Monday, 7 April 2014

Could global warming increase disease in shellfish farms?

Although we currently have a good understanding of how climate change will affect both physical and chemical processes within our oceans, we do not have the same level of knowledge when considering the effects of global warming on microbial agents responsible for disease.  The areas in and around the Irish Sea in particular are an important commercial source for a number of edible shellfish species such as lobster, edible crab and langoustines, and the farming industries in these areas are economically important.  These species are known to be susceptible to disease caused by marine microbes but it is not known to what extent these diseases affect sustainability in many cases, or how future environmental changes could alter these effects.

For diseases like haplosporidiosis, caused by bacteria of the Haplosporidium genus, which effects haemocytes, connective tissue and digestive gland epithelia in crabs and molluscs there is currently very little data, so possible future effects are unknown.  This disease causes high levels of mortality in infected species and this review highlights a definite requirement for research in this area to understand what conditions are required for this bacteria to thrive, or more importantly, not.

Hematodinium spp. are dinoflagellates that are known to be internal parasites affecting the hemolymph of crabs and lobster species, the disease is commonly known as pink crab disease due to the colour displayed in infected individuals.  It was recently discovered that the larvae of the hosts are also prone to infection and that it is not as previously thought, only the adults that can be infected.  This is of particular concern if global warming is to strengthen ocean currents possibly aiding the distribution of larval forms and therefore the parasites they may contain. 

A little more information is available for Vibrio spp. which include both human and shellfish pathogens.  We know that they generally prefer waters with temperatures of 15°C or above and salinities of less than 25ppt, meaning that costal areas and the species that reside there are at particular risk of infection.  The predicted increases in temperature of costal waters in addition to a decrease in salinity caused by an increase in rainfall due to a less stable climate will provide new areas for natural outbreaks.  It is also known that copepods in particular act as a reservoir for species such as V. cholerae and that they could also be expected to increase in numbers due to warming of costal waters.  There have already been cases of increasing sea temperatures being linked to Vibrio outbreaks in areas such as Chile, Peru and the Pacific Northwest of the United States, but again there are huge gaps in our knowledge of how widespread these effects are. 

It is unclear in many of these cases whether changes in environment will cause an overall increase in such diseases or if an increase in one area may be evened out by a decrease somewhere else.  There are certainly opportunities for research into not only the effects of temperature but also salinity and circulation / current patterns.  We must also consider the speed at which marine microbes may be able to adapt and evolve to allow them to survive in these changing conditions, this only increases the requirement to understand what part viruses may or may not play in gene transfer and control of outbreaks.  These problems could keep research facilities busy for many years, but can we get ahead of the game?

Rowley, A. F., Cross, M. E., Culloty, S. C., Lynch, S. A., Mackenzie, C. L., Morgan, E., ... & Malham, S. K. (2014). The potential impact of climate change on the infectious diseases of commercially important shellfish populations in the Irish Sea—a review. ICES Journal of Marine Science: Journal du Conseil, fst234.


  1. A paper I read diseases on macroalgae predicted that disease caused by bacterial pathogens will increase in frequency and severity with global climate change, and this is thought to be from opportunistic bacteria that colonize stressed/weakened individuals. Do you think that this could apply to shellfish populations aswell?

  2. I have also found a paper that looks at multiple types of organisms and the effect of global climate change on disease outbreak (Lafferty et al., 2004). It found that the incidence of disease seems to be increasing in many marine taxa due to climate warming (and greater introduction of terrestrial pathogens due to run-off etc.) because it facilitates the spread of warm-water parasites or weakens the defences of organisms that are struggling to cope at temperatures which are very close to their upper thermal limits, so this complements the study that George mentioned above. Examples included increased disease in turtles, coral reefs (and associated biota), marine mammals, and mollusks.

    However, it does also say that disease outbreak in fishes will also decrease in some cases with environmental degradation, as some pathogens and parasites are more sensitive to toxic pollution than their hosts. They also suggest that fishing and fisheries (ironically) may actually result in wide-scale losses of infectious diseases. This is a very interesting perspective considering the other studies that have been mentioned in this blog which suggest that aquaculture can induce more disease (or greater susceptibility to it).

  3. Georgia, I personally think that it could apply to shellfish populations, i also think that research needs to look into the possibility that shellfish may have to put more resource into shell formation for protection in warmer waters due to increased acidification and this may leave them prone to disease as some form of trade off. It could also be the case that locations of major fisheries may be moved over time to compensate for increase in temperatures, this could expose the farmed species to new pathogens for which they would have no immunity.

  4. Ellie,

    There were also suggestions and links to other papers within the review that suggested certain pathogens may die out or be less prevalent as you say due to increases in pollution. It makes me wonder if we might see an increase in archaea populations as they seem to have groups adapted to a whole host of environments or is it possible that bacteria could "steal" the genes for their own gain. As i mentioned in my blog the role of viruses in such a process may be key. There does not seem to be any clear pattern from current literature so this is a definite area of interest for future research and hopefully funding.