Seroprevalence of Brucellosis in Small Ruminants Across Three Agro-Climatic Zones

Abstract

Brucellosis caused by Brucella melitensis is one of the commonest infectious causes of abortion in sheep and goats, and the same animals are a main source of human infection through raw milk and contact with birth fluids. Surveys of antibodies in small ruminants are therefore widely used to describe the problem, and many of them are read as showing that infection is commoner in one kind of farming country than another. This narrative review asks how far that reading can be trusted when flocks from arid rangeland, mixed-farming plains and wetter hill country are compared. It draws on published work on the biology and transmission of the organism, on serological testing, on the design and analysis of prevalence surveys, and on the livestock systems typical of each zone, and it reports no new data. Differences between zones are biologically plausible, since mixing at water points, housing density, trade and the presence of cattle all change exposure. They are also entangled with the choice of test, the sampling frame, the unit of analysis, the vaccination history and the year. The review concludes that a zone effect is credible but rarely demonstrable from existing reports, and it sets out what a survey needs to record before its figures can be compared with those of another.