Thermal Inactivation of Human Norovirus Surrogates in Blue Mussels (Mytilus edilus)

Introduction: Mussels are able to filter large quantities of water in order to obtain food present in water. In addition to water, they also retain bacteria and viruses. Thus, mussels are a food that is commonly involved in foodborne norovirus outbreaks. It is therefore advisable to eat mussels only after a heat treatment. However, little is known about the thermal inactivation kinetics of viruses in mussels. Due to the absence of human norovirus infectivity assays, cultivable surrogates such as feline calicivirus (FCV-F9) and murine norovirus (MNV-1) have been used to begin to understand thermal inactivation behavior.

Purpose: The purposes of this study were (i) to determine thermal inactivation kinetics of MNV-1 at 60°C and FCV-F9 at 56°C in blue mussels, and (ii) to compare First-order and Weibull models in describing the data in terms of selected statistical parameters.

Methods: Thermal inactivation was performed using 2 ml vials on the surrogate viruses at titers of 2.5 x 10⁶ (MNV-1) and 7.3 x 10⁵ (FCV-F9) plaque forming units (PFU)/ml. Each treatment was replicated thrice. Inactivation data were fitted to Weibull and First-order models and statistical evaluation and linear and non-linear regression analyses were performed using SPSS Statistical package.

Results: The D-value for MNV-1 at 60°C was 2.48 ± 0.41 for Weibull model and 3.87 ± 1.07 for First-order model.  For FCV-F9, the D-values were 3.68 ± 0.67 and 3.59 ± 0.75 for Weibull model and First-order model, respectively. No difference was found in the D-values using either model (P > 0.05). In contrast, the Weibull model (R² = 0.93-0.99) had higher regression coefficients than the First-order model (R² = 0.84-0.91) for both norovirus surrogates. Thus, the model chosen for calculation of thermal inactivation parameters of norovirus surrogates during thermal inactivation is important.

Significance: Knowledge of the thermal inactivation kinetics of norovirus surrogates will allow development of processes that produce safer shellfish products and improving consumer safety.