One of the most confusing and controversial areas of equine management is
: what is a good deworming program.
You can ask ten different horse owners how they control parasites in their horses and may get ten different answers. However, they may all be right. Everyone needs to tailor their own deworming program to their own specific needs. Things to consider are the age of your horses, current parasite load and the environmental conditions in which they live.
The best place to start is to consider the environment in which they live. This determines how often one needs to treat their horses. If your horse is in your own pasture completely under your control then purge deworming every three months rotating products is sufficient. The goal here is to keep the worm burden down to an acceptable low level and not necessarily a zero level. The reason for this is that parasites are great at developing resistance to dewormers over time. Constant use of one particular dewormer has in the past been the cause of this resistance. Ivermectin has been relied upon heavily for the last couple of decades because there is no resistance in horses. However, this can change with overuse and may have already started. Also, ivermectin does not get all parasites, such as bots and tapeworms. Rotating between the three classes of dewormers is best. The classes are avermectins(ivermectin and moxidectin or Quest), benzimidazoles (fenbendazole, oxibendazole, oxifendazole) or pyrantal pamoate(Strongid). The rationale for aiming for a low level of parasites and not a zero level is because, in order to achieve this, one has to use frequent dewormings that actually will increase the levels of resistance in parasites. Our area here in the Gulf Coast Region is parasite heaven with its hot, humid, usually wet conditions. Every day is a continual battle with more and more worm eggs in the soil. These are the recommendations for adult horses. Under there conditions, foals should be dewormed at two months of age and every two months until one year of age then follow the adult schedule. Broodmares should be dewormed with ivermectin just prior to being bred then with the other classes of dewormers during the pregnancy. Ivermectin is labeled as safe for pregnant mares; however, I prefer not to give it during pregnancy as it is a strong medication. One month prior to foaling, deworm the pregnant mare and then five to seven days after foaling deworm with ivermectin to eliminate a worm that is passed in the milk.
If you board your horse at a facility that has excellent management of their pastures such as harrowing or removal of manure from the pasture AND all horses located there are dewormed at the same time, you may use the above schedule. If one or the other is not true or if the managers spread manure out in the pasture as their way of disposal then you may have to consider stepping up the program to deworming every eight weeks while still rotating or using the daily pyrantal dewormer. A worm's life cycle is eight weeks and since most of the above dewormers only get adults, treating less than eight weeks may not have any affect as they are not adults at six weeks and there is no residual effect from the dewormers. Removing manure is the single most important factor in reducing worm loads by preventing exposure rather than treating it. Harrowing is second best by allowing the sun to kill the larvae that are now exposed. If the facility has a high number of young horses, the foals will need to be dewormed at one month and then every six to eight weeks until one year of age.
If your facility has no pasture maintenance and every owner does her own deworming schedule or none at all, then it may be necessary to deworm every six weeks. With horses on different schedules then there will be a variety of stages of life cycles present and in this instance deworming at six weeks may be beneficial. However, this is going to set up for resistance and it is not a question of IF but WHEN. Daily dewormer would be at an advantage as the worms will not progress to the damaging stages of their life cycle with in your horse's intestines. Also, a better idea would be to attempt to get your horse owner neighbors all on one schedule and maybe even volunteer to deworm some of the horses that may never get dewormed (with their owner's permission of course). It may cost you some more money short term but benefit in the long term.
Just a few tips : with either the rotational scheme or the daily dewormer, you should use ivermectin plus praziquantal (Zimectrin Gold or Equimax) at least twice yearly to get bots and tapeworms. If you are uncertain of your new horse's worming history you should perform a five days in a row, double the usual dose of fenbendazole to get rid of any larvae that are encysted (hiding) within the gut wall. No other wormer can do this as effectively. If the horse is debilitated or if there is concern that it may be heavily parasitized do not give ivermectin or moxidectin (Quest) as these are powerful and very effective dewormers. This may cause a large kill of worms within the intestines and may cause colic or diarrhea. Try the gentler pyrantal (Strongid) or fenbendazole (Panacur or Safeguard). Then follow up in two weeks with the stronger dewormers.
If your horse is impossible to paste worm (some will spit it out when you leave the stall), consider using the daily dewormer or mixing the paste wormer in with a generous amount of sweet feed or if all else fails, then get your veterinarian to tube deworm your horse. Paste worming can be just as effective as tube deworming if you give enough and are positive that your horse swallows all the paste. Consider following the pasting with feed to insure the horse swallows the paste. Biannual fecal exams are recommended to evaluate your deworming program and to check for resistance. Collect 3-4 fecal balls ten days after paste worming and anytime if you use the daily dewormer. You can also purchase a fecal flotation kit and an inexpensive microscope if you have many horses and do them yourself. One source for this is farmsteadhealth.com or Google fecal flotation kit. Most importantly, pick a program and stick with it and just do it. The most common excuse I hear when I am treating colic and inquire about the deworming history
"I keep forgetting to pick up the paste dewormer at the feed store." Consider buying in bulk, enough for the year and keep it at your house. Just check the expiration dates before purchasing to insure that they are good for a year. As always, consult with your veterinarian regarding your particular situation.
Dr. Angela Chenault practices at La Paloma Equine Clinic in Waller County and the surrounding areas.