Parasites

By Robert K. Ridley.

Parasitism can best be considered a farm or herd problem and not a problem of individual horses. To control equine parasites, it is necessary to understand that not only are horses parasitized but that pastures, currycombs, blankets, horse trailers, etc., may be involved in the infection and reinfection of horses with some parasites.

To survive and propagate themselves, well-adapted parasites live in harmony with their hosts. If a parasite were always to kill its host, it would be responsible for its own demise, since by definition parasites are organisms which live in or on another organism of a different species for the purpose of obtaining food.

Implicit in this definition is the notion that the parasite harms its host to some extent. However, sometimes it is only when parasites "get out of control" that we become aware of their harmful effects, and seek to rid our horses of their unwelcome guests. Although parasites affect horses of all ages and are found in all parts of the country, they most commonly occur in young animals in temperate regions.

Internal parasites are those which are found on the inside of the horse and are single-celled animals (protozoa), roundworms (nematodes), or flatworms (flukes and tapeworms). These parasites usually are found in the gastrointestinal tract, but may be found in other internal organs. External parasites (lice, ticks, mites, etc.) are found on the skin, in the ears, etc.

Signs associated with both internal and external parasites depend on the type and number of parasites present and can range from no apparent effects to general unthriftiness, weakness, debilitation, and ultimately to death of the host.

According to our definition, many organisms satisfy the definition of a parasite, including bacteria, viruses, rickettsias, and yeasts, as well as more traditional "parasites" which as noted above include protozoa, nematodes, cestodes, trematodes, and acanthocephalans (spiny-headed worms).

Most of the parasites affecting horses are nematodes, or roundworms. Indeed, because the horse is infected with so many species of roundworms it is said to harbor more helminth (worm) parasites than any of our domestic animals. Fortunately many of these are similar and we can group them in order to discuss the effects they have on horses and the way horses become infested with them.

Protozoans are single celled animals which occur in the bloodstream and intestinal tract of horses. These organisms multiply by dividing and may be transmitted from horse to horse by an arthropod vector or simply by being ingested in food or water as a result of fecal contamination.

Most equine nematode parasites have a direct life cycle. This type of parasite does not require any other organism except the definitive host to complete its life cycle.

Typically, females which live in the digestive tract lay eggs which are passed to the outside with the horse's feces. The eggs hatch in 2 to 3 days, depending on temperature and humidity, into small wormlike organisms called first-stage larvae (L1). First-stage larvae develop and molt to second-stage larvae (L2) molt to third-stage larvae (L3).

It is the L3 stage that is infective to the final host. They migrate up blades of grass and the horse ingests them when grazing. These so-called preparasitic stages are much the same for most of the strongyle parasites of the horse.

When the horse ingests the third-stage larvae, it develops into a fourth-stage larvae which may wander extensively through the body of the horse before becoming an adult in the intestinal tract ("large strongyles"), or it may develop into an adult in the gut with no migration through other organs ("small strongyles").

Other nematode parasites require a second host in order to complete their life cycles. This second host is an invertebrate and is called the intermediate host. Typically the intermediate host eats the egg or first-stage larvae, which then undergoes its development in the intermediate host instead of on the ground. The definitive host then becomes infected when the intermediate host (flies, ticks, etc.) injects the infective stage of the parasite while it is taking a blood meal. Sometimes the host gets the infective stage by eating the infected intermediate host.

All cestodes (tapeworms) which occur in horses use pasture mites as intermediate hosts. The host becomes infested while grazing by ingesting the mite containing the infective cysticercoid. Mites become infected by ingesting tapeworm eggs which are deposited on the pasture with the host's feces.

Flukes also require an intermediate host, most often a snail. Although flukes do occur in horses, they are of minor significance and will not be considered here.

Roundworms are by far the most serious and economically important of the worms which occur in horses. These,as their name implies, are elongated, cylindrical worms ranging in size from 2 to 3 mm to 12 to 14 inches in length. Although the large worms cause significant problems, the small worms are far more important both from an economic and health point of view.

Some roundworm parasites damage the host by sucking blood, others cause damage by migrating through body tissues such as the lungs, and still others can cause severe colic in horses simply by forming a mass of worms in the intestine which interferes with intestinal motility and to some extent absorption of nutrients.

The tapeworm which occurs in horses is a large worm consisting of a head, which attaches to the intestine of the horse, and a long ribbon-like body with many similar segments called proglottids. In the nematodes the sexes are separate there are males and females and mating occurs to produce the next generation. In the tapeworms, both sexes are contained within the same worm.

Nematodes have a complete digestive system, that is, they have a mouth through which they suck blood or intestinal juices and excrete their waste through an anus. Tapeworms absorb nutrients through their skin, having no mouth or anus.

Large Strongyles

The most important parasites, as far as horses and horse owners are concerned, are nematodes comprising a group called the "large strongyles." These are the most damaging of all the parasites which occur in horses. Adult worms range in size from approximately 1/2 inch up to about 1 1/4 inches in length, and the adults live in the large intestine and cecum where they feed by eating plugs of the mucosal lining.

Far more damaging than the adult, however, are the larvae which migrate through internal organs of the host. Some prefer to live in one of the large arteries supplying the small intestine of the horse. These larval strongyles damage the artery's lining causing it to react and become very thickened, producing an aneurysm.