Worming and resistance to wormers

This  feature about worming has been written by Dr. S. Davidson, BVMS, MRCVS, Sussex Equine Hospital

Sarah Davidson, Veterinary Associate, Sussex Equine Hospital

There is regularly talk in the news about antibiotic resistance, how it affects us now and how it will affect us in the not-so-distant future.

The veterinary world is currently experiencing a similar problem regarding wormers and it’s called anthelmintic resistance. Resistance to wormers comes about when there are a large number of worms in any one horse’s intestines. A chance mutation occurs in the genetic makeup of one of those worms making it able to survive the the specific worming drug used.

This worm then has offspring that also carry the new gene and, therefore, the ability to survive worming treatment. Very quickly, the horse has a resistant population of worms.

The lifecycle of a worm involves time spent in a host animal and also on the pasture, so when these resistant worms are passed by the horse on to the pasture, its field mates are at risk of becoming burdened with resistant worms and the problem spreads.

The issue we are facing now is a result of many years of poor worming protocols:

  • Using the same wormer each time.
  • Blanket treatment of all horses. There is a common phrase that 20% of horses will carry 80 per cent of the total worm population, this means that not all horses need treatment all the time.
  • Not following the manufacturers’ recommendations, most commonly worming more frequently than necessary which increases the rate of selection of resistant worms, but also under-dosing.
  • Not using the right wormer for the particular worm likely to be present in the horse at that time.
  • Not taking into consideration the age of the horse, as horses get older they develop a degree of immunity to intestinal parasites. Younger horses need a higher level of care and attention.
  • Roundworms in horses

All these things considered, there are new ways of thinking that can help to minimise the spread of resistance and keep your horse’s worm burden under control at the same time. If you keep your horse on a livery yard, there may already be protocols in place, but if not, it might be worth a discussion because treating a whole herd with resistance in mind is a much more effective way to tackle the problem than each horse having a slightly different management plan.

  1. Collect faecal samples, keep them cool to prevent eggs hatching which will skew the result and submit them to a lab for worm egg counts (number of eggs per gram of faeces). The lab should also specify the type of eggs present and their distribution.
  2. Based on the number of eggs reported, a decision must then be made as to whether the horse is wormed or not. Sources vary but a suggested cut off is 200 eggs per gram. Any horse with a burden above this should be treated.
  3. Treatment should be influenced by the species of egg identified.
  4. It is then possible for those that wish to be extra-vigilant to work out whether resistance is a problem in their horse. A faecal sample can be taken two weeks later and again submitted to a lab. If the wormer has been effective, the number of eggs per gram should have decreased by 90%. If you have not seen a satisfactory result, worm with a different drug to target those worms that were not killed off by the first treatment.

On top of combating resistance, taking faecal samples instead of blanket treating all horses regularly can save a considerable amount of money.

An ideal plan for timing of faecal samples is to begin in spring and treat according to results. If you have not had to treat your horse, take another faecal sample three months later and so on. If you have treated your horse, ideally take the second sample two weeks later, but otherwise wait six months until autumn to take another sample.

A point to note here is that worm egg counts do not take into consideration tapeworms, due to their lifecycle. To test for tapeworm, a blood sample should be collected and submitted to a lab to look at antibodies and again, depending on the result, a decision is made whether to treat or not.

Good pasture hygiene remains very important in the control of worms, poo-picking to minimise pasture contamination, rotation between species, for example, cattle and or sheep and rotation of fields, if possible, will all help. If new horses are coming onto a yard, checking and treating them before they are turned out on to pasture is ideal, but understandably not always practical.

It is worth bearing in mind that the above is in relation to adult horses that are in good health and body condition. If you suspect that a horse in your care has a high worm burden, discuss an appropriate plan with your vet as a rapid kill of worms can cause your horse to become sick or even be fatal.

Foals have a different susceptibility to worms and should also be wormed with a careful programme. Foals are susceptible to ascarid worms which are thick, white worms that can be up to 15 inches in length. Adults have an inherent immunity.

Ascarids can block the intestines completely and, if killed all at once can cause diarrhoea, low protein and more seriously telescoping (scientifically termed ‘intussusception’) of the intestines and death.

Management of intestinal parasites is as important now as it ever has been. While most horses will live out their days without worms causing them a problem, it has been clearly demonstrated that an uncontrolled worm burden increases the risk of colic, diarrhoea and un-thriftiness in horses.

If we cannot combat worms with the drugs that we currently have due to resistance, these are problems that we are going to see more commonly in the future as there are very few or no new drugs being developed.



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