Bovine tuberculosis (TB), a contagious disease caused by the bacterium Mycobacterium bovis, is a serious threat to the cattle and deer farming industries. It poses an economic risk to New Zealand, as dairying is one of the country’s most valuable industries, with exports worth $6.5 billion in 2007.
Bovine TB arrived in New Zealand with the cattle of the first European settlers. Despite attempts to eradicate it, small pockets have always remained. In 2007 around 0.4% of national cattle herds and 1.4% of deer herds were infected.
The mystery of why some herds continued to be infected was solved in the 1960s with the discovery that possums carried TB. Cattle were sniffing or licking dead or dying possums that came out onto pasture, and so contracting the disease. Ferrets also spread TB.
Although on average only about 2% of possums actually have TB, there are pockets where the numbers infected may be up to 40%. Infected possums were present in about 38% of the country (10 million hectares) by 2003. Most die six months after contracting the disease. They become lethargic and stay out in the open during the day, where curious cattle lick the dead or dying animals and become infected. In their native Australia possums do not have TB, possibly because they have less chance of contact with cattle.
Bovine TB control
The Animal Health Board is responsible for managing the possum TB problem. In 2006 it spent $60 million on controlling TB, mostly by poisoning possums, and an additional $27 million on research, herd testing and compensation.
There are two phases in bovine TB control: first, possums are heavily reduced over a large area, usually by aerial poisoning. Then, in the maintenance phase, possum numbers are kept low. If this is not done, TB returns to its previous level within five to eight years. The disease has been eradicated so far in only a few small areas.
Possum control responsibility is divided between the Animal Health Board, the Department of Conservation, regional councils and private individuals or companies.
Possums are killed by either poison or traps. Researchers are also working on biological control solutions.
A biological-control vaccine to reduce possum breeding is being developed. Trials have shown that possums treated with the vaccine have reduced breeding by 70–80%. If permission is given for the vaccine to be used in the wild, it will be made into baits and fed to possums from bait stations.
Six main poisons are used against possums: 1080 (sodium monofluoroacetate), phosphorus, cholecalciferol, pindone, cyanide and brodifacoum. Each is used in different situations.
What’s your poison?
The active ingredient in 1080, fluoroacetate, occurs naturally in about 40 plants in Brazil, south and west Africa, and western Australia, which can be dangerous if eaten. There are low concentrations of fluoroacetate in tea, but not enough to be harmful.
The most common and most controversial poison for possum control is 1080, which has been used in New Zealand since the 1950s. It is especially lethal to mammals, although it also kills birds, amphibians and insects. In the US it is used to protect sheep and goats from coyotes, and in Australia to kill pests such as rabbits, foxes, wild dogs and pigs. In 2002–3 New Zealand used 80% of the world’s total 1080 production: 2.3 tonnes of powder.
1080 is usually added to cereal, carrot or gel baits and dropped from helicopters in areas where there is a high level of possum infestation, or where the terrain is too difficult for trapping or laying poison by hand. Using GPS technology, pest control experts are able to accurately drop the poison to within a few metres of target sites.
People oppose the use of 1080 for three main reasons: they consider it harmful to the environment, they believe it is inhumane, and it also kills other ‘non-target’ animals.
Supporters point out that 1080 degrades in the environment, especially in water. Being soluble, it is rapidly diluted. In soil it does not persist for more than a few weeks, especially at warm temperatures.
Dogs sometimes die when they scavenge on possum carcasses after 1080 drops, although owners are warned to keep their dogs away from poisoned areas. Pests such as deer, stoats and rats are also killed, either because they eat 1080 directly or they feed on possums.