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 survive, is often decreased as a result of the parasite´s presence. Many parasites are specific to a species of animal. For example, Oesophagostomum radiatum, the nodular worm, infects only cattle, whereas another species of the same genus, Oesophagostomum venulosum, infects sheep and cattle. Other parasites, such as Fasciola hepatica, the liver fluke, may infect several different animal species including cattle, sheep, horses, rabbits and goats. Each species of parasite has a predilection for a specific location within the host animal. The common names of many worms reflect location of the adult parasites. Two examples are the large stomach worm or Barberspole worm (Haemonchus contortus), and the stomach hairworm (Trichostrongylus axei).

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 animals in several ways. Cattle may be so preoccupied with the itching and irritation caused by lice that feeding is irregular, and consequently the host may fail to gain weight. 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. Heavy infestations of the cattle tick (Haemaphysalis longicornis) can cause serious blood loss and unthriftiness in young animals. Both ticks and lice can affect hide quality and result in poor leather quality.

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.

In very general terms, endoparasites tend to be a more serious problem in wet, temperate areas, whilst 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 cattle herds are greater than the damage caused by either type of parasite itself. For example, a herd that is heavily infested with lice will be more severely affected by a sudden build up in roundworm parasites than would a herd that is free of ectoparasites.

Because of the diversity of cattle 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 chemical treatments will provide savings in labour costs.

Cattle ticks cause serious economic losses. Ticks affect 80 % of the world cattle population, causing hundreds of millions of dollars in lost production. Indirect costs include the expense of tick control measures. In addition, there are very extensive regions of the world where cattle are not raised because of ticks and the diseases they transmit.

The gastrointestinal roundworms, including Haemonchus, Ostertagia, and Trichostrongylus, are important internal parasites. These parasites are widespread, and can cause heavy economic losses. Haemonchus is more prevalent in areas with hot, moist summers while Ostertagia tends to be more significant in cool, wet areas (northern temperate climate) and hot dry areas (southern temperate climate).

The cattle lungworm is an important parasite in different parts of the world where it can cause severe losses.

Many parasites of cattle have regional significance. For example, the liver fluke, Fasciola hepatica, is an important parasite in Europe and some parts of North America. In Australia and the United Kingdom the same parasite is less significant in cattle but is a problem in sheep. Most parasites have complex life cycles coordinated with climatic conditions. This seasonal incidence of parasites is a very important factor to consider when planning a total parasite control program. For that reason the accompanying charts show the approximate variations in activity of some of the major parasitic groups in different climatic regions.

Significant concentrations of cattle

Internal organs showing predilection sites