From Horse
Habronemiasis (swamp cancer)
Psoroptes equi mange in an adult horse
Anterior mesenteric artery with thrombus cause by Strongylus vulgaris
Bot eggs of Gasterophilus spp on horse's coat
Two neutrophils, each containing an Ehrlichia equi morula, in equine blood

The term parasitism may be defined as a two-species association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life and obtains nourishment from it. This is a commonly accepted working definition of parasitism and using it we can emphasize several important features of the host-parasite relationship. Parasitism always involves two species, the parasite and the host. Many of these parasitic associations produce pathological changes in hosts that may result in disease. Successful treatment and control of parasitic diseases requires not only comprehensive information about the parasite itself but also a good understanding of the nature of parasites' interactions with their hosts. The parasite is always the beneficiary and the host is always the provider in any host-parasite relationship.

This definition of parasitism is a general one but it tells us nothing about parasites themselves. It does not address which particular infectious organisms of domestic animals we might include in the realm of parasitology. The protozoa, arthropods and helminths are traditionally defined as parasites. However, there are members of the scientific community who designate all infectious agents of animals as parasites including viruses, protozoa, bacteria and fungi as well as the arthropods, helminths and protozoa. Within this broad definition, parasites are further divided into microparasites and macroparasites.

Immunity to parasites

Immunity to a parasite develops as a result of exposure to infection with that particular parasite. As a result, young animals that have had less exposure tend to be more susceptible and older animals more resistant to infection. This means that disease and death caused by parasites, especially Strongylus, Parascaris, and Strongyloides, occur more readily in foals. Some older animals that become immune rid themselves of most of the infecting parasites. This is called self-cure. In other cases, parasites are able to evade the host´s immune response and infection persists in the older animals, which results in continuous contamination of the environment. For this reason most older animals require treatment.

Common parasites of the horse

  • Skin
- Stomoxys spp and Tabanus spp - stable flies
- Hypoderma spp - warble flies
- Haematopinus asini - sucking louse
- Damalinia spp - biting louse
- Sarcoptes spp - sarcoptic mange mite
- Chorioptes spp - chorioptic mange mite
- Psoroptes equi - psoroptes mange mite
- Trombicula spp - trombicula mites
- Onchocerca cervicalis - Onchocerca worm larvae
- Ixodes pacificus - hard tick, causing ehrlichiosis
- Otobius megnini - spinose ear tick
- Amblyomma spp, Boophilus spp - hard ticks
- Dermacentor spp, Ixodes spp, Rhipicephalus spp - hard ticks
  • Eye
- Thelazia lacrymalis - eye worm
- Setaria equina - filariid worm
  • Lung
- Dictyocaulus arnfieldi - lungworm
  • Liver
- Fasciola hepatica - liver fluke
  • Blood
- Babesia spp - equine piroplasmosis
- Sarcocystis neurona - Equine protozoal myeloencephalitis
- Anaplasma phagocytophila - Ehrlichiosis
- Neorickettsia risticii - Potomac Horse Fever
- Theileria spp
  • Intestines
- Anoplocephala spp - stomach tapeworm
- Gasterophilus spp - stomach bots
- Gongylonema pulchrum - gullet worm
- Habronema spp - stomach worm
- Trichostrongylus axei - stomach hairworm
- Oxyuris equi - pin worm
- Parascaris equorum - large roundworm
- Setaria equina
- Strongylus vulgaris - bloodworm, S. equinus, S. edentatus, Triodontophorus spp - large strongyles
- Strongyloides spp - thread worm