For the first time, researchers have identified viruses that naturally infect nematodes in the wild (PLoS Biol. 9, e1000586;2011). Though scientists have extensively studied bacterial infections in the nematode Caenorhabditis elegans, they have only been able to carry out limited studies of antiviral responses in this microscopic worm. Scientists will now be able to use these viruses to study host-virus interactions in nematodes, which could lead to the identification of alternative tactics for fighting off viral infections.

Marie-Anne Félix of the CNRS-University of Paris-Diderot in France and colleagues isolated infected C. elegans and C. briggsae, a related worm, from rotting fruit. The infected worms did not get better after receiving antibiotics. The researchers homogenized tissue from infected worms and filtered this solution, leaving out any particles larger than a virus. These filtered homogenates infected healthy worms, leading Félix and colleagues to suspect that a virus might have been causing the infections.

Further analyses lead to the identification of two novel RNA viruses, one infecting C. elegans (Orsay virus) and the other infecting C. briggsae (Santeuil virus). These viruses are distantly related to known viruses in the family Nodaviridae, which infect fish and insects. The two novel viruses share only about 40% of their genetic material with previously identified nodaviruses. Orsay virus and Santeuil virus share a novel genetic sequence, called ORF δ, which is not found in any of the previously identified nodaviruses. The authors suspect that the protein encoded by ORF δ might play an important role in the viruses' ability to infect or replicate in nematodes.

Researchers can use a process called RNA interference (RNAi) to systematically block the expression of almost every gene in the genome of C. elegans. Some plants and animals use RNAi to target and silence gene expression of invading viruses. Félix and colleagues found that a strain of mutant C. elegans that is deficient in the RNAi pathway accumulated more viral RNA than did the commonly used wild-type strain of C. elegans.

These results indicate that the RNAi pathway likely plays a role in defending worms against Orsay virus infection. Study of the genetic differences between strains of C. elegans that have different levels of susceptibility to infection by the Orsay virus might lead to the identification of genes involved in halting the spread or replication of the virus. Results from such studies could help researchers develop antiviral treatments for humans.