Parasite Control In Sheep

Worm burdens can cause clinical disease including sudden death, anaemia and scouring in greater than 10% of your flock. Sub clinical disease will affect a greater proportion of your flock for a much longer time. These sheep will have a decreased appetite and weight loss reducing live weight gain by up to 50 %.  Can you afford this given the price of lamb?


To worm sheep effectively has become more important and more challenging with the development of anthelmintic (wormer) resistance. It is thought that 80% of sheep farms have resistance to at least one class of anthelmintic with triple class resistance present on some UK farms.  With the introduction of monepantel (orange drench) a new class anthelmintic, onto the market we should aim to integrate it into our worming protocol. It should prolong the effective life of our existing anthelmintics and allow for continued effective worm control. The following guidelines should be followed in order to allow this and to delay the development of worm resistance on farm;



                        i. Worm with Monepantel (orange drench) and Moxidectin (Macrocyclic lactones) one after another. Moxidectin injectible will also cover sheep scab.

                        ii. House for 48 hours before turning out onto dirty pasture.


                        i. Dose to the heaviest in group - weigh them.

                        ii. Ensure all equipment is working correctly.

                        iii. Restrict feed for non pregnant ewes prior to Benzimidazole or macrocyclic lactone drenching

                        iv. Ensure worming dose goes over the back of the tongue (so that it enters the rumen for greater efficacy)


                        i. Avoid lambs going onto same pasture for two successive springs (this will reduce the Nematodirus build up as it can survive harsh winters)

                        ii. Consider co grazing with cattle or grazing following cattle.


                        i. Look at your protocol; could you target the timing of your worming better or cut the frequency of dosing down?

                        ii. Mature sheep should have some immunity and are most at risk for 8 weeks around lambing (peri parturient relaxation of immunity).

                        iii. Look at monthly worm forecasts to asses risk period in your area, or ask your vet.

  1. FEC (faecal egg count monitoring)

                        i. Do you worm pre tupping?  Do a FEC to see if necessary. You may only need to dose lean immature ewes.

                        ii. Perform FEC to monitor your flocks worm burden and asses the most appropriate time to worm them.


                        i. Use Monepantel late in the season when worm burdens are high in order to kill worms resistant to the other anthelmintic classes. This will mean that you will use two different anthelmintic classes a year, moving away from the old recommendations of annual rotation.

                        ii. If dosing for fluke only, use a flukicide product that doesn’t contain another wormer.

                        iii. Do your wormers work? Perform a FECRT (faecal egg count reduction trial) to test for resistance on your farm.



Faecal egg count reduction trial (FECRT)

Collect faeces from 10 sheep and send for FEC, the following number of days post worming.

Benzimidazoles (white drench) 10-14 days        

Levimasole/Morantel(yellow drench) 7 days     

Macrocyclic lactones (clear drench) 14-16 days



                     You should expect an effective anthelmintic to reduce                      egg counts to zero                             

  1. PRESERVE NON RESISTANT (susceptible)WORMS by;

                        i. Treat and return to same pasture for 4-7 days beforeturning to ‘clean’ pasture allowing resistant worms to be shed.

                        ii. Leave a small number of thriving sheep untreated (10%) ensuring your wormer is highly efficacious.


  1.  BREEDING from worm resilient rams. This is only beneficial if breeding your own replacements.


Liver fluke (Fasicola hepatica) cannot be overlooked it is an important parasite in sheep causing acute, sub-acute and chronic disease. Over the last few years there have been increased reports of fluke due to an increase in the average temperature and rainfall. With no effective immunity all sheep need to be dosed and often repeat treatments are required.


Prevention and control should be tailored to each individual farm as it will vary according to location, weather, type of land and number of sheep.

A control strategy should be discussed with your veterinary surgeon.


Sian Robinson BVSc MRCVS, Allen & Partners