Protecting Your Horse from Equine Internal Parasites

Effective parasite control is one of the keys to successful horse management.  There are more then 150 internal parasites posing a threat to the health and vigor of your horse.  Parasites compete for nutrients and can cause extensive damage to tissues and organs as they migrate through the horse’s body.  Often horses are kept in stalls and paddocks or in close confinement with other horses - parasites thrive under these conditions. 

What are the signs of parasite infection?                                               

How do horses become infected with worms?

How does the environment become contaminated?

How often should I deworm?

What dewormers can I use?

What are the signs of parasite infection?

Signs of parasite infection can be extremely variable, ranging from a barely detectable reduction in growth or performance to a severe and painful disease, sometimes resulting in death.  Signs including dull hair coat, weight loss, unthriftiness, pot-bellied appearance, lethargy, tail rubbing, colic, diarrhea, and appetite loss are familiar to many horse owners.  However, contrary to popular belief, horses can have a dangerous, damaging parasite burden and appear externally healthy, fat, and shiny.

How do horses become infected with worms?

Every parasite has a unique life cycle but in general a horse becomes worm-infested by taking either worm eggs or infective larvae into its mouth.  This occurs when grazing contaminated pasture, eating contaminated feeds, drinking contaminated milk (foals), or licking a contaminated hair coat.  The horses’ environment should always be considered contaminated with worm eggs or larvae.  The opportunity for infection or re-infection is continuous.

How does the environment become contaminated?

Most horse parasites produce eggs that pass out of the horse in manure.  The eggs hatch releasing larvae into the environment.  The larvae develop and are ingested by the horse during grazing.  The larvae migrate through the body eventually maturing and releasing eggs repeating the vicious cycle.  Eggs and larvae can survive long periods of cool weather.  Heat and dryness can reduce their numbers but in favorable conditions pasture contamination increases over time.

Which parasites cause the most problems?

Of the many parasites able to infect horses there are a few groups that do the most damage.  Large strongyles (bloodworms) damage the arteries that supply blood to the intestine interfering with the flow of oxygen and nutrients.  They also ulcerate the gut wall.  Strongyle infestation can result in severe and often fatal colic.  Small strongyles (cyanthostomes) are very prolific-one horse may harbor hundreds of thousands of small strongyles.  The larval stages form cysts in the intestinal wall causing the wall to become inflamed and thickened.  A more severe reaction develops when the encysted larvae emerge and ulcerate the intestinal lining.  This can result in severe diarrhea, weight loss, and colic. 

The prevalence of anoplocephala (tapeworm) infestation varies between regions-one study of southern Ontario found that 14% of horses had tapeworms.  It’s been suggested that tapeworm infestations are more common in horses grazing in low, swampy areas.  Tapeworms colonize parts of the large intestine causing inflammation.

Ascarids (roundworms) are primarily a problem in young foals age 2-6 months.  Migration of roundworm larvae through the lungs and liver is often implicated in foal pneumonia.  Horses develop an immunity to ascarids at about 6 months.  Strongyloides (threadworms) cause disease in young foals with the larvae found in small intestine, skin, and lungs.  Foals can ingest larvae by suckling an infected udder.  Horses tend to develop an immunity at 15-23 weeks of age. 

The female oxyuris (pinworm) lays eggs in the skin around anus of the horse.  The horse’s anal area becomes itchy and the eggs are rubbed off.  The larvae feed on the lining of the cecum, part of the large intestine.  Gasterophilus (bots) eggs are obvious and often concern horse owners.  Larvae irritate the tongue and cheeks of horses and the adults live in the stomach but cause few serious problems.

How often should I deworm?

Parasite control in horses is primarily based on the use of effective drugs.  However, there are some management practices that are useful in limiting the build-up of infective larvae on pasture: 

• avoid overcrowding of horses on pastures

• manually remove manure – highly effective but  labour intensive

• compost manure as high temperatures kill eggs and larvae 

• harrow pastures to break up manure and kill infective larvae by exposure to sunlight 

• periodically plough and re-seed pasture

• group horses by age

• feed from a feeder rather than the ground

No single parasite control strategy is ideal for all horses.  The degree of parasite control necessary to maintain optimal health in a horse depends on age, use, management, stocking density, and exposure to other infected horses.  Seasonal variations in climate also affect the development and survival of larvae on pasture.  Areas with higher stocking density or milder climates require much more frequent parasite treatment (every 4-8 weeks) or feeding a daily preventative wormer to maintain optimal health.

Horses at different ages have varying requirements for parasite control.  Treatment of broodmares with ivermectin within 24 hours after foaling reduces the transmission of Strongyloides to foals in the milk.  Young horses are very susceptible to ascarid (roundworm) infections.  For the first 6 months of a horse’s life, parasite control is directed primarily against Strongyloides and ascarids.  Effective control of this parasite should begin before 8 weeks and require worming at 30 day intervals until young horses develop a natural immunity.  For mature horses, strongyles infections become the focal point of parasite control programs.           

With the safe, convenient products available it isn’t difficult to establish an effective worming schedule.  While everyone’s program will not be the same, the main features of a control program include:

• regular treatment of all horses

• quarantine and treatment of new horses before turning them out to pasture

• regular treatment of mares to protect foals

• alternate drug classes to decrease resistance

• following dosage recommendations to ensure effectiveness and avoid toxicity

• periodic fecal exams to check on the effectiveness of program. 

What dewormers can I use?

The effective use of drugs is an important part of a parasite control plan.  A large number of products are available for use in horses.  No single product is 100% effective at eliminating all parasites.  Many products contain the same drug under different labels so it’s important to read the label to find out what you’re treating with. 

Ivermectin (Eqvalan, Ivomec, Zimectrin, etc) has a broad range of anti-parasitic activity.  It is very effective against large strongyles (bloodworms) and its activity against all stages of Gasterophilus (bots) is unique.  However, ivermectin is not effective against Anoplocephala (tapeworms) or encysted cyathostome (small strongyles) larvae.  Small strongyle eggs may reappear in manure 8-10 weeks following ivermectin treatment.

Moxidectin (Quest) also has a broad range of anti-parasitic activity.  It too is very effective against large strongyles and 90% effective against bots but doesn’t get all forms.  Moxidectin is effective against encysted cyathostomes and reduces small strongyle egg counts for 10-12 weeks after treatment.  The product should not be used in horses less than 4 months old.

Pyrantel pamoate (Strongid P) is unique among other available products because it is effective against both Anoplocephala (tapeworms) and most roundworms. Effectiveness against tapeworms is increased by using pyrantel at a double dose rate. It has no activity against Gasterophilus (bots).

Benzimidazoles (Panacur, Safeguard, Benzelmin, Equicide, etc) are generally less effective against Strongyloides and ascarids then either ivermectin or moxidectin.  As well, resistance to benzimidazoles by cyathostomes is not uncommon. Benzimidazoles are effective against encysted cyathosomes (small strongyles) if used daily for five consecutive days.

Praziquantal (Dronci equine paste) is effective against Anoplocephala (tapeworms), including rare cases that may be resistant to pyrantel pamoate. It has no activity against strongyles, ascarids or bots and therefore must be used in combination with other deworming products.

Both ivermectin and moxidectin are good choices to base a worming program on. However, since neither are effective at treating Anoplochepala (tapeworms), use of  pyrantel pamoate or drontal once a year is recommended.  Parasites can develop resistance to many of the drugs used to kill them.  To prevent drug resistance it’s important to rotate wormers. 

Effectiveness of Common Dewormers against Equine Internal Parasites