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Parasites in sheep

Parasites are major contributors to lower productivity on sheep farms. The internal parasites of sheep can be subdivided into nematodes (roundworms), trematodes (fluke), cestodes (tapeworms) and protozoans (coccidian, Toxoplasma, Cryptosporidia).  Disease can vary from subclinical disease where sheep appear healthy but perform below their full potential, chronic disease resulting in various degrees of morbidity resulting in underperformance and perhaps death and  acute disease with high mortality.

Control is achieved principally through the use of anti parasiticide medicines e.g anthelmintics (gut worms), flukicides (liver and rumen fluke)and coccidiostats (coccidian) preferably in combination with management strategies.  Results from research shows that there is wide scale resistance to wormers (anthelmintics) on Irish sheep farms Treating lambs with products that do not give the desired control is not only a waste of time and money, but it also results in poor performance as the sheep end up carrying a heavy worm burden despite having been treated.

Up to 2009, farmers had three ‘families’ of broad spectrum wormers available for treating parasites found in the gut. These were benzimidazole (white drenches), levamisole (yellow drenches), and macrocyclic lactones (clear group e.g.  ivermectin, moxidectin, doramectin).  After over 25 years, two new family of wormers were launched on the Irish market; in 2010 Novartis Animal Health launched  the product called  Zolvix  which belongs to a totally new ‘family’ of wormers called the AADS ( the orange group) .  While in 2012, Zoetis launched a new product called STARTECT which is a dual active compound combining abamectin with derquantel which belongs to a new family of drugs Spiroindoles  (the purple group). These 2 wormers remain as prescription only medicines (POM) only and as new families of wormers, afford new opportunities for Irish flock owners if used judiciously  in prolonging the lifespan of reliance on the ‘older families’.

If using a wormer that belongs to the three older families, it is important to check that it is still working on your farm.  This can be achieved by carrying out a simple test called a drench check test in your lambs a specific number of days after drenching (see table below), sending it of the lab where they will check to see if there are any eggs in the dung. If the dung sample tests positive for worm eggs then a more detailed test is warranted to see if the worm population on the farm is resistant to the drench in question.  Samples should be taken from at least 10 lambs that have not been treated in at least 6 weeks (longer if ‘you have treated with a long acting wormer).  The length of time that should elapse between drenching and taking the dung sample depends on the type of wormer that is used. The table below outlines the various time frames for the different wormer groups.

Table1 Time frame for testing the flock post drenching with wormers from different group

Wormer classNumber of  days  before retest the sheep post treatment
Benzimidazole (white group) 10-14 days
Levamisole (yellow group) 5-7 days
Macrocyclic lactones (clear group) 14-16 days