The Paper: Changsun Choi and David H. Kingsley. “Temperature-Dependent Persistence of Human Norovirus Within Oysters (Crassostrea virginica).”Food and environmental virology 8, no. 2 (2016): 141-147. DOI: 10.1007/s12560-016-9234-8
Human norovirus. Chances are, you or someone you know has been stricken with this incredibly unpleasant gastrointestinal bug. One way humans can contract the human norovirus is by eating contaminated, raw shellfish.
The human norovirus is most often found in human waste. Unfortunately, after some torrential rainfall events, waste water treatment plants can get overwhelmed, resulting in an overflow of raw sewage into the ocean. Shellfish, such as the Eastern oyster (Crassostrea virginica), are filter feeders; so if any human norovirus was in the untreated sewage, those oysters can get contaminated and pass the illness back to humans who ingest the oysters.
Yes, this does seem like a “perfect storm” of events – the norovirus needs to be transferred from sewage runoff, into an oyster then into an unlucky human. However, the human norovirus has been detected in 3.9% of oysters in the United States, 9% in Japan, and a whopping 76% of oysters produced by aquaculture in the United Kingdom. But before you swear off eating oysters, take solace in knowing that the United States does have strict shellfish regulations. Bacteria like E. coli are routinely measured and shellfish beds will be closed down if certain threshold standards are exceeded. Before a shellfish bed can be re-opened for harvest, select oysters undergo a process called depuration. The oysters are held in “clean” water until the bacteria is no longer detected within the oyster’s tissue. This tells you how long it takes an oyster to clear the virus from its system.
But E. coli is a bacteria and the norovirus is, well, a virus. It turns out, not much work has been done in understanding how long it takes a contaminated oyster to purge the human norovirus. The U.S. regulations require that contaminated shellfish beds stay closed for at least 3 weeks after the waters are clean of bacteria…but is 3 weeks sufficient for the human norovirus?
Choi and Kingsley set out to measure the clearance rate of the human norovirus in the Eastern oyster under different temperatures, which could mimic how clearance may vary in different seasons.
The Study Approach
The researchers obtained active strains of the human norovirus by extracting it from voluntary, contaminated human stool samples. Science is not always glamorous! Meanwhile, mid-sized oysters of the same approximate size (about 5-8 grams if shucked) were all harvested on the same day from Cape May, New Jersey and acclimated to norovirus-free water for one week at 15°C.
Oysters were individually contaminated with the human norovirus for four days, then the oysters were put through the depuration protocol (meaning that oysters were held in sterile water without any detectable traces of norovirus). Individual oysters were kept at 7, 15, and 25 °C and were measured for the human norovirus weekly starting from immediate exposure (0 weeks) to 6 weeks, with 3 oysters being measured during each time step for each treatment. The human norovirus was detected by extracting RNA from the oyster (via blood cells and tissue) and amplified the RNA using real-time polymerase chain reaction (PCR).
The human norovirus was detectable after 6 weeks for contaminated oysters held at both the 7 and 15°C treatments but fell below detection limit after 4 weeks for the 25°C treatment. The authors proposed that this observation at 25°C was not simply a result of their small sample size of three oysters per measurement. Instead, it may have been the result of an internal effect within the oyster since there was a big, statistically significant, drop-off in human norovirus detection levels even after one week at 25°C. The oyster’s metabolism likely played an important role in the study since the oysters held at warmer water temperatures were able to clear the human norovirus out of their tissue faster. (In general, metabolism speeds up as temperature rises.) A similar temperature effect on clearance rate (higher clearance rate at higher temperatures) has been observed by other studies in other oyster species. So, clearly the temperature of the water seems to play an important role in the ability of oysters to flush out the virus – colder temperatures could mean lower clearance rates.
However, the researchers do note that detecting human norovirus above their method’s detection limit does not necessarily mean it is still infectious. Laboratory studies, such as this one, also may not necessarily reflect what is happening to the oysters in the natural environment. For example, the contamination method places individual oysters in a small volume of water at a fixed temperature. In nature, there would be hundreds of other variables to consider (such as day/night fluctuations in temperature, the pH of the water, the algal food source available, etc.). Furthermore, the level at which detected human norovirus becomes an issue for human consumption is still a much debated topic.
We often associate the human norovirus with the winter, which this study supports, since this virus can persistent in oysters longer when water temperatures are cooler (lower clearance of the virus). The temperature at which oysters are kept at have a big influence on how long the human norovirus can linger in the oyster’s tissue, thus still be a contaminating risk. Thus, placing a contaminated oyster in sterile waters at or below 15°C may not be as effective as keeping the oyster at a warmer temperature when testing for the human norovirus. The researchers here estimated that depurating oysters at 25°C could actually reduce the risk of human norovirus infection by 12-26%, which I would take any day over contracting the “winter vomiting disease” aka the human norovirus.