Rise of the 'superworm' a new threat to equine health
New strategies are needed to deal with drug-resistant 'super worms' in horses, writes Dr Vivienne Duggan
Published 24/02/2016 | 02:30
Intestinal parasites, or worms, are still a very common cause of disease in horses and foals. A number of fatal cases, with disease caused directly by worms, presented to the UCD Veterinary Hospital Equine Clinic in the last year.
In addition to causing damage to the gut lining with leakage of protein and blood and subsequent weight loss, equine worms have a propensity to leave the intestine and go off travelling all around the body, where they can cause significant damage and scarring to other organs like the liver, lungs, kidney and even brain.
This is not new information. Most horse people are aware that one of the most common causes of failure to thrive in a foal or adult horse is intestinal worms.
Traditional strategies to manage worms in horses include reducing the stocking density of horses on pasture, rotating pastures and harrowing them to expose worm eggs to the elements.
Additionally, de-worming horses on pasture every 6-8 weeks, while rotating de-wormers so as to limit the development of worms that are resistant to the drugs, is commonly recommended.
The strategy of bi-monthly de-worming has been reviewed and is no longer considered appropriate for all horses.
Foals, however, do need de-worming from about two months of age as they are exposed to worm eggs and larvae straight away from the mare, in her faeces or even in her milk.
The development of strong immunity to worms in the foal needs time to develop and therefore treatment every eight weeks is appropriate.
Most adult horses have developed a good strong immunity and are far less susceptible to worms.
For that reason, traditional strategies of routine de-worming of all adult horses every two months while at pasture are no longer appropriate.
In fact, it is now believed that only 20pc of horses in a paddock will be carrying 80pc of the total worms. The other 80pc have low numbers of worms and don't need such frequent de-worming.
Worryingly, de-worming horses that don't need it is leading to the development of 'super-worms' like 'super-bugs' that are resistant to the available drugs.
This has become a major problem for horses in Britain and resistant worms are also being found on Irish horse farms.
The 20pc of horses that do have high worm burdens, likely have them because of different levels of stress, frequent travel and exposure to other horses, temporarily depressed immune systems, concurrent diseases or pregnancy in mares.
But how do you know which horses they are? You can collect the dung of individual horses and have it analysed for worm egg numbers. This is a simple and cheap test, and could save you a bucket on unnecessary de-worming in addition to preserving the effectiveness of the de-wormers we have.
This counting of eggs in the faeces (a faecal egg count) can be done twice a year, in the spring and the autumn to categorise the horses into high, medium and low shedders. This will direct the frequency of de-worming that will be necessary for individual horses.
The next question is which de-wormer to use in which horse at which time of the year? Not all de-wormers are equal as some worms have devised complicated strategies to evade destruction.
Red-worms, in particular, burrow into the walls of the gut and become dormant and very resistant to most de-wormers.
Red-worms are important in young and adult horses alike, and all grazing horses should be treated in the autumn and/or spring for dormant red-worms, regardless of whether the horse has a high, medium or low worm burden.
Tapeworms are also notoriously resistant to regular de-wormers and are an important cause of colic in young horses. All foals and young horses should be treated in the autumn for tapeworms.
It is, of course, important to be sure that the horse is getting the proper dose. In the past, when the vet used to drench the horse with the de-wormer, you could be sure it was getting where it was needed.
These days we use pastes, and some horses are very adept at spitting out what they don't like. If your horse doesn't like the taste, you could mix it with something they do like, such as molasses or apple sauce.
Make sure there is no big wad of hay or grass in the mouth before you try giving the paste.
The best way to give the paste is to insert the tube into the horse's mouth, in the gap between the incisor and cheek teeth and then slide it over the tongue towards the back of the mouth.
Slowly depress the syringe to allow the paste out gradually, giving the horse time to swallow a bit at a time. Pushing the whole lot in at once is less pleasant for the horse, and makes it more likely he will resist the next time, or just spit the whole lot out.
Some horses may get a little colicky after being de-wormed so a good time to treat them is after feeding in the morning. Then you can watch for signs of discomfort over the day and get veterinary assistance if it is needed.
Horses that are properly de-wormed and not allowed to develop high worm loads are less likely to have complications than those with high worm loads which might obstruct the bowel as they die.
Also, there is no need to stop exercising the horse just because he has been de-wormed. Tack up and continue your usual routine.
Your vet is the best person to advise you on developing a de-worming strategy that is appropriate for your horses and your yard.
The School of Veterinary Medicine in UCD also has specific worm (parasitology) and equine specialists that are willing to advise and help.
If problems are resistant to treatment or need a herd health approach the School of Veterinary Medicine can offer a multi-disciplinary approach in conjunction with your local veterinarian.
Later this year at the UCD Veterinary Hospital Conference, two staff members, Dr Theo de Waal and Aoife Quigley, will present their research on worm disease in horses.
Dr Vivienne Duggan is a senior lecturer in Equine Medicine, School of Veterinary Medicine, UCD