They conducted trials on 14 farms throughout New Zealand, and say while pour-ons and injectables are easier to use, they do not deliver the same benefits.
“Based on overseas data, I would not have been surprised if the pour-on product was generally less effective than the other two routes as there are issues with drug penetrating the skin and animals either licking the drug off their own backs or their neighbours,” says Dr Leathwick.
“We also thought the oral drug would work pretty well, while injectable macrocyclic lactones (the drug family which includes moxidectin, ivermectin, abamectin etc.) are regarded as the gold standard when treating cattle parasites on a global basis.”
He says when the results were analysed, the study confirmed how ineffective the pour-on product was, reducing the number of worm eggs shed in faeces by only around 50%.
“What came as a surprise, however, was that the injectable product performed no better than the pour-on. In comparison, the much cheaper oral product reduced worm egg output by over 90%.”
The parasite surviving treatment was predominantly Cooperia, which on most farms showed a level of resistance to these drugs.
However, this was not always the case and the presence of resistance does not explain the difference between the routes of administration.
Dr Leathwick says the results are probably related to how the drug reaches the target (i.e. the worms) after it is administered. Drugs given as injections or pour-ons have to be absorbed into an animal’s bloodstream, and then re-circulated to be released into the gut tissue where the worms live. This is easier for an injectable than for a pour-on product, as the latter has to get through a hide, which has evolved to keep things out.
“This became obvious when we measured the amount of drug in the bloodstream in the treated cattle – the levels were far higher in the animals given injections than in those treated with either of the other two routes.”
He says despite these results, the oral drench was still better at killing worms. “We don’t know for sure why this is the case, but some recent overseas work suggests that the oral drug does not need to rely on absorption and transport around the body in order to reach worms living in the gut.
“Instead, the drug gets bound to material in the gut and passes directly to the organs where the worms live. It appears that this results in higher overall levels of drug reaching the target worms - hence higher efficacy.
“What we have already proven is that using drugs with higher efficacy against worms lifts animal productivity, while killing more worms by using an effective drench reduces the selection pressure for resistance to develop, promoting the sustainability of worm control."
Dr Leathwick says follow-up studies have confirmed that this is not unique to moxidectin and that other pour-on and injectable products were no more effective.
“The next steps are to repeat the study against different worm species and also develop techniques to measure drug concentrations in the tissues where the worms live.”
He says there is four to five years of research ahead to determine whether the research findings apply equally to all worm species, and to assess the likely implications for long-term, effective worm control should farmers continue to use pour-on or injectable products.
Media contact Alex Fear, Senior Communications Advisor, AgResearch. Tel: 07 834 6636 / 021 809 183 email@example.com
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