Smart Deworming

Equine parasites can be resistant to dewormers, explains Hagyard Equine Medical Institute’s Dr. Luke Fallon, who outlines a strategy that can discourage resistance while managing your horses’ health.

For centuries, horsemen have battled the scourge of worms, and one has only to crack open a horse-care manual from previous centuries and read the bizarre measures they took — including blood-letting or administering mercury, turpentine, or tobacco — to sense their desperation to get rid of parasites.
So when modern deworming drugs, or anthelmintics, hit the livestock market in 1940, they were met with enthusiasm. Between 1940 and 1970, chemical compounds like phenothiazine, piperazine, and

thiabendazole revolutionized managers’ ability to treat their horses’ common and potentially fatal intestinal parasites. But the relief that greeted phenothiazine’s introduction as a livestock dewormer in 1940 only lasted a couple of decades before troubling evidence emerged that worms were developing resistance to it — and so began a cycle of innovation and resistance that continues to this day.

These days, roundworms (ascarids), large strongyles (Strongylus vulgaris), and tapeworms are largely well controlled. But more worry centers around small strongyles, whose rapid life cycle helps them develop resistance relatively quickly, even as our range of dewormers to fight them remains limited.

“As the number of horses as stock animals has decreased over the last 50 years, the investment in research and development of anti-parasitic medications has dropped dramatically,” explained Dr. Luke Fallon of Hagyard Equine Medical Institute in Lexington, Ky. “That’s why we have to be very careful to maintain the efficacy of the anti-parasitic medications we currently have and decrease or minimize the resistance within the parasites in the herds.”

In today’s war against equine intestinal worms, your best weapon against parasite resistance is not to deworm more; overusing dewormers can contribute to resistance. The current best practice is to target your deworming to horses that need it when they need it, and to monitor your horse’s or herd’s parasite load by lab-testing their manure for the presence and number of parasite eggs.

“With fecal egg counts, you’re assessing your herd and determining which horses in that population tend to carry a heavier parasite load than others who typically carry a low parasite load because their own immune system is kicking the parasites out,” Dr. Fallon said. “By doing more fecal egg counts, you can assess those horses that are chronically infected with parasites and therefore need to be dewormed more frequently, versus the majority of the adult population that doesn’t need to be dewormed as frequently and can be dewormed maybe just two or three times a year.”

Targeted Deworming

Conducting fecal egg counts instead of simply deworming every horse on the property on a set schedule — the rotational worming practice that was common until the 1990s and which helped bring large strongyles under control — should not add much expense, and in the long run, it can help better protect horses.

“To run a fecal egg count is typically less than $20 here in central Kentucky,” Dr. Fallon said. “It gives you not only a quantitative assessment, but also a qualitative assessment, i.e., whether they have particular parasite eggs.”

Fallon suggests that horse owners and managers, in partnership with their veterinarians, use fecal egg counts to get a good picture of their horse population that can guide their deworming program going forward.

“If your farm has 100 head of resident horses, over a period of about six months with about three rounds of fecal egg counts, I can give you with fairly strong certainty which ones are chronically infected or are reinfected quickly after being dewormed,” Dr. Fallon said. “At that point, we can deworm those horses every 60 days, and I don’t need to run fecals any more on them; I’ve determined their patient profiles. If the rest of the horses in that field stay negative for fecal eggs between the first and second counts, then we know they can be dewormed in the spring and fall for tapeworm and small strongyles. We don’t really have to worry about ascarids (roundworms) in the adult horse, because it’s not very common for them to have ascarids. Typically, ascarids become less of a problem once a horse becomes 20-24 months of age.”

Fallon’s suggested regimen to set your herd’s baseline is simple:

Step 1: Begin by conducting fecal egg counts on all resident horses’ manure

You want to know a count, and you want to know what kinds of eggs are being found,” Dr. Fallon said. “We’re typically looking

for five different types in a routine fecal egg count: small strongyles and large strongyles; ascarids and tapeworms, which can be missed on a fecal egg count because they’re intermittently shed; and also strongyloides (Strongyloides westeri, an intestinal threadworm). Your goal isn’t necessarily a zero small strongyles parasite count; you want a horse to be exposed so they develop immunity, but you just don’t want them to be overwhelmed with parasites.”

The fecal egg count will help your veterinarian identify your herd’s low, moderate, and high shedders. As a general reference, low shedders have an egg count of less than 200 eggs; moderate shedders have 200-500; and high shedders have more than 500.

“So if I test your horse that was wormed 90 days earlier and say you have a low count of small strongyles — say, a count of 150 — I’d suggest sitting tight and then running another fecal egg count in 45-60 days to see where we are, rather than just deworming the horse,” Fallon said. “If it stays at 150 next time, keep sitting tight. We’re not trying to get it down to zero, even though we probably could. I’m trying to practice more judicious use of the medications and use the lab result to practice good medicine.”

Step 2: After the first count, deworm those “high shedder” horses who showed evidence of heavy parasite load

“Those horses are likely being turned out into paddocks that are being used by other horses and where they are going to be picking up parasite eggs,” Dr. Fallon said. “So after the first fecal egg count, we’d deworm those horses.”

Step 3: Thirty days after deworming the high-shedder horses, conduct another fecal egg count on them

This will help you identify horses who are persistently high shedders and will be candidates for deworming every 60 days to keep their parasite loads under control.

Step 4: At 60 and 120 days after the initial whole-herd test, conduct new fecal egg counts for the entire herd, including both dewormed high shedders and unwormed horses

Multiple rounds of tests will provide you with a picture not only of your high, moderate, and low shedders, but also of the specific parasites that are present in the group and how well your deworming protocol is controlling them. That information can help you target your deworming more effectively to tackle both the types of worms you’re seeing evidence of and the horses who more frequently carry a high parasite load.

“You’ll know you’ve got to keep a particular horse on a regular deworming schedule every 60 days or 90 days, even if the rest of the herd doesn’t need that,” Dr. Fallon said. “Use the fecal egg count to determine if that horse needs to be wormed. If you know your horse is one that doesn’t typically carry a parasite load, then I’d be comfortable advising you to save the money on the fecal egg count and just deworm in the spring and fall.”

An adult horse’s status as low, moderate, or high shedder is unlikely to change, so once you’ve established a baseline for a group of horses it should remain fairly stable, Fallon said.

“Generally, once you get beyond the juvenile period (past two years of age), horses typically will stay pretty true to form as far as how their parasite counts are maintained, unless they’re in a particularly stressful situation or have a debilitating health issue that compromises their immune system,” he explained. “Once they get beyond that juvenile period, they typically stick to that pattern, whether they’re a reinfector or a low-parasite load-carrier that only needs deworming twice a year.”

In the long run, this kind of strategic deworming can help reduce the chances of parasite resistance in your operation. That’s important: consider a world without effective dewormers to combat the internal parasites that can cause chronic weight loss and diarrhea, migrate into the lung and cause respiratory problems, damage or kill tissue, or cause colitis or blood clots leading to fatal colic.

“These are like antibiotics that get overused,” Dr. Fallon said. “You will develop resistance within these parasites. The stakes are very high.”

Photos: Shelley Paulson photography and Paula da Silva