Detection of Norwalk-like virus in shellfish implicated in illness

In the 1990s, Norwalk-like viruses (NLVs) were identified in patient specim ens as the primary pathogen associated with shellfish-borne gastroenteritis in the United States. Identification of these viruses from implicated shel lfish has been difficult due to inefficient recovery of viruses, natural po lymerase chain reaction (PCR) inhibitors in shellfish, and low virus contam ination. Recent improvements to the method of detecting NLVs in shellfish i nclude enhanced processing of virus and shellfish samples, application of n ested PCR and nucleotide sequencing, and increased knowledge of NLV genetic diversity. Using a newly developed and sensitive method, an NLV G2 strain was identified in 2 oyster samples implicated in a 1998 California outbreak involving 171 cases. NLV capsid primers demonstrated a greater specificity of PCR detection than did polymerase primers. The 175-base viral capsid nu cleotide sequences derived from oysters were 100% identical to those derive d from a patient stool sample. This finding supports the epidemiologic asso ciations indicating that contaminated shellfish serve as the vehicle for NL V transmission.