Worms in Horses
Internal parasites, or worms, are silent thieves and killers. They can cause extensive internal damage without you even realizing that your horses are heavily infected. The effects of internal parasites range from a dull haircoat and unthriftiness to colic and death. Internal parasites lower the horse’s resistance to infection, rob the horse of valuable nutrients and in some cases, cause permanent damage to the internal organs.
In terms of management priorities, establishing an effective parasite control program is probably second only to supplying the horse with clean, plentiful water and high-quality feed. It’s that important.
Types of Internal Parasites
There are more than 150 species of internal parasites that can infect horses. The most common and troublesome are the following:
- Large strongyles (bloodworms or redworms)
- Small strongyles
- Roundworms (ascarids)
- Tapeworms
- Lungworms
- Pinworms
- Threadworms
- Bots
Probably the most important in terms of health risk are the first four: large and small strongyles, roundworms and tapeworms.
The lifecycle of most internal parasites involves eggs, larvae (immature worms) and adults (mature worms). Eggs or larvae are deposited onto the ground in the manure of an infected horse. They are swallowed while the horse is grazing, and the larvae mature into adults within the horse’s digestive tract (stomach or intestines). With some species of parasite, the larvae migrate out of the intestine into other tissues or organs before returning to the intestine and maturing into egg-laying adults.
Large strongyles
Large strongyles, as larvae, penetrate the lining of the bowel and migrate along the blood vessels that supply the intestines. Even small numbers of these larvae can cause extensive damage and possibly death.
Infection with large strongyles can cause unthriftiness, weight loss, poor growth in young horses, anemia (low numbers of red blood cells) and colic. In most cases, colic caused by these parasites is relatively mild, but severe infections can result in loss of blood supply to a portion of the intestine, leading to severe and potentially fatal colic. Fortunately, large strongyles can be effectively controlled by most available dewormers for horses.
Small strongyles
Small strongyles have become a group of major importance. Unlike the large strongyles, small strongyle larvae do not penetrate the intestinal wall or migrate through the tissues. Instead, they burrow into the lining of the intestine and remain dormant, or “encysted” (enclosed in a cyst-like structure), for several months before completing their lifecycle. During this time the larvae are resistant to most dewormers.
Small strongyle larvae can cause severe damage to the lining of the intestine, especially when large numbers of larvae emerge from the encysted stage all at once. Colic and diarrhea are common in heavily infected horses. These parasites also cause weight loss, slowed growth in young horses, poor coat condition and lethargy, or lack of energy. While lighter infections are not obvious, it is common for a horse’s general health and performance to improve after treatment for these parasites.
The early and late larval stages (before and after they burrow into the lining of the intestine) and the adult parasites are susceptible to several dewormers. But currently there are only two types of dewormers that are effective against the encysted larval stage— the stage that causes the most damage. Strategic use of these products is called larvicidal therapy, as it is targeted at the encysted larvae.
Roundworms
Roundworms, or ascarids, are most often a problem in young horses (especially foals, weanlings and yearlings). Adult roundworms are several inches long and almost the width of a pencil; in large numbers they can cause blockage (or impaction) of the intestine. In addition, roundworm larvae migrate through the internal organs until they reach the lungs. They are then coughed up and swallowed back into the digestive tract to complete their lifecycle. Large infections can lead to damage to the liver or lungs due to migration of these larval forms.
Roundworm infection in young horses can cause coughing, poor body condition and growth, rough coat, pot belly, and colic. Colic is most likely in older foals (over 3 months of age) that are heavily parasitized with roundworms when dewormed for the first time. By this stage the roundworms can have matured into adults that could cause an impaction. In this situation, it is a good idea to have your veterinarian deworm the foal or recommend a deworming plan for the foal.
Tapeworms
Until recently, tapeworms weren’t considered to be a significant problem in horses. We now know that tapeworms can cause colic, ranging from mild cramping to severity requiring surgical treatment. The tapeworm lifecycle involves a tiny mite as an intermediate host, and horses are at risk of developing tapeworm infection when they eat this mite in grass, hay or grain.
Until recently, no equine dewormer was approved for use against tapeworms. Praziquantel has been demonstrated to be highly effective against tapeworms. Several pharmaceutical companies have developed combination products that offer a complete antiparasitic spectrum of activity. Horses should be dewormed for tapeworms annually. Consult your veterinarian for advice on the best product to use for your situation.
Other internal parasites
Lungworms cause chronic coughing in horses, ponies and mules. Donkeys are the natural host of this parasite, so typically they don’t show any obvious signs of infection. Pinworms lay their eggs on the skin around the horse’s anus. The irritation they cause makes the horse repeatedly rub its tail. Threadworms are mostly a problem in young foals, in which they can cause diarrhea. Bots don’t usually cause major health problems, although they can damage the lining of the stomach where they attach. Since ivermectin has become such an easy deworming medication to obtain, bots are rarely found in properly dewormed horses. They may also cause small areas of ulceration in the mouth, where the larvae burrow into the tissues for a time after the eggs are taken into the mouth.
In the next article we look at signs of paratism, fecal egg counts and designing a deworming programme
© Copyright Bayer AG 2013. Article reproduced with permission from Bayer
