Discussion

The problem of tuberculosis in wild possums in New Zealand bears many similarities with tuberculosis in the badger Meles meles in South-west England. In 1971 M. bovis was isolated from a badger carcase found on a farm in Gloucestershire where bovine Tb existed in cattle (M.A.F.F. 1976). Since then studies on tuberculosis in wild badgers have been made (Muirhead et al. 1974, Gallagher et al. 1976), and experimental work on transmission of infection, viability of the bacillus and badger ecology and behaviour have been initiated (M.A.F.F. 1976).

Behavioural changes have been reported in some badgers with advanced lesions. These badgers may leave their sets in search of more easily available food supplies or they may be rejected by the remaining set occupants (Muirhead et al. 1974). In possums I think the former reason most likely as, at least in experimental studies, they do not appear to reject sick animals and have continued to sleep with the carcase of a recently dead animal.

Inhalation is thought to be the main route of infection in badgers because of the high prevalence of lung or pulmonary lymphnode lesions, the dusty conditions that can prevail in sets, the finding of dust aggregations in lungs and the knowledge that dust particles can render the lung more susceptible to tuberculosis (Gallagher et al. 1976). The possum similarly has a high prevalence of lung infections but it is unlikely that dust would be a factor in the pathogenesis of the disease. The possibility of aerosol infection directly from infected possums occupying the same denning site is obviously high.

This intimate association in the denning site may also account for the high prevalence of superficial lymph node lesions in the possum: there could be direct contamination of wounds with infection localising in the regional lymph nodes. Bite and scratch wounds inflicted during the mating season may also be a factor in this method of spread of the disease.

The kidney is considered to be a predilection site for lesions following haematogenous spread in the badger (Gallagher et al. 1976). Such a site has not been defined in the possum. The superficial lymph nodes have been suggested (Animal Health Division 1974). The lungs should also be considered because of page 172 the high prevalence of lung lesions in possums and the finding that lungs were always affected following subcutaneous inoculation. Unfortunately no correlations have been made between the different sites affected in possums so we do not know the usual distribution of lesions within an individual animal. This information could give some information on the method of transmission of tuberculosis in the possum.

There is similarity between the gross lesions in the two species though it appears as if the badger develops miliary lesions in the lung more often. Histologically there appears to be no great differences between the two species.

As in the possum the tuberculin test was found not to react in experimentally infected badgers (M.A.F.F. 1976). The mild cellular reaction in field cases of badger tuberculosis has suggested that the badger has a low hypersensitivity rating, that the development of lesions is likely to be slow and the clinical course more prolonged than might be expected in the bovine. This hypothesis is supported by the finding that only two-thirds of the infected badgers examined had distinct visible lesions (Gallagher et al. 1976). The possum could also be considered to have a low hypersensitivity rating but experimental work has shown that the clinical course of the disease is rapid following inoculation with large numbers of organisms. It will require more work to see if the above hypothesis holds true when low numbers of organisms are inoculated.

The large numbers of possums examined in surveys have precluded the practicality of examining tissues from all animals microbiologically as was done in the British survey. But, in light of their finding one-third of infected animals with no distinct visible lesions, I feel that it is now necessary for us to do such a detailed survey. This does not mean that a full microbiological examination is necessary in all surveys as infected animals without gross lesions can hardly be considered a significant source of infection for cattle. This information is necessary to better understand the pathogenesis and epidemiology of the disease.

Parallels can also be seen in the epidemiology of the disease in the two situations. Both species were most likely originally infected from tuberculous cattle and now are both maintaining the disease in their own populations and serving as a reservoir of infection for cattle. The mechanics of transmission between the wild species and cattle has been suggested to be pasture in both instances.

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These comparisons have been made to give a better understanding of the problem of tuberculosis in wild animals. By pooling information from both sources one is then in a better position to formulate a policy aimed at solving such a problem.