Principles for the use of macrocyclic lactones to minimise selection for resistance

Abstract

The primary recommendation is to use a mixture of effective drugs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, reliance on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low efficacy drugs such as naphthalophos when appropriate. Anthelmintic treatment should be given as part of a strategic worm control program. It is suggested that IVM-oral and IVM-capsules should not be used when ML resistance is present. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen ML is effective against the resistant parasite. It is essential to monitor the efficacy of ML and drug combinations by post-treatment worm egg counts, particularly when ML resistance has been detected.

Domestication of ruminant livestock and the impact of nematode parasites:possible implications for the reindeer industry

“…If the proportion of the population that escapes exposure to drug selection (refugia) is very small, then those individual parasites that survive treatment will have an enormous survival advantage to pass on resistance genes to their progeny (Dobson et al, 2001). Although the chanc-es of this occurring within parasite populations of reindeer might seem remote, the threat will always be present, particularly if ivermectin alone remains the drug of choice.…”

The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants

“…The two key approaches to deal with anthelmintic resistance currently used by producers are firstly to prevent the introduction of resistant worms onto farms and secondly to slow the development of resistance. With respect to the former, stringent quarantine measures can be used to limit the risk of introducing resistant worms (Dobson et al, 2001). However, in terms of the latter, the need to conserve the efficacy of current drugs and any anthelmintic families that may be developed in the future must be balanced with the ability to maintain high levels of animal production and welfare.…”

World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) Guideline: Anthelmintic combination products targeting nematode infections of ruminants and horses

“…The benefits of maintaining a population of nematodes in refugia, as a means of slowing the development of drug resistance, were first advanced by Martin et al (1981) and should not be underestimated; van Wyk (2001) and Dobson et al (2001) proposed that refugia could be the most important factor in determining the rate at which AR develops. Full consideration should be given to refugia when designing and implementing nematode control programs.…”

“…These infections are addressed as individual cases, unlike the population treatment paradigms typically employed in anthelmintic treatment strategies for ruminant livestock (and sometimes horses). Indeed, a key principle for slowing the emergence of AR in parasitic nematodes of ruminant livestock or horses is to obtain the highest possible removal of the parasites in significantly infected animals while ensuring that surviving parasites are diluted into a pool of susceptible, untreated parasites in the local environment as a refugial population (Dobson et al, 2001(Dobson et al, , 2011bBartram et al, 2012). This principle applies to singleconstituent active and combination products containing two or more distinct constituent actives (e.g., anthelmintics from different chemical classes with distinct mechanisms of resistance, or at least demonstrably different mechanisms of action).…”

“…The principles of continuing education are the same whether producers use singleconstituent active or combination anthelmintic products. This includes testing for AR to identify suitable constituent actives, estimating (however inadequately) nematode burdens and species by fecal egg counts (FEC) and preferably larval culture (or PCR) to determine appropriate treatment regimens, and the management of pasture exposures to reduce the overall parasite challenge in balance with the maintenance of drug-susceptible populations in refugia, which can help slow the development of AR in nematodes (Barger, 1999;Dobson et al, 2001Dobson et al, , 2011bvan Wyk, 2001;Baker et al, 2012;Bartram et al, 2012).…”