Three serotypes of Norwalk-like virus demonstrated by solid-phase immune electron microscopy

Solid phase immune electron microscopy (SPIEM) was used to investigate the serological differences between Norwalk-like virus (NLV) strains from five different outbreaks within the United Kingdom. The existence of two previously demonstrated serotypes, Lewis et al. (Journal of Clinical Microbiology 26:938-942, 1988), was confirmed by the use of whole convalescent sera and purified IgM. A third serotype was found to be the agent of two recent hospital outbreaks and could similarly be typed by use of whole sera or pure IgM. Paired sera were available for two of the three serotypes and demonstrated rising antibody levels. These antibody rises were also specific for the infecting serotype. However, two serum pairs from a later outbreak gave antibody rises to all three serotypes, although much higher counts were produced with the infecting serotype. SPIEM is a useful method for distinguishing NLV serotypes and can also be used to detect specific IgM and to demonstrate seroconversion. Cross-reacting antibodies, possibly anamnestic in origin, can occur after natural infection and could cause confusion in typing virus unless further evidence of the identity of the infecting agent is obtained.