IUCN Species Survival Commission Veterinary Specialist Group Newsletter Number 12 Autumn 1996

ISSUES

1. Working together
(Summary: Report on conversations between VSG and officials of the World Association of Wildlife Veterinarians on future cooperative efforts.)

2. Mad deer disease? A bit of a stir among our members and friends
(Summary: Report on Chronic Wasting Disease observed in captive deer and wapiti in north-central Colorado and southeastern Wyoming since the 1960s)

3. Domestic dogs as reservoirs of rabies and canine distemper in the Serengeti
(Full text: Article by Sarah Cleveland of the London School of Hygiene and Tropical Medicine and the Institute of Zoology, London, UK)

During the past decade, rabies and canine distemper virus (CDV) have emerged as significant pathogons of wild carnivores in the Serengeti-Mara ecosystem. Outbreaks of rabies in the late 1980s and early 1990s caused high mortality in the endangered population of African wild dogs (Lycaon pictus) (Gascoyne et al., 1993; Kat et al., 1995) and from 1986-1988, accounted for 90% of adult deaths in one population of bat-eared foxes (Otocyon megalotis) (Maas, 1993).

Historically, several epidemics in Serengeti's wildlife have been attributed to CDV, including disease outbreaks in African wild dogs in 1968 (Schaller, 1972) and in black-backed jackals (Canis mesomelas) and bat-eared foxes in 1978 (Moehlman, 1983). Confirmation of the significance of CDV as a pathogen for Serengeti's wildlife came in 1994 during an epidemic that affected lions (Panthera leo), hyaenas (Crocuta crocuta) and bat-eared foxes (Roelke-Parker et al., 1996).

In Tanzania, as in many other part of Africa, there are several reasons why rabies and CDV may be an increasing threat to wildlife. First, dog populations are growing rapidly in many countries, at a rate often exceeding that of human populations. As dogs become more abundant, rabies and CDV are increasingly likely to persist in reservoir populations large enough to maintain infection. Second, increased mobilization of the human population has led to rapid expansion in rural areas, which is likely to result in higher rates of contact between dogs and wildlife. Third, the Serengeti lion epidemic suggests that new biotypes of CDV may be emerging that are pathogenic for an increasing range of wildlife species (Roelke-Parker et al, 1996).

For both rabies and CDV, antigenic and genetic characterization of isolates from wild carnivores and domestic dogs suggests that a single "canid" strain of rabies and CDV is circulating in Serengeti's carnivores (Gascoyne et al., 1993; Kat et al., 1995; Harder et al., 1995, Roelke-Parker et al., 1996). However, although many species may become infected, not all are able to act as reservoir hosts. The key to controlling these diseases is the identification of reservoir populations and this has been the focus of recent studies in the Serengeti (Cleaveland and Dye, 1995).

Throughout much of sub-Saharan Africa, rabies is most frequently reported in domestic dogs. However, in part of southern Africa, there is evidence that some wildlife populations are able to maintain independent cycles of infection. For example, in the central plateau of South Africa, one distinct group of viruses ("viverrid" rabies) is maintained principally in yellow mongooses (Cynictis penicillata) (King et al., 1994). The other type ("Canid" rabies) appears to be maintained independently both by domestic dogs (e.g., Natal) and, in some areas, by wild carnivores (for example jackals in Transvaal and bat-eared foxes in the south-western Cape) (Swanepoel et al., 1993; Thomson & Meredith, 1993).

In the Serengeti, domestic dogs are found in all areas surrounding the National Park and are kept for guarding property, herding livestock and, occasionally, hunting wildlife. Domestic dog densities vary from less than one dog per square kilometer in pastoralist areas to the south and east of the park (Ngorongoro District) to more than 5 dogs per square kilometer in agropastoralists areas in the west (Serengeti District), a density higher than that recorded for any of Serengeti's wild carnivores (Cleaveland and Dye, 1995). Historical data, combined with cross-sectional and cohort studies, indicate that rabies and CDV persist only in higher-density dog populations (over 5 dogs per square kilometer) and that the Serengeti District population is the most likely reservoir of both infections.

... (Four lines of evidence providing support for this view) ...

If CDV and rabies do indeed persist only in higher-density dog populations to the west of the National Park, control of infection in this population should reduce or eliminate infection in all other species. This hypothesis forms the basis of a vaccination trial soon to be [now being] conducted in Serengeti District under the auspices of the Tanzanian Ministry of Agriculture, with the support of Tanzania National Parks and with funding from Project Life Lion.

A key issue in the design of any vaccination program is the critical percentage of the population that needs to be immune to prevent outbreaks of disease. For rabies, both epidemic theory (Coleman & Dye, 1996) and practice (Beran, 1991) indicate that it should be enough to vaccinate 70% dogs. This is likely to be a rigorous estimate, since it was established from experience with high-density, urban dog populations, where contact rates are probably higher than those in Serengeti.

For CDV, analysis of age-seroprevalence data suggest that a 50% vaccination coverage should prevent CDV outbreaks in Serengeti District dogs (Cleaveland, 1996. This preliminary estimate is lower than that obtained for measles and rinderpest (about 95%) and raises questions about our assumptions about CDV transmission dynamics in nature.

The aim of the vaccination trial in the Serengeti is to immunize 70% of dogs in the Serengeti District population, to test the prediction that high-coverage vaccination will control rabies and CDV in domestic dogs and, hencee lead to a reduced risk of infection for wildlife. In agropastoralist areas, accessibility of dogs appears to be high, public attitudes to vaccination positive, and attaining 70% coverage should be feasible by central-point vaccination (Cleaveland, 1996). This contrasts with pastoralist areas, where accessibiity of dogs is much lower. Mass vaccination of dogs is due to start ... within a 20 km vaccination one immediately adjacent to the western and northwestern borders of the park. An unvaccinated control zone will be located further from the parks toward Lake Victoria. The impact of vacccinaion will be monitored by comparing incidence and sero-conversion rates in immunized and control zones. Birth and death rates will also be closely monitored to assess the impact of disease emergence of a stray dog population.

In the Serengeti , there remains an important question of sustainability of disease control programs. The limited financial resources available to government veterinary services are unlikely to be sufficient to maintain long-term vaccination programs and viable strategies for funding need to be explored that reflect the needs of public health, conservation, tourism, and hunting. In the Serengeti, rabies and distemper present problems for local people, wildlife and domestic animals, and an integrated approach to their control is likely to be the best way forward.

(References for this article and omitted material may be obtained through Dr. Michael Woodford in conjunction with a paid subscription to the newsletter.)