Norwalk-like Viruses Associated With A Gastroenteritis Outbreak Following Oyster Consumption

Area of Science:

• Public Health
• Virology
• Foodborne Illness Epidemiology

Background:

• An outbreak of gastroenteritis affected over 200 people in Quebec, Canada, in October 1991.
• The outbreak was associated with the consumption of raw oysters.
• Initial bacteriological analysis of oysters showed low levels of fecal coliforms.

Purpose of the Study:

• To identify the etiological agent responsible for the oyster-associated gastroenteritis outbreak.
• To investigate the antigenic relationship of the causative virus to other known Norwalk-like viruses.

Main Methods:

• Direct electron microscopy of stool samples from affected individuals.
• Immunoelectron microscopy using acute and convalescent sera.
• Solid-phase immunoelectron microscopy for antigenic characterization.

Main Results:

• Electron microscopy revealed 27-34 nm, small, round Norwalk-like viruses in stool samples.
• Immunoelectron microscopy confirmed the presence of antibodies against these viruses in patient sera.
• The outbreak virus was antigenically similar or related to a Norwalk-like virus from a previous Ontario outbreak.

Conclusions:

• Norwalk-like virus was identified as the causal agent of the Quebec gastroenteritis outbreak.
• Contaminated oysters were the likely source of infection.
• The specific source of oyster contamination was undetermined but likely involved proximity to human fecal sources.