It's Deworming Season: What Vets Actually Recommend for Spring 2026

It is mid-March, the ground is a muddy disaster, your horse has been living in a paddock that looks like a swamp for the past three months, and you are standing in the feed room staring at a tube of Zimectrin wondering if you are doing this right. If that sounds familiar -- welcome to spring deworming season. It comes every year, it confuses almost everyone, and the recommendations have actually changed significantly in the last few years in ways that a lot of horse owners have not caught up with yet. So let's talk about it.

This is not going to be a lecture. It is going to be the conversation I wish someone had with me earlier -- the actual current thinking from veterinarians and equine parasitologists, translated into plain language so you can walk into your barn this week with a real plan instead of just guessing.

Everything You Were Taught About Deworming Has Changed

If you grew up being told to deworm your horse every eight weeks on a rotating schedule -- a different drug each time, like clockwork -- you were taught what was considered best practice for decades. That approach is now officially outdated, and the American Association of Equine Practitioners (AAEP) updated its Internal Parasite Control Guidelines most recently in 2025 to reflect this.

The old calendar-rotation method felt simple and thorough. The problem is that exposing parasites to the same drugs repeatedly, on a fixed schedule, created exactly the conditions for drug resistance to develop. The AAEP now explicitly recommends discontinuing the practice of deworming all horses with fixed intervals year-round -- every two months, for example -- and stopping the blind rotation of drug classes. What replaced it is something called targeted selective treatment, and it is genuinely smarter.

Here is the core idea: most adult horses are low parasite shedders and many only need one or two deworming treatments per year, often timed for spring and fall. This is the group most commonly over-dewormed in the past. The goal is no longer to eliminate every single parasite in every single horse. The true goal of parasite control is to limit parasite infections so that horses remain healthy and disease-free -- not to completely eradicate all parasites from every horse, because doing so would accelerate drug resistance even faster.

Why March Is the Right Time -- and What to Give

Spring deworming timing is not arbitrary. Treatments should be administered during times of peak transmission, typically spring and late fall for those in the northeastern United States. Peak transmission times can vary by region, so your local veterinarian should always be consulted for region-specific recommendations. For most of the Midwest and Northeast -- including here in Illinois where we are right now -- March is the window.

Here is what the current veterinary recommendations look like for adult horses broken down by shedding category:

The recommended spring products are ivermectin -- sold as Equell, Zimectrin, Rotectin, and IverCare among others -- or moxidectin, sold as Quest. In the fall, the recommendation shifts to ivermectin with praziquantel (Equimax or Zimectrin Gold) or moxidectin with praziquantel (Quest Plus) to cover tapeworms. The praziquantel component in those fall products is specifically what targets tapeworms -- a parasite that does not show up reliably on standard fecal egg counts but can cause serious colic if left unaddressed.

So what is Ivermectin actually doing?

Ivermectin belongs to the macrocyclic lactone drug class and works by targeting the nervous system of parasites -- specifically interrupting nerve signal transmission in a way that paralyzes and kills them without affecting the horse. Ivermectin and moxidectin are the most effective drug classes currently available against strongyles and bots. Moxidectin has the added advantage of killing encysted small strongyle larvae -- the ones hiding in the gut wall -- which ivermectin alone does not reach as effectively.

The products you see at the feed store -- Zimectrin, Eqvalan, IverCare, Equell -- are all ivermectin in paste syringe form.

Fecal Egg Counts: The Step That Actually Tells You What Is Going On

Here is the honest truth: most horse owners skip fecal egg counts because they feel like an extra step, and historically the old every-eight-weeks protocol made them seem unnecessary. They are not unnecessary. They are actually the most important part of a modern deworming program -- and they are not complicated or expensive.

A fecal egg count (FEC) is exactly what it sounds like: a lab analysis of a fresh manure sample that counts the number of parasite eggs per gram and tells you what you are actually dealing with. The recommendation is to do fecal egg counts on all horses in the spring prior to giving any dewormers. This identifies horses who are heavily infected and may require more frequent deworming throughout the season. It is estimated that only about 20% of horses will be heavy shedders who need to continue on a regular deworming program every 8 to 12 weeks throughout spring, summer, and fall. The other 80% are low shedders who are perfectly fine with twice-yearly treatment.

How to collect a sample: one manure ball per horse is adequate. Place it in a labeled plastic bag or container with the horse's name, deworming history, your name, and the date. The sample can be refrigerated but should be less than 24 hours old and not frozen. Your veterinarian can run the test in-house or send it to a lab. Many equine vets now offer FEC testing as a routine spring service -- call yours and ask.

One critical timing note: the sample must be collected a minimum of 12 weeks after giving any ivermectin product, or 16 weeks after giving moxidectin. You need to allow enough time for the parasite population to rebuild so eggs can actually be detected in the feces. If you just dewormed in January, wait until March or April for your spring FEC.

Why Drug Resistance Is the Real Issue Here

This is the part that should motivate every horse owner to take the new approach seriously. Years of frequent, blanket deworming exposed parasites to the same drugs repeatedly. As a result, many common parasites -- especially small strongyles -- now survive treatments that once worked reliably. Resistance to ivermectin, fenbendazole, and pyrantel has been documented across the United States.

The AAEP specifically recommends performing fecal egg count reduction tests annually to ensure that the dewormers you are using are actually effective in your specific barn or herd. No anthelmintic will eliminate all parasitic stages from a horse. A fecal egg count reduction test (FECRT) involves running an FEC before deworming, then running another one 14 days after. A reduction of at least 95% is expected for ivermectin or moxidectin. Results lower than 95% for those drug classes indicate resistance or incorrect dosage. If your horses are not hitting those numbers, your veterinarian needs to know.

Small strongyles are the main parasite of concern in adult horses because they are the most common and because of growing anthelmintic resistance. There are no new classes of antiparasitic drugs currently on the horizon. That last part is important. We are working with the drugs we have. Preserving their effectiveness is not optional -- it is urgent.

A Different Protocol for the Young Ones

Everything above applies to adult horses over two years old. Foals, weanlings, and yearlings follow a completely different protocol because their primary parasite concern is different.

Foals should be dewormed at 2 to 3 months, 5 to 6 months (specifically just before weaning), 9 months, and 12 months of age. The first two treatments should target ascarids -- roundworms -- with the AAEP advising benzimidazoles as the drug class for those early treatments. Ascarids are the dominant parasite concern in young horses and require a different treatment approach than the strongyle focus in adults.

Moxidectin requires extra caution in young stock, small ponies, and miniature horses. Always read the label, never guess the dose, and speak with your veterinarian before treating foals and young horses. This is not a product to administer to a foal without veterinary guidance -- the dosing window is narrow and the consequences of overdose are serious.

The Part of Deworming That Has Nothing to Do With a Tube

Here is something the tube of Zimectrin cannot do on its own: manage pasture contamination. Parasite control is only half chemical. The other half is environmental, and in muddy spring conditions it matters enormously.

What to Actually Do This Week

Deworming Is Not Complicated -- It Just Requires a Plan

The shift from calendar-rotation deworming to targeted selective treatment felt overwhelming when I first heard about it. Another thing to test, another thing to track, another conversation with the vet. But in practice it is actually simpler -- and cheaper -- than what most of us were doing before. Two fecal egg counts a year, two to four treatments depending on your horse, one conversation with your veterinarian about what is working on your specific farm. That is it.

The horses that need more treatment get more treatment. The horses that do not need it are not bombarded with unnecessary chemicals. The drugs that we all depend on stay effective longer. Everybody wins -- especially the horses.

It is mid-March. The mud is real and the larvae are moving. Go call your vet.