The Blastocystis-colorectal cancer hypothesis: correlation is not causation [version 2; peer review: 2 approved, 1 approved with reservations]

Although superficially persuasive, claims suggesting a causal link between Blastocystis and colorectal cancer (CRC) lack robust scientific support. As Blastocystis is the most common gut protist found in human populations globally, its detection in CRC patients is unsurprising and does not imply pathogenicity. Current claims championing a causal role for Blastocystis in CRC are based on speculative correlations, a single poorly controlled animal study, and inconsistent subtype associations. We argue that linking Blastocystis to CRC is premature, misleading, and may give rise to unnecessary concern in patients that are colonised by or test positive for Blastocystis. We emphasise the need for rigorously designed investigations to establish causal roles for any microorganism in disease and the importance of conclusions being based on solid evidence, particularly in matters of public health.