Puzzling and ambivalent roles of malarial infections in cancer development and progression
The latest Parasitology Paper of the Month is “ Puzzling and ambivalent roles of malarial infections in cancer development and progression” by Eric Faure.
Many reasons were at the genesis of this article. The first one was that, even if they remain relatively poorly understood, the anti-carcinogenic mechanisms involving eukaryotic parasites, including malarial parasites (Plasmodium), may open future pathways for new therapeutic approaches to treat cancers and to better understand mechanisms of tumour induction. The second one was that the old thought suggesting an inverse correlation between malaria and cancer may be reassessed at the light of the most recent molecular data. The third and last one was that I wanted to bring new arguments against the traditional Manichean view of the parasitic world.
In this review, the most relevant studies showing pro- or anti-carcinogenesis roles of malarial infection have been analysed. To date, Plasmodium spp. appear to be the best example of the ambivalent roles of a eukaryotic parasite infection in cancer development and progression. The pro-carcinogenesis role of malarial infections seems to be primarily indirect (via virus reactivation). As for the plasmodial anti-carcinogenic role, malarial-induced fever alone can play a positive role; however, experiments using mouse models have provided evidence that infections by Plasmodium can suppress tumour growth via the induction of both innate and adaptive anti-oncogenic responses.
Since at least the Hippocratic Corpus, dated back to the 5th-4th centuries BC, it was believed that malaria (the “intermittent fevers” reported by ancient literature) might be beneficial to patients suffering from other diseases (but not cancers). Approximately two millennia later, European physicians were convinced of an inverse correlation between malaria and cancer. Their conclusions were based on case series and empirical observations without statistical rigor; furthermore, biases were numerous as the low life expectancy in malarious areas. However, the large number of citations issuing from different regions suggested that this old thought should not be rejected outright. In the same way, the supposed therapeutic value of malarial infection against some bacterial infectious diseases, including syphilis, often evoked until the Second World War, has been confirmed by modern science; indeed, the relative effectiveness of malaria against these diseases is principally attributable to the increase in body temperature.
Parasites are generally only known for their pathogenic impact, but some of them can be beneficial to their hosts. For example, there is the now well known inverse association of many infections with autoimmune and allergic disorders. Eukaryotic parasites exhibit anti-tumour potential and some of them, other than malarial parasites, might also be associated with the induction of oncogenesis. Studies of these last infectious agents present a triple interest. Indeed, this would allow:
- to develop new ways to fight against these parasites,
- to understand possibly new or unrecognized processes of carcinogenesis, and
- to lead to future pathways to develop novel strategies for anti-cancer therapies, based on anti-tumourigenic mechanisms involving parasites.
Read the paper “Puzzling and ambivalent roles of malarial infections in cancer development and progression” in full for free until 31st October 2016.