Thermal Inactivation of Human Norovirus Surrogates in Oysters Homogenate

Introduction: Human Norovirus (HNV) is considered most frequent causative agent of foodborne diseases in the United States. Raw and undercooked oysters are commonly involved in outbreaks caused by HNV. The FDA recommends that shucked oysters should be boiled for at least three minutes, but it is not clear this thermal treatment can inactivate HNV.

Purpose: The objective of this research is to evaluate whether this recommendation is sufficient to inactivate two HNV surrogates, Murine Norovirus (MNV-1) and Tulane virus (TV) in oyster meats.

Methods: Oyster homogenates were inoculated with ~5 log PFU/ml MNV-1 and ~3 log PFU/ml TV, respectively. The inoculated samples were treated in a circulating water bath maintained at 49, 54, 58, 63, 67°C for different durations. First-order model was used for calculating the D values of MNV-1 and TV at different temperatures. The oyster homogenates were also treated in a boiling water bath for 1, 2 and 3 min.

Results: D-values were 28.17 to 0.88 min for MNV-1, while they ranged from 18.18 to 1.56 min for TV in oyster homogenates treated at 49 to 67 °C. The kinetics demonstrated that TV was much more heat sensitive (P<0.05) compared with MNV-1, which might make it a less comparable virus surrogate as HNV in cooked oyster meats. One minute of boiling water had 0.73 and 1.22 log reduction for MNV-1 and TV, respectively, while 2 minutes of the treatment produced 3.64 log reduction for MNV-1 and below detect limitation (1 log PFU/ml) for TV. After 3 minutes of boiling, the titer of MNV-1 also dropped <1 log PFU/ml.  

Significance: MNV-1 is more heat resistant than TV in oyster homogenates. Boiling the oyster meats for 3 minutes may be effective to inactivate HNV to below 1 log PFU/ml.