Epidemiology

To date tuberculosis has not been recorded in wild possums in Australia so we presume that infection occurred once they became established in New Zealand. Thus there can be little doubt that the possums were originally infected from cattle. When this occurred it is impossible to say. It is likely that cattle-to-possum transmission occurred independently in widely separated areas in New Zealand (Davidson 1976).

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Possum-to-possum transmission of tuberculosis has been found to occur readily under experimental conditions. It has occurred between possums in direct contact (Bolliger and Bolliger 1948, O'Hara et al. 1976), close contact in adjacent cages and by aerosol over distances up to 180 cm (O'Hara et al. 1976).

It is possible to infect possums experimentally by intraperitoneal and intramuscular injections or by feeding infected material (Bolliger and Bolliger 1948), also by subcutaneous inoculation and intranasal instillation (O'Hara et al. 1976). All these factors suggest that transmission of tuberculosis could readily occur in possum populations especially when possums are in close association such as in the denning site. It is highly probable that the disease can maintain itself in a wild population and, on occasions, result in spread to neighbouring populations.

There are two ways in which man can disseminate tuberculosis from one population of possums to another; by the transportation of infected carcases to a part distant from the area of collection, and by introducing tuberculous cattle into a previously uninfected possum habitat (Davidson 1976).

The potential risk of infected possums contaminating pastures which could then be grazed by cattle has been realised for some time (Ekdahl et al. 1970). The importance of possums as foci of infection for cattle is now more clearly understood with infected possum communities discovered in some 23 general localities in New Zealand (Davidson 1976), and compelling circumstantial evidence linking tuberculosis in possums to high and persistent reactor rates in cattle sharing the same environment (Animal Health Division 1976). The transmission between possum and cattle is presumably through contamination of cattle pasture and possibly also by investigation of possum carcases by cattle (Davidson 1976).

Possum trappers and people involved in autopsies on possums in survey work are obviously exposed to a significant health hazard (Ekdahl et al. 1970, Davidson 1976).

To date only one case of human tuberculosis has been ascribed to handling diseased possums, this was a case of a wound infection in a trapper (Davidson 1976). Protective clothing is made available to all people involved in survey work. Possum trappers can minimize the risk by limiting their handling of obviously diseased carcases and by maintaining good hygiene during and after page 171 handling carcases. It is important that trappers are taught to recognise lesions which may be tuberculous in possums.