The Battle Against Equine Dewormer Resistance

Thirty to 40 years ago, the major parasitic scourge was large strongyles, also known as blood worms. These parasites would migrate through both the wall of the intestines and the intestinal blood supply itself, causing both local damage and in many cases extensive clot formation in the vessels feeding the intestine or those supplying the hind legs. Chronic colics and even death related to these parasites were common. The drugs available then were only effective against adults living inside the interior of the intestine.

With the advent of the modern dewormer drugs effective against more life stages — and the development of inexpensive, easy-to-use and effective paste dewormers — strongyles have been virtually eliminated from horses. That’s the good news. The bad news is that niche is now being occupied by the small strongyles, or cythastomes. And they can do some impressive damage of their own. Add to this the concerns among researchers rising about drug resistance, and you’ve got a challenge on your hands.

Larval Stages

Larvae of cyathostomes spend some of their larval life stages inside the wall of the cecum and colon. This species does most of its damage when penetrating the wall and then again on leaving it to mature to egg-laying stages inside the intestine.

A pecularity of cyathostomes is that the larvae have a stage, L3, where they form a cyst inside the wall of the intestine and can become dormant for as long as two years. It’s believed the larvae receive ”messages” from the interior of the intestine and don’t mature if there are large numbers of parasites already in the bowel.

A clinical syndrome has emerged called cyathostomiasis. It’s seen when horses are heavily infested with cyathostomes, or when the dormant stages are triggered to mature and emerge in large numbers simultaneously. The damage to the intestinal wall can be severe enough to cause diarrhea, colic and a loss of blood protein (albumin) into the intestine, causing edema of the legs and abdomen.

Young horses, which have poor natural immunity to parasites anyway, as well the very old or otherwise debilitated animals, are most susceptible. However, any horse can pick up a large load of these parasites and have problems.

Only moxidectin (Quest) or a double-dose of fenbendazole for five days straight (like Panacur Power Pack) can kill the larvae when they’re in this dormant state. Ivermectin can remove late-stage larvae as well as adults.

Drug resistance

If you look at the rack of dewormers at your local tack store, you’ll notice there are a host of different drugs out there. Price ranges vary a lot, too, with many of the older drugs costing a lot less than newer choices. Price shouldn’t be your only guide, though, as many of the older deworming drugs are surrounded with a strong proof of developing parasite resistance.

The benzimidazole class of dewormers, with thiabendazole the flagship drug, was initially effective at both regular doses and higher larvicidal doses for removing immature larvae. Now we’re seeing some evidence of resistance to this drug by cyathostomes. A related class of drugs, called substituted benzimidazoles (e.g. cambendazole, oxibendazole), continued to fight the infestation for a while but, again, some resistance from cyathostomes is being seen there, too.

The pyrantel drugs — pyrantel pamoate (Strongid T) and pyrantel tartrate (Strongid C, the daily dewormer) — were also highly valuable weapons against this stage of small strongyle. With time, though, resistance is again developing there, too.

An older drug, piperazine, is still available, but cyathostomes are highly resistant to that as well.

Moxidectin, which is a comparatively new drug (approved by the FDA for use in horses in 1997), can remove the encysted L3 stage of cyathostomes, as well as later stages and adults. However, it can’t be used in foals, and it has a comparatively narrow safety margin. This means you need to be extra careful to get an accurate weight determination before using the drug, to avoid a potential overdose. It’s a strong choice, though, provided you heed these precautions.

While ivermectin can’t remove the encysted larvae that moxidecin can, it’s highly active against late L3 and L4, as well as adults. Regular deworming with ivermectin will remove these life stages and prevent the environment from being contaminated. Ivermectin has a favorable safety margin and is effective against a wide range of parasites, including the strongyloides infections in lactating mares and young foals. It sounds like the answer, but since its release in the early 1980s, there’s some question whether parasites are getting savvy to it, too.

Ivermectin Studies

An alarming paper was released in the journal Parasitology Research. A study group headed up by Dr. Eugene Lyons from the Gluck Equine Research Center in Kentucky found that horses with documented cyathostome infestations treated with ivermectin began showing eggs in their manure again four weeks after treatment instead of the usual eight that is expected with ivermectin. Reduced time for eggs to reappear was also found in a 2003 study from North Carolina State, published in the May issue of the Equine Veterinary Journal. These studies have led to initial claims that small strongyles are becoming resistant to ivermectin, but are they really’

In the Gluck study, all the horses treated with ivermectin showed a 100% reduction in fecal egg counts (no eggs in the manure) after treatment, and they stayed that way until the four-week mark. When parasites are resistant to a dewormer, it usually means that the egg counts don’t drop satisfactorily after they’re treated. This was different, though, because they did drop — it’s just that they then returned earlier than expected.

The paper mentioned one possibility for the quick return was that the capacity of the cyathostomes to lay eggs may be temporarily paralyzed, but Dr. Lyons said there’s no proof that occurred. It’s also possible that there are parasite species that can mature to egg-laying status from the encysted L3 form much quicker than the recommended eight-week deworming interval used for ivermectin, so the ivermectin schedule would give those particular cyathostomes a survival advantage.

The Gluck study’s findings are only preliminary, and further study is definitely needed to identify the specific strain/type of small strongyle that’s re-appearing so soon and why.

A similar question for roundworm — ascarids, primarily a problem for young foals and older horses — developing resistance to ivermectin has been raised on the basis of studies looking at egg counts alone in a limited number of study horses. Does this mean we can’t count on ivermectin either’

We need confirming studies in larger numbers of horses before anyone pushes the panic button. The situation with roundworms is also more complicated because these are typically parasites of very young horses, and a young horse’s poor immunity to parasites complicates these studies.

Because moxidectin works similarly to ivermectin, concern has been raised over possi ble resistance to it as well, although it doesn’t seem to be a major problem for horses — yet.

Human Error

Most of us have long believed that rotating dewormer drugs will prevent parasite resistance in our horses. Unfortunately, this may no longer work, partially because owners don’t rotate the drugs correctly and because rotation systems include older drugs.

As we’ve seen, the older the class of drug, the more likely there is documented evidence of resistance. Some companies believe this may be due to a serious lack of understanding about dewormer drug activity, drug classes and timing doses.

”Parasite resistance is becoming an ever increasing problem,” said Wendy Valla, DVM, of Intervet/Schering Plough Animal Health. ”Some horse owners rotate between products within the chemical class without realizing that they are administering the same medicine over and over again, which can lead to drug resistance among the parasites that survive.”

It’s easy to focus on the brand name of the product you’re looking at instead of the drug name when you choose a dewormer. It’s the brand is what catches your eye. However, you need to look at the active ingredients list on the package back or under the brand name to see what drug is in that paste. In order to properly rotate dewormers, you must rotate actual classes of drugs.

In addition, many horse owners are unaware of their horse’s actual weight and guess what size dose to administer, usually on the lower end because they don’t want to harm the horse. This situation is similar to the rising resistance to antibiotics. Drug dosages are specifically tailored to get the job done, whether you’re talking about dewormers or antibiotics. Not finishing your full course of antibiotics just makes the bugs you’re fighting that much stronger.

Finally, not all dewormer drugs are effective for 60 days. Some have longer and or shorter retreatment periods:

??? Moxidectin: 90 days

??? Ivermectin: 60 days

??? Pyrantel: 42 days

??? Oxibenazole: 35 to 42 days

??? Fenbendazole: 42 days.

Therefore, you have to consider which drug you’re giving and mark your calendar for the next dose, based on specific drug activity times, which is not necessarily the same number of days for every drug class.

Bottom Line

About the only thing clear at the moment is that there is no ”best” path in deworming. The jury is still out on the value of rotating drugs, moxidectin is a worrisome choice, and ivermectin may or may not be losing the battle against small strongyles. Therefore, we recommend that you:

??? Pay attention to the recommended retreatment times on drug packages if using a rotational system or an all-ivermectin schedule.

??? Supplement daily deworming programs with twice-annual ivermectin paste dewormings.

??? Ask your veterinarian to do a fecal-egg-count check both before and two weeks after deworming to be sure your program is working.

??? Consider targeted deworming (see sidebar on page 10), especially for healthy adult horses at low risk of parasite infestation.

Article by Eleanor Kellon, VMD, our Veterinary Editor. She has written articles and books, including the new ”Horse Journal Guide to Equine Supplements and Nutraceuticals.”

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