For 40 years now, veterinarians have recommended that horses be dewormed every 8 weeks all year round and rotation of dewormers has been recommended for nearly that long. This is considered by many vets and horse owners to be “the recipe” for adequate and appropriate deworming of horses. However when you look at the research that has been done in the last 15 years you really have to question this practice. “’The recipe’ no longer represents an acceptable program for strongyle control” according to Dr. Craig R. Reinemayer, DVM, PhD of East Tennessee Clinical Research, Inc during a webinar presented via www.thehorse.com
. This means that deworming recommendations for adult horses need to be reconsidered because strongyles are the only significant nematode pathogen of mature horses. Continuing to use the same old deworming program will likely lead to heavily parasitized horses and further drug resistance. Instead, each horse and each situation should be evaluated to determine the reinfection rates and an appropriate deworming program for the individual.
There are 4 classes of dewormer on the market: benzimendazoles --fenbendazole, oxibendazole, other chemicals that end in -azole (there is a long list) pyrantels---pyrantel pamoate (paste) and pyrantel tartrate (daily dewormer) avermectins---ivermectin and moxidectin praziquantel Of those 4 classes, all but praziquantel are "broad spectrum" meaning that they kill several types of parasites. It is not necessary (or at least was not prior to the developement of parasite resistance) to rotate dewormers using these products to kill the most common parasites of horses. The "rotate to kill the different types of parasites" was necessary when we only had the much older drugs which were often only effective against one or two types of parasites. Praziquantel is the only one of the current drugs that is not broad spectrum and it kills tapeworms which the other drugs are not effective against unless you use pyrantel at twice the normal dose. But, these drugs are not all as effective as they used to be because they have been over-used and mis-used for many years. Now we have parasites that are becoming and have become resistant to these drugs so we have to change our deworming strategies to help slow the build up of resistance while still minimizing the parasite load in our horses.
It's now known that all adult horses living in the same situation do not necessarily need to be dewormed on the same schedule. 50% of horses in a herd will control parasite loads on their own due to natural resistance. Only about 20-30% of horses carry heavy parasite loads.
Thus each horse should be dewormed based upon an understanding of his own personal resistance to parasites. The best recommendation is now 2-4 dewormings a year based upon knowing which horses carry lots of parasites and which tend to carry little parasite load.
Rotational deworming is no longer an adequate or appropriate deworming program for adult horses. There are too many issues with strongyles developing resistance to 2 of the 4 most commonly used dewormers on the market---fenbendazole (more than 90% of areas tested have resistant strongyles) and pyrantel (around 1/2 of areas tested have strongyles resistant to this drug). And resistance is starting to be seen in strongyles to ivermectin---1st study showing it was done in KY in the last couple of years.
All adult horses in the continental US/Canada should be dewormed spring and fall with ivermectin/praziquantel or moxidectin/praziquantel. Other than those 2 standard dewormings, the rest of the deworming program should be based upon location and the horse's own resistance to parasites. The new recommendation is 2-4 dewormings per year based upon fecal egg counts used to determine the normal amount of egg shedding each horse does during the time of year when the weather in your area is most conducive to strongyle larva development and environmental survival. In the northern states in the US and in Canada, this means running a fecal egg count in the middle of summer (3 months after spring dosing if you used ivermectin or 4 months after spring dosing if you used ivermectin). In the southern US and Mexico you would be looking at testing in the middle of the winter (same time after spring deworming as listed above). Then based upon the number of eggs per gram of feces you can determine if you need more than the spring/fall dewormings and if so if you need 1 or 2 more dosings.
In the northern US and Canada, deworming should be discontinued during the winter months because the environmental conditions are not conducive to reinfection---that time of year has been proven to have extremely low reinfection rates. In the southern US and Mexico the opposit is true....deworming can be discontinued during the heat of summer because temps over 85 degrees lead to the infective strongyle larva dying quickly in the environment so the reinfection rates are lowest then.
For more detailed information check out the deworming webinar that was aired via The Horse magazine's website in April. Be prepared to sit for a while because it is an hour long presentation, but it's well worth the time. The veterinarian gives you all the information on strongyles and deworming in adult horses that you've always wanted to know and then some. It is a wonderful lecture. (And have plenty of paper and a pen.) http://www.thehorse.com/Video.aspx?vID=1…
(Craig R. Reinemeyer, DVM PhD --parasitologist)