Worms & Worming!


After a number of claims associated with worms, we look at the importance of worming correctly and the problems we can be faced with…

We have recently had three claims submitted for serious diarrhoea associated with small red worm (strongyle) infections, an infection which is preventable through the correct worming management. Worming, as with vaccinating against equine influenza and tetanus, is likely to be a condition of your insurance policy, as part of your fulfilment to ensure your horse’s health is maintained, so it is important to check this stipulation carefully.

At KBIS we state that the insured horse “should be regularly wormed or assessed for worm infestation by egg count, and treated in the event of a positive result in accordance with veterinary guidelines.” Two owners, therefore, unfortunately, found that their horses were not covered as they had failed to comply with this condition. The third horse was a new purchase which had had a pre-purchase examination where the vet had recommended purchase, so the worm infestation was not the insured’s fault and so the claim was valid.

The symptoms result from encysted larvae in the gut all “hatching” at the same time, resulting in massive irritation and inflammation of the gut lining. It is usually something in the spring grass that triggers this- it is an evolutionary advantage for the worms to emerge in the early spring- they then mature in the gut to adult worms who will lay lots of eggs to get eaten up in the lush spring grass! The larvae can “sleep” in arrested development in the gut wall cysts for several months over the winter until conditions are right for emerging. It is, therefore, possible that a moderate level of infestation over the summer months could lead to a massive problem the following year.

Establishing a correct and effective worming routine and management system to prevent such infections should be straight forward. But with changing guidelines it can get confusing and for those keeping their horse in a livery yard environment without a standardised policy across the whole yard it can pose a number of problems.

Where it all starts.

Every horse owner will be aware that horses on small paddocks are more at risk of re-infecting themselves by eating the early stage larvae on the grass. This is why we are advised to pick up droppings at least twice a week to keep the pasture clean. In fact, faeces on pasture that is left for 48hrs or more will taint the pasture and, in larger areas, horses will avoid grazing these areas. Naturally, horses will divide their pasture into dunging areas, which we recognise as roughs and lawns. This is nature’s way to try to prevent parasite spread. Unfortunately, a large percentage of the equine population has limited grazing and so there is a huge risk of parasite contamination.

Resistance, a rapidly increasing problem.

We are verging on the brink of disaster with parasite resistance, in rather the same way as with antibiotics. No drug kills 100% of its target, be it bacteria, parasites or cancer cells. An 80% efficiency rate would be very good, which leave a certain number of the targets left. The body’s immune system can normally deal with these. However, due to injudicious use of drugs, mostly due to underdosing as owners have underestimated their horses’s weight over many years, the worms have been exposed to a less than optimal dose of drug, and they have adapted to become resistant. Resistance has been slowly emerging over the 30 plus years that I have been in practice and much research has gone into finding new drugs and the best way to control intestinal parasites in our horses. Unfortunately, there are no new drugs on the horizon. There is a potential new drug for sheep which may be available in five or so years, but that is assuming it passes all the required clinical trials. Even then it has to be licenced and will not necessarily be of any use to horses, so equine parasitologists have concentrated on how best to reduce the risk and the advice has been constantly evolving.

So what should we do to minimise our horse’s worm burdens?

In the 80s the advice was to worm regularly every six weeks and use a different wormer each time. Next, we had to use the same wormer over the entire grazing season and to pick up droppings. Then the advice was to worm twice yearly for tapeworms, then to give a high dose of one group of drugs in the spring, to new horses or when moving and so on – it is little wonder that the average horse owner is confused!

The British Horse Society has produced an excellent detailed booklet, which can be downloaded from their website – British Horse Society – Worm Control and Worming Booklet, with accurate up to date information. Also there is excellent information available on the websites of the commercial laboratories. Each individual premises will have different needs and as such your veterinary surgeon should be involved in tailoring a yard -specific parasite policy.

Management can play an important part.

Poo picking and changing onto new paddocks once the grass is grazed to 5cm will help enormously as worm larvae tend to lie nearer the roots of the grass. If you are farmers graze the cattle first- they eat the long grass, then the horses and finally the sheep. Horse parasites do not develop in sheep and using horses can prevent cross-contamination of ruminant parasites. Keep all horses in set groups and only introduce a new horse to the herd once its parasite status is known. I would advocate taking egg counts from all new horses and routinely 4 times a year, only worming those horses with counts of over 200 eggs per gram. If the horses have been wormed, leave a few days before moving paddocks. You might find certain horses have a persistently high count – these should be kept separately as they are a risk for the others. Young horses also tend to have higher counts, which should decrease over the years as a certain amount of immunity does develop. Of course where there is no pasture contamination at all the foals will not get worms! Sadly that is rarely the case in modern situations.

How do I know which wormer to use?

There has been, and can still be conflicting advice on which wormers to use. There are numerous brands but it is important to know which active ingredient you are using. The main groups used in horses are Benzimidazoles and Macrocyclic lactones used against red worms with Pyrental and Praziquental for tape worm. There is known to be massive resistance to Fenbendazole, a Benzimidazole found in Panacur Equine Guard, but unfortunately, there is no knowing if the worms in your horse are resistant so if using this I would advocate a worm egg count afterwards. If used for encysted redworm larvae in the winter I would recommend a worm egg count is done in the early spring to test efficacy. I once dosed a pony who was straight off the New Forest with this drug- she produced masses of dead worms in her faeces but we then did an egg count and she still had 3000 eggs per gram!As a first line wormer for those horses with high egg counts I would use Ivermectin- a first generation of macrocyclic lactone, available under many brand names and also in combination with praziquantel if you need to worm against tapes as well. There is, however, an emerging resistance to this drug so if you are suspicious an egg count to show reduction should be done about 10-14 days after dosing.

In my opinion it is important to avoid the use of Moxidectin, a second generation macrocyclic lactone (available only as Equest; Zoetis) as a first wormer as this is the only drug which at present has no proven resistance problems, (although suspected by some laboratories) and as such it should be reserved for those cases which have not responded to other treatments. Moxidectin can be used against encysted larvae in the winter but I would advise discussing with your veterinary surgeon whether this is necessary in your own particular circumstances. The decision will be taken based on the history over the grazing season. You may not need to worm at all over the winter if all has been clear during the grazing season, or perhaps fenbendazole would be appropriate initially.

In multi-horse, multi-owner yards it is therefore vitally important that there is a co-ordinated worming program and this is best organised by the yard manager who should keep a record of all the egg counts and which horses were wormed and when.

So what went wrong for the two poor ladies whose horses went down with severe strongyle diarrhoea?

Their horses both had a clear egg count when moving to a new yard. This was done correctly at their livery owners request prior to bringing in a new horse. But the egg counts needed to be repeated during the grazing season. In addition, these were young horses, with little immunity and they would have been picking up the worm larvae deposited on the pasture by the older horses (who would have been tolerant to a low worm burden) and then subsequently re-infected themselves. One owner told me that the horses were all together in a big field and there was no removal of droppings. It is therefore highly likely that at the end of the summer these horses would have had considerable worm burdens, which would have encysted in the gut wall for the winter.

If identified as being at risk and received a larvicidal dose of wormer during the winter months, the infection could have been prevented, which only highlights the importance of correct management and seeking veterinary advice on your worming programme – The time to take action is NOW!

KBIS are one of the largest independent horse insurance providers, offering all types of insurance for the equestrian. Our products include horsebox and trailer insurance, breakdown insurance, personal accident cover, liability insurance and horse insurance – including a wide variety of veterinary fee options. You can get a horse insurance quote online or call 0345 230 2323.