Parasitism is part of a spectrum of intimate zoological relationships between unrelated organisms which includes:

1. Commensalism: two species living together for the benefit of one or both, but without detriment to either party, and without any metabolic dependence (e.g. cattle egrets and cattle).
   
2. Symbiosis: two species living together, each dependent on the other for their mutual well-being and survival (e.g. cellulose-digesting organisms in the caecum of a horse).
   
3. Parasitism: two species living together, where one of the pair (the parasite) is living at the expense of the other (the host).
   
4. Parasitoidism: two species living together as in parasitism except that the host invariably dies (or is at least rendered incapable of functioning) once the parasitoid has extracted the sustenance it needs for that stage of its development. Familiar examples include parasitoidal wasps used in horticulture that lay their eggs on or in other insects to provide a food-source for their larvae.

Parasitism implies nutritional dependence on the host for at least part of the life-cycle. It also involves a high degree of specialised adaptation as the animal body is not a passive ecological niche (like a rotten tree-trunk harbouring beetles, for example) but is responsive and hostile to foreign invasion. A parasite must be able to overcome host defences and evade immunological attack. Mechanisms must also be in place to ensure transfer of infection, both geographically from host to host (‘horizontal transmission’) and temporally from generation to generation (‘vertical transmission’). This often entails an intricate integration of the life-cycle of the parasite with that of its host.

Parasites can themselves be victims or beneficiaries of invading organisms. Fleas, for example, are exploited by larval stages of both tapeworms and nematodes, while the canine heartworm, Dirofilaria, is metabolically dependent on a symbiotic bacterium, Wohlbachia.

The unwise student could approach every parasitic infection as a separate entity, but this would be an enormous task and a very inefficient approach to learning. It would soon become apparent that similarities exist between some diseases and this would prompt the question: ‘what are the common factors?’ So, classification is an inherent attribute of human curiosity. It has been noted already that Veterinary Parasitology embraces at least three types of arthropod, several types of protozoa and at least three types of parasitic worm, and so the value of classifying aetiological agents of disease is already becoming apparent.

Taxonomy is a powerful and essential component of biological understanding, although, from a clinician’s viewpoint, it is a tool rather than an end in itself. Knowledge of the relationship between parasites often allows similarities in life-cycle, epidemiology, pathogenesis and drug susceptibility to be predicted. Thus, if used intelligently, classification provides a valuable framework for learning and reduces considerably the amount that has to be committed to memory. The classification in this book is kept at the simplest level compatible with this objective.

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The identity of every organism is defined by using a combination of its genus and species names. Thus, the protozoan parasite that causes redwater fever in northern European cattle is Babesia divergens, while the related species Babesia bovis and Babesia bigemina cause similar diseases in warmer regions. By international agreement, the ending -osis is placed on a parasite name to indicate the disease caused by that parasite, e.g. babesiosis. By tradition, the ending -iasis is sometimes preferred in human medicine and may occasionally be found in veterinary publications.

It is sometimes useful in Veterinary Parasitology to refer to the common characteristics of a larger grouping of parasites such as a family, which always has a technical name ending in -idae (e.g. the Ixodidae, which is anglicised as ‘ixodid ticks’), or even a superfamily with the suffix -oidea (e.g. the Trichostrongyloidea, which becomes ‘trichostrongyloid worms’).

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Parasites and their hosts have evolved together over many millions of years. Every host is vulnerable to infection by several, if not many, parasitic species. Thus, there are many more parasitic species on this planet than host species! It is not surprising, therefore, that a great diversity of host–parasite relationships exists. These are often amazingly intricate and are part of the fascination of parasitology, as will become apparent when the life-cycles of individual parasites are described in later chapters.

Hosts have evolved many behavioural and other strategies to reduce the risk of succumbing to parasitism. Herbivores, for example, will not eat the lush grass close to a faecal deposit where the greatest concentration of infective worm larvae occurs (the ‘zone of repugnance’). The most powerful form of defence, however, is the immune system. This comprises a battery of chemical and cellular weaponry used to combat invasive organisms. Immune reactions may completely or partially disable the attacker or they may alleviate the clinical consequences of infection.

Ideally, immunity should protect against reinfection after the invading parasites have been eliminated. This is called ‘sterile immunity’. It can last for a lifetime but often wanes with time. Sometimes, however, such protection persists only as long as a few parasites survive to continually boost the immune processes. This is known as ‘premunity’.

In some cases, parasite evasion has gained an evolutionary advantage that renders host immunity relatively ineffective, so the host remains vulnerable despite being repeatedly exposed to infection (e.g. sheep with liver fluke). Some immune reactions directed at a parasite can produce collateral damage to host tissues. Hypersensitivity and allergy are well-known examples.