This blog will discuss the most common worms in foals, how they get worms, at what age foals need to be dewormed, and the treatment of worms.
An important component of your foal’s overall wellness is parasite control. Foals and weanlings are more susceptible to roundworm infections and are at higher risk of illness secondary to parasite infection. Foals start to develop immunity at 12-24 months of age. Knowledge of common parasites in foals will help you keep your foal healthy. There are several different parasites in horses, but this blog will focus on three parasites that are more specific to foals and weanlings.
Parascaris equorum aka ascarids
Cyathostomins aka small strongyles
Strongyloides westeri aka threadworms
Parascaris equorum aka ascarids
Parascaris equorum is the equine roundworm and is the most common and important type of worm in foals (as young as 2.5-3 months) and young horses (weanlings). These worms can measure 30-50 cm in length. Foals become infected by ingesting larvated eggs in the environment (dirt paddocks, pasture, and the stall).
Life cycle- The larvae hatch in the small intestine and migrate through the liver and the lungs via the circulation. The larvae migrate up the airways, are coughed up, and swallowed back into the gastrointestinal tract, where they further develop and reproduce in the small intestine again. Finally, eggs are expelled through the feces and reach the environment. This process takes approximately 75-80 days postinfection.
Horses acquire excellent immunity; ascarids are generally uncommon in horses 1-2 years or older. However, patent infections can occur in adult horses highly exposed to infection, such as broodmares or adult horses with reduced immune systems.
Clinical signs range from no clinical signs, ill-thrift, poor hair coat, cough, weight loss, pot-bellied appearance, and colic.
Colic is one of the more important clinical signs because it can lead to impaction colic. This commonly occurs in heavily infected foals that have recently been dewormed. Essentially, several worms have died at once and cause a blockage within the small intestine that, if severe enough, can lead to small intestinal rupture and death.
Cyathostomins aka small strongyles
Cyathostomins are also referred to as small strongyles. These worms generally become the predominant parasite group in horses 1-2 years and older. Strongyles are ubiquitous in the environment, and all grazing horses are exposed to infection. The mare can be a source of infection for the foal.
Life cycle- Strongyle eggs are passed in the manure and will hatch, and the larvae develop on the pasture. The third larvae stage is the infective stage and when consumed by the horse, the larvae encyst in the colon and cecum. They can be encysted for a variable time (some for years) before migrating back to the intestinal lumen to be passed out in the manure.
Clinical signs range from no clinical signs, mild colic, weight loss, loose manure, or, in rare cases, colitis with accompanying colic, diarrhea, weight loss, protein loss, and ventral edema. Colitis occurs when a large number of strongyles concurrently emerge from the cysts, causing severe generalized inflammation in the colon and/or cecum.
Strongyloides westeri aka threadworm
Strongyloides westeri, the equine threadworm, parasitizes the small intestine of suckling foals. Foals become infected by ingesting infective larvae in the mare’s milk during the first week of lactation. Additional routes of infection are penetration of the larvae through the skin and ingestion of larvae from the environment. Infections are common but rarely cause a problem.
Clinical signs- usually no clinical signs, but occasionally enteritis, including diarrhea, fever, weight loss, and colic.
After weaning, S. westeri is almost never seen because horses acquire excellent immunity.
Treatment with a specific dewormer is recommended for clinical cases only.
Most Common Worms in Foals-Treatment of Worms
Treatment/reduction of parasite burden- Treatment targeting ascarids should be timed around the age of 2.5 to 3 months of age. Then again, at 5 months of age. Anthelmintic resistance to ivermectin is highly prevalent in ascarids, and should be considered in treatment decisions. Benzimidazoles are a valid treatment option, but monitoring efficacy is recommended.
Every farm and foal operation is different, and parasite control designed in one geographic area may be neither efficient nor effective in another location. There is also evidence of resistance to the current dewormers. Therefore, working with your veterinarian on an appropriate parasite control program tailored to your individual location and horses is important.
General principles for parasite control
-Remove manure from the environment.
-Practice good sanitation and hygiene.
-Clean feeders, pans, water buckets, feed off the ground, and reduce crowding of animals.
-Perform fecal egg counts to identify the type and amount of parasite egg present to target which dewormer would be most effective and appropriate.
-In foals, perform fecal egg counts at 4-6 months of age. This requires submitting a fecal sample to your veterinarian. Foals on green pastures will likely require deworming at 7-9 months of age and then again as a yearly targeting treatment for small strongyles and tapeworms.
Resource on internal parasites of horses- for horse owners by American Association of Equine Practitioners (AAEP)
Client Education Presentation: Internal Parasites – AAEP
Most Common Worms in Foals-Take Home Points
Discuss with your veterinarian deworming recommendations for your foal.
P. equorum aka Ascarids are the most important and pathogenic parasite infecting foals and weanlings.
Anthelmintic resistance in ascarids and strongyles is a factor to consider when choosing an appropriate dewormer. Determine anthelmintic resistance by performing fecal egg reduction tests (FECRT).
Horses under 3 years of age require a different deworming protocol than adults because they are more susceptible to parasite infection and developing disease.
Worms in Foals Frequently Asked Questions (FAQ)…
At what age do you deworm foals?
Between 2.5-3 months of age. That time frame also matches the prepatent period for the most important worm in foals (ascarids). Foals should not be dewormed earlier than 60-70 days of age. Treating foals bimonthly (i.e., approximately every 60 days) is considered the maximum dosing interval for controlling ascarids. That frequency may not be needed if the farm has a low worm burden. Deworming at more frequent intervals will worsen worm resistance to the current dewormers. Treatment with Benzimidazoles (Panacur or Safe-Guard) is recommended. These are dewormers that are given by mouth.
What is the most common parasite in foals?
Parascarid equorum – ascarids – roundworms
How can I tell my foal has worms?
Poor growth, rough haircoat, diarrhea, pot-bellied appearance, colic, ventral edema, worms in the feces, and cough. Sometimes, there are no clinical signs.