Cancer-causing pathogens: an update

When the Scottish pathologist William Russell published his article “The Parasite of Cancer” in The Lancet over a century ago, it was met with complete incredulity by his medical colleagues who ‘knew’ that cancer was a non-communicable disease. Indeed it was not until the latter half of the 20th century, when the link between Epstein-Barr virus and Burkitt’s lymphoma was elucidated, that medical scientists finally gave some credence to Russell’s claim and conceded that there may be a very few cancer-causing pathogens. In the fifty years since then there has been a steadily expanding number of different viruses, bacteria and parasites found to cause cancer, the most serious in terms of numbers of cases being some of the Human Papilloma Viruses (cervical and anal cancer), Hepatitis B and C viruses (hepatocellular carcinoma) and Helicobacter pylori (gastric cancer). Such organisms can evade the host’s immune system, establish persistent infections of many years duration and ultimately trigger abnormal cell growth followed by tumour development. An article published a couple of years ago in The Lancet Oncology estimated that globally 16.1% of new cancer cases were due to infections.
Two years on this estimate might well be higher. The incidence of oropharyngeal cancers in developed countries had begun to decline with the decrease in tobacco use (smoking is a major risk factor), but this decline has not been sustained. Much current research is now being directed towards oropharyngeal cancers and HPV infection. Recent data suggest that over 40% of cases in Europe are attributable to this virus, and robust data from the Danish national database for head and neck cancers reveal an alarming 12-fold increase in oropharyngeal cancer in Denmark during the past 35 years, with HPV-positive disease increasing from 37% to 74% of cases during this period. Recent studies suggest that HPV may account for up to 80% of oropharyngeal cancer cases in North America. And now a team from the University of California has suggested that there may be a link between the common sexually transmitted parasite Trichomonas vaginalis and aggressive prostate cancer. It is known that this parasite can colonise the prostate, and the team found that it secretes a protein very similar to human macrophage migration inhibitory factor (HuMIF), a proinflammatory cytokine that is elevated in prostate cancer and that triggers inflammation and cell proliferation. Of course more work is needed to establish this and similar links between pathogens and cancer, but if vaccines and antimicrobial drugs could reduce the world’s cancer burden, then surely such research is crucial.