VN July 2023

Vetnuus | July 2023 8 Prof Gareth Bath Using ParasiticidesWisely All veterinarians have been alerted to the worldwide problem of AR – Antimicrobial Resistance – but are we paying sufficient attention to the other AR – Anthelmintic Resistance, or ER – Ecto-Parasiticide Resistance? These drugs are very costly to discover, test, formulate and register, and new drug ‘families’ only rarely come onto the market. They should therefore be used wisely and to best effect when they can do the most good and the least harm. We should not be relying only on drugs to control endo- and ectoparasites if there are other methods available. Resistance to drugs is, in the long term, inevitable because of the huge genetic diversity in parasite populations – the major factor is how fast the drug resistance develops. This, in turn, depends largely on the selection pressure – and this is strongly dependent on the survival of untreated parasites (‘refugia’) that remain susceptible to the drug. Once the genes that code for drug resistance become more common in a parasite population, the increase in clinical resistance becomes exponential. The following advice to farmers applies especially to anthelmintic use, but some underlying principles can apply to ectoparasiticides as well. Practices to avoid: • Be wary of relying exclusively on drugs to control parasites – this is a powerful cause of AR and ER. Overuse and misuse must be avoided • Blanket treatment of the whole flock or herd when only some animals will benefit • Frequent treatments in pre-planned programmes that increase selection pressure for resistance • Immediately moving the herd or flock after treatment to a fresh ‘clean’ pasture – the only parasites that survive this are drug-resistant. Practices to implement: • Implement Targeted Selective Treatment (TST) for helminths by using FAMACHA and the FIVE POINT CHECK • If the entire herd or flock is treated, then leave them in the same pasture for a few weeks to pick up untreated worms before moving them to a new ‘clean’ pasture. • Monitor the flock/herd parasite situation using FAMACHA and/ or Faecal Eggs Counts (FECs) • Check periodically (every three years) for drug efficacy using the FEC Reduction Test. • Quarantine all introductions of livestock and treat them with the best drugs available, then test for efficacy before releasing them onto an infected pasture that contains the local farm varieties of parasites. • Mark (identify) animals that need repeated treatments and cull them from the breeding programme. Select rams that are most resistant and resilient. • Manage the grazing system to benefit the livestock and not the parasites. • Identify and eliminate ‘hot spots’ where parasites survive and increase. • Alternate livestock species (like sheep vs cattle) for sequential grazing to lower the infection rates for both species. v