Vet’s View

Worm wor­ries

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An­thelmintic re­sis­tance in sheep

An­thelmintics, other­wise known as worm­ers are gen­er­ally used to treat in­ter­nal par­a­site in­fec­tions in an­i­mals. Ru­mi­nants can be in­fected with a num­ber of gut worms which can cause clin­i­cal dis­ease such as di­ar­rhoea and de­hy­dra­tion, anaemia and in ex­treme cases death. In less se­verely af­fected ru­mi­nants, gut worms will slow growth rates in young an­i­mals. In most cases, once the an­i­mal reaches adult­hood it has some im­mu­nity to par­a­site in­fec­tions and gut worms are less of a con­cern.

In sheep, there are both pro­duc­tion and wel­fare con­cerns with gut worm in­fec­tions. Slow growth rate and clin­i­cal dis­ease will de­lay the time lambs take to reach fat weight or breed­ing weight in re­place­ment an­i­mals. In ad­di­tion, di­ar­rhoea can cause mucky back-ends in sheep, which will at­tract flies and lead to fly strike.

One of the big­gest chal­lenges cur­rently fac­ing the UK sheep in­dus­try is an­thelmintic re­sis­tance. This is when the par­a­site de­vel­ops re­sis­tance against the prod­uct used to kill it, which means worm­ing treat­ments aren’t as ef­fec­tive as they should be.

How to slow re­sis­tance de­vel­op­ing

There are cur­rently 5 groups of worm­ers; un­for­tu­nately, wide­spread re­sis­tance to 3 out of 5 of those wormer groups has been well doc­u­mented in re­search. Pro­duc­ing new wormer groups a long process, so we can­not rely on new drugs to tackle an­thelmintic re­sis­tance. Ev­ery time a worm­ing drench is used it in­creases the chance of re­sis­tance de­vel­op­ing so we need to be care­ful about how we ad­min­is­ter worm­ers, which sheep we ad­min­is­ter them to and look at other ways to con­trol par­a­sites in our flocks. Some key points in­clude:

QUAR­AN­TINE DRENCHING – bring­ing new an­i­mals onto your hold­ing is the eas­i­est way to con­tam­i­nate your pas­ture with “re­sis­tant” worms. Con­sult your vet when buy­ing-in to en­sure you have a quar­an­tine pro­to­col in place

DRENCH WHEN RE­QUIRED - the less fre­quently we use worm drenches, the slower re­sis­tance will build. This needs to be care­fully bal­anced with avoid­ing dis­ease in our flocks. Us­ing worm egg counts can be an ex­tremely use­ful tool to de­cide when worm­ing is re­quired

DRENCH COR­RECTLY – en­sure you have an ac­cu­rate weight for your sheep and al­ways dose for the heav­i­est in the group (of the same age group). If us­ing a drench gun, cal­i­brate be­fore each use.

AVOID­ANCE – ro­tat­ing graz­ing with other species such as cat­tle or horses can be very use­ful to “mop up” worms Con­sult with your vet to cre­ate a strate­gic par­a­site con­trol plan for your hold­ing; more in­for­ma­tion about worms and worm­ing can be found at

Wormer re­sis­tance threat­ens the health and wel­fare of UK sheep.

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