you're reading...

Human impacts

Oysters going viral

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

Figure 1: One way to contract the human norovirus is by consuming raw oysters that have been exposed to raw sewage, often caused by overflow events after large storms. Credit: Wikimedia https://upload.wikimedia.org/wikipedia/commons/3/37/Oysters_p1040741.jpg

Fig. 1: One way to contract the human norovirus is by consuming raw oysters that have been exposed to raw sewage, often caused by overflow events after large storms. Credit: Wikimedia

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

Figure 2: A scanning electron microscope image of the norovirus, which is one culprit of the stomach flu. Each norovirus cell ranges in size from 26 to 35 nanometers, aka, really small! Credit: Public Domain Image http://www.public-domain-image.com/free-images/science/microscopy-images/norovirus-infection/noroviruses-are-a-group-of-viruses-that-cause-the-stomach-flu-or-gastroenteritis-725x500.jpg

Fig. 2: A scanning electron microscope image of the human norovirus, which is one culprit of the stomach flu. Each norovirus cell ranges in size from 26 to 35 nanometers, aka, really small! Credit: Public Domain Image

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 Findings

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.

Figure 3: Oysters held at cooler temperatures had a slower clearance rate of the human norovirus. Credit: Flickr (https://c2.staticflickr.com/8/7032/6755685221_f717c1e69f_b.jpg)

Fig. 3: Oysters held at cooler temperatures had a slower clearance rate of the human norovirus. Credit: Flickr

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.

The Significance

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.




No comments yet.

Post a Comment


  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com