Parasitology. Alan Gunn

one would expect it to be common where parasites are abundant and challenge frequent. By contrast, asexual reproduction should be favoured where parasites are absent, or the level of challenge is low. Although there are several instances of exactly this in the literature, they remain remarkably few. The best‐known example is that of the snail Potamopyrgus antipodarum that originated in New Zealand and has since spread to many parts of the world. It exists as sexually reproducing populations, asexually reproducing populations, and mixed sexually and asexually reproducing populations. Positive correlations have been described between the extent of parasitism by parasitic flatworms and the frequency of sexual reproduction. Sexual reproduction is rare where flatworm parasite challenge is low, and conversely, it is common where the parasite challenge is high (Lively and Jokela 2002). Another commonly cited example is that of certain minnow populations living in Mexico (Lively 1996). These minnows exist as both asexually reproducing and sexually reproducing populations, but those reproducing sexually tend to have lower parasite burdens (except where inbreeding has resulted in reduced genetic diversity). Most multicellular parasites reproduce sexually themselves, although some combine it with asexually reproducing larval stages, such as schistosomes and the tapeworm Echinococcus granulosus. Even some parasitic protozoa, such as the trypanosomes, exhibit something akin to sexual reproduction. This suggests that even endoparasites living in protected environments such as the gut or bloodstream of another animal remain vulnerable to infections. However, although there is experimental evidence that parasitism influences the evolution and maintenance of sexual reproduction (Auld et al. 2016), there are almost certainly many other factors involved. For example, sexual reproduction may help protect against transmissible cancer cells (Thomas et al. 2019).

1.6 Parasitism as a ‘Lifestyle’: Advantages and Limitations

      Provided one can get away with it, stealing something is easier than making it oneself or earning money to purchase it. Therefore, it is unsurprising that so many organisms have adopted a parasitic lifestyle to some extent. If one takes the view that the main purpose of an organism’s existence is to transfer as many of its genes as possible into the next generation, then all organisms should maximise their reproductive output. However, an organism must trade the costs of reproduction against other activities such as finding food and then digesting and absorbing it, finding a mate, and protecting itself against competitors, predators, and the environment. By living upon or within a host, a parasite can reduce many of these ‘other costs’ and thereby devote more of its time and energy to reproduction. Most parasites 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