Dewormer resistance in sheep

There is a growing problem of parasitic worm resistance in sheep. Even if it is often not im me diately recognisable by farmers because of its insidious nature, it can lead to significant productionloss in sheep on af fec ted farms. However, resi - stance can be controlled through appropriate worm treat - ment and flock management strategies.

Background to worm resistance in sheep Resistance to treatment for round worms amongst sheep has been identified as a gro - wing problem in many coun - tries where sheep are produced. Often the problem is not recog - nised in sheep flocks until it has reached significant proportions, as the loss of production may not be noticed until such time as a threshold or tipping point is reached.

By the time resis tance is clinically apparent, it has become well and truly established in the flock and on the pasture. By then, the appli ca tion of resistanceprevention strategies might not be effective. At that stage too, treatment with most classes of commonly-used wormers may be of little value, as resistant worm-eggs have become dominant on the pasture, where they remain as a reservoir for future generations of sheep which may graze on them.

The resistance problem can be traced to the practice of wor - ming sheep year-on-year with wormers that contain chemicals of the same chemical class. Such routine preventative di -

OVK NUUS | Maart 2020 Information from HPRA

sease approaches for worm control that were practiced for the past number of decades are now out-of-date scientifically. These approaches were aimed at reducing the parasitic burden in individual animals and in the flock at large. However, they had the effect of selecting for resistance amongst the parasitic worms ingested from the pasture.

In particular, the practice of treating all sheep on a farm at the same time and with the same wormer and moving them to a clean pasture immediately after treatment, and repeating this practice at frequent intervals during the grazing season, has created the ideal scenario for the development of worm resistance.

The reason for this is that once susceptible worms are killed in the sheep, they can no longer shed eggs which are passed out in the dung to the pasture. This means that the way is clear for eggs from resistant worms to take over; in accordance with the established life-cycles of the worms concerned, the resistant worms lay eggs that contain the genes coding for resistance to the class(es) of wormer being used become the dominant strain on the pasture. In extreme cases, these resistant eggs become the only source of new infestation amongst naive sheep. Once ingested by the animal they

take up residence, develop into worms and begin to lay eggs. In this way, the level of pasture contamination with resistant eggs increases significantly over time. Moreover, infested sheep may be unknowingly traded and can act as incubators to spread resistance to new farms that previously did not have the problem.

Problem identification Stock owners and keepers should ideally monitor the status of their flock for worm resistance as this will influence the choice of appropriate treatment and control strategies. Resistance to wormers can be suspected where: l The overall liveweight gain of the flock is falling yearon-year for no obvious reason. l Individual animals in the flock fail to thrive despite adequate nutrition or obvious disease being present, l Individual animals continue to scour even after treatment with a broad-spectrum wormer.

Resistance can be determined by faecal testing. This can be done either on: l A representative sample of animals in the flock, or l Individual animals e.g. bought-in animals.

Where a problem is suspected, professional advice from a veterinary practitioner or other animal health specialist should be sought and presence of resistance confirmed by appro - priate laboratory tests.

Treatment goals Effective treatment and control strategies are based on: l Targeted treatment of se - lected animals with an ap - propriate drug,

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Biosecurity measures to prevent the introduction of resistance into the flock, e.g. by appropriate treatment and quarantine of new animals (e.g. rams) before they join the flock, Monitoring the health and production of the flock, and Managing the pasture to minimise the threat of the resistant worm-eggs beco - ming dominant over sus - cep tible worm-eggs.

The goal for modern worm strategies is primarily to manage the pasture worm burden, ba - lancing the need of individual animals for suitable therapy with the goal of ensuring the survival of susceptible parasites which can competitively inhibit the number of resistant parasites available on the pasture.

In this way animals at pasture will not be exposed exclusively to re sistant worm eggs but rather will ingest susceptible worm eggs. These animals can be treated before their worm bur den leads to significant produc tion loss. In practice, this means that treatment is targeted to wards: l Scouring or soiled animals only, or l Animals with confirmed high infestations, or l Animals with markedly poor body condition.

Animals with light worm infes - tations of susceptible parasites should not be routinely treated (so as to maintain the pasture supply with susceptible worm eggs).

Choosing an appropriate wormer There are a considerable num - ber of veterinary medicines authorised for the treatment of worms in sheep and it can be difficult for a lay-person to de - termine which to use. This dif fi - culty is compounded by the fact that the same, or very si - milar, wormers are marketed un der different brand names. If the user were to switch be - tween brands of drug from the same chemical class of chemi - cals, he might not in fact be switching to an appropriate drug, but might conversely be ac centuating the problem.

The objective in choosing an appropriate wormer is to pick one which will kill the parasites that are present and avoiding those for which resistance has been identified e.g. so-called white drenches are not effective to control parasites that are benzimidazole-resistant. There - fore where benzimidazole resis - tance has been identified, users who switch from e.g. alben - dazole to fenbendazole contai - ning brands will be wasting their money and adding to the pro - blem of resistance on their farms, likewise where resistance to other chemical classes has been identified.

Recommendations for using wormers Whatever the chemical class of the drug that is to be used, sole reliance on that chemical as the only wormer will, inevi - tably, hasten the develop ment of worm resistance in a flock. The following general principles apply to minimise the deve - lopment of worm resistan ce:

Use a wormer from a chemical class which is likely to be effec - tive. Where resistance to a parti - cular chemical has been confir - med, avoid using that class of chemicals again.

Treat only those animals in the flock that: l Are scouring, or l Are suffering from ill-thrift, or

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Have a high level of parasitic infestation as confirmed by faecal tests.

Weigh the animals before treat - ment and apply the correct do sage. Too low a dosage in - creases the chances of resis - tance developing.

Rotate the chemical classes of wormers used each year, avoi - ding those to which resistance has been found.

Monitor the productivity of the flock during the year. Where poor thrive is suspected, discuss the position with your veterinary practitioner or animal health advisor.

Recommendations for worming new sheep prior to joining the flock Sheep that are purchased to join an existing flock, which is known to be free of resistant worms, may contain worms that are resistant and therefore act as a source of anthelmintic-re - sistant eggs which could in fect the flock. Two control mec h a - nisms exist: l The imported sheep may be quarantined in a shed or concrete yard and tested for the presence of resistant worms. Only if the sheep is found not to harbour resis tant worms, should it be allowed to join the flock. l Where the imported sheep have either not been tested for the presence of resistant worms, or have tested po sitively for resistant worms, they should be quarantined in a shed or concrete yard and wormed with an ap - propriate wormer. In this case the appropriate wor - mer is one that is: l Of a different chemical class to that of which resistance has been found,