Parasite biodiversity: A parasitologists perspective

This month is being celebrated as biodiversity month and many seminars, webinars and other events are conducted throughout the educational institutions, particularly the higher educational institutions of J&K highlighting the importance of biological diversity. But ironically parasite biodiversity is not finding a place for deliberations and discussions in these forums, despite the tremendous significance. We know that around 50% of life on this earth is parasitic. Being a student of parasitology, I can’t stop myself from writing this article, emphasising that parasite biodiversity too matters, and deserves attention and conservation in the similar way as is given to other forms of biological diversity.

Parasite is a strategic, smart and prudent natural agent primarily created by nature to balance the energy dynamics of life in varied ecosystems on this earth, though for us they mean noxious, irritating, annoying, pathogens and harmful agents. Parasite evolutionary history enables us to understand that nature has created every form of life on this planet with a reason and purpose to enrich the earth with diversity of species ecologically interlinked and mutually beneficial too. This ecological linkage overall balances and maintains the ecosystem functioning. Parasite is one such biological agent that has been entrusted with multiple ecological roles.

Basically, parasite (either a macroparasite or a microparasite) which is the focus of this article is a biological organism which lives in or on the body surface of other living organism (different types of hosts) for shelter and to fulfil its metabolic and reproductive needs. As students of parasitology we are made to understand that co-evolution of a host and its parasite/s is a stable and specific relation guided by natural selection, however, this relationship is not complete for all parasites, but is in evolutionary transition for many parasites. As laws of ecology guide us to understand that the aim of a parasite is never to harm its host and the resulting pathogenic aspects (if any) of parasitic infections in the host are actually the outcome of the parasite-host relationship manifested through morphological, physiological, biochemical and immunological mechanisms. Thus it keeps the problem open for man to consider whether a parasite is a pathogen or a nature’s connector or balancer of food web dynamics.

However, if can not be denying that alhough parasites are integral components of an ecosystem, but pathogenesis due happen in the host when infected by a parasite species or species of parasites. And as a rule, there is a density dependent ecological principle governing host-parasite relationship which naturally stops the destruction or damage process in the host after a certain tolerance threshold. As a matter of fact, when the density of a host reduces, parasite transmission rates should also drop, thereby when parasite abundance drops, it potentially prevents the host destruction. Then why is man after parasite destruction and elimination rather than looking for their protection and conservation just like other components of biodiversity. What actually prompts man to think of parasite extermination or eradication is in fact the human greed and self-interest. The phenomenon of parasite species eradication is, therefore, an unnatural offshoot of human selfishness.

From the next few lines, let us try to understand the useful aspects of parasites in the ecosystems, thereby their ecological relevance and then to seek justification for the parasite diversity protection, management and conservation.

Parasites function both as predators and preys in an ecosystem, which shows that considerable energy may directly flow through parasites in food webs. In simpler words, it is in the evolutionary interest of the parasite that its host shall thrive not perish, but the host always tries to get rid of the presence of a parasite by a variety of physiological and immunological mechanisms. However, as this relationship progresses, either the parasite evolves to become less harmful for its host or the host evolves to cope with the unavoidable presence of a parasite or both adjust to a mutual living. Second important thing is that the distribution and abundance of parasites in physical space (both in host as well as outside the host) and time and the factors (host, parasite and environmental) ultimately regulate the host-parasite interaction and dynamics. Thus parasite ecology assumes a greater significance and has increased two way curiosity in the minds of biologists, first to understand the environmental impact on parasites, and second the utility of parasites to assess environmental health and pollution.

This should incite our mind to think and know, as why parasites are ecologically important too if they are dangerous and frightening as pathogens. For time being let’s forget the parasitic nuisance and associated problems and recall that parasites with varied host spectrum is such an important ecological and evolutionary adaptation of animal world that it drives the existence of diversified land and aquatic ecosystems. Evidence is accumulating day by day about the beneficial environmental relevance of parasites, because parasites are important components of life on earth in terms of their diversity and beneficial roles in the ecosystem being the part of food chains, biomass, ecosystem detoxifying, host evolution, indicators of environmental health, bio-monitoring, etc. Their significance can be understood by the fact that about 50% of the life is in the form of parasites with great evolutionary and ecological importance rather than pathogenic significance. If latter would have been the main focus of parasites, then the other 50% of non-parasitic life would have seized to exist, but that actually does not happen.

It is unfortunate that despite the tremendous ecological significance of parasite species, man has initiated the process of elimination and eradication of parasites. But the question is, why should man be after the parasite extinctions? Before trying to answer this question we should first answer the second question, is there any example of a parasite in the whole biological world which has caused extinction of its host species ever? In true sense a parasite does only a limited or less damage to the host population before the latter learn and adapt to the ill effects of the former. In other worlds it means that the goal of a parasite is never to kill or permanently harm its host, because the moment a parasite will result in the destruction of its host, it means the end to its own life. Or the parasite could go extinct itself. Another interesting thing about the parasitic world is parasite itself is the host of other parasites, a phenomenon called as hyper-parasitism. In that instance if one parasite is altered or irritated or eliminated it means the destruction of so many other biological objects. The latter can be of very beneficial role at various levels in the food chain and food web because, parasites have very high diversity in the world. For example, Oligochaete parasitic worms feed on trematode parasite hosts that emerge from their infected fresh water snail hosts.

Under natural conditions, the extinction of the host species is not caused by the parasite itself, most of the times these are the events like population bottleneck in the first place that leads to the extinction of the host species (consisting of unhealthy populations, which are ecologically constrained due to low genetic diversity) infected by extremely virulent parasites? Further a species extermination by a natural selection is a normal process. But to do so deliberately merely for human benefit seems to be an extraordinary sin. Some of the most destructive diseases known to mankind are now written in past tense because they’ve been eradicated and are no longer a threat. Small pox and rinderpest also known as cattle plague, is the first animal disease that was ever eradicated. It may be possible to eradicate some of the world’s deadliest parasitic diseases, but should we? The battle to wipe out debilitating guinea worm parasite (Dracanculus medinensis) is ongoing and the target for its eradication is 2030, although the earlier target was 2020 to wipe this disease off the face of the earth.

But the word extinction can be of ordinary meaning for a layman but it sends red waves across the minds of a parasitologist and an ecologist. Eradicating any organism has serious consequences for ecosystems. We have to understand and believe that parasites are not disgusting but are very indispensable in regulating the populations of their hosts and the overall ecosystem. The phenomenon of parasite elimination and eradication will ultimately lead to co-extinction, which means loss or decline of a parasite species results in the loss or endangerment of other species that depends on it. Because organisms are interconnected in ecosystems at energy level, finally it may lead to a potential cascading effects across trophic levels. Simply co-extinction will mean loss of biological biodiversity and irreparable loss of history of co-evolution of species. Therefore, we need to adopt realistic paths in coordination with nature for effective management of parasitic diseases and protection of parasite diversity because parasites are well worth to be protected and not eliminated or eradicated keeping in view their useful ecological services.

The author teaches Zoology at Islamia College, Srinagar.