Sheep Worm Resistance
Worm resistance is the ability of parasitic worms causing scour and ill-thrift to survive treatment with anthelmintics.
Common causes of resistance are:
Under-dosing: All sheep should be dosed for the heaviest weight in the group.
Inappropriate dosing: the traditional dosing times at housing and pre-tupping are not effective at reducing worms and actually increase resistance.
Reinfection: after dosing, sheep should be put onto dirty pasture to pick up resident worms to dilute the resistant worms that have survived dosing.
Continual use: all anthelmintic groups should be rotated to clear the resistant worms. Continually using only one group of drugs will speed up resistance.
Lambing: around lambing, the immunity of the ewe is reduced , resulting in more worm eggs being shed onto the pasture. Worming at this time will reduce the pasture contamination if carried out properly.
Most sheep farmers rely heavily on the use of anthelmintics to control diseases associated with these worms due to relatively high stocking rates.
Traditionally white drenches (benzimadazoles) have been a problem specifically with resistance from strains of T.circumcincta, but now resistance to levamisole and ivermectin and even cases of triple worm resistance have begun to emerge, i.e. worms that are resistant to all three major groups of anthelmintic.
Three major groups of anthelmintic
Benzimadazoles (white drenches)
Avermectins (Vectin, Dectomax, Cydectin)
There is now a new 4th generation group 4-AD orange drench class called Zolvix available from the vets which needs to be protected for use against known cases of resistant worms.
The best advice is to rotate use of the three groups annually, using strategic dosing for parasites such as liver fluke and tapeworm when necessary. Remember it is the worms that become resistant - not the sheep.
Once you have resistant worms on your premises, it is very difficult to get rid of them, if not impossible. Our best advice to you to avoid the problem is the following:
Maintain a closed flock as far as possible.
Double fence your boundary and avoid stray sheep.
Quarantine sheep that enter the flock (including purchased tups) and treat them with an appropriate dose of Moxidectin (Cydectin) and Levamisole, only releasing them after it has had a chance to work (24-48hrs), then put group onto dirty pasture.
SCOPS now recommend the use of Zolvix with moxidectin as the quarantine dose.
A test can be done using faecal egg counts before and after worming to determine the farm's resistance status.