Last Updated: 24/08/2026
Horse worming guide
Our vet explains why "every 6-8 weeks" is outdated, how faecal egg counts help, and how often to worm adult horses, foals and pregnant mares.
Author: Dr Teagan Lever BVSc (Hons)
Reading Time: 8 minutes - short read
Worried about worming your horse? It's a common concern among horse owners. While it might seem like a messy chore, regular worming is crucial to your horse's health. Let's dive into the importance of worming and explore some helpful tips to make the process easier.
Why worming your horse matters
Right now, paddocks across Australia are carrying a hidden load of worm larvae and eggs, just waiting for a horse to wander past and pick them up. Depending on the parasite and how heavy the burden is, an infestation can cause anything from no visible symptoms at all through to weight loss, ill thrift, poor condition, diarrhoea, colic and in severe cases, death. Here are the main culprits worth knowing.
Large strongyles
Also known as bloodworms or redworms, large strongyles (Strongylus vulgaris) are thankfully far less common now than they once were, thanks to effective modern wormers. Their larvae migrate from the gut into the intestinal arteries, where they can cause colic or, in rare cases, fatal haemorrhage.
Small strongyles
Often referred to as cyathostomes, these small worms are really common and a very important parasite to take care of. After being ingested in feed, cyathastome larvae encyst in the walls of the intestine, emerging after a period of between one month and two and a half years to mature into adults in the intestines. Infestations with cyathastomes can cause ill thrift and potentially fatal colic, particularly if the larvae erupt 'en masse' from the gut wall.
Large roundworms
Large roundworms, or ascarids (Parascaris equorum), mostly affect foals. Once swallowed, the larvae take a genuinely wild route through the body, from gut wall to liver, then via the heart to the lungs, where they're coughed up, swallowed again, and finally settle in the small intestine as adults. That detour through the lungs can leave lasting tissue damage, and a heavy adult worm burden in the gut can cause life-threatening blockages. Their eggs are also remarkably tough, surviving on pasture for up to ten years.
Tapeworms
Horse tapeworms (Anplocephala perfoliata) affect horses of all ages and classes, and are suspected to be a common cause of ill thrift, diarrhoea and colic. These worms begin their lives as larvae within the pasture mite, which is then inadvertently eaten by horses while grazing. Once ingested tapeworm larvae develop into adults in the horse's intestine, potentially resulting in colic. Adult worms shed their eggs in segments in the manure which can be easily seen by the naked eye.
Pinworms
Named for their slender, pin like appearance, equine pinworms (Oxyuris equi) are less serious, generally just causing irritation and itchiness under the tail area. Horses with these worms are often seen madly rubbing their hindquarters against fences and trees to get some relief from itching.
Bots
Bot flies lay small, sticky yellow eggs on a horse's legs, flanks and face over the warmer months. Once licked or bitten off, the eggs hatch into larvae that burrow into the mouth (causing ulcers) before travelling to the stomach to mature. They're eventually passed in the manure, where they pupate into adult flies and start the cycle again. A heavy bot burden can cause colic, weight loss, mouth ulcers and a noticeably grumpier horse.
Resistant worms and how to avoid them
For decades, horse owners were told to pick a worming paste and give it every six to eight weeks, no questions asked. It sounded thorough. In reality, that level of overuse has driven many worm populations to become resistant to some of the most commonly used anthelmintic drugs, meaning those drugs now work less well, or not at all, against certain worms in certain areas.
Modern worm control leans on a much smarter idea: treat based on evidence, not the calendar, and leave a genetic "escape hatch" for drug-susceptible worms so resistant ones don't take over the population entirely. In practice, that means combining targeted medication with solid pasture management.
- Remove manure from paddocks at least twice a week
- Avoid feeding hay or hard feed directly off the ground
- Harrow and spell paddocks during hot, dry weather, when conditions are toughest on larvae
- Graze paddocks in rotation with cattle or sheep, horse parasites can't survive in these species
- If you're breeding, keep foaling boxes and stables scrupulously clean, and wash the mare's perineum and udder just before foaling
Rotational worming
Rather than switching wormers with every single dose, a straightforward and effective approach is rotational worming, alternating between two drug classes, benzimidazoles (BZs) and macrocyclic lactones (MLs, or "mectins"), on an annual basis. This keeps your horse protected against the full range of worms of concern while reducing the odds that resistance builds up against any one drug class.
Macrocyclic lactone horse wormers
Benzimidazole horse wormers
Whichever product you choose, dose accurately based on your horse's actual weight, a weighbridge, equine vet clinic scale or weight tape will all get you a reliable number, and underdosing is one of the quickest ways to accelerate resistance.
A faecal egg count (FEC) is one of the best tools available for worming smarter. It's a simple, affordable test that tells you which worms are actually present and in what numbers, so you're treating a real problem rather than a hypothetical one. Many adult horses turn out to need just two treatments a year once you're working from FEC results instead of guesswork.
Vet tip: Ask your vet about a faecal egg count reduction test alongside your regular FEC, it checks whether your current wormer is actually still working on your property, which is the clearest early warning sign of resistance you can get.
How often and when should you worm your horse?
When searching for a recommended worming schedule for horses, it is important to note that there is no 'one size fits all'. The frequency, timing and type of wormers needed will depend on a number of factors including where you live, how many horses you have their age and individual susceptibility to parasites. While we recommend checking with your local vet, see below for a general recommended worming schedule for horses.
Adult horses
As a general rule, for most adult horses, worming once or twice yearly is sufficient to keep worms under control. Aim to worm your horse at times during the year when the larvae in the environment are most infective. Parasite larvae and eggs are less likely to survive in the very hot dry summer or freezing cold winter, so worming in spring and autumn makes sense in subtropical and temperate climates. Be sure to worm all new horses on arrival at your property and keep them separated from the herd for at least forty eight hours afterwards.
Some adult horses seem to be more susceptible to parasites than others, shedding higher number of eggs in their faeces and increasing recontamination of the pasture. Higher shedding horses will need to be wormed more frequently to help keep worms under control Regular faecal egg count testing of all your horses is a simple, non invasive and cost effective way to identify horses that may be higher shedders and can also help to identify any potential problems with resistance. Routine health visits for dentistry or vaccination are a great opportunity for you to discuss faecal egg count testing and worming strategies with your veterinarian.
Foals and Pregnant mares
In order to reduce the risk of disease, particularly that caused by large roundworms, special care should be taken to ensure that pregnant mares and foals are wormed regularly and with the right products. Ensure that wormers which are effective against roundworms are used in all horses under two years of age. Pregnant mares should also be wormed for roundworm just prior to foaling (always check that the wormer is safe for use in pregnancy first), and foals should be wormed every 6 weeks from 6 weeks of age to 6 months.
Vet tip: If you're not sure whether a wormer is pregnancy-safe, don't guess. Call your vet or check with us before dosing a pregnant mare, the wrong product at the wrong time is a risk not worth taking.
FAQs
Whichever product you choose, dose accurately based on your horse's actual weight, a weighbridge, equine vet clinic scale or weight tape will all get you a reliable number, and underdosing is one of the quickest ways to accelerate resistance.
A faecal egg count (FEC) is one of the best tools available for worming smarter. It's a simple, affordable test that tells you which worms are actually present and in what numbers, so you're treating a real problem rather than a hypothetical one. Many adult horses turn out to need just two treatments a year once you're working from FEC results instead of guesswork.
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History
Our experts continually monitor the health and wellness space and we update our articles when new information becomes available.
Mon Aug 24 2026
Edited by Dr Belinda Stancombe BVSc (Hons)Dr Teagan Lever BVSc (Hons)
Head Veterinarian, BVSc (Hons)
Pet Circle's Head Veterinarian, Dr Teagan graduated from the University of Queensland in 2010 and went on to work in small animal and mixed practice in various locations around QLD & ACT before joining Pet Circle in early 2016. Dr Teagan has special interests in dermatology, nutrition and preventative health care. She feels privileged to witness the special bond people share with their pets on a daily basis and enjoys forming lasting relationships with pet parents and their fur children.
