INTERNAL PARASITES
Dr. Angie Struck

Internal parasites can be a very serious issue. Whenever we usually think of a wormy horse, we think of a horse that has difficulty shedding in spring or has dropped a bit of weight. The effects of parasite load can range from an unthrifty dull hair coat to colic and death. Parasites can decrease the immune system, decrease feed and nutrient availability, and physically cause internal damage. Horses that have compromised immune systems or are under stress are more susceptible to parasites. Pastures that have increasing numbers of infected horses, are not well kept in terms of manure, or are in warm environments are more of an issue as well.

There are more than 150 different species of parasites. The principal equine parasites are Arthropods=bots(larval stages of Gasterophilus), and Helminths=1)Nematodes=roundworms(Stongylus, Cyathostominae, Trichostrongylus, Dictyocalus, Parascaris, Oxyuris, Strongyloides, Habronema, and Onchocerca), and 2)Cestodes=tapeworms(Anoplocephala)

Many of these parasites have a fecal-oral route of transmission. That means that the adult worms live and reproduce within the horse, the eggs are passed into the horse’s manure, and the next horse comes along and eats the egg off of the ground, becoming infected. One parasite that works differently is the Bot Fly. The larvae that are passed in the feces actually crawl into the soil and develop and hatch into flies. The flies lay eggs on the hair of the horse and the horse ingests the eggs. The other one that works differently is the tapeworm. The horse ingests a mite that is infected with larvae, the larvae grow into adults in the intestine, the adult sheds segments of body that are full of eggs, the eggs are released and ingested by forage mites, and the eggs turn into the larvae in the mite’s body cavity.

Large Strongyles (Stongylus spp.)
Adults live in cecum and colon. Larvae migrate through mesenteric vasculature from the intestine. Adults are 1.5cm-2.5cm in length and are dark red in color. They have large mouthparts with teeth, which allow them to plug feed from the horse’s intestine. Immature larvae that migrate through vessels can cause compromise to the horse’s intestinal blood supply, leading to necrosis. This becomes a surgical problem at that point.

Small Strongyles( Cyathostominae)
Similar to the large strongyle, but are usually less than 1.5cm in length. Adults live in cecum and colon. In the summer, life cycle is very fast, taking 1-3months for larvae that they ingest to develop inot egg-laying adults. In the winter, larvae are able to encyst in the mucosa to insure the infection persists to the next season. They can remain until spring when they reactivate and develop into egg-laying adults. Spontaneous eruption of large numbers of larvae causes enormous damage to the intestinal wall and can result in dehydration, diarrhea, colic, and chronic weight loss. The immature larvae can migrate to liver and lungs, which creates great potential for problems.

Large roundworms(Parascaris or ascarids)
Ascarids can be up to 40cm in length!!! They don’t have teeth, but they do have a mouth with sucking lips. They are very good egg layers. They can lay from 100,000-200,000 eggs each day. They are very thick shelled eggs that can handle adverse weather. They can remain viable in the environment for up to 10 years!

Pinworms(Oxyris equi)
Females and males are different. Females are 10cm and males are 1mm-5mm in length. They spend their entire lives in the lumen of the intestine and don’t migrate through any organ tissue. Because of this, they cause very little physical damage to the horse.

Threadworms (Strongyloides westeri)
Tiny hair like worms 4-6mm in length. These guys live in the small intestine and feed on intestinal blood. Horses can either be infested by skin penetration or ingestion, because of this parasite’s ability to migrate through the bloodstream to different body tissues before making it to the intestine. This parasite can be transmitted to foals through the mare’s milk.

Lungworms (Dictyocaulus)
White 6cm-10cm in length worms. They can only reach adulthood in donkeys, so horses that share pastures with donkeys are more likely to be infested. They settle in the lungs, rather than intestine. Eggs travel up the trachea, are swallowed, and passed out in feces. Infective larvae are swallowed by horses or donkeys and then can cross the intestinal wall and enter arteries and pulmonary circulation.

Habronema
Adults are 1cm-2.5 cm and found in the stomach. Eggs of this worm are ingested by flies and develop into larvae. When flies feed on horses, they deposit larvae onto their skin. (Summer sores)

Onchocerca
Adults can be up to 30cm in length. These worms burrow in connective tissues of horses and can cause lameness. The early stages can cause dermatitis.

Cestodes (tapeworms)
Most horses show few outward signs. Infested horses do not often have poor hair coats, diarrhea, or low body weight. Rarely are they visible in feces. It has a predilection for the ileocecal junction. Enlargement of the distal ileum, ulceration, and edema are common at the site of attachment. Small intestinal rupture, intussusceoptions, intestinal torsions, ileal obstructions, and spasmodic colic have all been reported as associated with tapeworm infestation. 22% of gas colic cases in a case study were associated with tapeworms. Young horses(6 months to 2 years) carry the highest levels of tapeworm infestations. These levels drop in middle aged horses and start increasing again in older horses(>15 years).

DIAGNOSING INTERNAL PARASITES
Clinical signs may not be revealed until the parasite burden becomes very severe. The subclinical damage can lower the horse’s immune system, not allow them to utilize their feed efficiently, and may cause the horse to colic. Nematode infestations are found by detection of eggs in feces. This method does not detect migrating larvae or encysted stages of small strongyles. Stomach worms and eggs are found via endoscopy. Pinworms can rarely be visualized at the horse’s rectal opening. I have heard of people looking with a flashlight late at night and seeing them. Many parasite infections are found at surgery, unfortunately.

TREATMENT AND PREVENTION:
See attached tables for lists of class of dewormer and parasites that they are effective against. There are a few dewormers not listed. Equimax has recently been approved in the United States for use in horses. This dewormer contains a combination of ivermectin and praziquantel. The praziquantel acts on tapeworms. Another dewormer not on the list is Strongid C or Strongid C2X daily dewormer for horses. This dewormer contains pyrantel tartrate and is designed for daily continuous use. The benefits of this product are that it prevents accumulation of parasites within animals. The negative is that it has to be given every day, so it is high maintenance, and you still have to deworm for larval stages. This dewormer is not effective against larval stages of the parasites. Quest gel is also a dewormer not on the list. Quest contains moxidectin, which controls most worms other than tapeworms. This dewormer is only labeled for horses older than 6 months of age and is not to be used in thin or debilitated horses. It is approved for ponies and horses ONLY. Between 1997 and 2002, 111 reports of ataxia for horses less than 6 months of age were reported. 196 reports were made for horses of all ages. Other reactions include depression, lethargy, and recumbency. Horses that were less than 250 pounds and horses that were older in age also had increased rate of adverse reactions. YOU must know your horse's body weight prior to using this product. Accidental overdose can lead to very serious complications and even death!!!

WHEN TO DEWORM AND WITH WHAT PRODUCT
You have likely heard a significant amount about resistance to common dewormers. Resistance is a term used to describe the development of parasites within our horses that are no longer controlled by certain deworming products. Resistance to the benzimidazole family is widespread. Cyathostome resistance to pyrantel pamoate is becoming increasingly prevalent as well. Some parasite populations that show resistance to these dewormers are also showing some tolerance to ivermectin as well. These parasites continue to live and breed more resistant parasites to add to the population. We attempt to prevent resistant populations by not using the same dewormer over and over. Another very important aspect to preventing resistance is to attempt to eliminate eggs in the environment. We can do this mainly through strict pasture management.

Because small strongyle larvae encyst during the winter so that they can ensure the survival of another population in the spring, one of our major goals is to prevent this from occurring. Not all of the dewormers we use are effective against the encysted larvae. Ivermectin has limited efficacy against encysted stages. Moxidectin, with all of its possible risks, does have moderate efficacy against some larvae. Fenbendazole (Panacur) at 10mg/kg orally every day for five consecutive days does have larvicidal properties. So, in September, using the Panacur Power Pack will effectively eliminate larval stages before heading into winter. Ivermectin is the only dewormer that kills bots. Since bots are a problem when flies are around, we should use ivermectin in the spring so that we kill any leftover bots that made it through the winter, and in the fall so that we kill all bots that made it to the stomach during the summer. In between these important deworming times, a person can rotate pyrantel pamoate (strongid) with an anthelcide (ex =fenbendazole) every 2-3 months, depending upon your parasite burden. You should have your horse’s feces checked for parasites so that you are deworming them appropriately. People that use Strongid C daily dewormers should still treat for bots and encysted larvae. Care should be taken with pregnant mares and foals. Avoiding the use of ivermectin before the mare is 45 days along has been common practice. Deworming one month prior to foaling and the day of foaling with ivermectin is recommended. People often times forget about foals. Foals need to be dewormed starting at one month of age with an ivermectin product. Rotating ivermectin with fenbendazole every other month until the foal is weaned should protect against S. westeri and ascarids.

ALWAYS READ LABEL TO MAKE SURE THE DEWORMER IS SAFE IN FOALS!! DO NOT USE MOXIDECTIN IN FOALS LESS THAN 6 MONTHS OF AGE.

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