Scientists have discovered more than 200 known viruses and thousands of previously unidentified viruses in samples of raw sewage from 3 continents, according to a study published today in mBio, the journal of the American Society for Microbiology. Among the viruses identified were several that may have implications for human health.
Only about 3000 viruses are known, although scientists believe there are probably millions. To probe this diversity, an international team of scientists used a new set of techniques known as metagenomics or environmental genomics to investigate the viruses inhabiting samples of untreated sewage from Pittsburgh, Barcelona, and Addis Ababa, Ethiopia. The techniques allow high-volume sequencing of genetic material from the samples, which were prepared to eliminate nonviral sources of DNA such as bacteria. Unsurprisingly, the scientists found a host of plant viruses, likely from foods eaten by humans, as well as viruses known to infect cockroaches, mice, algae, and other creatures that may live in sewer systems.
Overall, the samples proved to be home to the greatest diversity of viruses described to date, according to the authors. This included 234 known viruses from 26 virus families, including 17 viruses that infect humans. Among those found were viruses known to cause gastroenteritis, such as Norwalk virus and the Aichi virus. The sewage samples also contained several recently described viruses, including human bocavirus, which causes respiratory illnesses, and human polyomavirus 6, which has been found in rare cases to cause disfiguring skin disease in immunocompromised patients.
The investigators also discovered a host of previously unknown viruses. One of the most abundant viruses detected was similar to a virus that has been detected in stool samples from patients with hepatitis.
Some of these unknown viruses likely play a role in human health or environmental processes, noted study editor Michael Imperiale, PhD, of the University of Michigan, Ann Arbor, in a statement. Some may cause opportunistic infections in individuals with weakened immune systems, while others may have health benefits. For example, a herpes virus that infects mice offers some protection from bacterial infections. “There’s a theory out there that we may be infected with viruses that don’t cause any disease and may have beneficial effects,” he said.
Imperiale believes that further studies of samples from other environmental sources will identify even more virus diversity. “The ocean is going to top raw sewage by orders of magnitude,” he predicted.