Most of the intestinal parasites of horses are acquired through grazing (or even eating hay from pastures where horses deficated). Tapeworms are contracted from eating grass mites that have ingested tapeworm eggs. So, horses in dry lots or stalled and fed hay are at lower risk for parasite infection (other than bots). So, how your horse is managed definitely plays a part in risk of parasite infection--including the old stand-by "deworm every 6-8 weeks rotating dewormers" plan which is not a good choice any longer.
However, strongyles can encyst in the walls of the GI tract and remain there for months or years and then erupt and mature into adult worms. You can also have parasite infections that persist because of resistance to certain dewormers or because of under dosing.
Here is my long-winded deworming bit:
A deworming plan depends on lots of variables--age of horses, type of management (stalled vs. pasture vs dry lot), pasture maintenance practices, history of the pasture (have there been horses on the pasture previously), local environmental conditions. Some horses only need to be dewormed once or twice a year while others require a much more stringent deworming program. Once these variables are known, then you can formulate a deworming plan that fits the situation. It's always best to contact your vet to discuss these issues however we can help you lay out a plan and then you can run it by your vet.
Rotating dewormers is an old plan that was necessary back when the drugs we had available where not broad spectrum--one might get ascarids, another strongyles, etc--so rotation was done so that you were treating for all the different types of parasites. Then it was continued once we got better drugs available for deworming even though it's not necessary to be able to target the different parasites but in the hopes of preventing the parasites becoming resistant to the drugs. However, this has not been the case and with the situation we are now in there is very little "room" for rotating drugs (which in fact means rotating classes of drugs not just brands or chemicals) because the benzimendizole class (fenbendazole-Safeguard, Panacur, etc) is of very little use because of resistance issues in 95% of the world, and the pyrantel paste (Strongid, Equicide, Rotectin, Strongyle Care) has resistance issues in around 50% of the areas where it is used. The avermectins (moxidectin and ivermectin) are the only class of dewormer that doesn't currently have any resistance issues in strongyles which is the main issue in adult horses (though ivermectin resistant ascarids have been seen in certain areas and this is an issue for foals). 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
As for appropriate dosing intervals, you want to treat according to the egg reappearance period so that you are dosing to prevent continual recontamination of your pastures. This method means that you will be cutting back on your horse's risk of parasite infestation significantly after a year appropriate dewormings because you will have cut the number of parasite larva on your pastures. It also means that you won't be deworming at a time when there are no parasites in the system that will be susceptible to the product you are using next which happens if you dose too soon after ivermectin or moxidectin with pyrantel or fenbendazole because at normal doses these two chemicals only kill adults in the GI tract and if you've dewormed with moxidectin last 8 weeks ago there aren't adults there to kill and you've missed the migrating larva which will then start shedding eggs 4 weeks after you treat with either of these drugs and shed for the 4 weeks until you deworm again. Or if you dewormed with ivermectin 6 weeks ago and treat with either pyrantel or fenbendazole you will start seeing egg shedding into your pasture in 2 weeks because you didn't have adults in the GI tract when you dewormed last but they will be there in a couple of weeks. Or in the case where you actually do have adults for the pyrantel or fenbendazole (which again is not likely to be working) to affect because it's been 8 weeks since your last dose of ivermectin you will again have parasite ova being shed in 4 weeks because that is how long after the use of either of those drugs you start seeing adult egg shedding parasites in the GI tract. So deworming based upon Egg Reappearance Periods, you would deworm and then deworm again based upon what drug you used last--4 weeks later for pyrantel (or Fenbendazole, though the next day would probably be more useful LOL), 8 weeks after ivermectin or 12 weeks after moxidectin. In this manner, you can significantly reduce the parasite load on your pastures in a year's time.
Deworming every 6-8 weeks routinely and rotating products is also not the best way to go. Again, we need to focus on appropriate use of the deworming products we have to help prevent the development of parasite resistance and blindly deworming every 6-8 weeks no matter what product you last used doesn't take into account how long it is until you actually have parasites that are capable of shedding eggs again.
Besides deworming, there are pasture maintenance practices that can help to minimize pasture contamination with parasite larva. Rotating pastures with other types of livestock (cattle, sheep, goats) can allow time for the parasite larva to die off before you put horses back on it but you have to have many months between rotations. Picking up feces at least every 2-3 days will also greatly reduce the parasite load on your pastures. (Doing this daily will keep the amount of feces you have to shovel at one time down ) Dragging pastures to break up feces during the very hot, dry southern summers will also help lower parasite burden on pastures, but if you do it during moderate weather it will only help to spread the larva around.
Whatever deworming information you read, make sure it's up-to-date. Many articles and recommendations are based upon old information and that can lead to wasting money and providing poor parasite control for your horses.
Licensed Veterinary Technician