Characterization ofa Variant Strain ofNorwalk Virusfroma Food-Borne Outbreak ofGastroenteritis on a Cruise ShipinHawaii

A gastroenteritis outbreak affecting atleast 217(41%)of527passengerson a cruise ship was causedbya variant strain ofNorwalk virus (NV)thatisrelated tobutdistinct fromtheprototype NV strain. Consumption offresh-cut fruit served attwobuffets was significantly associated withillness (P< 0.01), anda significant dose-response relationship was evident between illness andthenumberofvarious fresh-cut fruit itemseaten. Seven(58%)of12paired serum specimens fromill personsdemonstrated atleast fourfold rises inantibody responsetorecombinant NV capsid antigen. A32-nmsmallround-structured virus was visualized byelectron microscopy in4(29%o) of14fecal specimens, butnone ofthe8specimens thatwere examined byan enzyme immunoassay forNV antigen demonstrated antigen. Four(40%)of10fecal specimens were positive byreverse transcriptase-PCR byusingprimer pairs selected fromthepolymerase region ofNV.Ina 145-bp region, the PCR product shared only72%nucleotide sequenceidentity withthereference NV strain and77%nucleotide sequence identity withSouthampton virusbutshared95%nucleotide sequenceidentity withUK2 virus, a UnitedKingdomreference virusstrain. Inaddition, theoutbreak viruswas serotyped as UK2 virusby solid-phase immuneelectron microscopy. Thegenetic andantigenic divergence oftheoutbreak strain fromthe reference NV strain highlights theneedformore broadly reactive diagnostic assaysandforimproved understanding oftherelatedness oftheNV groupofagents.