Deworming Protocols

Horses, ponies, donkeys and mules all get internal parasites. Unfortunately, the overuse of wormers, particularly daily dewormers, has meant that there is an increase of parasite resistance, and it is getting harder to treat for parasites as a result. Until recently, barns would deworm on a rotation schedule, alternating between a benzimidazole, a pyrantel and an invermectin, every 2 months. The new way of thinking is to have your vet perform a fecal count of a sample of manure from each horse, and to deworm accordingly. Known as a targeted deworming strategy, this method will aid in combating the parasite resistance to dewormers. It is negligent to just go to the feed store and buy a tube of a random dewormer, or worse, a daily dewormer, without knowing exactly what you are parasites you are targeting.

Horses that are not dewormed on a regular schedule will look unhealthy: they will exhibit a dry coat, loss of weight and listlessness. Worms will damage the blood vessels, heart, intestines and lungs. It is estimated that 90% of colic cases are related to parasite damage, especially by large strongyles. Internal parasites can also result in anemia, lethargy, diarrhea and even death.

The object of parasite control is not so much as to eliminate the parasites entirely, but to maintain the parasite burden at a low level. This helps the horse to maintain a partial immunity to overwhelming infection. The ultimate goal in deworming is to minimize the risk of parasitic infestation, control parasite egg shedding, maintain effective drug control and avoid further resistance to dewormers. By utilizing fecal egg count tests, you can determine exactly how infested each horse is. By testing horses in a large boarding facility, you will have an indication of the parasite levels within the groups of horses if they are pastured together.

The life cycle of parasites is rather simple. Eggs can lay dormant for years in a pasture until environmental conditions are favorable and the eggs hatch into the first stage (L1) as larvae. These grow and molt into the second stage (L2) and progress to the third stage (L3). Horses cannot be infected at the egg, L1 and L2 stages, even if ingested. Infestation occurs when L3 is ingested, where they will burrow into the mucosal lining of the gut wall and molt into stage 4 (L4). If they are not ingested within a couple of days, they will die. To prevent death, they will migrate up blades of grass where they will most likely be eaten. In wet conditions, this is more favorable, whereas in dry conditions, they will most likely die before being able to migrate in the grass. Once ingested, L4 will finish their development in the gut as well as in the liver, heart and lungs, arteries and intestines, depending on the type of larvae. The larvae will return to the gut, become adults and lay eggs, which are passed through the horse and out to the pasture, to repeat the cycle.

 A fecal egg count will establish the number of parasite eggs that are present in each gram of manure (EPG). EPG values of 200-500 would indicate treatment for the parasites presenting, while an EPG value of 100-300 is considered normal and no treatment should be required, or it may not be needed as often as horses with higher EPG values. Testing will also provide insight into whether or not a horse or a herd is developing resistance, at which case your veterinarian will discuss the next step for treatment.

The most common parasites infecting horses are ascarids, strongyles, tape worms and bots. Different wormers treat different parasites, which is why the best method to manage internal parasites is via fecal egg count.

Other worms affecting horses include large and small redworms, roundworms, pinworms, threadworms and lungworms.

Performing fecal egg counts is relatively simple, in that all is needed is some fresh manure, gathered in a plastic baggie. The manure ball should be collected about 3 months after the most recent deworming, as this will indicate the shedding rate of parasites. Horses with low EPG numbers need to be dewormed no more than twice a year if even that often, usually at the beginning of the grazing season, after the last frost, such as during the month of April and again at the end of the grazing season, in the autumn, before the first frost. Moderate “shedders” will probably benefit from a 3^rd treatment during the main seasons of pasture transmission (spring through autumn in the north and autumn through spring in the south) and all high shedders should be treated intensively throughout the main seasons of transmission (3-4 times per year.) The goal is to prevent high shedders from contaminating the environment, therefore manure needs to be removed from pastures and not spread over them.

Foals are the exception to using egg counting, and they should receive 3-4 deworming treatments by the time they turn one. Treat at 2 months with fenbendazole or oxibendazole and repeat at 4-5 months. A fecal count should be done at this time to track the occurrence of ascarids versus strongyles. Treat at 5 months with ivermectin and before the horse turns one, treat with ivermectin plus praziquantel. Yearlings should be treated for strongyles three times with ivermectin and followed by a course with moxidectin plus praziquantel. Egg counts should be done as a yearling to check for the presence of ascarids and should be treated with fenbendazole or oxibendazole if needed. Always consult your vet before deworming any foal.

Typical Adult Horse DeWorming Schedule:

Low Shedder: <200 EPG of manure:

Spring (March-April) use ivermectin or moxidectin.

Fall (Sept-Oct) use ivermectin w/praziquantel or moxidectin w/ praziquantel.

Moderate Shedder (200-500 EPG):

In addition to the above schedule, add in a late summer deworming of Ivermectin, typically in July.

High Shedder (>500 EPG):

Add a 4th deworming in late fall, typically November, of Moxidectin.

In the end, fecal count is far cheaper than buying random tubes of dewormer every 8 weeks and hoping that it worked. There are labs that allow you to mail off your sample for an analysis, however most vets perform the test. It costs about $20-25 per test.

While parasites are present in all soil, there are ways to help prevent over infestation of parasites. These include:

• Avoid keeping too many horses on small acreage.

• Avoid overgrazing.

• Rotate pastures.

• Pick up manure from pastures and paddocks regularly

• If you spread manure in your fields, keep horses off those pastures for several weeks.

• Do not spread manure of high shedders.

• Keep hay, feed and water from becoming contaminated with manure.

• Horse parasites do not infest sheep or cattle, so if you graze these animals, rotating pastures with them will destroy the life cycle of horse parasites.

• Drag or harrow pastures to break up droppings. Sunlight will kill the eggs, especially in extreme heat. If the weather is not hot enough, remove the horses from that pasture for the rest of the growing season, as the parasites need to be exposed long enough to the elements in order to die.

• Compost manure before spreading on pastures. Both ascarid and small strongyle eggs are eliminated through composting.

• Treat for tapeworms once a year with a praziquantel classification.

• Treat for encysted small stronglyes in the fall or near the end of the grazing season, before going into winter. Use a moxidectin.

• Rotate fields with broodmares and foals, to reduce the build-up of ascarid eggs, which will infect new foals in the springs. Foals should never be turned out on pastures where known high shedders have been grazing.

• Do not feed directly on the ground.

• Mow pastures regularly and keep the height between 4-8 inches tall. Mow again at the end of the grazing season to reduce the chances of parasites surviving the winter. Parasites can survive if they are in fecal balls, insulated from the cold.

• Never spread foal manure.

Dewormer Classifications

Benzimidazoles (fenbendaxole, oxibendazole) target large strongyles, small strongyles (not encysted), ascarids and pinworms.

Macrocyclic lactones (ivermectin/moxidectin) target large strongyles, small strongyles (including encysted- moxidectin only) ascarids, bots and pinworms

Pyrimidines (pyrantel) target large and small strongyles, ascarids, tapeworms (must double dose) and pinworms

Anthelmintics work by targeting specific internal parasites by preventing them from obtaining nutrients. The parasites starve to death and are expelled from the horse’s body.

As a barn owner/manager, it is imperative that all horses be tested regularly and treated accordingly. Preventative measures should be taken to reduce infestation. Horse owners should be wary of any barn which requires a daily dewormer, as these do not work and are a waste of money. Good barn owners and horse owners will take the necessary steps to stop over treating horses and contributing to parasite resistance with unnecessary deworming.

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