Author: G. Nugent & J. Whitford
Objectives: To determine the sensitivity and relative utility of common hosts as sentinels for TB.
Main Findings: Possum density was estimated to average 55-110/km2 and home range size was estimated at 0.03 km2. Tb mainly affected adults. Possums have a 49-69% chance of 'detecting' (being infected by) any infected possum sharing their range.
Pig density was estimated at 1.9-2.8/km2, and home range size was assumed to be 3km2. Most pigs (85%) were infected, and it is estimated that pigs were capable of detecting at least 38% of infectious possum carcasses within their home range.
Ferret density was estimated at 0.9-1.5/km2, and home range size was assumed to be 1.2km2. Between 10% and 36% of ferrets were infected, and it is estimated that ferrets were capable of detecting at least 30% of the infectious possum carcasses within their range.
Depending on year, TB was confirmed in 0.62–2.51% of the cattle tested (approx 7400 per annum) on Molesworth Station. TB incidence in cattle exposed to infected possums was estimated at 0.026 per annum, and assuming a cattle home range size of 2 km2, it is estimated that cattle were detecting just 0.9–1.9% of infected possum carcasses within their ranges.
One of seven deer necropsied was infected - an incidence rate of 0.08 cases per annum. Assuming a density of 1/km2 and home range size of 2.5 km2 it is estimated that deer were detecting 3–6% of the infected possum carcasses within their range.
There was no TB found in the other 127 animals necropsied.
Comparison of the estimated cost of detecting each TB-infected individual indicate TB-testing of cattle was probably the most effective tool for measuring trends in TB prevalence on the station, but necropsy of pigs and ferrets was not much more expensive. As sentinels for determining the probability of freedom from TB, pigs were a more cost-effective detector than ferrets, wild deer, cattle or possums, in that order.
Conclusions: Possums and pigs are the main hosts of TB on Molesworth Station. It is estimated that approximately 1000 of each were infected, compared with <500 infected ferrets, even fewer infected deer, and only 45–143 infected cattle. TB appeared to be present in possums only where local possum density was above average (i.e. >9% Trap Catch Index). Patches of habitat capable of supporting such densities of possums are sparsely and discontinuously distributed, and it is suggested that spillback of infection from pigs may play a role in transferring TB between patches. The same did not seem to apply to ferrets even though they appeared to amplify and spread TB infection. While possum control is essential to eliminate TB from Molesworth, pig and ferret control is not crucial but could help increase the speed with which the risk to cattle is eliminated.
Although pigs and possums may be similar in their likelihood of becoming infected when they share their habitat with TB-infected possums, greater home range size and other factors make pigs many times more sensitive as sentinels. Ferrets are less sensitive than pigs and cattle and wild deer are far less sensitive sentinels than pigs or ferrets.
Despite much lower sensitivity, the cost-effectiveness of cattle testing was similar to that of necropsy of pigs and ferrets for monitoring disease trends in wildlife. If (or when) TB becomes rare, however, pig and ferret necropsy will become more cost-effective as tools for quantifying the likelihood of TB absence. In simple and very conservative terms, it can be said that necropsy of a 2-year old pig equals four 1-year-old ferrets, 100 annual tests of cattle or farmed deer, 50 biennial tests of cattle or farmed deer, or 50 necropsies of 2.5-year-old wild deer.
Recommendations: AHB should continue to increase its use of pig and ferret necropsies as cost-effective surveillance tools.
You can read the final report here.