Lakhasly

Importance of Parasitology
12.1 Introduction: The Goodness of Parasites?

Parasites are normally considered unwelcome guests whose presence is invariably harmful. Although, as we have seen in Chapter 1, defining and measuring harm is problematic, it can still appear perverse to consider parasites as beneficial. The term ‘the goodness of parasites’ was initially coined by David Lincicome whilst he was Professor of Zoology at Howard
University, Washington, DC, USA. Lincicome and his co-workers performed numerous experiments using various protozoan and nematode parasites
of rats and mice that are summarised in Lincicome (1971). He observedthat the parasitised animals often performed better in terms of body weight gain, longevity, and certain physiological parameters than those
that were parasite-free. He therefore felt that parasite–host relationshipsshould not be considered one-way and that parasites could (sometimes) provide paybacks to their host. However, some of his conclusions were
criticised for being based on animals fed very specific diets and low levelsof parasitaemia. Nevertheless, it is increasingly apparent that parasites sometimes prove useful to their hosts and that low parasite burdens may help maintain a healthy immune system. In some cases, a parasite may even contribute to its host’s survival during difficult environmental
conditions. For example, the rickettsia Anaplasma phagocytophilum enhances the survival of its tick vector, Ixodes scapularis, by inducing the expression of a gene that codes to produce antifreeze glycoproteins (Neelakanta et al. 2010). Consequently, infected ticks survive the harshwinters of the Northeast and Upper Mid-West of the United States of
America better than those that remain uninfected. Anaplasma phagocytophilum is an intracellular parasite that infects humans, dogs, and various other mammals. It causes lameness and flu-like symptoms, and infections are sometimes mistaken for Lyme disease. Another instance of

parasites enhancing the survival of their host is that of worker caste ants of
Temnothorax nylanderi infected with the larval stage of the
tapeworm Anomotaenia brevis (Beros et al. 2021). In this case, infected ants often live considerably longer than those that are uninfected. This is probably because the larvae induce changes in gene activity. Longevity is probably further facilitated by changes in the behaviour such that infectedants remain within their nest rather than venturing outside. These
changes probably benefit the tapeworm because the definitive hosts are woodpeckers that drill into ant nests to feed on the ants within. Therefore,

the longer an infected ant remains within its nest, the better the chancesare that it will be eaten by a woodpecker.
Within this chapter, we will consider some of the ways in which parasitesare useful to us. For example, some helminths have potential for the
treatment of certain autoimmune diseases whilst blowfly larvae andleeches are commonly employed in biosurgery. Parasites are also
potentially useful sources of novel pharmacologically active substances. Within agriculture, parasites are used in the biological control of pest invertebrates and plants. Parasites have even been implicated in criminalinvestigations and some workers have seen fit to coin the term ‘forensic parasitology’. Thankfully, parasites have not proved particularly useful as biological warfare agents although that is not for want of people trying.