First of all, let’s talk a little bit more about viruses. Viruses consist of genetic material (DNA or RNA) surrounded by a protective protein coat and, in some cases, an envelope of lipids. As such, they are considered as the smallest type of infectious agents. They are not able to live by themselves outside a host and they therefore need to infect other organisms in order to be able to replicate themselves. They are able to infect all other living organisms including animals, plants and even microorganisms. Once they have infected a host, genetic material contained in the virus particles can be integrated in the host’s DNA and persists there for many years. Interestingly, some integrated viruses’ DNAs have coevolved many hundreds or thousands years with their host.
Viruses are present everywhere, in all types of living species, including parasites. However, viruses infecting parasites are specific to them, meaning that a virus infecting a parasite cannot infect the human host (of the parasite). Furthermore, viruses infecting parasites are usually quite specific to one parasite’s species and are usually not able to infect other parasite’s species. As a matter of fact, viruses infecting one species of parasite are usually named after it and are numbered according to the sequence of their discovery. For example, Trichomonas vaginalis are infected by trichomonasvirus, Giardia intestinalis are infected by giardiavirus, and so on. Even within the same genus, some parasite species are more prone to carry viruses. For example, it is well known that Leishmania species present in the Americas are frequently infected with viruses while Leishmania species from outside the Americas only rarely carry viruses.
Scientists have known for many years that viruses are able to infect parasites, but new viruses infecting parasites are still discovered nowadays and this research area is currently evolving. As an example, it is interesting to note that a new class of viruses, the megavirus, has been discovered in the last dozen of years. The particularity of these viruses is that they contain a lot of genetic materials coding for up to a thousand of genes (usually viruses code only for a few genes). It is noteworthy that the amoeba Acanthamoeba is the only living species so far known to be infected with this new type of viruses.
It is only very recently that consequences of viruses on the parasite and its host have begun to be investigated. As such, many more research efforts will be required to better understand these complicated relationships. The protozoan parasite Leishmania was the first published case in the scientific literature-linking parasite infected with a virus and the consequences for its host. Studies suggest that a species of Leishmania infected with a leishmaniavirus cause a more severe mucocutaneous leishmaniasis than the same species without a leishmaniavirus infection. The host’s immune system reaction to the virus is thought to be the cause of this more severe disease. In fact, even though the host is not infected by the leishmaniavirus, some viruses may be released within the host when parasite dies. Then, the host’s immune system recognizes the intruder and tries to fight it, causing inflammation that aggravates the leishmaniasis.
It seems that Trichomonas vaginalis infected with viruses also trigger an exaggerate response of the immune system causing more aggressive symptoms. Furthermore, viruses may modify the response of the parasite to the treatment. However, more research is needed to confirm these hypotheses.
According to these very recent studies, the interactions between a virus infected parasite and the parasite’s host seem to be more complex than previously thought. As viruses infect many parasites, it is currently a very interesting research area. However, more efforts will be required to understand precisely these mechanisms and to find the precise roles of viruses in other parasitic infection. The next years will hopefully shed some light on the roles of parasite’s viruses in human infections.
Finally, some very recent studies also suggest that viruses could be used as a novel treatment against parasites. Actually, as viruses infecting parasites are very specific for one parasite species and are usually unable to infect humans directly, it is a very promising therapeutic avenue. Such viruses could specifically kill parasites inside their hosts. However, more research is needed to elucidate issues such as finding a mechanism to infect parasites with a virus while inside its host and alleviate potential side effects for the host.
Source: Ives A et al. (2011). Leishmania RNA virus controls the severity of mucocutaneous leishmaniasis. Science, 331 (6018), 775-8.