The importance of worming - keeping parasites under control

The use of homespun and herbal remedies may have been superseded by wormers formulated after lengthy research programmes, but the control of worms in the horse remains as important for horsemen today as it was when the significance of these unwanted passengers was first understood.

The main internal parasites of the horse are small red worms (Cyathstomins), large red worms (Strongyles), round worms and tapeworms. The worms undergo similar lifecycles: Larvae and eggs are ingested by a grazing horse and they mature within the gastrointestinal tract. The adults pass out eggs and immature stages in the dung which reinfect the pasture, allowing the cycle to be completed. Infestations with Bot Fly larvae may also be seen.
The development of all these parasites within the equine gut has the potential to cause clinical problems, including colic and ill thrift. However, the lifecycle of the cyathastomins can be particularly destructive.  Cyathostomin larvae actually grow and develop within the wall of the horse’s intestine, causing disruption to the highly specialised intestinal cells. In addition, the larvae have the ability to arrest their own development, entering an encysted or hibernatory phase within the gut wall. Importantly, during this encysted phase the larvae are relatively impervious to a number of common antheImintics (wormers) and over time the parasite burden on the horse may accumulate, with large numbers of larvae entering the encysted phase.
Following the encysted phase, the larvae continue their development by growing and literally bursting out through the gut wall to mature into adults within the lumen of the gastrointestinal tract. However, a cruel twist to the cyathastomin lifecycle is that thousands of encysted worms appear to coordinate their emergence from hibernation, usually in the spring. Large numbers of larvae emerging at once can give rise to a variety of clinical signs from slight lethargy, anaemia and weight loss through to spasmodic or obstructive colic.  Large areas of damaged gut may be replaced by scar tissue instead of the specialised, absorptive cells of the intestine, potentially resulting in weight loss and diarrhoea. Our equine athletes must be able to utilise the high quality (and expensive!) feeds we offer them, and this necessitates a healthy gastrointestinal tract. Thankfully, it is unusual to hear of parasite-associated mortality in racehorses but it would be interesting to know the contribution  made by infestations to sub-optimal performance or training days lost.
It has been accepted for many years that the routine worming of horses is important for their health. This is especially true in establishments with a young and constantly changing population of horses, or pastures which are heavily stocked or grazed by multiple horses. Although all of these conditions are likely to prevail in racing yards, parasite-associated problems could formerly have been dismissed as irrelevant to the well-organised yard with a sound worming policy. Unfortunately, things are now not so simple and it appears that the worms are fighting back. Keen to ensure the survival of their own  kind, they are evolving new strains that are resistant to some anthelmintics. It is not scaremongering to say that some horsemen may soon have no effective means for controlling the internal parasites affecting their charges.
Resistance can occur when any chemical is regularly used to control an infective organism, hence the problem of bacteria resistant to several types of antibiotic found in hospitals e.g. MRSA. In some cases ‘operator error’ may be to blame for encouraging the development of resistance. Incorrect dosing (particularly under-dosing) with anthelmintics may promote the evolution of resistant worms.
Only three classes of anthelmintic are licensed for use in the horse and red worms resistant to the benzimidazole group are common in thoroughbreds. Pyrantel forms the second class. Strongyles resistant to pyrantel developed in the USA where it was used as a feed additive. They are increasingly recognised as a problem in Europe. More worryingly, resistance is developing to the third and final class of wormer, the macrocyclic lactones (ivermectin/moxidectin). Round worm control in foals is not guaranteed by their use and cyathostomins resistant to them are now present on a donkey sanctuary in the UK. Evidence for  cyathstomin resistance has also emerged from Brazil and Germany. It may be the case that resistance has not been detected in more countries due to lack of testing, rather than no resistant parasites being present.
Clearly, planning the worming regime is of the utmost importance and requires detailed knowledge of the strengths and weaknesses of different worming products. However, in a telephone survey of English racehorse trainers in 2002, only 42% stated that their choice of anthelmintic was based on veterinary advice. Furthermore, the same study suggested that strategies used for the treatment of new arrivals were unlikely to prevent the introduction of resistant worms or the development of encysted red worms in the majority of cases.
It is also known that the parasite burden of horses in a yard is not distributed evenly. Most horses will be relatively worm-free. However, one or two ‘wormy’ individuals will be contributing the majority of eggs to the pasture. Identifying these individuals is done by performing faecal worm egg counts (FECs) regularly on all horses within the yard. This could also facilitate a change in the way wormers are used on training yards, moving away from pre-planned blanket dosing of the whole yard to treating only those individuals which require it. Current thinking would suggest that only horses with FECs in excess of 200 eggs per gram (epg) should be treated. An important point to make regarding FECs is that they do not detect encysted/immature red worms.
It is also possible to establish the resistance status of the worms in the horses on the yard. FECs are performed at the time of treatment and repeated afterwards to ensure that the wormers have worked, the faecal egg counts have been reduced and that the horses don’t harbour resistant populations of worms. In the case of pyrantel, the FEC should be repeated seven days later and resistance should be suspected if the FEC is reduced by less than 90%. For benzimidazoles, the count is taken 14 days later and the FEC should be reduced by over 95%. The interval for ivermectin is 21 days and FECs should be less than 1% of the previous level if resistance is not to be suspected. The persistence of Moxidectin makes it unsuitable for this type of test.
Tapeworms have been implicated as a factor in cases of colic. Work at Liverpool University has lead to the development of a test for the presence of tapeworm which can be performed on a blood sample. This indicates if treatment is necessary and can be repeated to check that anthelmintic treatment has been successful.
Although this monitoring may appear to be time-consuming, it would allow a very accurate picture of control programme efficacy to be established. The use of expensive anthelmintics is curtailed and selection pressure for resistance on the parasites is reduced.
As previously mentioned, a protocol for new horses on the yard is extremely important. Recent arrivals should be confined to their box, or allowed access only to a quarantine paddock. An FEC should be performed. It is best to assume that the animal is carrying encysted red worm larvae and to treat for these with moxidectin or five daily doses of fenbendazole. If later FECs suggest the presence of resistant worms, the horse should be assigned its own paddock, or returned to where it came from.
When worming any horse, it is important to follow some basic guidelines to ensure the correct dose is administered. Anthelmintics, or any other drugs, should only ever be given by the route prescribed on the data sheet. An accurate weight should be obtained for each horse to be treated and the full dose for that weight given. If there is any doubt about the accuracy of the weight i.e. obtained by measuring tape, then it is best to slightly overestimate the dose. Ensure that each horse ingests their full dose of paste by holding the head up until it is swallowed. Giving inadequate doses of wormer may hasten the selection of resistant parasites. Animals identified as requiring an anthelmintic treatment which share grazing should receive synchronised treatments. This will help to prevent an immediate major reinfection. It is now advised that, where more than one class of wormer is still effective, they should only be rotated on an annual basis.
Worm control is not all about the use of anthelmintics and these alternative strategies assume an even greater importance with the advance of resistant parasites. They mainly involve reducing the level of contamination on the pasture and so preventing the worms from completing their lifecycle in the gut of the horse. The most direct method is to remove faeces from the grazing, ideally twice weekly during the summer and once per week over the winter. This can be done manually or by machine.  Sheep and cattle will ingest the equine parasites, but are not themselves affected and so clean the grass for horses. Simply lowering the stocking density on the pasture will also help.
Thoroughbred breeders may also have a role to play in worm control. Faecal egg counts may not be the first thing that comes to mind when planning matings, but that may have to change. Resistance is developing to our third and final class of anthelmintic and no new wormers licensed for equines are likely to be on the market in the near future. We know the debilitating effects of an untreated, or possibly untreatable, worm infestation. A horse carrying a heavy infection would never be able to realise its full potential. So, without a major re-evaluation of anthelmintic use, it may be that the classic winners of tomorrow are descended from the innately parasite-resistant individuals of today.  

Dr Philip K Dyson BVMS Cert. EM and Barry Sangster BVMS MRCVS
 (19 May 2007)