The epidemiology and socio-economic impact of rift valley fever in Tanzania: a review

Abstract

Rift Valley Fever (RVF), caused by RVF virus (RVFV), is an acute, mosquito-borne viral disease that has a significant global threat to humans and livestock.  The causative agent belongs to the genus Phlebovirus of family Bunyaviridae. It is an emerging pathogen, whose range has recently expanded from East Africa, across sub-Saharan Africa to North Africa, and to the Arabian Peninsula. Tanzania is amongst the countries of the sub-Saharan Africa that have encoutered a number of disease epidemics. Relatively little is known about the trend of occurrence, epidemiological factors and socio-economic impact related to previous epidemics in the country.

This review was conducted to provide comprehensive update on Rift Valley Fever (RVF) in Tanzania, with particular attention devoted to trend of occurrence, epidemiological factors, socio-economic impact and measures which were applied to its control. Ultimately, the information gathered in this study will help to improve better understanding of epidemiology of disease and areas at higher risk important for contingency plans for outbreak management, proper allocation of resources to prevent further occurrence and spread of this disease. Information presented in this paper was obtained through extensive literature review. RVF was documented for the first time in Tanzania in 1977. This was followed by epidemics in 1997 and 2007. Contrary to the latest epidemic in 2007 sporadic cases of RVF during the previous epidemics were confined to mainly livestock and mostly affecting northern parts of Tanzania. The latest disease epidemic expanded to cover wider areas of the country involving both human and domestic ruminants. During the latest disease outbreak 52.4% (n=21) of regions in Tanzania mainland were affected and majority (72.7, n=11) of the regions had concurrent infections in human and animals.

Phylogenetic comparison of nucleotide and amimo acid sequences revealed different virus strains between Kenya and Tanzania.  Epidemiological factors that were considered responsible for the previous RVF epidemics in Tanzania included farming systems, climatic factors, vector activities and presence of large population of ruminant species, animal movements and food consumption habits. Majority of the RVF positive cases in the latest epidemic were livestock under pastoral and agro pastoral farming systems.

The disease caused serious effects on rural people’s food security and household nutrition and on direct and indirect losses to livestock producers in the country. Psycho-social distress that communities went through was enormous, which involved the thinking about the loss of their family members and/or relatives, their livestock and crop production. Socially, the status of most livestock producers was eroded in their communities.

Cessation of lucrative trade in ruminants resulted in serious economic losses to the populations who were totally dependent upon this income. Livestock internal market flows drastically dropped by 37% during latest epidemic. RVF epidemics had dramatic impact of RVF outbreak on the international animal trade in which there was a 54% decline in exports equivalent to loss of 352,750 USD. The estimate of loss as a result of deaths for cattle was 4,243,250USD whereas that of goats and sheep was 2,202,467 USD.

Steps taken to combat epidemics included restriction of animal movements, ban of the slaughter of cattle and vaccination of livestock and health education.