In this chapter, we introduce the concept of parasitism as a lifestyle and explain why it is such a difficult term to define. We also introduce some of the terms commonly used by parasitologists. Like all branches of science, parasitology uses specialist terms such as ‘intermediate host’, ‘definitive host’ and ‘zoonosis’ that one must understood before one can make sense of the literature. We explain why the study of parasites is so important and why parasitic infections will remain a problem in human and veterinary medicine for many years yet to come.

1.2 Animal Associations

All animals are in constant interaction with other organisms. These interactions can be divided into two basic types: intra‐specific interactions and inter‐specific interactions.

Intra‐specific interactions are those that occur between organisms of the same species. They range between relatively loose associations such as those between members of a flock of sheep and highly complex interactions such as those seen in colonial invertebrates. For example, the adult (medusa) stage of the Portuguese man o’ war ‘jellyfish’ (Physalia physalis) may appear to be a single organism but is actually composed of colonies of genetically identical but polymorphic individuals. These colonies divide labour between themselves in a similar manner to that of organ systems within a non‐colonial organism. For example, some colonies are specialised for reproduction, whilst others are specialised for feeding. The term ‘jellyfish’ is in inverted commas because although P. physalis superficially resembles a jellyfish and is a member of the Phylum Cnidaria, it is taxonomically not a true jellyfish. Instead, it belongs to the order Siphonophora within the class Hydrozoa. The true jellyfish belong to the Class Sycphozoa within which there are several orders but in all of these, the medusa stage is a single multicellular organism.

Inter‐specific interactions are those that take place between different species of organism (Figure 1.1). As with intra‐specific interactions, the degree of association can vary between being extremely loose and highly complex. Odum (1959) classified these interactions on the basis of their effect on population growth using the codes ‘+’ = positive effect, ‘−’ = negative effect, and ‘0’ = no effect. This leads to six possible combinations (00, 0−, 0+ etc.), and these too can be broken down into further subdivisions. Some authors also include a consideration of the direction and extent of any physiological and biochemical interactions between the two organisms. Many terms have been suggested to compartmentalise these interactions (e.g., phoresis, mutualism, predation), but these are merely convenient tags, and they cannot be defined absolutely. This is because there is a huge diversity of organism interactions, and even within a single interaction there are many variables, such as the relative health of the two organisms, that will determine the consequences of the interaction for them both. It is therefore not surprising that there is a multiplicity of definitions in the scientific literature, and it is not unusual for two authors to use different terms for the same type of interaction between species. In this section, we will discuss symbiosis, commensalism, phoresis, mutualism and finally parasitism, with some examples of each.

Photo depicts a horse co-operating for mutual benefit.

Figure 1.1 Different species will sometimes co‐operate for mutual benefit.

1.2.1 Symbiosis

The term symbiosis derives from the Greek συμβίωση and is usually translated as ‘living together’. It was originally used in 1879 by Heinrich Anton de Barry to define a relationship of ‘any two organisms living in close association, commonly one living in or on the body of the other’. According to this definition, symbiosis covers an extremely wide range of relationships. Some authors state that both organisms in a symbiotic relationship benefit from the association (i.e., it is [++]) although this is clearly a much more restrictive definition, and it is more appropriately referred to as mutualism. However, some authors consider symbiosis and mutualism are synonymous – this only adds to the confusion. For the purposes of this book, we will keep to de Barry’s original definition.

1.2.1.1 Symbionts

Strictly speaking, a ‘symbiont’ is any organism involved in a symbiotic relationship. However, most scientists tend to restrict the term to an organism that lives within or upon another organism and provides it with some form of benefit – usually nutritional. The association is therefore referred to as a host: symbiont relationship and most symbionts are microorganisms such as bacteria, algae, or protozoa. Where the symbiont occurs

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