The role of scavenging in the spread of bovine tuberculosis in free-living ferret populations in New Zealand

Abstract

Ferrets (Mustela furo) exhibit high prevalences of bovine tuberculosis (Tb) in some areas of New Zealand. Whether the disease will persist in ferret populations in the absence of external sources of Tb infection is unknown and subject to extensive debate. The pathology of the disease in free-living ferrets suggests that in this species infection arises primarily through ingestion of tuberculous carrion. This thesis investigated the scavenging behaviour of ferrets with a view to determining their importance as a wildlife reservoir for the spread of bovine tuberculosis in New Zealand.

A field trial was undertaken on farmland in North Canterbury, New Zealand, to assess the relative contribution that scavenging of brushtail possum, ferret and hedgehog carcasses is likely to make to the observed tuberculosis prevalence in ferrets. A total of 108 carcasses were laid out in the field for 4-week periods in various seasons. Ferrets visited 35 percent of all the carcasses, with 16 percent of these visits resulting in scavenging. Ferrets were more than twice as likely to visit ferret carcasses as carcasses of other species (p<0.001), but did not prefer to scavenge them (p>0.05). The number of visits by ferrets declined in winter, but the scavenging rate per visit did not vary seasonally.

In a second trial, an outdoor enclosure and time-lapse video equipment were used to investigate the feeding behaviour of 10 adult wild-caught ferrets in response to ferret, hedgehog and possum carcasses. Ferrets fed more on possum carcasses than on the other carcass types, whereas there was no significant difference in the number of feeding events on hedgehog and ferret carcasses. On at least 15 of 22 occasions, ferrets ate parts of carcasses that would have put them at high risk of exposure to Mycobacterium bovis, had the carcass been infected with Tb. Even when they were not actively feeding, ferrets often had considerable physical contact with ferret carcasses. Few differences in feeding behaviour were observed between sexes, although male ferrets tended to eat more of a carcass than did females. Ferrets fed more frequently during winter but these feeds tended to be shorter in duration.

The implications of the observed scavenging behaviour for Tb transmission are discussed. Scavenging rates in North Canterbury were markedly lower than those observed by other researchers in Otago. It seems unlikely that scavenging alone is capable of maintaining Tb within North Canterbury ferret populations, although in other areas that have higher ferret densities and scavenging rates this remains a possibility.