In recent years it has become more and more evident that intestinal worm parasites in sheep are increasingly resistant to the anthelmintic (worming) treatments.
Although there is a lot of evidence of anthelmintic resistance in the UK, we are fortunately still in a position to combat the development of wormer resistance.
However it will require a radical change in the approach to controlling parasites in sheep.
Alnorthumbria Veterinary Group is endeavouring to ascertain the level of resistance within flocks across Northumberland.
Controlling internal parasites is an essential component of livestock husbandry and currently good control is highly dependent on anthelmintic usage. Failure to control parasites effectively will lead to clinical signs such as diarrhoea and dehydration which, left uncontrolled, results in poor weight gain and eventually death.
Existing practices such as the use of inappropriate wormers at inappropriate times, under dosing, and the ‘dose and move’ strategy are promoting the development of resistant worms. In order to preserve the efficacy of the five anthelmintic groups currently available, it is these practices that we must change.
The distribution of worm species across the UK has changed in recent years, possibly due to change in weather patterns. Haemonchus, the so called ‘barber’s pole worm’ due to its microscopic appearance, was once restricted to the South East but is now seen in northern Scotland, and Alnorthumbria have experienced problems on farms in north Northumberland.
Nematodirus was once a problem only seen in lambs in the spring, but poor weight gain, diarrhoea, and death due to Nematodirus infestation can now be seen in sheep at any time of year with the first case of resistance seen in 2011. The Tricholstrongylus species historically caused issues in store lambs in the Autumn but can be seen all year round in mild winters.
Resistance to an anthelmintic can be defined as the ability of a parasitic worm to survive exposure to the normal dose of anthelmintic. This ability is a heritable trait therefore worms which are resistant can pass this ability on to their offspring, and it is this fact that allows resistant worms to proliferate whilst their non-resistant companions are killed by worming.
Vets are most interested in the part of the life cycle of a worm that takes place within the gut of the sheep, they also need to be aware of the life cycle stage on the pasture or ‘in refugia’. Only a very small proportion of the total worm burden on a pasture is found within the livestock grazing the pasture. Most of the worms are found as either larvae or eggs on the pasture. Clearly, worming a group of sheep regularly with a wormer which has a high level of resistance rapidly selects for the resistant worms and their population massively increases on the pasture as they are no longer in competition with non-resistant worms which have been wiped out by the anthelmintic administered.
One of the problems faced by both farmers and ourselves as vets, is that the presence of resistance on a farm is unknown until a group of sheep fail to respond to worming. By this time, the resistant worm population will already be well established.
SCOPS (Sustainable Control of Parasites in Sheep) recommend that the new anthelmintic groups are integrated into the parasite control plan for the farm rather than left in reserve for when the other groups are no longer working.
Therefore it is vitally important all farmers spend some time with their vet to devise a parasite control plan as part of the flock health plan. This needs to be a bespoke plan which takes into account the presence or absence of resistant worms, the replacement policy of the flock and the control of external parasites such as ticks, lice and scab. The use of worm egg counts is an invaluable tool in assessing the level of parasitism on the farm and should be used regularly to aid the decision-making, including when to worm and what product to use.
It is important also to consider other methods of controlling parasites in sheep. For example, the New Zealand Romney breed of sheep have been selectively bred to develop genetic resilience to worms, thus reducing the requirement for anthelmintic treatment. The use of grazing management to reduce the exposure of susceptible sheep to high worm burdens should always be considered. Studies have demonstrated that feeding of bioactive forages such as chicory have conferred reduced worm burdens and increased growth rates in lambs.
In Northumberland I am pleased to report that we are seeing a gradual change in the worming habits on our beef, dairy and sheep farms. However, we would like to accelerate this change and promote the appropriate control strategies on farms. To help with this acceleration we are offering free worm egg counting when purchasing any sheep wormer from any of the Alnorthumbria practices.
The plan is to perform a drench check on all of the sheep farms in the area in order to map the level of wormer resistance in Northumberland. This requires pre and post worming egg counts which can be easily collected, first when the sheep are gathered for worming and second at the time appropriate to the wormer type given. When resistance is suspected, Alnorthumbria plan to follow up with faecal egg count reduction tests in order to ascertain the level of resistance on the farms.
The future of British sheep farming is dependent on the ability to control parasites. Farmers and vets must work closely together to formulate sustainable methods of control in order to preserve the efficacy of the products available and safeguard the health, welfare and productivity of sheep flocks across the country.