A parasite is a living organism that 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 to survive, is often decreased as a result of the parasites' presence. Many parasites are specific to a species of animal. For example, Oesophagostomum dentatum, the nodular worm, infects only swine, whereas another species of the same genus ie. Oesophagostomum columbianum, infects sheep. Other parasites such as Trichostrongylus axei, the stomach worm, may infect several species, including pigs, horses, cattle and sheep. Each species of parasite has a predilection for a specific location within the host animal. The common names of many worms reflect the location of the adult parasites. Two examples are the kidney worm (Stephanurus dentatus) and the lungworm (Metastrongylus apri).

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 an exo-skeleton. Arthropod ectoparasites fall into two classes, arachnids and insects.

Ectoparasite infestation affects the health of the host animal in several ways. Pigs may be so preoccupied with the itching and irritation caused by ectoparasites that feeding is irregular and, consequently, the host fails to gain weight. Such "parasite worry" is a problem in almost all infestations. Animals have decreased feed efficiency and are susceptible to various other sicknesses such as bacterial and viral diseases. Heavy infestations can cause serious blood loss and unthriftiness, which can greatly reduce income from swine. One of the serious aspects of ectoparasite infestation is the ability of some ectoparasites to transmit other serious diseases. For example, the swine louse (Haematopinus ) can transmit hog cholera and swinepox.

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

The effects of both endo and ectoparasites acting together on swine are greater than the damage caused by either type of parasite by itself. For example, animals that are heavily infested with lice will be more severely affected by a sudden buildup in roundworm parasites than would swine that are 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 potentially will save labour costs.

The roundworms, particularly Ascaris, Oesophagostomum, Metastrongylus, Stephanurus, and Strongyloides, are important internal parasites of swine. These parasites are widespread and cause heavy economic losses. 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.

Significant concentrations of swine

Internal organs of swine showing predilection sites of endoparasites