Microsoft word - august 2010 newsletter

Dr. Christina Mohos 43 Simpson Street East Alma, Ontario N0B 1A0 Phone: (519) 846-1800 Fax: (519) 846-1635 [email protected] August 2010 Newsletter
The office will be closed Monday, August 2, 2010 for the Civic Holiday, as well as Monday,
September 6, 2010 for Labour Day. If you are having an emergency please call the pager at
(519) 829-6825. Please note this number is for Emergencies Only.

Deworming – Are we creating a problem???
Many of you know that we hate worms. We feel strongly about avoiding parasite resistance through fecal monitoring and strategic deworming protocols. We believe that horse owners should be aware of the resistance issues we are facing with dewormers so that they can implement an effective parasite control program. Unfortunately, the drugs available to control parasites in horses are limited, so; if we take a proactive approach, hopefully we can avoid a complete resistance to all classes of drugs. The resistance issue with worms appears to have been created by an excessive and inappropriate use of dewormers. This overexposure gives the pathogen more opportunities to find a way to avoid the effects of the medication. The most well known case is the resistance of small strongyles to benzimidazoles (e.g. Panacur). This is a particular problem within North America, but also occurs around the world. In parts of the southern U.S. resistance to Pyrantel, a drug used to kill strongyles and other intestinal parasites of horses, has also been documented. Recent studies in Holland have identified incidences of total resistance of roundworms to the class of drugs called macrocyclic lactones (eg. Ivermectin). A farm being studied here in Ontario has also presented evidence suggestive of resistance of roundworms to macrocyclic lactones. One of the biggest issues is the availability of dewormers in tack shops. Most people will buy and use a dewormer without consulting the veterinarian. Owners administer it, hope for the best and assume it is working. However, this practice will not necessarily target the type of parasite your horse is harbouring. It may also be unnecessarily overexposing harmless low-levels of parasites to the medication, thereby increasing their chance of becoming resistant. Here are some suggestions for a ‘Strategic Regime’ to slow the progression of parasite resistance as described by Dr. Andrew Peregrine, a leader in equine parasitology. 1. Use strategic deworming treatments based on the parasite’s epidemiology.
Cyathostomes are acquired by horses at pasture. Therefore, horses only need deworming while on pasture. The viability of cyathostome larvae is greatly reduced in freezing temperatures, so a typical horse living in Ontario does not require deworming in the winter months. 2. Use inter-treatment intervals based on the drug’s duration of action.
Each drug has a certain length of time when a horse’s feces will remain negative for eggs. This is called the ‘egg reappearance time’. This interval varies among dewormers. Dewormer Chemical
Compound Commercial
Egg Reappearance
Benzimidazoles Fenbendazole,
4-5 weeks
Tetrahydropyridimine Pyrantel
Strongid, Exodus
4-5 weeks
Macracyclic Lactones
Eqvalan/Panomec 6-8 weeks
So when selecting a treatment interval (time between treatments), do not use a time less than the figures given above. Treating every 4 weeks with Ivermectin is not a good idea. Ideally, intervals should be used that are as long as possible. Ivermectin should be given every 8 weeks. 3. Carry out regular monitoring of deworming effectiveness.
This is where people get slack. It is not safe to assume that your deworming protocol is working without monitoring the efficacy of dewormers. A Fecal Egg Count (FEC) should be performed annually during the summer months in Ontario. A FEC assessment will tell you if your deworming protocol is working or if changes should be made. Your vet can outline for you what is involved in FEC testing. 4. Ensure correct dose of dewormer is administered.
Under dosing is a major risk factor for development of dewormer resistance. Owners are encouraged to use a weight tape measure so that they can dose accordingly. Overdosing can lead to toxicity. 5. Rotate dewormer class on an annual basis.
It is recommended to use a slow rotation program in which one dewormer class is used for an entire year or two. The following year a different class of dewormer will be used. Depending on the drug used, bots or tapeworms may not be controlled so inclusion of these drugs at appropriate times of the year is acceptable. If resistance to a particular class of drug has been identified, that drug should not be used on the farm for 5 years. 6. All horses on the farm should be on the same deworming program.
7. New arrivals.
Dewormer resistance most commonly establishes on a farm with the arrival
of a new animal that is infected with drug-resistant parasites. All new horses should be
isolated for 14 days and dewormed when they arrive. A FEC should be performed before
introducing the new horse into the herd.
8. Targeted treatment ?
Within any group of horses only a minority of animals harbour the majority of the parasites. An increasing number of veterinary parasitologists are suggesting that one only needs to treat horses with high fecal egg counts. Such targeted treatment regimens are dependent on fecal examinations being carried out on all horses every 4-8 weeks and typically are associated with a substantial (>50%) reduction in the total amount of dewormers used on a farm. Although the cost of fecal monitoring is increased, targeted treatment can be economically viable and has been used to control multi-drug resistant parasites. In the end the horse owner saves on the cost of dewormer and sometimes may not have to treat with dewormer at all! 9. Sound pasture management.
Situate manure piles far from water sources and grazing areas; follow composting procedures (high temperatures inside compost will kill parasites); muck stalls on a daily basis to avoid exposure due to confinement; remove manure from pasture twice per week; on larger acreage harrow the land only in dry, hot weather or below freezing (but snow-free) to kill parasites by exposing them to unfavourable elements. 10. Avoid overcrowding your pasture.
Overcrowding increases the concentration of shed parasites. One horse per two
acres is an ideal maximum.

11. If adequate grazing is available, pastures should be rotated.
12. Adolescent horses
Young animals are more susceptible to gastrointestinal parasites than older horses and have the potential for high fecal egg counts prior to the development of immunity. Such animals should therefore be given the cleanest pasture available and not grazed with other age groups. In addition, the pasture they have used should be assumed to be heavily contaminated with parasites after their withdrawal.


Annex i

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