Monitoring Ferrets for TB Control

Ferrets are scavengers and experience has shown that if they are infected with TB then vectors such as possums are likely to be infected. In areas where herds are TB free but very occasional outbreaks may occur, monitoring of ferret TB status is a cost- effective method for identifying locations that may need more extensive vector control programmes. From time to time ferrets have “runs” and this is the most productive time to trap them. The Lower McKenzie Ferret Survey involves farm staff monitoring sentinel traps to determine when ferrets are on the move. When this happens staff notify the Animal Health Board, which arranges for contractors to bait and monitor more traps in the location. Carcasses are collected for post mortem examination and if TB is detected then the information is used in planning future vector control activities.

When TB infection is detected in cattle or deer herds, it can cause serious disruption to farm business. Movement control is put in place and this limits the options for sale of stock from the herds and hence affects profitability. Some years ago this happened in the Lower McKenzie basin and local farm owners and managers sought the help of the Animal Health Board in eliminating the problem. Culling of infected stock and a vector control programme (mainly possums) was effective and there are currently no infected herds in the area.

However, in past decades there have been spasmodic herd breakdowns (infections detected) indicating the presence of the disease in the vector population, according to Grant May, senior field supervisor with the Animal Health Board (AHB).

“Herds are tested annually and we found where there was a TB outbreak in a herd there was also TB detected in ferrets, so there was a distinct correlation between infected ferrets and infected herds grazing the same location,” says Grant.

“Ferrets are efficient scavengers and will pick up TB if it is about, so we are using them to test whether there are any pockets of infection in the wider vector population.”

Ferrets are a secondary vector for TB. The primary one in the area is the possum, and possums are controlled in the Basin, which covers an area of about 500,000 acres. However, attempting to survey possums for TB is very difficult, but because of the scavenging nature of ferrets they have about 100 times the TB sensitivity of possums and so are a much more cost effective species to target for TB surveillance purpose.

While they constitute a more sensitive litmus test, there are only a few times of the year when ferrets roam and are likely to have more contact with possums and livestock. Setting traps and checking them year round would also be very expensive, but an alternative programme devised by the AHB with support of farmers is proving very cost effective.

Lines of traps are placed on each farm (there are only six very large farms involved) in appropriate areas. For much of the year they remain unused. However, in specific locations where farm staff are likely to pass each day, there are sentinel traps permanently baited and set. When staff see a ferret caught in a trap they notify the AHB. If it is thought likely that a ferret “run” is happening a contractor goes out to the location and baits all the traps in the surrounding area, monitoring them frequently until no more ferrets are caught.

Each ferret caught is tagged with the GPS location of the trap and frozen for future post-mortem examination.

The process makes efficient use of time and resources by ensuring that much higher numbers of ferrets are caught without too much effort and unnecessary expense.

Noel Tait, a contractor involved, has had great success using wooden traps because he believes they retain the ferret smell and that attracts more ferrets and makes them less trap-wary. He says that the overall programme is unusual but effective because it revolves around farmers being proactive.

“Farmers call the AHB if they see ferrets in the sentinel traps, and once we work out that there is a pulse coming through then we open some main lines around those sentinels,” says Noel.

“The job is all about high capture rates, and Grant and I work out which lines should be opened and which ones closed. It’s all about timing and knowing when the ferrets run, it’s a bit different to any other method that we use for ferrets.”

Generally speaking the most likely time for ferret movement is in the breeding season from January to May but on Haldon Station the manager, Paddy Boyd, has noted runs in early and late spring as well. For this reason he and his staff make sure the sentinel traps are watched all year round.

“I’m very keen on this project because we have no herds in the Basin that are infected and we want it to remain that way. Ideally we would like to eliminate the last vector carrying TB, so we are more than happy to be part of this programme,” says Paddy.

“We have had good buy-in from the other farmers in the area and we are all prepared to monitor traps so that we see when the ferrets are moving and can let the AHB know. This makes it a very cost-effective way of monitoring and controlling infection.”

Paddy believes that the approach could well be useful elsewhere in the country.

Brent Rohloff, the AHB’s programme manager for vector control, is also pleased with the project which he says has just begun its third year.

“It differs from our normal ferret surveys where trapping is done between January and the end of May and the contractors go along when it is reasonably convenient for them to do the job and just catch what happens to be around at the time. In this case we target the ferrets when we know they are on the move,” says Brent.

“Around 1000 ferrets were caught last season in the survey but none had gross TB. This contrasts with earlier surveys where around 2-3% of samples had gross infection. This is a sign that our possum control programme is being effective in reducing TB infection rates in other wild animals, which we are also seeing reflected in the herd testing.”