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.