Horsemen need to deworm for small strongyles (cyathostomes) at the proper time to thwart massive egg-laying that contaminates spring pastures. “If your pastures don’t start till late spring, you can wait till later to deworm. If horses are stabled and won’t be turned out until May, there’s no need to deworm until just before that time,” he says.
“The early spring deworming is the most important. Depending on where you are, and how much rain you have, summer may or may not be a problem. When it’s hot and dry, worm transmission is drastically decreased. They do best when it’s warm and moist. The two times of year most important for deworming are late winter/early spring, and again in late summer/early fall – before autumn rains begin. A cool wet fall will bring on more worms,” he says.
If horses pick up large numbers of small strongyles in the fall, there will be a tremendous emergence again next spring. “This is why late winter – before spring really starts in your area – is important for deworming. My choice of drugs at this time would be moxidectin. Ivermectin gets the adults and any larvae that are active, but is not as effective against the inactive ones that are hiding,” he explains.
Another effective drug is fenbendazole. “If you give a high dose, over several days, this usually does the same thing as moxidectin. It kills adults with the first high dose, and this signals immature worms to come out, since this is their chance to take up space vacated by the eliminated adults. Then if there is still an adequate blood level of fenbendazole, it kills the younger worms as well. But I’ve seen cases where fenbendazole won’t kill them, so you need to determine whether that drug works on your farm.” On farms where worms are resistant, the only thing to use would be moxidectin.
Roundworms in foals
Ascarids primarily affect foals, weanlings and yearlings. Older horses have developed some immunity against these worms. The problem today is the beginning of ascarid resistance to the macrolides (ivermectin and moxidectin).
“On one farm where I do some experimental work the ascarids are resistant to ivermectin, but if we use a double dose of pyrantel, it is effective. We also see tapeworms in foals in late summer and autumn, and a double dose of pyrantel will kill them, too. But pyrantel has very limited effect on cyathostomes, even at the double dose. At this farm we also compared the fenbendazole product (double dose for 5 days) which was very effective in removing ascarids but had only limited efficacy against cyathostomes,” he says.
“We have two different parasites in these foals at the same time, but if we use one drug we only kill one of them. On some farms we now recommend using a combination of ivermectin and pyrantel at the same time, on weanlings and yearlings.” These two drugs are in different families and work in completely different ways, and it is not likely that a worm will have the genes for resistance to both at the same time.
“By giving full therapeutic dose, we are able to kill the targeted parasite. Yet because the two drugs are working in different ways, there is no increase in toxicity to the horse. If you were to use two drugs in the same family, this would be an overdose. Since these are in different families, working in different ways, they are safe to use at the same time,” says Craig.
“On farms where ivermectin has been the only dewormer used since it was introduced in the 1980’s, we compared use of ivermectin alone, ivermectin and pyrantel given at the same time, and fenbendazole at double doze for 5 days. Ivermectin alone was effective against cyathostomes but failed to control ascarids. Fenbendazole was effective against ascarids but only marginally effective against cyathostomes. The combination of pyrantel and ivermectin was acceptably affective against both families of worms.”
Drug resistance is usually a local problem, but when horses are moved around the country, this spreads resistant worms around. “Dewormers may kill a few wimpy worms, leaving the stronger, more resistant ones to reproduce,” he said. Extensive use of dewormers speeds resistance development. It’s better to judiciously use dewormers just two or three times a year, at proper season to hit a targeted species at the best time to control it, rather than routinely using dewormers throughout the year.
“Rotating drugs – switching families of anthelminitics – to keep ahead of resistance problems, doesn’t always work. You may need to use different dewormers against different parasites however, such as in foals, or at a time of year horses might have tapeworms in large numbers.”
The big problem is that some immature worms (such as cyathostomes) can remain in a quiescent state in the intestinal wall for up to two years, and then emerge. “In theory, using fenbendazole or moxidectin should kill these inactive worms. If they are 100 percent effective in killing immature as well as mature worms, there would be no problem. But if only 99.9 percent are killed, the survivors have only other survivors to mate with (resistant worms) and in a few years the drugs will fail. We’re now seeing a shorter time from treatment until eggs are seen again in horses treated with ivermectin,” Craig explained.
Many horses develop some resistance to worms. “If you check egg counts in adult horses, you’ll find that many of them don’t have egg levels that would cause problems. Yet another horse in the same pasture may have a tremendous number of worms,” Craig said.
“Thus it makes sense to just treat the wormy individuals (diagnosed with fecal egg counts). Probably all foals, weanlings and yearlings need dewormed, because this group is highly susceptible. But for adult horses, selective deworming is practical and effective. You are not aiding development of resistant worms as much as when you continually deworm the whole herd.”
One reason for continual deworming (every two months) – to control Strongylus vulgaris (the major parasite of domestic horses in the past) – is no longer valid, since this parasite nearly disappeared with use of ivermectin. “A mustang off the range might have a few bloodworms, but probably below a level that would cause disease. If that horse came onto a farm and lived in confinement, however, it could spread those worms and they would become reestablished unless steps were taken for control,” Craig added.