equine parasite control

Consider your property’s equine traffic, geographic location, and parasite control goals to create an effective program

By Alexandra Beckstett, The Horse: Your Guide To Equine Health Care

The hardy Icelandic horses housed at Swallowland Farm, in Shelbyville, Kentucky, live as natural a lifestyle as possible. Owner and trainer Carrie Lyons Brandt ensures her 30 charges live in groups, consume a forage-based diet, are allowed to grow winter coats, and spend most of their time turned out. Although her herd is fairly low-maintenance, Brandt knows there’s one area of their care she must pay close attention to and manage carefully: parasite control. Brandt’s Icelandics are primarily pastured and are exposed to high numbers of parasitic worms that exist naturally in soil and on grasses, waiting to be consumed, so she has worked with her veterinarian to customize a parasite control program for her farm.

With internal parasites developing increasing resistance to anthelmintic (dewormer) drugs, horse owners can no longer deworm willy-nilly and assume their animals are protected. Conscientious farm owners and managers like Brandt now consider their property’s equine traffic, geographic location, and parasite control goals to create a program that works for them.

We teamed up with Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, co-author of The Handbook of Equine Parasite Control, chair of the AAEP Parasite Control Subcommittee, and an equine parasitologist at the University of Kentucky’s Gluck Equine Research Center, in Lexington; and Claudia True, DVM, from Woodside Equine Clinic, in Ashland, Virginia, who helped The Virginia-Maryland Regional College of Veterinary Medicine develop its parasite control protocols, to guide you in doing just that.

The Purpose of Parasite Control

You can go ahead and breathe a sigh of relief, says Nielsen, because “horses don’t drop dead left and right from untreated parasite populations.” The truth is, all horses harbor internal parasites and no amount of deworming is going to eliminate them completely.

So why is parasite control such a crucial element of horse care? The point is to prevent horses from amassing extremely high parasite burdens that can eventually cause colic, diarrhea, and weight loss.

Nielsen says the foundation of most parasite control programs for mature horses is twice-yearly deworming—once in the spring against strongyles and again in the fall against strongyles and tapeworms—bookended by pre- and post-treatment fecal egg counts. Our sources explain that the egg count’s purpose is not to reveal exactly how many worms a horse has, but to determine which horses are high shedders and might need additional treatments and to evaluate if the dewormer used worked.

Because the current dewormers we have to rely on are limited and parasite resistance is increasing, “it is very important for farms to know what products work (on a property),” says True.

To do this you must perform a fecal egg count reduction test (FECRT), which involves running egg counts on the day of deworming and again two weeks later on manure from a sampling of horses on your farm. And you don’t necessarily need to test each horse at every deworming, says True, since studies and anecdotal evidence have shown that 80% of horses are low shedders and the rest are high shedders. For this reason, Nielsen suggests a practical strategy of retesting just the animals with the highest pre-treatment samples. Then calculate the percent reduction in egg counts to determine drug efficacy. For most dewormers, he says you want to see at least 90% reduction.

Fecal Egg Counts

These tests can also help you identify the horses that are consistently shedding higher or lower numbers of eggs. “If you run some repeated egg counts over time to monitor your horses you will see the pattern,” Nielsen says. “If you see one horse’s FECs are always above 1,000, you might want to give him an extra treatment in the middle of summer—not because he’s at risk of getting sick, but because there may be another horse that comes onto the property at some point that’s more susceptible (to infection). That’s the horse we’re thinking of when we’re deworming the horse with the high egg counts.”

Are Your Horses Frequent Travelers?

That leads us to one of the factors you must consider when customizing your parasite control program: horse traffic on and off the farm. The stress associated with relocating can cause parasite egg shedding to rise suddenly.

“This doesn’t mean these horses all of a sudden acquire more worms,” Nielsen says. “We think it has to do with stress and immune suppression. They have the same worms, but because their immune system is challenged, the worms are allowed to shed more eggs (especially in horses younger than 4 that are at highest risk).”

So if a horse is moving on or off your farm, you might want to suggest he receive an anthelmintic about two weeks before transport to prevent an egg count increase right after he arrives at his destination, Nielsen says. Then follow this with a post-treatment FECRT.

But this doesn’t necessarily mean you should deworm your performance horse every time he travels to a competition. True explains that show horses accustomed to travel—particularly if they are low shedders—likely won’t exhibit the same egg count fluctuations as, say, a 2-year-old hauling to a sale. She suggests performing FECRT as needed on these performance animals a few times a year and deworming with the appropriate drug in the spring and fall.

Similarly, Nielsen performed a study in 2011 evaluating parasite load in Standardbred racehorses and found that the winningest horses had the highest egg counts. “Horses that are good racers probably go to more races, travel more, and are under more physical pressure,” he says, theorizing that the elevated levels might just reflect stress. “This might be justification for additional treatments in these horses.”

Introducing New Horses

Our sources remind us that parasites are everywhere, and new horses are very unlikely to introduce parasite species that are not already present on a farm.

But consider this scenario: Say you have an effective deworming program in place on your farm, but a horse comes in from a farm that uses a different strategy, and he’s loaded with worms resistant to the drug you use most. “These are the parasites you would rather not have introduced on your farm,” Nielsen says.

Quarantine any incoming horses to prevent their parasites (or any infectious diseases) from running amuck in your resident herd. Then treat the horse with your dewormer, employing the use of pre- and post-treatment egg counts to determine if the drug is effective. If you find the horse is harboring parasites resistant to this drug, you know to try a different dewormer.

This is the tactic Brandt says she uses at Swallowland, where horses pass through regularly for training and sales. She says she also receives horses imported directly from Iceland. For international travelers such as these, Nielsen suggests a similar practice: arranging for the horses to be dewormed before they experience the very high stress levels of shipping and acclimating to their new home.

Weather and Region Matter

Parasite transmission begins and ends with the grazing season. So depending on where you live and how warm or wet it is, the ideal time to perform FECRTs and deworm your horses will vary greatly.

“Try to get those egg counts done when you know the horses are consistently shedding eggs—think about the warm but not hot times of year,” True suggests.

As far as administering treatments, “the key is when your grazing season starts,” Nielsen says. “Infection pressure increases through the summer as horses are grazing, ingesting worms, and shedding eggs. So by late summer/early fall (worm burdens) are at their peak.”

For a Kentucky farm like Swallowland, this means administering horses’ second basic treatment in September or October. If owners wait until late fall or the first frost, says Nielsen, the parasites will have essentially gone into hibernation already—at which point the drug will be less effective. “If you treat earlier but still within the grazing season, at the last part of it, then you interrupt that dynamic,” he explains.

But what if you live in an arid region with little to no grazing season, or your horses live on a drylot or in stalls (such as racehorses)? These horses are exposed to fewer pasture-borne parasites. For adult horses under such circumstances, Nielsen recommends still following the basic two treatments per year schedule and letting the parasite egg counts determine whether your horses need additional treatments.

Because prime infection pressure periods also depend on precipitation levels and temperatures (extremely hot and dry summers lead to lower parasite loads, whereas mild and rainy summers are very favorable for parasite transmission), Nielsen says he and colleagues at AgResearch in New Zealand are currently developing a weather-based parasite prediction model for deworming planning. Owners will be able to input weather conditions from the previous month and determine whether it’s a high- or low-risk season for parasite infection.

Property Management Measures

With increasing parasite resistance to all anthelmintic classes, owners should focus on more than just drugs when constructing a parasite control program.

Manure removal from turnout areas, either with pitchfork or pasture sweeper, is a very effective parasite control practice, especially for owners with limited pasture and/or high stocking rates. “Picking up manure is most important thing owners can do (to reduce parasite loads),” True says. If you can remove manure from your paddocks once or twice a week in most climates, you will stay ahead of the parasites’ development—they take at least a week to develop from egg to infective stage.

Pasture rotation dividing a pasture into smaller fields and rotating horses through them—has been shown to reduce parasite burdens in fields, Nielsen says. How long you rest your pasture between rotations, however, depends on the weather. “If it never gets hot enough for parasites to die off during the grazing season, then the larvae will survive in that field regardless of rotation,” he explains. “But under very hot, dry conditions you could rest a pasture for two to three weeks and a large majority of the parasite population will die.”

Manure spreading and mowing are two parasite-reducing techniques that work through similar means. “If you disrupt the (parasite-containing) fecal balls, the larvae will be much less protected,” Nielsen says. “And, if you mow the grass, there is less shade and moisture to protect the larvae, and they’re exposed to harmful UV light. A couple studies have shown very consistently that if you mow and spread fecal balls at the end of the grazing season, survival of larvae was much lower because they weren’t protected.” But, just as with pasture rotation, if conditions are mild you will be wasting time and energy just spreading the larvae around. True says, “Spreading manure is great from a fertilizer standpoint, but horrible for spreading parasites unless it’s really hot and desiccates (dries out) and kills the parasites.”

Take-Home Message

After a discussion with her veterinarian, Brandt came up a customized parasite control program for her farm that includes twice yearly deworming and FECs, more frequent deworming of high shedders, communication with owners of incoming horses to ensure they’re dewormed prior to arrival, and pasture rotation and management.

Your farm might not require this same protocol, however, so take the aforementioned information into account, and work with your own veterinarian to devise a plan that’s most effective for your situation. Your horses’ health and the future of parasite resistance depend on it!

Breeders Beware

Foals are susceptible to a class of parasites that mature horses are not: ascarids, or roundworms. If you have foals on the ground—particularly if they are on pastures where foals have grazed year after year—you need to be treating them with a dewormer effective against these problematic parasites.

Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, equine parasitologist at the University of Kentucky’s Gluck Equine Research Center, in Lexington, suggests first deworming foals for ascarids no earlier yet no later than 2 to 3 months of age because dewormers used against these worms only kill the adults, and that’s about how long it takes for acquisition and maturity. By 5 or 6 months, foals are at a critical phase with regard to ascarid treatment, and this is when most cases of parasitic disease occur.

“Deworming at this age should primarily target ascarids, but one needs to be aware that strongyle parasites are beginning to enter the picture too,” Nielsen warns. “The challenge is that the dewormers that work well against ascarids may not work against strongyles. Proper (fecal egg count) testing will help guide you to making the right treatment choices.”

After weaning, when foals reach about 8 to 10 months of age, strongyles will be the main parasite category to guard against. Tapeworms can also enter the picture at this age, so Nielsen advises using a dewormer with good efficacy against both parasites. Perform a fourth deworming, targeting strongyles, when the horse is a year old.—Alexandra Beckstett 

Reprinted with permission from The Horse: Your Guide to Equine Health Care/TheHorse.com. Find more veterinarian-approved horse care information at TheHorse.com.

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