Worming your horses

All horses have worms. It is well known that heavy burdens (=an increased amount) of internal parasites, or worms, can have a severe effect on the health and performance of all classes of horses, especially foals, working and aged horses. The side-effects of worm burdens are related to the species of worm that are present, their relative numbers and the age or use of the horse.

Worms or internal parasites can cause a variety of problems in your horse. The simplest signs include not putting on weight, dull coat, reduced appetite, mild colic or anxiety and an itchy backside. The next phase includes diarrhoea, anaemia, lowered ability to exercise, susceptibility to infections, non-healing sores ("summer sores"), coughing and significant or recurrent colic. If things get out of hand these parasites can cause pneumonia, emaciation, severe and debilitating diarrhoea and colic, gut emergencies such as torsion, intussusception and perforation, and it is possible that some of these can lead to the death of your horse.It must be emphasised that, for the well cared for horse, the milder signs are by far the more likely to occur.

Types of worms in horsesIn terms of equine health and management priorities, creating an effective parasite control program is probably second only to supplying the horse with clean water and high quality feed and roughage. While there are numerous parasites that can infect horses the most common internal parasites seen in horses are strongyles (large and small), large roundworms (ascarids), pinworms, bots and tapeworms.


Adult large strongyles (sometimes called bloodworms) are found in the large intestines either attached to the walls or in the intestinal contents. There the females deposit large numbers of eggs that are excreted in the manure. These eggs then hatch and the larvae develop and climb blades of grass. Horses then consume these larvae while grazing and ingested larvae penetrate the intestinal walls and migrate to various organs and arteries. Large strongyles mature about 6 to 11 months after larvae are ingested. Small strongyles larvae migrate into the intestinal wall but not into other organs or blood vessels.

Small strongyles develop to maturity about 6 to 10 weeks after larvae is consumed. All horses can be affected by strongyles but young horses are most vulnerable. Signs of strongyle infection are loss of appetite, weight loss, fever, depression, weakness, anaemia, diarrhoea and death.


Large Roundworms

The adult stage of the large roundworm (ascarid) is found in the small intestine of the horse. The female lays large numbers of eggs in the intestines which are passed out in the manure. It takes approximately 2 weeks for these eggs to be infective and the horse picks them up while grazing. The eggs hatch in the stomach and intestines and migrate into the blood circulation where they are carried to the liver and lungs. A complete life cycle takes 3 months and eggs can remain infective in the environment for several years if not properly removed. Large roundworms cause digestive upset and damage the liver and lungs, if the burden is large enough, rupture can occur in the small intestine leading to death.



Pinworms mature in the large intestine and rectum of the horse. The eggs of pinworms are picked up by horses from contaminated feed, water, bedding, stall walls, fences etc. These worms are irritating and cause the horse to rub its tail, resulting in hair loss. The eggs are smeared onto any surface the horse touches and some are passed in manure. Pinworms can affect all ages but young animals are most susceptible, with symptoms including digestive disturbance, slow growth, irritation and tail rubbing. The life cycle is completed in five months.



Bots are the immature stage of the bot fly. The female bot fly lays their yellow eggs on the hairs of the horse, typically on the throat, front legs and underline. The eggs containing first stage larvae hatch after a 2- to 5-day incubation period either freely or after being licked by the horse. The larvae then migrate to or enter the mouth and attach to the lips, tongue, bums or other parts of the mouth and burrow into the tissue where they about 3 weeks.

About three weeks later they emerge as second stage larvae and progress down the throat, attaching to the stomach lining. In the stomach they develop to third stage larvae and remain for up to 7 months. Damage in the stomach occurs due to obstruction of food flow and irritation of the stomach lining. In severe case stomach rupture can occur. These larvae finally pass through the horse and hatch from manure by entering the soil below the manure pile and pupate in approximately 2 weeks to 2 months depending on the season (from spring to autumn).



Tapeworms attach to the horses intestinal lining where they rob the horse of nutrients and damage tissues. Horse tapeworms require a host (forage mite). Tapeworm eggs are passed with the manure of infected horses onto pasture, where forage mites ingest them. The immature tapeworm develops within the body cavity of the mite and is ingested by the grazing horse. When the horse digests the infested forage mite, the tapeworm is released and within 6-10 weeks develops into an adult that attaches to the horse’s intestine and continues the cycle.


Parasite Control

Pasture management

The control of internal parasite of the horse is based on cleanliness, management and deworming drug treatment. Appropriate removal of manure from stalls and pastures is paramount to parasite management. In small pastures (less than 3 acres) manure should be removed from the paddock at least twice a week and placed in a compost pile. The larvae in composted manure will be destroyed if sufficient heat is built up. In large pastures frequent mowing, chain harrowing (dragging), and rotation of pastures along with separating age classes of horses and avoiding overcrowding should be practiced.

A new approach to De-Worming

Faecal Egg CountTraditional parasite control programs involving rotational treatment with de-wormers at regular intervals were commonly recommended. This approach is based on concepts and strategies developed more than 40 years ago when large strongyles were the most common and damaging internal parasite in horses. The rationale for this parasite control scheme was to eliminate large strongyles before they could mature and lay eggs that would contaminate the environment. Since it took about two months for strongyle eggs to reappear after treatment, treatment every two months prevented large strongyle eggs from being excreted on pastures. This approach was very successful in controlling large strongyles, and they are now relatively uncommon in managed horse groups. The American Association of Equine Practitioners (AAEP) have stated that “Decades of frequent de-wormer medication use has selected for high levels of drug resistance in small strongyles and ascarid populations, which emphasizes that the traditional approaches for parasite control are not sustainable and that new strategies are needed.” Drug resistance means the parasites within an animal have the capability of surviving treatment with a particular class or family of de-wormer.

Faecal Egg Count

Treatment for worms with de-worming medications should be based on faecal egg count examination and may vary from farm to farm. Periodic faecal examinations assessing faecal egg count (FEC) are the most accurate way to determine deworming medication needs. A faecal egg count measures the number of strongyle or ascarid eggs your horse is passing in each gram of his manure. When you send a sample to your veterinarian or independent laboratory, you get back a number like 50 EPG (eggs per gram) or 500 EPG.                                                                                    Faecal egg count contamination levels Faecal Egg Count;Low contamination Less than 200,Moderate contamination 200 to 500, High contamination More than 500.

How often should I do an FEC on my horses?

All new horses should have an initial FEC test conducted and then biannual to annual tests (in the spring) should be conducted in mature horses depending on their exposure to other horses. Horses under the age of 2 may need more frequent FEC test conducted (every 6 months). Initial FEC will determine worm burden and a follow-up FEC 14 days after de-worming will evaluate medication effectiveness (Table 2). If used correctly, a FEC can decrease your reliance on de-worming medications reducing the risk for drug resistance. Consult with your veterinarian to assist you in conducting a FEC on your farm and developing a specific de-worming program.

De-Worming Programme

What do I look for when selecting a de-wormer?

The four main de-worming chemical groups are benzimidazoles (white drench), Tetrahydropyrimidines (clear drench), macrocyclic lactones, and pyrozine. It is recommended that chemical groups be rotated every 12 months to delay a build-up of resistance in worms to a particular drench chemical. Praziquantel is the only de-wormer effective at killing tapeworms.

Chemical Group                                          Active ingredient

Benzimidazoles                                          Oxibendazole, Oxfendazole, Fenbendazole                                                Tetrahydropyrimidines                                  Pyrantel, Morantel                                                                                    Macrocyclic Lactones (Avermectin)               Ivermectin, Abamectin                                                                             Macrocyclic Lactones (Milbemycin)              Moxidectin                                                                                               Pyrozines                                                   Praziquantel (Tapeworms)

There is a well know myth surrounding parasite control in horses – “Change your de-wormer every time you de-worm”. This used to be common advice, but is now known not to be the best practice as it increases parasitic resistance.    While you should change the chemical group used, it should not be done every time but every 3rd time or based on your veterinarians assessment of the type of worm infestation your horse has. Be aware that changing the brand of de-wormer does not mean you are changing the de-wormer, make sure to check the active ingredients.

Which active ingredient should I use and when?

In mature horses focus on control of small strongyles. Depending on climatic conditions, one or two yearly treatments are sufficient to prevent occurrence of strongyles. Treatment is best done towards the end of the grazing season when internal parasite burden is at its peak (spring in tropical and subtropical climates). Include treatments against bots (along with removal of bot eggs from the horses’ hair coat), ivermectin & moxidectin are the only available medications for horses with activity against bots. Include a tapeworm treatment (Praziquantel) at least annually if they are a problem in your region.

De-worming foals and weanlings

In young horses during the first year of life foals should receive a minimum of four deworming treatments.                    First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against large roundworms. Second deworming is recommended just before weaning (approximately six months of age). An extra treatment can be justified before weaning if the time period between the two treatments exceeds 3 months. At weaning FEC are recommended to determine whether worm burdens are primarily strongyles or large roundworms, to facilitate the right choice of drug class. Third and fourth treatments should be considered at about 9 and 12 months of age, respectively, and treatment should primarily be targeting strongyles. Tapeworm treatment should be included on one of these latter treatment occasions. Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite burdens.

Correctly dosing your horse according to weight is a vitally important part of worming.Studies show that most people underestimate a horse’s weight by approximately 20%. Underestimating, and therefore under-dosing, can leave your horse at risk of serious disease including colic, diarrhea, weight loss and even death.             

Important weight awareness tips

If you intend to share a syringe of wormer between 2 animals, make sure you correctly estimate the weight of the ponies, foals or young horses in question. Re check your horse’s weight every time you worm, as factors such as work, age, pregnancy or retirement can impact their weight.

Round up the weight estimation to the nearest 50kg calibration on the syringe.                                                        Never round down as you risk under-dosing.

Did you know?

- 1 in 6 Australian horses over 14.2 hh weigh more than 600kg.                                                                                   - 1 in 2 horses over 16.2 hh weigh more than 600 kg.                                                                                                  - 1 in 2 ponies less then 14hh weigh more than 300 kg.                                                                                         Check your horse’s weight Horse scales or a weighbridge are the most accurate way to weigh your horse, however not many horse owners have an access to such.Therefore, the best alternative is to use a weight tape of to use a weight estimation formula.

Weight tape – place the tape around the horse’s girth and read the estimated weight in kilograms from the tape.

To measure girth correctly, the tape (weight tape or conventional measuring tape) should be snugly positioned around the horse’s barrel, directly behind the elbow and passing over the back at the lowest part of the withers.To measure length correctly, place one end of the measuring tape on the point of the shoulder, keep the tape taut and run it diagonally to the point of the buttock. Stand by the horse’s back leg and read the tape without peering around the horse’s body.