A parasite is a living organism which lives upon or within another living organism, known as the host. The parasite gains an advantage from the host, usually without providing any compensation to the host.

Endoparasites live within a host and must obtain nutrients from that animal in order to survive and reproduce. The host´s ability to thrive, or even survive, is often decreased as a result of the parasite´s presence. Many parasites are specific to a species of animal. For example, Dictyocaulus arnfieldi, the lungworm, infects horses and donkeys, whereas another species of the same genus, Dictyocaulus viviparus, infects cattle. Other parasites, such as Trichostrongylus axei, may infect several species, for example, cattle, sheep, and horses. Each species of parasite has a preferred site or specific location within the host animal. This site is referred to as the site of predilection for the species of parasite. The organ of preference provides the common names of many parasites, such as the eyeworm (Thelazia), the ligament worm (Onchocerca), and the intestinal threadworm (Strongyloides).

Parasites that feed or live on the body surface of a host animal are called ectoparasites. Most of them are arthropods, that is, invertebrates with jointed legs and hard external skeletons. Arthropod ectoparasites fall into two classes, arachnids and insects.

Ectoparasite infestation affects the health of the host animal in several ways. The horse may be so preoccupied with the itching and irritation caused by ectoparasites that feeding is irregular and, consequently, the host fails to thrive. Such "parasite worry" is a problem in almost all infestations. Animals may become emaciated and susceptible to various other sicknesses such as bacterial and viral diseases. Loss of blood resulting from heavy infestations can be serious and greatly reduce a horse´s stamina. One of the most serious aspects of ectoparasite infestation is the ability of some ectoparasites (particularly ticks) to transmit other serious diseases, including redwater fever (Piroplasmosis).

Various parasites are found wherever horses are raised. The worldwide incidence of parasites in horses and their economic importance are greatly influenced by geographic location, season of the year, and climatic conditions.

Significant concentrations of horses

In very general terms, endoparasites tend to be a more serious problem in warmer, wetter, temperate areas, while ectoparasites tend to cause the greatest losses in drier areas. However, there is considerable overlap in the incidence of endo and ectoparasites. The effects of both types of parasites acting together on horses may be greater than the damage caused by either type of parasite by itself. For example, a horse that is heavily infested with lice will be more severely affected by a sudden build up in roundworm parasites than would a horse that is free of ectoparasites.

Because of the compound effects of two or more classes of parasites, the use of a broad-spectrum parasiticide will provide more effective treatment than the use of a product that kills only one group of parasites. Additionally, any reduction in frequency and total number of treatments will save labour costs.

Horses can be divided into those kept for working (such as handling livestock) and those kept for recreation, including racing. The former usually graze on open country and may live in close association with cattle and sheep, thus being subjected to attacks of those parasites that are not host specific. The problem is essentially due to overgrazing. Recreational horses spend all or a portion of their adult lives in stables, but are usually on pasture as foals. The susceptibility of horses to certain parasites is determined by these management techniques. The numbers of horses being grazed together can contribute to the level of the worm burden.

The nematode parasite Strongylus vulgaris is universally the most significant parasite of horses. In more recent times small strongyles or Cyathostome worms have received greater attention. It is found wherever horses are raised and causes serious damage. Strongyloides and Parascaris species can cause serious damage in foals. The confining conditions of stables assist in the spread of these worms, but, as stabled horses can easily receive regular treatment with anthelmintics, these horses can generally be expected to have low worm burdens, provided an effective treatment programme is being used.

Ticks are the most important ectoparasite of horses and are most frequently found in the hotter areas. In Central and South America ticks can transmit the disease piroplasmosis, which causes severe economic losses. The stablefly (Stomoxys calcitrans) is a damaging pest that inflicts painful bites, causing irritation and nervousness. Horses subjected to large numbers of these stableflies will not thrive.

The horse botfly (Gastrophilus spp ) is common in most parts of the world where horses are raised. Large numbers of the bots in the stomach sometimes cause damage and even blockage, and the adult flies irritate horses. However, it is not a pest that causes extensive economic losses. Lice can be a problem among horses that are not well cared for.

Most parasites have complex life cycles coordinated with climatic conditions. The seasonal incidence of each parasite species varies in different regions. This seasonal incidence of parasites is an important factor to consider when planning a total parasite control program.

Internal organs showing predilection sites