CHARACTERIZATION OF A VARIANT STRAIN OF NORWALK VIRUS FROM A FOOD-BORNE OUTBREAK OF GASTROENTERITIS ON A CRUISE SHIP IN HAWAII

A gastroenteritis outbreak affecting at least 217 (41%) of 527 passeng ers on a cruise ship was caused by a variant strain of Norwalk virus ( NV) that is related to but distinct from the prototype NV strain. Cons umption of fresh-cut fruit served at two buffets was significantly ass ociated with illness (P less-than-or-equal-to 0.01), and a significant dose-response relationship was evident between illness and the number of various fresh-cut fruit items eaten. Seven (58%) of 12 paired seru m specimens from ill persons demonstrated at least fourfold rises in a ntibody response to recombinant NV capsid antigen. A 32-nm small round -structured virus was visualized by electron microscopy in 4 (29%) of 14 fecal specimens, but none of the 8 specimens that were examined by an enzyme immunoassay for NV antigen demonstrated antigen. Four (40%) of 10 fecal specimens were positive by reverse transcriptase-PCR by us ing primer pairs selected from the polymerase region of NV. In a 145-b p region, the PCR product shared only 72% nucleotide sequence identity with the reference NV strain and 77% nucleotide sequence identity wit h Southampton virus but shared 95% nucleotide sequence identity with U K2 virus, a United Kingdom reference virus strain. In addition, the ou tbreak virus was serotyped as UK2 virus by solid-phase immune electron microscopy. The genetic and antigenic divergence of the outbreak stra in from the reference NV strain highlights the need for more broadly r eactive diagnostic assays and for improved understanding of the relate dness of the NV group of agents.