Balancing benefits of using lambing-time wormer

IT IS not too early to start thinking about worming programmes for this season. Traditionally it starts with ewes around lambing time, but according to independent sheep expert Lesley Stubbings producers must balance the possible benefits against the pressure that such treatments apply to the development of resistance in worms.

The advice is particularly important because injectable moxidectin is increasingly being used for worm control in ewes.

While this is effective in the short term and reduces ewe egg shedding, it is vital that flock keepers do not overuse the product if they wish to sustain its efficacy in the future, says Mrs Stubbings, who is a spokesperson for SCOPS (the Sustainable Control of Parasites in Sheep group).

“I would strongly advise against the use of Moxidectin year-on-year,” she said. “It is advisable to switch between alternative groups of worming products for the lambing treatment.

“A drench test should also be part of the regime. It involves taking faecal samples from ten sheep after dosing and sending them off to a laboratory; the timing of sampling will depend on anthelmintic type and is usually in the following seven or 14 days.

“The drench test is an indicator of wormer efficacy and one of the issues is possible resistance. The accuracy can be improved by also taking faecal samples from the same group of ten sheep as they are dosed, as it will give a guide to the actual reduction in faecal egg counts that has been achieved.”

Another important recommendation is to dose only 90 per cent of the flock. The untreated ten per cent should be made up of the ewes with the highest condition scores and they should be spread between groups.

Some farmers will select the ten per cent group from mainly ewes with singles, but if these are grazed separately from ewes carrying twins this system is inadequate, she stressed.

“The impact on the development of resistance in worms can be further reduced by choosing the right time to dose ewes,” said Mrs Stubbings.

“On a practical level, the timing of wormer treatments around lambing time will be close to the point of lambing or in early lactation and when the ewes can become reinfected by larvae on the pasture, which will produce a ‘dilution’ effect.”

Some farmers are under impression that wormers are given to ewes at lambing time to protect the animals themselves, she added. In fact the aim in most cases is to reduce the number of worm eggs shed by the ewes, which will pose a challenge to their vulnerable young lambs. She explained how the peri-parturient rise puts young lambs at risk.

“For the majority of the year, the worm burden of a healthy adult ewe is kept under control by the animal itself and only a small number of worms are present, producing low egg numbers.

“In late pregnancy, the ewe’s immune system is challenged and this allows the worms in her gut to produce more eggs. At this point, she will be unable to exert her previous control of the parasites that are picked up while grazing. The net result is an increase in egg output from ewes.

“These eggs contaminate the pasture and they hatch into large numbers of infective larvae, which the lambs then pick up later in the season. This series of events is known as the peri-parturient rise, or PPR.

“The precise timing will vary, according to the weather and individual systems, but it will usually begin during the final weeks of pregnancy and continue through to early lactation. After that period, the ewe’s immune system will recover and the peak risk time will have passed.

“There is a case to be made for singling out only the youngest ewes and ewes whose body condition has not met the target for worming treatment and research has shown that there will be no negative effect on the health of their lambs.

“There are benefits to worming some ewes at lambing, but programmes must be managed carefully.

“We need to carry out more research on the contribution that lambing wormers make to the wider picture of wormer resistance, but we believe that the problem is on the rise,” she said.

Sheep scab

Moxidectin is routinely used for the control of sheep scab, but Mrs Stubbings urged producers who adopt this practice to avoid using the same product on ewes at lambing time.

“Some flocks on common land in the North of England in particular are treating animals with moxidectin for sheep scab control,” she said.

“In these instances, it is important that it is not used for a second time to treat worms in lambing ewes, because this is one of the most common methods of the development of resistance to the product.

“A good alternative is to return to OP dipping for scab control.

“Producers have moved away from this system in recent years, but there are some excellent mobile sheep-dipping specialists who will carry out the task effectively.

“There are plans in the pipeline to develop an accreditation system for mobile dipping companies and this will be a step forward for the industry,” she said.