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Equine Parasite Control

By: Dr. Philip Johnson

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What is a Daily Deworming Program?

In recent years, a popular and effective program for the prevention of parasite problems in horses has entailed the administration of a specific anthelmintic drug every day. The drug used in this program is pyrantel and it is marketed as Strongid-C. Horses in this program must first be dewormed using either ivermectin or moxidectin (the most highly effective anthelmintic drugs). Immediately following this primary treatment, the horse receives a small dose of pyrantel in the feed every day. This dose of pyrantel is all that is needed to kill new incoming larvae before they invade the intestinal tract. The use of daily pyrantel in this manner is an effective preventive strategy for the strongyle parasite group (the most important group) but it is probably not effective against other categories of parasites. This program is also very good for minimizing problems with roundworms (ascarids) in young (< 10 months of age) horses. Not all horses are managed in a system that readily allows for individual horses to be treated with a dose of pyrantel every day. This is a good method for reducing parasitism in those horses that are looked after as individuals and are fed individually each day.

In the southern US, the timing is different. The high risk for infection occurs at the end of the summer. In spring-summer, it is too hot for eggs to develop into infective 3rd-stage larvae (even if there are lots of eggs). Risk of infection increases during fall – a result of newly-deposited eggs which are now able to mature. Therefore, pasture contamination should be prevented by deworming just prior to the time pasture contamination will start occurring (Sept-Oct).

The practice of SUPPRESSIVE DEWORMING is intended to prevent pasture contamination. The climate is allowed to "clean-up" the pasture at a time when all grazing by horses is done only by horses which are not shedding eggs. The frequency must be based on the ERP (see above) and you must deworm within the ERP of the previously-used drug!

By using this SUPPRESSIVE DEWORMING practice, fecal egg counts remain low and the pastures stay clean. Eventually, the cycle of reinfection is broken and the transmission of small strongyles is attenuated. It takes 2 to 3 years to deplete horses of their pool of encysted (hypobiotic) cyathastomes.

In the northern US, wintered horses should be dewormed just prior to being turned out onto pastures in the spring. For horses in the northern US, it is only necessary to suppress contamination past the beginning of July. After this time, there is insufficient grazing time left for significant pasture infectivity to develop. A final deworming is advocated when the horses are stabled for winter (in combination with a bot treatment).

In the southern US, the transmission season begins sometime around September, at which point horses should be dewormed to prevent pasture contamination. Repeated suppressive treatments should be continued until late winter (February/March) and then suspended until the following September. Some eggs will be passed by horses after March, but the weather takes care of them. Hot dry summer prevents eggs from developing into infective 3rd-stage larvae.

Resistance to the benzimidazoles and pyrantel has been reported (sometimes to both). Resistance (which occurs naturally) is promoted by the exclusive use of one drug (or group). Although controversial, the practice of rotating between different anthelmintics has been advocated to minimize the risk of promoting resistant strains. Rapid rotation entails changing the drug every time a deworming is done. Slow rotation entails changing the drug every 6 – 12 months. You should be rotating between classes (do not rotate between 2 drugs in the same class).

Ungrazed areas ("roughs") should not be dispersed into grazing areas ("lawns") by use of harrowing or dragging pastures, unless the pasture is to be left unoccupied by horses for a long period.

Other Strategies - Integrated Control

Incorporate pasture management with deworming program. Remember that there are 2 parasite populations on every farm – those in the ground and those in the horse. Remember that a pool of encysted ("hypobiotic") larvae always survives each deworming treatment. Obviously, there are limitations based on the availability of space.

  • "Treat-and-move" principle – Deworm and transfer to a "clean" pasture. Very good idea!

  • "Pasture rotation" – Ideally need to leave pastures vacant long enough to allow larvae to die out. The only area where this is really useful is in the southern USA during the summer.

  • "Pasture hygiene" – Remove feces from pasture before infective 3rd stage larvae emerge. Very effective, but labor-intensive and you will need to perform dung removal twice weekly.

  • "Alternate species grazing" – no significant cross-species infectivity – therefore, other animals can eliminate infective 3rd stage larvae when they graze horse pastures!

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