Pasture contamination in the spring arises both from over-wintered infective third stage larvae on pasture and from nematode eggs shed by recently lambed ewes.
The relative importance of these sources depends on farm management, climate and weather. Midsummer sees the highest pasture larval burdens (see figure 1) which partially depends on how well the periparturient rise in the ewe was controlled at lambing.
Adult sheep acquire natural immunity to worms and do not generally harbour large burdens of adult trichostrongyle worms, but ewes suffer a transient loss of immunity to worms in late pregnancy and early lactation.
This transient loss of immunity or periparturient rise (derived from ‘peri’ meaning around, and ‘parturient’ meaning lambing) lasts for six-eight weeks beginning a couple of weeks before lambing and the duration is determined by a number of factors including:
Age - worse in younger animals versus mature ewes.
Fecundity - worse in ewes carrying twins or triplets versus single bearing ewes.
Nutritional status - worse in thinner ewes.
Anthelmintic treatment to prevent the periparturient rise in faecal egg output by ewes can be given at various times depending on farm management system.
Effective anthelmintic therapy of ewes during the fourth month of pregnancy should eliminate most of the worm burdens present at this time including arrested larval stages and in the case of ewes on extensive grazing, where nutritional status is frequently low, this treatment often results in improved general body condition.
Treatment around lambing or turnout will significantly reduce the ewe contribution to pasture contamination
In light of this, the general recommendation is to use a product with licensed persistence against worms (e.g. CYDECTIN® drench or injection) to treat ewes at lambing, but ideally leave 10% of the mature ewes carrying singles which are in good body condition untreated. Leaving a few untreated animals will add little to contamination levels, but significantly reduces selection for anthelmintic resistance.
Cydectin 20mg/ml Long Acting Injection for Sheep delivers longer persistency compared to the 1% injection, which has potential benefits for their lambs. The persistent effect of Cydectin 20mg/ml Long Acting Injection for Sheep leads to a ‘Hoover’ effect for up to 111 days.
A reduced level of pasture challenge to lambs means lower levels of worm infestations in the lambs, which has potential to optimise lamb growth rates and reduce the need for anthelmintic treatments.
Moxidectin is inherently potent and persistent. The concentration and formulation of Cydectin 20mg/ml LA for Sheep leads to the long action with persistency up to 111 days against worms and 60 days against scab.