Several factors must be considered when developing a schedule to meet your horse’s needs. With good management practices in place, and armed with the right facts, you can keep internal parasites in check. Controlling internal parasites in horses is a worldwide endeavour, yet it is becoming ever clearer that regional differences in protocols are necessary. In order to avoid over-medicating our horses and to prevent resistance to deworming medications, highly targeted deworming programs are being recommended by researchers and veterinarians. Each farm, stable or breeding operation may well require a tailored program suited to the needs of the individual horses living there, based on use, exposure and age.

Canadian Research

Dr. Chris Clark, Assistant Professor, Large Animal Clinical Sciences at the Western College of Veterinary Medicine (WCVM) at the University of Saskatchewan recently completed a 12-month study on 60 mares and their foals on a Saskatchewan farm using monthly fecal egg counts and considered clinical signs to determine if an individual horse should receive deworming treatments.

The study examined 60 mares and their foals on one farm for a period of one year with only one fall deworming with ivermectin (with owner permission and with close veterinary supervision for clinical signs) in order to determine the ‘natural’ course of infestation with internal parasites within a herd. Fecal egg counts (FECs) were performed monthly throughout the summer and twice during the winter months.

All of the foals were infected two to three months after birth with roundworms (parascaris equorum), a heavybodied worm up to 50 centimetres in length. It is assumed that the foals ingested eggs from the pasture left there from previous years. The ivermectin treatment was not effective in eradicating p. equorum in the foals, but no clinical signs of infestation were observed (debilitation, poor growth) and none of the adults shed roundworm eggs.

The ivermectin was very effective against strongyles, which did affect the adult horses based on FECs. In an attempt to target individual animals more specifically, grad student Ela Misuno, who collected the samples, divided the animals into low shedders and high shedders. The plan was to try to identify which animals were in greater need of deworming. The FECs fluctuated so widely among the adults, however, that no initial patterns have yet been discovered. At this time, it appears that individuals go through different periods of egg shedding and that multiple FECs may need to be performed on individual horses to determine deworming needs.

Strongyles, while a serious problem for horses in many areas, may be less significant in western Canada due to susceptibility to weather. Larvae disappear rapidly from pastures during hot, dry weather and freezing is fatal to strongyle eggs. In the Saskatchewan study, early results do seem to show that, even with only one annual deworming, adult horses were not clinically affected by strongyle infestation nor did high shedders remain high shedders. More research is needed to determine if this state would continue past the one-year period.

Another parasitology study is being performed by Dr. Emma Read of the University of Calgary. Dr. Read is comparing the deworming needs of pasture-kept horses (broodmares, yearlings and foals) versus competition horses kept stalled or in indoor/outdoor turnout at a large Warmblood farm in Alberta. FECs were completed to compare all the groups with regard to the types of parasites shed and the level of infestation. Early findings indicate that, as expected, indoor housing without grazing opportunity “did result in lower transmission and lower parasite load,” stated Dr. Read. “The stalled and small paddock group with no opportunity to graze had no individuals with elevated fecal egg counts (>200 eggs per gram feces) despite 12 months without being dewormed. All these horses were older than three years of age.”

The pastured horses, on the other hand, showed one individual with a high FEC over the 12-month period. The pastured young horse group showed eight horses with high egg counts – some had this occur only once in the study period and some had repeat high counts despite deworming. These findings confirm that the minority of a herd harbours the majority of the parasites and that most are carried by young horses.

Dr. Read qualified the findings with, “It is important to note that there were no horses less than three years old in the stall-confined or paddock-confined groups. This would have been a nice comparison, but this was not the management of this particular herd.”

The FECs taken in this study did not account for bots or tapeworms, so Dr. Read feels it is still important to deworm once a year in the fall after the first frost despite low FECs. One interesting note is that in neither of the western studies was any sign of ivermectin resistance shown. This is not the case for other areas of Canada, however.

The Problem with Resistance

In the past, it was commonplace to deworm horses every two to three months. Since infestation was ongoing in horses, frequent deworming was believed to be necessary. This approach is one reason why resistance has been found in some herds to the common anthelmintics (dewormers). How common is resistance in Canada and why is it a problem?

In order to fully answer this question, it is important to note that all the drugs licensed in Canada belong to just four chemical classes. Since drugs in the same class have the same mechanism of action, if parasites develop resistance to one drug, they generally become resistant to all drugs in the same chemical class. If a particular population of parasites in a particular herd becomes resistant to the effects of fenbendazole, for example, using any of the other benzamidazoles will also be ineffective in eliminating the targeted parasites. Since some drug classes are less effective on certain worms than others, eliminating the use of an entire class can severely limit the effectiveness of a deworming program.

Parasite Location Damage

Resistance in the parasites develops because no drug is 100 per cent effective on the parasites it targets. The remaining parasites either need higher doses to kill them or a different class of drugs. Benzimidazole resistance is now a common occurrence in North America. A 2004 US study of 786 horses of various breeds on 44 horse farms in the southern US found resistance to benzimidazole on 98 per cent of the farms. The authors concluded that ‘singledose use of fenbendazole is now ineffective on virtually all farms in the southern USA.’ In 2005, Dr. Andrew S. Peregrine, in Rounds of the Department of Large Animal Clinical Sciences at the WCVM, stated, “In light of the high prevalence of resistance to benzimidazoles around the world, and the extensive movement of horses around North America, it is likely that a sizeable proportion of farms in Canada have benzimidazole-resistant cyathostomes [encysted strongyle larvae which are highly resistant to deworming drugs]. Anecdotal evidence also suggests this is the case.”

More recent studies have since proven this to be the case. In fact, Canadian horses have now shown parasitic resistance to other classes of anthelmintics as well. During a period of several years, a number of foals were examined at the Ontario Veterinary College (OVC) with ascarid impactions despite regular treatment with ivermectin. Ivermectin resistance is now considered clinically significant, at least in eastern Canada. Dr. Peregrine stated, “Resistance to pyrantel has also been demonstrated in Ontario. It should be noted that the prevalence of pyrantel resistance observed in North America is significantly higher than other parts of the world, and appears to be associated with daily feeding of low-dose pyrantel. This treatment regimen is not used elsewhere.”

New Deworming Protocols

What this means for owners is that we must take many factors into consideration when treating our horses against internal parasites including age, regionality and housing. Veterinarians should be consulted regarding the prevalence of any parasite as well as any known resistance in your area. It is important to alternate anthelmintic classes to avoid building up a resistant population of parasites. Always refer to and follow label directions for the most effective treatment and do frequent FECs to determine the need for treatment. Routine, frequent dewormings are no longer desirable, especially in areas and herds where conditions do not foster dense infestations. As shown in the University of Calgary study, stalled horses or horses in dry lots may require less intervention than pastured animals, especially with regard to strongyles. The most important risk factor for the development of anthelmintic resistance appears to be the frequency of treatments.

The primary objective for a sound parasite program is to minimize the number of treatments and to minimize environmental contamination with parasites. Several management tactics can be undertaken to reduce parasite burdens in pastures and, hence, infection to grazing animals and the need for treatment.

Parasite prevention management practices include:

1. Manure should be removed daily, if possible, from pastures, stalls and dry lots. Spreading the manure on ungrazed areas will break the cycle of worm infestation. Composting the manure results in heat that kills parasite eggs and larvae.

2. Practice frequent mowing and chain harrowing of pastures. Exposing eggs and larvae to heat and sunlight will decrease their survival rate.

3. Rotate pastures as much as practical to reduce parasite build-up and interrupt their life cycles (which require the horse as a host to complete). If possible, alternate with cattle or sheep since the parasites of one species rarely infect others.

4. Consider keeping horses off grass pastures until late spring. Freezing winter conditions kill the majority of cyathosome larvae on pastures. Those that manage to over winter outdoors, can become a source of infection for early grazing horses. Eventually, the parasites burn up reserves and die off and by the end of June, pastures can be considered clean.

5. Avoid overstocking, which increases parasite build-up and encourages over grazing, resulting in increased ingestion of parasite eggs and larvae.

6. Graze weanlings and yearlings separately from adult horses to prevent cross-contamination since different ages appear to be susceptible to different parasites. Young horses should be given the cleanest pastures possible until they have developed some natural immunity to parasites.

7. Do not feed hay or grain on the ground.
Bunks, feeders or mangers keep feed from being contaminated by manure.

8. Ensure the water supply is free of manure contamination.

9. Have your vet perform fecal egg counts as often as practical and always after a treatment to evaluate your control program and to prevent over-treating. If FECs do not show a reduction of 90 to 95 per cent in egg shedding post-treatment, the parasites on your farm are resistant to that medication and will need to be re-treated with a different class of drugs.

10. Read the label and alternate classes of dewormers to prevent resistance. Too low a dose or a loss of effectiveness in a drug are critical components in developing anthelmintic resistance.