THE EPIDEMIOLOGY OF MYCOBACTERIUM-BOVIS INFECTIONS

Mycobacterium bovis has an exceptionally wide host range, but until re cent years there was little concern about infection in species other t han cattle and man. Diversification of farming enterprises has led to cognizance of the need for control in other domestic animals, notably deer. There has also been recognition that self-maintaining infection is present in wildlife hosts in some countries - notably the European badger in the United Kingdom and Ireland, the Australian brush-tailed possum in New Zealand, and various species of ungulates in limited are as of a number of countries. Although transmission of M. bovis can occ ur by a number of different routes, control measures imposed on cattle and to a lesser extent on other species have reduced a number of the routes to insignificance. Hence the vast preponderance of transmission within host species is now by the airborne route, and predominantly b etween species as well. Transmission of infection from badgers to catt le may be an exception, with evidence remaining equivocal about the re lative importance of pasture contamination by excretion in badger urin e and airborne transmission. In general, contamination of feed and pas ture appears to be unimportant in transmission of the disease, because survival times of infective doses of organisms on fomites are relativ ely short under realistic conditions and because animals are not commo nly exposed to a dose high enough to be infective by the alimentary ro ute. Infection through the ore-pharyngeal mucous membrane may be signi ficant, although the infective dose for this route is not known. While many species of animals can become infected with M. bovis, only a few act as maintenance hosts and the rest are spillover hosts in which in fection is not self-maintaining. With the exception of cattle and deer , other species have become maintenance hosts only within part of thei r ecological range. For both badgers and possums, maintenance of infec tion within a local population is due to pseudo-vertical transmission from mother to young, and horizontal transmission linked to breeding a ctivity. Transmission from possums to domestic animals appears to occu r mainly during atypical behavioural interactions between the species, and this may well be important for badgers as well. Difficulties in c ontrolling the disease adequately in domestic animals generally result from administrative problems since the necessary technical procedures are available and have been shown to be effective. Where there is int erplay between infection in wildlife and domestic animals, eradication of the disease becomes impractical. Although herd prevalence will the n be inescapably higher and characteristically clustered in patches in both time and space, reduction of the incidence rate to a low level i n domestic stock is nevertheless achievable with current methods. Furt her reduction of incidence in the presence of a wildlife host should b e possible through application of ecologically designed management pro cedures at farm level, but greater gains could be made if new control measures could be developed, especially vaccines.