the affected districts. Whereas the first two national epizootics in 1951 and 1955 were confined to eight districts in the Rift Valley province, there was a sustained epizootic between 1961 and 1964 that spread the virus to over 30% of the districts across six out of eight provinces. The Western and Nyanza provinces, located on the southwestern region of the country, had never reported RVF infections by 2007. The probability of a district being involved in a national epizootic was fivefold higher (62%) in districts that had previously reported disease compared to districts that had no prior disease activity (11%). These findings suggests that once introduced into certain permissive ecologies, the RVF virus becomes enzootic, making the region vulnerable to periodic epizootics that were probably precipitated by amplification of resident virus associated with heavy rainfall and flooding.

Rift Valley Fever Virus Infection in African Buffalo (Syncerus caffer) Herds in Rural South Africa: Evidence of Interepidemic Transmission

“…[1][2][3] Consequent animal export embargoes create significant economic hardship for affected local ranchers and pastoralist communities. Because of the ability of RVFV to infect many vector and animal species, and the likelihood of its regional persistence once introduced, it is essential to learn more about how RVFV is spread and persists within its transmission zones.…”

“…This pattern appears more subtle than the large outbreaks of zoonotic RVFV disease seen after extreme rainfall events. 3,6,7 It is possible that these lower levels of seroconversion may represent echo events of heavy RVFV transmission that occur during extremely heavy rainy seasons. These findings suggest the likelihood of continuing low-level transmission of RVFV to buffalo and other animals during non-flooding off years in suitable RVFV vector habitats.…”

Dynamic risk model for Rift Valley fever outbreaks in Kenya based on climate and disease outbreak data

“…The three sites are at varying altitudes with Garissa at 138 m, Murang'a (Thika) at 1501 m and Kwale 422 m. Meteorological data for Murang'a were obtained from Thika Meteorological Station (0 o 01'S 37 o 06E). The three sites were selected because they are key RVF-prone geographical areas of Kenya that are known to have experienced serious RVF outbreaks in the past as they are endemic for the disease (Murithi et al, 2010).…”

“…In Kenya, the spread of RVF has been rising systematically, from the confines of a single district in the Rift Valley area in the 1912-1950 period to 55% of the national districts in 2007. As a result, various parts of the country have gradually become enzootic, resulting in periodic epizootics following the first report of the symptoms connected with this disease in 1912 (Murithi et al, 2010). The areas in Kenya where RVF is enzootic include Nakuru, Nairobi, Thika, Maragua, Laikipia, Uasin Gishu, Trans Nzoia, Kiambu, Machakos, Kilifi, and Kwale.…”

“…The areas in Kenya where RVF is enzootic include Nakuru, Nairobi, Thika, Maragua, Laikipia, Uasin Gishu, Trans Nzoia, Kiambu, Machakos, Kilifi, and Kwale. Maragua sub-County in Murang'a has had 23 outbreaks since the first report in 1951; Garissa Central sub-County in Garissa has had 21 outbreaks since 1961, when the disease was first reported there, while Kwale in Kwale County has experienced 21 outbreaks since 1961 (Murithi et al, 2010). Early warning messages for RVF outbreaks, especially in East Africa, are often given by international institutions such as the Emergency Prevention System (EMPRES-i), the National Aeronautics and Space Administration (NASA) and the World Health Organization (WHO).…”