Drench Resistance Research

August 2007
A national survey of the prevalence of drench resistance on sheep and beef farms has shown that the spread of drench resistance is 20 years ahead of levels predicted three years ago. Lowered efficacy of drenches can result in significantly lower stock growth rates, loss of profitability; and in cases of total drench failure, clinical parasitism and death of the animals. Sheep farming has become impossible on some farms overseas due to an inability to control worms, so this is a big issue for New Zealands pastoral sector.

Hopkirk Research Institute Parasite Ecology team leader, Dave Leathwick, says the survey benchmarked an issue which was far more widespread than anybody thought and this has raised awareness among researchers and the farming community as a whole. When Dave started doing research on the issue 19 years ago, drench resistance was regarded as a fairly minor problem.

Hopkirk staff have been running a field trial (funded by Meat and Wool NZ) at Flock House, near Bulls since last December, where they aim to recreate real farming systems on a small scale. Currently they are working with 400 lambs divided into 12 farmlets, testing whether using combination drenches rather than single action drenches will slow the rate at which drug resistance develops in parasitic worms. They are also researching the effectiveness of leaving a small proportion of animals undrenched, thus allowing some drench-susceptible worms to survive. Identifying practical ways to retain some susceptible worms and slow the development of drug resistance is the main aim of the trial.

What farmers can do to minimise resistance.

One of the big issues is that many farmers never test the effectiveness of the drenches they use. Two to three years ago, no more than 5% of farmers had ever carried out tests on their farms. Although this is starting to change, Dave says that in his experience farmers think that if there is a problem they will notice it and that testing is not required. However this can sometimes be too late and result in increased costs. Dave suggests that farmers should increase their awareness of the problem of drench-resistance, talk to their vets, carry out drench efficacy assessment to see if the drenches they are using are working. The cost is a fraction of what it could cost them.

The Parasite Ecology research team have identified some high risk areas in farm management which result in worms becoming resistant faster. For example the practice of using long-acting drench products pre-lambing is convenient for farmers but carries a higher risk.

There are ways of avoiding the risks by targeted use of these drenches. For example you can use these drenches in the most vulnerable animals (lambing hoggets and 2-tooths) and leave the least vulnerable animals which are more able to deal with worms by themselves, says Dave. Also better-fed animals will result in better animal health and nutrition and therefore reduced susceptibility to parasitic worms.

Grassroots liaison

While the Hopkirk Institute has put in place the Hopkirk Foundation to advise and liaise with the farming sector, the Parasite Ecology team has had such a mentoring group in place since 1999. Consisting of farmers, a vet, a farm consultant and a Meat and Wool NZ representative. Dave says they have been fantastic and are the best thing they could have done as the group provides a valuable industry perspective. They ask questions about things we may not have thought of and they are also staunch advocates for our work among the farming community.