Neither of those are quite correct. It used to be that you used different dewormers because they treated for different types of parasites at different times of the year. However, that was before the drugs that we currently use were on the market. Before, you had to switch drugs to target specific parasites because they generally got only 1 or two parasites. Now the drugs are broad spectrum and kill multiple species of parasite. The only time you have to choose a drug based upon season now is for your spring and fall dewormings to target bots and tapeworms---either ivermectin or moxidectin are used at these times.
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.
The difference between ivermectin and pyrantel specifically:
ivermectin kills a few more types of parasites and kills migrating larval stages where pyrantel does not. Both drugs have resistance problems when used to treat ascarids (but these aren't a big concern in adult horses) and pyrantel has moderate resistance issues with strongyles. Pyrantel can be given at twice the normal dose to treat tapeworms where ivermectin is never effective against tapeworms no matter the dosing.
Licensed Veterinary Technician