Deworming: How to Develop an Effective Program&Avoid Drug Resistance

As a follow-up to the article on new deworming practices in the Canadian Horse Health Annual, Dr. Ela Misuno, former internal medicine resident and master’s student at the Western College of Veterinary Medicine, provides additional information about drug resistance and effective deworming practices, by highlighting key findings of her recent study.

Parasitic Resistance to Dewormers

Resistance among equine internal parasites to deworming drugs is a farm-specific issue, not country or region specific, says Dr. Misuno. Horses on the same premises mix and exchange their parasites, resulting in the same gene pool within horses in contact with one another. “Of course, small amounts of eggs can be carried from one farm to another via fomites – such as people, their boots, clothes, brushes, buckets and more. If a particular farm experiences intensive horse movement, such a racing facility, then we possibly could extrapolate farm resistance into a region,” said Dr. Misuno.

Reports of ineffective deworming due to resistance are found among all three classes of dewormers and among all common equine parasites worldwide. It is important to understand that despite numerous different brand names, drugs contained in dewormers belong to only three chemical groups.

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In her study, Dr. Misuno found ivermectin resistance in large roundworms (Parascaris equorum) in foals at two out of three test farms. Other parasites (strongyles, also known as ‘redworms’) residing in the same foals did respond to ivermectin treatment, however. Parascaris equorum is a major pathogenic parasite for foals, hence, if resistance arises, the foals may not be protected despite administration of a dewormer. Dr. Misuno stressed that this underscores the need to test the efficacy of a dewormer used in a herd by performing Fecal Egg Count sampling (FEC) and a Fecal Egg Count Reduction Test (FECRT). This is a simple test comparing the pre- and two weeks post-deworming parasitic egg counts in a horse’s manure. Acceptable reduction in egg count differs between dewormer classes, but a satisfying result provides nearly 100 per cent drop in egg count post-deworming.

Without this type of testing, horse owners can’t be assured that the dewormer they use actually does the job. If resistance to one class arises, it does not mean the other two classes will be effective. Nor does it mean that the same class will be ineffective at the farm next door. The upshot is that advice from a veterinarian and a FECRT are necessary to effectively treat your horses.

Routine rotation between the classes was once commonly recommended, but Dr. Misuno says this can be misguided. “Go to your vet and ask if he/she performs Fecal Egg Counts. Have your herd tested to all three classes of dewormers before blindly rotating between them. Imagine rotating from a 100 per cent effective class to a 50 per cent effective one. This gives a false sense of good management, as dewormer was administered, but results in lack of protection or treatment of your horse and intensifies spread of resistance.”

How Often to Deworm

Dr. Misuno and Dr. Chris Clark, the study’s supervisor, emphasize that the results did not support annual deworming, even if, on the surface, it might seem possible to decrease frequency of deworming in western and northern Canada.

“Horses are very individual in their sensitivity to parasitic infections,” said Dr. Misuno. “We can identify them as low, moderate and high shedders based on the number of parasite eggs found in the horse’s manure. It’s just like with any other infection; some individuals are more prone to get sick and others are healthy despite everyone around them getting sick. Interestingly, even on farms with high stock density with no use of dewormers at all, there are horses that consistently test negative (zero eggs) on FECs. Therefore, deworming, as part of a parasite control program, should be scheduled individually for every horse based on his/her manure sample result and, in some cases, a physical exam.

“Dewormers are drugs, and just like antibiotics, their overuse, underuse, improper use and the use of a type of drug that already does not work (resistance) leads to serious problems. It is very desirable for the health of the whole herd to identify high shedders and treat them with higher frequency than low shedders. High shedders not only contribute the most to the contamination of the pasture, but having poor immunity to parasitic infections, they are the most susceptible to actually suffering from clinical signs such as colic, diarrhea, cough, weight loss or even death. On the other hand, low shedders have developed their own mechanisms to fight parasitic infections and are less likely to be severely sick, as their internal parasite load tends to stay low. They will require less frequent deworming.”

According to Dr. Clark and Dr. Misuno, generalized schedule recommendations for a herd or a region are inappropriate. The only recommendation that may be uniform, for the prairie regions, may be that there is no active transmission of new parasitic infections throughout the winter, as the eggs on the pasture do not have conditions in which they can develop to infective stages. Further, every horse must be looked at individually by taking under consideration their health. Certain diseases may be linked to increased or decreased sensitivity to internal parasites such as age, management and deworming history.

Management of a horse’s environment is a key element in good parasite control program. Picking up manure simply decreases the infective ‘dose’ consumed by your horse. Low stock density and rotation with cattle may be helpful, but most of the parasitic eggs survive outside the horse for a year, with large roundworms being a leader, having five to 10 years presence on your pasture!

“A modern parasite control program takes all these factors into account; it is far more complicated than administering an anti-parasitic product,” said Dr. Misuno. “Hopefully, we will not run into a situation of losing all dewormers due to resistance, when pasture management will be the only advice a vet can give to a horse owner. I strongly believe that educating our horse community and encouraging vets to provide FECs as a standard test will help slow down the resistance issue.”

Dr. Ela Misuno (DVM, MVSc) conducted the study on equine parasite prevalence and resistance in the western Canadian prairies as part of her master’s degree at the the Western College of Veterinary Medicine, University of Saskatchewan. Her research was supervised by Dr. Chris Clark and Dr. Lyall Petrie from the Large Animal Medicine faulty. The study was performed at three breeding facilities around Saskatoon, involving 90 horses. The farms differed in breeds, herd sizes, herd management, stock density and deworming history.
Dr. Misuno, of Vetoquinol, noted that the veterinary pharmaceutical company is currently sponsoring a series of parasite control ‘lunch and learn’ talks for horse owners as well as hands on fecal testing labs for veterinarians and veterinary technicians (mainly in western Canada). Ask your vet for this free-of-charge opportunity to learn, and don’t miss out on parasite control seminars during horse fairs.