Vibrio Saga, Part II — Managing a Current Threat to the Oyster Industry

This post can also be found on the Environmental Change and Aquatic Biomonitoring (ECAB) Lab website.

This post is continued from Part I of this series on Vibrio parahaemolyticus (Vp). If you’re interested, you could read the first part of this series, which helps frame a lot of the policy discussion in this post.

Vp Policy and What It Means for Oyster Growers

In the simplest terms, there is a limit set on how much Vp can be found in oysters before the CFIA considers them unsafe. This limit is currently set at 100 MPN/g or CFU/mL.¹ MPN/g, or Most Probable Number per gram, is a statistical measurement of bacterial growth based on DNA tests, while CFU/mL, or Colony Forming Units per millilitre, is a measurement of bacterial growth made by actually counting the number of growing colonies on a bacterial culture plate. These two measurements are made to be interchangeable, and the CFIA uses techniques that use both MPN/g and CFU/mL to test for Vp in oysters.²

Image: Matthias M. on Wikimedia Commons. Vibrio parahaemolyticus colonies appear green on specially designed growth plates.

The CFIA, when testing for Vp in oysters, takes several batches of oysters, a batch being 5 or 6. This means that every time a Vp test is done, around 20 or 25 oysters are taken. Each batch of 5 or 6 oysters has the meat and liquor (the ‘brine’ or seawater in the oyster) removed, and it is all blended together and cultured for Vp growth.³ Each batch is cultured independently to make sure that the final result isn’t a fluke. First, the batches are cultured and counted with a method that measures in CFU/mL, which is faster, but less precise. If there is an indication of Vp infection (having looked at the data, this appears to be above 10 CFU/mL or so), then subsequent batches are analyzed with qPCR that measures in MPN/g. This second round of testing takes longer but it is more accurate. This switching of methods raises one main question: are higher test results encountered because the more precise method is revealing that higher number, or is it the effect of switching measurements itself? Because the two methods are made to be interchangeable, though, it is thought that the switching in testing methods is inconsequential. Any test result of 100 or more MPN/g or CFU/mL in any batch is considered a test failure (meaning oysters cannot be sold), even if other batches from the same site, on the same day, test below 100.

The limit at 100 MPN/g is stringent, but infections from Vp in Canada are rare because of it.⁴ Australia and New Zealand also set their limit at 100 MPN/g,⁵ as do many countries globally. Having the same limit as many foreign markets for oysters allows Canadian oysters to be easily exported and pass import regulations, because any oyster that passes Canadian regulations will also pass the others. This all makes perfect sense.

Image: Part of Figure 2 from Archer et al. (2023). Most northerly cases of Vp sibling Vibrio vulnificus by year.

The United States sets its limit at 10,000 MPN/g,⁶ 100 times higher than Canada, but the FDA allows lower jurisdictions to set their own lower limits, whereas the CFIA’s approach is more of a blanket limit. The United States and Canada consider each other ‘partners’ in shellfish trading,⁷ which essentially means that both countries trust that the other country’s regulations are satisfactory and effective in preventing foodborne illness. Though the US regulations do not necessarily align with international regulations, a CFIA audit of FDA Vp control practices found that the FDA’s procedures were satisfactory, so long as oysters exported to Canada meet Canadian guidelines or are labelled as requiring cooking. I’ll acknowledge here that there seems to be at least some degree of tension between growers in the two countries because of the differing limits, which may hinder exports on both sides, when Canadian exporters are closed for exceeding count limits, and US exporters must meet a limit hundredfold lower than they are used to.

With climate change, the waters in Atlantic Canada appear to be becoming more conducive to Vp, and the CFIA is catching Vp outbreaks when the environmental conditions make for good growth. The answer is probably not to revise the Canadian limit higher. This could have health implications – although Vp is rare in the US, infections are more common than in Canada.⁸ It would also harm the ease with which Canadian oysters are exported to the global market, because the regulations would no longer be in line. It is generally not a good look to loosen regulations on a foodborne pathogen, anyways.

If the CFIA keeps the limit at 100 MPN/g, there are a few options to help improve the testing system. A DFO horizontal evaluation of the Canadian Shellfish Sanitation Program, jointly run by the CFIA, DFO, and ECC, finds that, while the program is effective in its goals of minimizing health risks to the public, it is significantly under-resourced to be able to meet current demand, especially with respect to the amount of microbiological/chemical labs and the large amount of coastline that the program covers. So, an expansion of the system to make it more agile in response to Vp outbreaks is logistically unlikely. However, if we can anticipate with a certain degree of confidence when and where the next outbreak is likely to occur in a season, or if there are certain noted Vp hotspots, testing can be streamlined to target these areas and improve the agility of the system. This means that sites that are known to have outbreaks can have those outbreaks caught when they happen, instead of having to wait for the current all-at-once testing to make it to them. It could also mean the quicker removal of closures to producers by giving information about when Vp counts might be expected to fall. The prediction of Vp outbreaks in Atlantic Canada is what I am working on, in order to help the CFIA, oyster growers, and the public.

This issue is one of policy that must consider public health and the interests of the oyster aquaculture industry in the face of a changing climate. It must also consider the global context in which these regulations exist: when an outbreak occurs, who can sell, who can’t, and where. Right now, these regulations are based on alignment with international standards and preventing outbreaks, which are both universally good things, but the global context of Vp is changing. Understanding where, when, and why Vp outbreaks have occurred in Atlantic Canada is an important step in reinforcing the relatively local context of Canadian public health and the interests of the local aquaculture industries.

An Aside: MSX in Eastern Oysters

MSX, or Multinuclear sphere X (X for unknown), is a disease of oysters caused by the eukaryotic parasite Haplosporidium nelsoni.⁹ MSX, unlike Vp, does not pose a threat to consumers of oysters by compromising food safety. Instead, MSX is extremely lethal to oysters. Outbreaks in the past have devastated oyster populations, with mortality rates approaching 100%, for instance in Delaware Bay in 1957.¹⁰ It is clear, though, that MSX, like Vp, is on the rise in Canada. The first outbreak detected in Canada was in the Bras d’Or lakes of Cape Breton in the early 2000s, an outbreak from which the oyster population in that area has yet to fully recover. Prevalence of H. nelsoni, like Vp, is influenced by both water temperature and salinity, and this implies the possibility that climate change is playing a role in expanding the region where H. nelsoni is found northwards to include the Canadian Atlantic region, which previously did not suffer from MSX outbreaks. A new outbreak of MSX has been detected in Bedeque Bay, PEI (first on July 13, 2024),¹¹ an area which has never seen an outbreak before. Since then, the outbreak may have spread to New Brunswick by November 2024. The oysters are susceptible to it, not having experienced this parasitic infection before, and so little is known about the life cycle of H. nelsoni outside of shellfish that it is hard to know how to control it. Governments at various levels have responded to the outbreak by announcing funding for MSX research and holding MSX science summits.

Vp and MSX are not the only threats to oyster populations and the oyster aquaculture industry in Atlantic Canada. They are only two of many organisms – bacteria, viruses, eukaryotic parasites, and more – that are changing in distribution and prevalence across the world, in large part due to the changes in large-scale environmental conditions brought about by anthropogenic climate change. As these effects continue to worsen, which they almost certainly will, it is still unknown how existing threats will evolve, and which of many new threats will emerge. Both wild and farmed oyster populations in Atlantic Canada, and the industries that rely on them, are not just susceptible to the impacts of climate change in the future — they are clearly already feeling its effects.

Notes

1^. CFIA/Health Canada limits.

2^. As per CFIA methods MFLP-39 / BAM Chap. 9 and MFLP-102, whose documentation is available from Health Canada upon request.

3^. Applies to both MFLP-39 / BAM Chap. 9 and MFLP-102.

4^. There were noted outbreaks in British Columbia in 1997 and 2015 that led to increased testing and more stringent regulations.

5^. FSANZ Compendium of Microbiological Criteria for Food (see p. 18).

6^. Table A-5 in Appendix 5 to the FDA’s Fish and Fish Products Hazard Control Guidance.

7^. The US is listed as an approved country for all shellfish here, and Canada is listed as an approved foreign fish producer here.

8^. US numbers (2015), Canadian numbers (2022).

9^. Information from DFO.

10^. Fact sheet from the University of Massachusetts, Dartmouth.

11^. Statement of Detection by the CFIA and additional detections on July 24, 2024.