Ewes generally show low worm burdens, which produce only a small number of eggs, due to natural immunity to worms developed over time.
But as SRUC veterinary investigation officer David Gibson says, around lambing time, ewes temporarily lose their immunity to worms, causing worm egg numbers in their faeces to rise.
He says: “This can heavily contaminate pastures, threatening the health and growth of lambs later in the season. Because of this, many farmers have a policy of worming around lambing time. However, treatment at this time can also have serious consequences for the development of anthelmintic resistance.
“This is because a lower proportion of the total worms on a farm are living on pasture during winter and early spring – most surviving worms are inside the sheep you are planning to treat. A higher proportion of worms on the farm will therefore be exposed to the wormer used at this time, increasing the pressure to select for anthelmintic resistant worms on the farm.”
Mr Gibson says as a result, the timing of dosing and the choice of wormer are both ‘very important’.
“If the immunity of ewes is still low when the effect of the lambing worm dose wears off, they are likely to become heavily re-infected quickly, particularly if pastures have high levels of infective larvae.
“This will reduce the risk of resistance developing, but the benefit of treatment in terms of reducing pasture contamination will be very limited. Therefore, dosing ewes when immunity is low will only reduce their output of worm eggs for a short time.”
To avoid increasing the risk of developing anthelmintic resistance when dosing ewes at turn-out, Mr Gibson advises following two protocols.
“Firstly, leave a proportion of ewes [about one in 10] in good condition untreated to allow some contamination of the pasture with eggs of non-resistant [susceptible] worms to dilute the number of resistant worms on the pasture.
“The best ewes to leave untreated are the ones which will not be under pressure and will be less likely to have a high worm egg output. Good candidates to select include those carrying single lambs and mature ewes in good body condition which are carrying twins.
“Secondly, treat early post-lambing or at lambing to ensure ewes become re-infected with some susceptible worms before their immunity is fully restored. Do not treat them too early, for example at housing well before lambing, or much later in lactation.”
Mr Gibson says the desired effect is a ‘balancing act’ between protecting lambs from a high worm burden later in the season and protecting the farm from developing anthelmintic resistant worms due to over-exposure to wormers.
As anthelmintic resistance is now very common, it is essential to know which groups of wormers are effective. This can easily be done by checking faeces samples from ewes around lambing time when infection levels are likely to be high. Depending on the wormer being used, post-dosing checks should be carried out at the appropriate time (see panel).
Mr Gibson says some worm products for sheep have a persistent action, preventing re-infection with particular worms for a period after treatment, for example.
He says: “Use of a persistent product has been popular as it can take the place of two treatments of a short-acting wormer when pasture contamination is high and sheep are becoming re-infected with susceptible larvae. They should not be used when pasture contamination is low, for example, when sheep are being turned out to fairly clean grazing.
“However, using such products expose worms to the drug for a long period and this may increase the likelihood of anthelmintic resistance developing. Additionally, the ‘long action’ only applies to certain types of worms – nematodirus, for instance, is not included in the persistent action of moxidectin.”
Mr Gibson says care needs to be taken when choosing a product for ewes around lambing time. He advises:
“Worming to avoid developing anthelmintic resistance is becoming increasingly urgent, particularly illustrated by the recent reports of development of resistance to the new wormer monepantel in Holland in late autumn last year. Farmers should seek advice from the Sustainable Control of Parasites in Sheep group and discuss any concerns directly with their vet.”