Organisms that live in association with another animal are usually referred to as a host and symbiont, and the relationship is referred to as symbiosis . In the case of wildlife disease, the microparasites (e.g., viruses, bacteria, and fungi) or macroparasites (e.g., protozoa, helminths, and arthropods) are the symbionts , and the wildlife (but in some cases livestock and humans) are the fi nal or defi nitive hosts. In the examples here, arthropods are often intermediate hosts. The nature of symbiosis varies, with the most common types:
• Mutualism . The host and symbiont are dependent on one another, and the relationship is mutually probeneficial. The microbial fl ora found in the digestive system of ruminants is a good example of mutual dependency.
• Commensalism . The host provides the habitat and food for the symbionts, which live without benefit or harm. In this case, the symbionts are dependent on the host, but the host does not depend on the symbiont. Many microbial parasites of wildlife have co - evolved with their normal hosts and cause no measurable harm, so under normal conditions they qualify as commensals. However, this relationship can shift when the host is stressed, and under these conditions may cause injury.
• Parasitism . The host supplies physiological support for the symbiont by providing habitat and sustenance, and often transport, and the symbiont is harmful to the host. The symbionts discussed here are parasites of wildlife to some degree, and sometimes to livestock, pets or humans. Their effect on wildlife may be severe or mild, and sometimes nearly imperceptable. There are several forms of parasitism, with parasitism classifi ed according to where the parasites are found, their temporal occurrence, and their relationship with their host. Some major forms of parasitism include:
• Ectoparasites . These organisms live on the external surface of the host, or cavities that open directly onto the surface. Foremost among the ectoparasites are the arthropods, such as lice, mites, and ticks.
• Endoparasites . These live within the body of the host, including the digestive system, lungs, liver, tissues, cells and freely in the body cavity. Viruses, bacteria, fungi, protozoa, tapeworms, and nematodes are good examples of endoparasites.
• Temporary parasites. These visit the host only briefl y, usually for food. Bloodsucking arthropods are a good example of such parasites.
• Stationary parasites . These spend a definite period of development in association with the host, either on or in it. Some are periodic parasites , leaving to spend another portion of their life in a non - parasitic mode. Examples of this include ticks and botflies. Others are permanent parasites , spending all of their life on a host except for a brief period when transferring from host to host. The viruses, bacteria, protozoa and helminths are examples of this (although in some cases they may persist for long periods, in the absence of a suitable host, in a resting stage).
• Incidental or aberrant parasites . These are parasites that occur only occasionally in a host, usually because there are behavioral or ecological barriers that keep the parasite from the prospective host.
• Obligate parasites . These parasites cannot continue with their life cycle unless they have access to a certain host (or hosts). Most of the parasites discussed here have a certain host or a restricted range of hosts to which they must gain access.
• Facultative parasites . These are parasites that do not require a certain host, but sometimes are found in association with it.
Parasites display many adaptations that make it possible to have a symbiotic relationship with their host. Although they have the same basic nutritional requirements as the host, the symbiont must have adaptations that allow them to enter or attach to a new host after they have escaped from their old host. The tarsal adaptations of lice that allow them to cling to the body hairs of their host are a readily visible adaptation, as are the hooks and suckers of helminths living in association with the host ’ s digestive tract. The symbiont must also escape the host, or at least its progeny must escape, in order to fi nd new hosts. The avenue of escape may be direct , as when eggs, immature stages of nematodes, or body segments (proglottids) of tapeworms are released from the host’s anus. Escape may also be indirect , as when an arthropod feeds on the blood of a host, takes up the symbiont, and transports it. Symbionts escaping both directly and indirectly may have one or more additional hosts that provide transport, maintenance, or an opportunity for propagation.
Organisms with direct escape from the old host and direct transmission to the new host can be said to have a direct life cycle . Those with indirect escape and indirect transmission can be said to have an indirect life cycle . The latter condition is more complicated, as host, symbiont, vector, and environment must all coincide for infection and development to occur. Here we are concerned mostly with disease organisms with an indirect life cycle, and specifically those that involve arthropods.
