Addressing Drench Resistance in Cattle

Scientists in the AgResearch Animal Nutrition and Health Group have trialled the efficacy of applying moxidectin drenches to cattle either orally, as a pour-on along the back or by subcutaneous injection and compared the efficacy of internal parasite kills, followed by publication of the findings. They are extending this work to compare combination drenches (more than one active ingredient) administered in different ways and the implications for drench resistance in both Cooperia and Ostertargia parasites.  This work has taken on new importance for the industry now that resistance to the highly pathogenic parasite, Ostertagia ostertagi, has been confirmed on both heifer grazing and dairy beef operations in the North island

AgResearch set up a study comparing the efficacy of anthelmintics (drenches) applied to young cattle in three different ways – orally, topically (as a pour-on) and injected. Groups of 15 calves were used on each of 14 commercial farms throughout the country. They were sampled for faecal nematode egg count and then treated with ivermectin administered orally, or with moxidectin administered orally, as a pour-on or as an injectable. Faecal egg counts were done again 14 days after treatment and drench efficacy was calculated as a percentage reduction in-group mean egg count between pre- and post-treatment samples. On five farms untreated control groups were also run.

The results showed that the reduction in faecal egg count was significantly greater after treatment with moxidectin oral (91.1%) than with moxidectin injectable (55.5%) or moxidectin pour-on (51.3%). The low efficacies were invariably against Cooperia oncophora because drench resistance against this parasite is now widespread in New Zealand.

When publishing the results of their work, Dr. Dave Leathwick from AgResearch Grasslands and colleague Chris Miller said “Based  on  these  results, along with those of other studies,  it is proposed that oral administration of macrocyclic lactone anthelmintics results in higher  concentrations of active reaching the target worms in the gastrointestinal  tract than following either administration by injection or by pour-on.”

“Based on overseas data, we would not have been surprised to see that oral administration is much more effective than pour-ons because there are issues with the drug penetrating the skin and cattle licking the product off the backs of their herd mates, but the low results for injection were a surprise. Injectable macrocyclic lactones (which include ivermectin, doramectin and moxidectin etc.) are thought to be very effective elsewhere in the world. However, in the AgResearch study the much cheaper oral products were more effective than pour-ons or injectables. The Cooperia resistance does not explain the differences in methods of administration. The results are probably related to how the drug reached the target worms in the gut, either directly from oral drenching, or by a roundabout route through the blood supply in the case of the pour-ons and injections. Using drugs with higher efficacy against worms lifts animal productivity but it also reduces the selection pressure for drench resistance, promoting the sustainability of worm control.”

This issue has risen considerably in importance recently with confirmation of drench resistance in the parasite Ostertagia on several commercial farms in the North island. Drench resistance in the parasite Cooperia is extremely common in New Zealand cattle but has not become too much of an issue because it is the least pathogenic of the cattle parasites.  Ostertagia however is by far the most important of the cattle worms and widespread resistance in this species would have serious implications for farmers.  We now know that resistance exists in this worm species but we don’t know how severe it is or how widespread.  The obvious point is that the time for farmers to start taking action about resistance in cattle is now.  Blindly carrying on with what they have done in the past, which in most cases means using pour-ons, is not a wise move.

Further trial work is underway for 2014 when various combination products which are commercially sold and promoted for effectiveness against developing resistance will be compared, both as orals and as pour-ons. The trial into the pharmacology and efficacy of combinations is to test the proposition that some formulations are delivering less effectiveness than would be expected.

Scientists are investigating the use of combinations of drugs to combat resistance. Combinations of drugs not only slow the development of resistance, they also help farmers maintain worm control in the presence of resistance to one or more classes of anthelmintics.  However, the benefit of combinations relies on them delivering a full dose of all the actives to the worms, if they don’t do this then their benefit is reduced.