stay in association with their host for the duration of a life cycle stage, and therefore, having located and infected their host, the need for sensory apparatus and locomotion are reduced because the parasite has access to a guaranteed food source. This guarantee also means that the parasite does not have to extract as much energy as possible from each ‘unit of resource’. Instead, it can afford to be wasteful, and many parasites have reduced metabolic pathways. Furthermore, there is no need to lay down metabolic reserves beyond those required for the next life cycle stage. Parasites rarely need well‐developed food gathering apparatus and, in some cases, such as the tapeworms, they have dispensed with a mouth and gut altogether, relying on nutrients being absorbed across the body wall.
Because parasites live within or upon their host, they have less need to maintain body surfaces and behaviours that protect them from desiccation, heat, cold because this is done by the host. Similarly, the parasite is to a large extent protected from predators and pathogens, because these must overcome the host’s immune system before locating the parasite. Even ectoparasites receive protection to some extent because hosts cannot always distinguish between a predator attempting to take a bite out of them from an animal solely interested in removing a flea or louse.
A parasite will be transported wherever the host goes and therefore the limits of its dispersal depend upon the dispersal powers of its host, coupled with whatever other special needs the parasite must complete its life cycle (e.g., the presence of a suitable vector or environmental conditions). Consequently, a parasite does not have to devote energy to dispersal.
Table 1.1 Summary of advantages and disadvantages associated with the parasite lifestyle.
| Advantages | Disadvantages |
|---|---|
| Once host located, no need for further searching | Extreme host specificity can increase vulnerability to extinction |
| Food permanently available | |
| Limited requirement for complicated food capturing mechanisms | Must locate at optimal site on/in host to ensure food/survival |
| Reduced need for food processing | |
| Protection from environmental extremes | Must adapt to host’s internal physiological environment (internal parasites only) |
| Protection from predators and diseases | Must overcome host’s immune defences |
| Reduced need for dispersal because host (+ vector) carries the parasite. | Spread limited by host’s geographic range |
| Can devote larger proportion of energy intake to reproductive output than a free‐living organism | Transmission can be extremely risky and most offspring die before establishing in a new host |
If the benefits of parasitism are so enormous, this therefore begs the question why there are not more highly specialized parasites and why parasitism tends to be extremely common among some groups of organisms but rare among others. For example, there are comparatively few parasitic higher plants, Lepidoptera, or vertebrates.
Any would‐be parasite must first overcome the putative host’s immune defences and adapt to its internal physiological environment: this involves many physiological modifications, and therefore most parasites are host specific. However, host‐specificity places the parasite in a difficult situation because its existence then becomes dependent upon that of its host. Should the host become extinct, then its parasites will follow suit unless they are able to infect other organisms. Furthermore, for the individual parasite, finding hosts is seldom easy. Although many parasites produce huge numbers of offspring, the chances of any one of them managing to locate a suitable host, establishing an infection, and reproducing successfully are extremely small. The advantages and disadvantages of the parasite lifestyle are summarised in Table 1.1.
1.7 The Economic Cost of Parasitic Diseases
The morbidity (illness) and mortality (death) associated with parasitic diseases causes financial losses to both an individual, their family, and to the wider society. These losses divide into the direct costs and indirect costs, and these are used in ‘cost‐of‐illness’ studies to prioritise healthcare funding decisions (Onukwugha et al. 2016). The direct costs include factors such as the costs of diagnosis and treatment. They are therefore relatively easy to identify and calculate because they consist of purchase costs and wages. By contrast, the indirect costs are much more wide‐ranging and nebulous. For example, they include the costs associated with the infected individual’s inability to work or reduced efficiency/productivity. They also include wider and often unappreciated costs that are borne by the family and/or the community. For example, the death of someone results in their family incurring the funeral costs (which can be considerable), as well as debilitating psychological stress that may impair their ability to work. Because most parasites cause chronic infections that persist for months or even years, the indirect costs associated with them often exceed the direct costs. For example, a study in China found that one case of malaria cost $US 239 (1,691.23 Chinese Yuan) of which the direct costs constituted 43% and the indirect costs 57% (Xia et al. 2016). Furthermore, the costs were equivalent to 11% of a household’s income. Similarly, in southern India, lymphatic filariasis costs in the region of US$ 811 million per year and cause productivity losses as high as 27% in the weaving sector (Ramaiah et al. 2000). Parasitic diseases that cause disfigurement often results in social exclusion that further traps the sufferer in poverty and mental ill health. People suffering from lymphatic filariasis can become so isolated that they will not venture out to seek freely available treatment at government clinics, let alone to look for paid employment (Wijesinghe et al. 2007). Although it is not a financial calculation, experimental studies indicate that for wild animals living communally, it is also the indirect costs of parasitism that impact most upon the group (Granroth‐Wilding et al. 2015).
For domestic animals, there are the direct costs of diagnosis and treatment along with mortalities but the losses that result from lost productivity (e.g., milk yield, live weight gain) and/ or work capacity (e.g., draught oxen, camels, donkeys) are much greater. Unfortunately, the calculation of losses associated with parasites in the agricultural industry is problematic, and there is a lot of variation between individual farms. In addition, published figures can rapidly become out of date through currency fluctuations, changes in farming practices and the value of stock (amongst many other factors). Therefore, we provide just a few figures to illustrate the potential of parasites to cause financial losses. In the United Kingdom, gastrointestinal parasitic infections in lambs are estimated to cost the British sheep industry ~£84 million per year (~USD$ 102.4 million); the costs associated with infections in breeding ewes are not known but the combined figure would obviously be much higher (http://beefandlamb.ahdb.org.uk/wp‐content/uploads/2013/04/Economic‐Impact‐of‐Health‐Welfare‐Final‐Rpt‐170413.pdf). Brazil is a much larger country with a huge cattle industry, and the financial impact of parasitic diseases is correspondingly massive. They are estimated to cause losses of approximately US$13.96 billion per year; gastrointestinal nematodes are responsible for ~51% of these losses and the tick Rhipicephalus microplus a further 23% through direct effects and as a vector of other parasites (Lopes et al. 2015a). In the United States, the protozoan parasite Neospora caninum is estimated to cause in the region of US$ 546 million per annum in the dairy industry alone. The losses it causes in agriculture on a worldwide basis could be as high as US$ 2.38 billion per
