Rotation of sheep louse treatments
Infestations of sheep body louse, Bovicola ovis, have been controlled by the application of chemicals to the host animal's fleece for as long as sheep have been farmed in New Zealand.
Chemicals used historically and now abandoned include arsenic, nicotine, and organochlorines. More recently used chemical classes include organophosphates, synthetic pyrethroids, benzoyl phenyl ureas (diflubenzuron and triflumuron), ivermectin, spinosad and imidacloprid.
While this list appears comprehensive, offering many treatment choices, the reality is that the number of effective options is limited, largely because of louse resistance to many chemicals.
Of the available chemicals, resistance has been identified in Australian sheep louse populations to synthetic pyrethroids, diazinon, and benzoyl phenyl ureas.¹ Louse resistance to spinosad, ivermectin and imidacloprid has not been recorded.
In New Zealand, resistance to synthetic pyrethroids has been recorded, however, the true situation in this country is unclear following the dis-establishment of the AgResearch Entomology Group in 2000 which monitored resistance in this country.
Based on Australian recommendations², the approach sheep farmers should adopt is the rotation of effective treatments.
Resistance is heritable, meaning that resistance is passed on from one louse generation to the next. Repeated use of the same chemical group can cause ongoing selection of the more resistant parasites, while the dwindling number of susceptible lice are killed. If the relative proportion of resistant lice in the population increases compared to susceptible lice, and the resistant lice come to dominate, then the effectiveness of the chemical to control the parasite continually diminishes.
Importantly, resistance to different chemical groups occurs through different mutations within louse populations. As a result, resistance to one chemical group does not usually equate to resistance to other chemical groups. However, resistance against an active within a chemical group confers some level of resistance to all other actives within the same chemical group. Avoiding repetitive use of actives within the same chemical group helps reduce the build-up of resistant individuals within populations.
Having a good knowledge of the active ingredients in the chosen louse product is essential. An integral component of any louse control rotational programme is using products based on the active ingredient spinosad, such as Expo™ Extinosad™ and Cyrex™.
It is also important that along with rotation of effective products, other louse control measures are implemented. This includes correct application of products, quarantine of brought-in stock, biosecurity measures and animal management strategies.
For assistance planning a louse control programme, contact your local PGG Wrightson Technical Field Representative.
1 Heath, A.C.G., & Levot, G. (2014). Parasiticide resistance in flies, lice and ticks in New Zealand and Australia: mechanisms, prevalence and prevention. New Zealand Veterinary Journal 63(4):1-32.
2 Pesticide Resistance for Sheep. Retrieved 18 March 2024 from https://liceboss.com.au/pesticide-resistance-2/.
Registered pursuant to ACVM Act 1997, Expo™ Extinosad™ Pour On #A010205 and Cyrex™ Liquid #A09917. Always read the registered label before use.
