High worm egg counts caused by a mild winter combined with the decreased immunity in pregnant ewes is the perfect storm for large worm burdens in growing lambs, experts are warning.

Pregnant ewes represent the major source of gastro-intestinal parasites for growing lambs. Around two weeks before lambing, until six weeks post lambing, ewes are more likely to shed worm larvae due to decreased immunity. This is known as the spring rise, and the reason why ewes should be prioritised for worm control now.

Hefin Rowlands, winner of Farm SQP of the Year in 2018, has seen worm egg counts as high as 1000 eggs per gram (epg), the threshold for treatment is at 250 epg or above.

“This is a very high figure so early in the season, farmers really should be looking at worming their sheep prior to lambing,” he said.

Sheep farmer Andrew Phillips, Windsor Farm, Lamphey, Pembrokeshire, controls the spring rise by using Cydectin long acting injection, which he has been using it for the past six years and says it has been able to reduce worming in the flock by controlling the spring rise.

“We have been using Cydectin LA for the past six years on all the ewes pre-lambing. There’s a noticeable difference in the ewes and lambs at turnout, both are a lot cleaner and healthier, they look great. We have been able to reduce worming in the lambs because the ewes are cleaner, which has saved money and time as we are not having to re-treat for worms.”

Findings from a study with FAI Farms1 found injecting ewes with the wormer led to low worm egg counts in their lambs. Ewes treated with the short-acting clear wormer; doramectin or were not treated at all (control group) showed higher worm egg counts in their lambs.

The study also found that single-bearing ewes produced fewer eggs and therefore contaminated less pasture compared to twin, or triplet-bearing ewes.

As a result, vet Dr Dave Armstrong from Zoetis suggests it may be worth leaving single-bearing ewes untreated and instead monitor their worm egg counts.

This is because the twin and triplet-bearing ewes are potentially drawing more protein for milk production they have a greater immunity drop compared to single-bearing ewes.

Dr Armstrong said: “By injecting ewes with long-acting moxidectin prior to lambing farmers can minimise the use of other anthelmintic treatments in that breeding year and can reduce pasture contamination for those sheep which will graze the pasture in the next year.

“With the annual cost of stomach worms to the British Sheep industry estimated to be about £84m, sheep farmers need to be looking at ways to reduce losses on farm due to worms, but in a responsible way.

“Cydectin long acting injection for sheep has been proven to control the spring rise and reduce lamb worm egg counts throughout the season,” he added.


When used correctly, long-acting products such as Cydectin LA will not contribute to anthelmintic resistance any more than using multiple wormer treatments.

Dr Armstrong said: “Farmers not using a long-acting product may be required to use multiple treatments to prevent reinfection. Every time a product is used, the efficacy starts declining, eventually the time will come when not all the worms will be killed.

“If using multiple treatments there is more chance resistant worms will survive as there are increased opportunities when the efficacy of the product may not be enough to kill off all the worms.”