The amoeba, Entamoeba coli (not to be confused with the gastro‐intestinal bacterium Escherichia coli, which is also abbreviated to E. coli) is a common commensal that lives in our large intestine. Unlike its pathogenic cousin, Entamoeba histolytica, E. coli feeds on bacteria and gut contents and does not invade the gut mucosa or consume red blood cells. Therefore, E. coli is of little interest per se, although a study in Mexico suggested an association between moderate‐heavy infestations and childhood obesity (Zavala et al. 2016). The most important feature of E. coli is that its morphological similarity to E. histolytica means that one must be careful to distinguish between the two species in microscope surveys of faecal samples.
1.2.3 Phoresis
The term ‘phoresis’ derives from the Greek verb ϕέρω (‘phero’) meaning to bear/carry. This association involves one species providing shelter, support, or transport for another organism of a different species and may be temporary or permanent. For example, apart from during their first instar, the larvae, and pupae of the blackfly Simulium neavei attach themselves to the outer surface of freshwater crabs. The larvae feed by filtering out phytoplankton and detritus from the water and the crabs act as a firm yet mobile substrate on which to attach. An appreciation of this association is important because adult S. neavei are vectors of the filarial nematode Onchocerca volvulus that causes ‘River Blindness’.
1.2.4 Mutualism
Mutualistic (Latin, mutuus meaning ‘reciprocal’) relationships are those in which both species benefit from the association in terms of their growth and survival (++). Some authors further restrict the definition to one in which neither partner can live on its own, whilst others are less prescriptive. The association between Wolbachia bacteria and O. volvulus is clearly mutualistic. The bacteria live within the cells of the reproductive tissues and hypodermis in the adult female worms and provide them with essential metabolites. In the absence of the bacteria, the worms cannot establish themselves in their host and grow and adult females become infertile. The bacteria are therefore a potential target for the chemotherapy of filarial nematode infections (Jacobs et al. 2019; Taylor et al. 2019).
Whether the relationship between the Cnidarian Hydra viridissima and its algal partner Chlorella is mutualistic depends upon the strictness of one’s definition. Hydra viridissima can grow and reproduce in the absence of their algal partner, but it is uncertain whether the strains/species of Chlorella associated with H. viridissima can survive independently. The algae live within vacuoles in the endodermal cells of the Hydra and thereby impart it with its characteristic green coloration. Whether this provides camouflage that is beneficial is uncertain. When the Hydra reproduces by budding, its algal partner is passed on to the offspring; the algae are not essential to the budding process, but H. viridissima seldom undergoes sexual reproduction if the algae are absent. Experiments in which the algae are removed from the Hydra by exposure to high light intensities (Habetha et al. 2003) indicate that the nature of the relationship depends upon the environmental conditions. Like other Hydra species, H. viridissima obtains its food by capturing prey on tentacles that are armed with nematocysts, whilst the alga carries out photosynthesis and releases the sugars maltose and glucose‐6‐phosphate that can potentially be used by H. viridissima. If there is suitable illumination and plenty of prey for the Hydra, the growth of H. viridissima with and without algae is similar. This indicates that, under these conditions, the sugars released by the algae have little importance for the Hydra. If, however, there is illumination but no food for the Hydra, then those lacking algae die after a few weeks, whilst those containing algae shrink but can survive for at least 3 months and commence feeding again if presented with food. Therefore, the symbiotic algae play an important role in the survival of H. viridissima whose normal food supply is low/absent. By contrast, if H. viridissima are kept in the dark but with plenty of prey available, those lacking algae grow much better than those containing them. Furthermore, the algal population declines by about 60% although they are not lost entirely and the H. viridissima remain pale green. This indicates that under these conditions, the algae receive nutrients from the Hydra to such an extent that the relationship changes from mutualism to one akin to parasitism.
1.2.5 Parasitism
Parasitism is a surprisingly difficult term to describe, and there are numerous definitions in the literature. We have adopted the definition that: ‘parasitism is a close relationship in which one organism, the parasite, is dependent on another organism, the host, feeding at its expense during the whole or part of its life (− +)’. Parasitism is frequently a highly specific relationship that always involves a degree of metabolic dependence of the parasite upon its host and often, though not always, results in measurable harm to the host. The association is usually prolonged, and although it may ultimately result in the death of the host, this is not usually the case. It is therefore distinct from predation in which the predator usually quickly kills and consumes its prey. However, owing to the complexities of animal relationships, there are always ‘grey areas’ in which any definition starts to become unstuck. This is particularly apparent in the case of animals that feed on blood. Mosquitoes and tsetse flies are not considered parasites because they only feed for a few seconds or minutes before departing. By contrast, hookworms and crab lice are parasites, because they live in permanent associated with their host. Blood‐feeding leeches and lampreys, however, are free‐living organisms that attach to their victim for several hours whilst taking a blood meal; some authors consider them parasites, whilst others define their feeding as a type of predation.
From Welcome Guest to Villain: The Derivation of the Term ‘Parasite’
The word ‘parasite’ derives from the Greek παρά (‘para’) meaning ‘beside’ and σῖτος (‘sitos’) that means ‘food’. In Ancient Greece, the term ‘parasite’ had religious connotations and nothing to do with infectious organisms. According to a stone tablet discovered in the temple of Heracles (Hercules) in Cynosarges, the priest was required to make monthly sacrifices in the presence of parasites who were to be drawn from men of mixed descent. Declining a request to act as a parasite was a punishable offence. (Cynosarges was an area near to the city walls of Athens. In addition to the temple there was also a gymnasium, and it was here that the Cynic philosophers taught.) Subsequently, the word came to mean someone who shared one’s food in return for providing amusement and flattery. The ‘parasitus ridiculosissimus’ was a popular character in Greek and early Roman comedies and they even had joke books to help them should they run out of witticisms. The greed of the parasite was a constant source of fun for dramatists, and he was often given crude nicknames such as ‘little brush – because he swept the table clean’. Double entendres were as popular over 2000 years ago as they are today and the Latin for little brush ‘peniculus’ is also a diminutive for a penis (Maltby 1999).
An obligate parasite is one that has no alternative but to develop as a parasite of its host. On the other hand, a facultative parasite can develop as a parasite or a free‐living organism depending upon the circumstances. For example, the larvae of the warble fly Hypoderma bovis must develop as parasites of cattle and are therefore obligate parasites. By contrast, the larvae of the blowfly Lucilia sericata are facultative parasites. This is because if the female fly lays her eggs upon a live sheep, the larvae will feed on living tissue and therefore be parasites. Conversely, if she lays her eggs on a dead sheep, the larvae will feed as free‐living detritivores. Similarly, the amoeba Naegleria fowleri can live as a free‐living organism in ponds and lakes but if it enters the nostrils of someone swimming in the water, then it can become an opportunistic parasite and infect their brain.
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