First, understand that you should always involve your vet in planning your deworming program and that's even more important now because strategic deworming should be done rather than following the old "deworm every 6-8 weeks rotating dewormers" which will no longer provide effective protection for your horse and will only help to build resistance. Along with the development of resistance is the fact that we now know that 20% of horses carry 80% of the parasites and those are the ones that need a more stringent deworming regimen while the other 80% of horses will need less frequent dewormings because they have a better resistance to parasites and thus don't carry big burdens even without frequent dewormings. . Add in the fact that environmental conditions vary all over the world and they have a direct affect on environmental contamination with parasite larve and on when infection rates are going to be highest in each situation. All of these facts mean that there is no "one-size-fits-all" deworming program. Some horses may only need deworming twice yearly while others like foals require much more frequent dewormings. So, rather than just following the old plan it's now recommended to practice strategic deworming--plan a program based upon the specifics of each horse and use diagnostic testing to ensure that the program is appropriate or to determine when deworming is necessary. This is to help reduce the number of dewormings to help slow the development of resistance while still providing adequate deworming for each horse. This is important because there aren't any new deworming drugs that will be hitting the market anytime soonIt is important to take all factors into account and know which horses are more resistant and which are less resistant in order to plan a deworming program that is going to be effective for minimizing parasite loads, minimizing the frequency of treatments and also minimizing the risk/rate of parasite resistance developing to the drugs in use.
So, rather than just following the old plan it's now recommended to practice strategic deworming--plan a program based upon the specifics of each horse and use diagnostic testing to ensure that the program is appropriate or to determine when deworming is necessary.
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
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.
When planning a deworming program for adult horses, your main concerns are strongyles, tapeworms and bots in that order. (And this is where you can really see the big problem with deworming based on that link above.) Strongyles are the parasite with the most resistance issues--in more than 90% of areas tested these parasites are now resistant to fenbendazole and in more than 40% they are resistant to pyrantel. There has even been 1 study showing strongyles becomeing resistant to ivermectin. So, the standard rotation in that link you are likely not going to be effectively killing strongyles for 1, maybe 2 and even as many as 3 out of 3 dewormings.
For foals, your main concerns are ascarids, strongyles, tapes and bots. So you have the same concerns as with adults---resistant strongyles, but you also have the added concern of ascarids which are shown to be resistant to ivermectin in some areas of the country. Rotating is still a good idea in foals so that you balance possibly not killing ascarids with one treatment and then the next treatment killing ascarids but possibly not killing strongyles. However, it's probably best to stick to rotating either pyrantel or fenbendazole with ivermectin.
So, rotation options are limited at best and it's really not the rotation that is most important for preventing parasite resistance but appropriate dosing and treatment intervals.
You need to be SURE you are not under dosing your horses so always use a weight tape or measure your horse and calculate his weight. In studies even many vets were way off on weight estimations and they have the benefit of spending a few years working in situations where they get to walk horses onto scales daily to see what 900 lbs LOOKS LIKE.
Weight calculation information: Body Weight Estimation of Horses
KG calculation shown as well as a chart based upon heart girth measure (remember, the heart girth only assessment may be off by as much as 200 lbs just like weight tapes) Horse Weight: Estimate It Easily
How to calculate in pounds
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.
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.