MAINTENANCE OF A MICROPARASITE INFECTING SEVERAL HOST SPECIES - RABIES IN THE SERENGETI

Whether and how microparasites such as rabies persist in their host po pulations are among the fundamental questions of infectious disease ep idemiology. Rabies is a fatal disease of all mammalian species, but no t all mammalian species can maintain the infection as reservoirs. The approach to control depends on which of the affected species do act as reservoirs. Bringing together old and new data, we examine here the r ole of wild and domestic animals in maintaining rabies in the Serenget i region of Tanzania, presenting our findings in two parts. In Part I, we argue that domestic dogs are the likely reservoirs because: (1) ra bies has been continuously present in the dog population since its (re )introduction in 1977, whilst (2) wildlife cases have been very rare o ver this period, despite intensive study of Serengeti carnivores; (3) outbreaks of rabies in wild canids (jackals) elsewhere in Africa (Zimb abwe) have followed, rather than preceded, outbreaks in the dog popula tion; (4) all viruses isolated from wild carnivores in the Serengeti e cosystem (including the Kenyan Masai Mara) are antigenically and genet ically indistinguishable from the typical domestic dog strain; (5) dog rabies control in the Serengeti between 1958-77 apparently eliminated the disease from both dogs and wildlife. Having identified dogs as re servoirs, Part II explores some possible mechanisms of maintenance in dog populations. In theory, infection is more likely to be maintained at higher dog densities, and we provide evidence that rabies is mainta ined in one district with a dog density > 5/km(2), but not in two othe r districts with densities < 1/km(2). Because 5 dogs/km(2) is much low er than the expected density required for persistence, we go on to inv estigate the role of atypical infections, showing: (1) from serology, that a substantial proportion of healthy dogs in the Serengeti have de tectable serum levels of rabies-specific antibody; (2) from mathematic al models that, whilst we cannot be sure what seropositivity means, pe rsistence in low-density dog populations is more likely if seropositiv es are infectious carriers, rather than slow-incubators or immunes.