While it is unlikely for horses to suffer harmful side effects from deworming too frequently, the health of the horse can be compromised in the long run when parasites start to develop resistance against horse wormers.
In the 1960’s, horse worming strategies were developed and followed simple protocol – to treat the horse for 8 weeks using the available wormer benzimidazole (an anthelmintic drug used to kill end expel intestinal worms). There was a significant improvement on mortality from parasitic infections. In the following decades, new wormers have been developed and veterinarians recommended rotating different classes of equine dewormers, but using the same strategy for every horse.
However, equine parasites responded to dewormers in a different way – they have developed resistance. For instance small strongyles, the most prevalent equine parasite in adult horses at present, developed resistance to two major classes of dewormers, the benzimidazoles and pyrantels. Small strongyle resistance resulted from the rotation of drugs, some of which still exhibit efficacy against this particular parasite, and some of which do not.
Is too much worming old news – new stratagies.
Experts believe that it is time to abandon the old practices of deworming a horse. Horses shed worm eggs via their feces; therefore, it simply doesn’t make sense to deworm a horse with the same 8-week treatment course. Once you have determined the frequency at which you need to treat your horse, it is important to make sure that you choose products that actually exhibit an efficacy against parasites found on your farm. This practice is also called strategic deworming and is considered a better way to control parasites and help avoid the risk of drug resistance in your area.
Strategic deworming works best when you get your veterinarian involved. The first step to be done by your veterinarian is to conduct a fecal egg count on each horse, this will determine the type of parasite your horse has and which among your horses are high or low shedders. With the results, your vet will recommend to you the frequency of worming. Most likely, the vet will perform follow-up fecal egg count reduction tests to check whether your chosen wormers still work against the culprits on your farm. In the end, you might find that it is just appropriate for you to stop using some of the dewormers that were once part of your rotation program.
Strategic deworming can help you save money in the long term as broad spectrum agents may be not be required as often as they used to for some horses.