Current climate patterns are changing: temperatures and the frequency of extreme weather events are likely to increase. The air temperature across the UK may rise between 0.5° and 1.5°C by 2020 and between two and 3.5°C by 2080. Winters are likely to become wetter and summers drier.
A qualitative risk assessment model was used, based on previously published work (Gale et al. 2009), to assess how these changes may influence the threat from both exotic and non-exotic infectious diseases of freshwater fish in England and Wales.
The six-modules cover the introduction, establishment and spread of pathogens and parasites. The impact of climate change on the host-pathogen interaction, the distribution and abundance of intermediate hosts, and transmission pathways are considered.
The key issue (module 3) was the host-pathogen interaction:
- Fish are poikliothermic, thus their physiology is directly affected by the ambient temperature. In general, the fish immune response increases with temperature and is optimal at the species’ normal summer temperature.
- The replication rate of parasites and pathogens is also affected by temperature. At higher temperatures, the generation time of bacteria, fungi and parasites with direct lifecycles is shorter, whilst each virus has its own optimal temperature range for replication.
As a result there are optimal temperature ranges for infection and clinical disease; increasing water temperatures will shift the balance in favour of either the host or pathogen, changing the frequency and distribution of disease.
A clear conclusion was that increases in water temperature and other changes in environmental water quality parameters, caused by climate change, are likely to adversely affect salmonids more severely than cyprinids.
Atlantic salmon are a cold water species and tolerant to a narrow thermal range. Their increased susceptibility to disease as water temperatures rise has been most clearly illustrated by the emergence of proliferative kidney disease (PKD) in wild salmon in both Switzerland and Norway (in part due to proliferation of the alternative host).
In the UK, high water temperatures in the river Tyne during 2002 resulted in opportunistic bacterial infections which caused high levels of mortality in migrating salmon.
Other non-exotic diseases of farmed salmonids which are likely to become more difficult to control as water temperatures increase include enteric red mouth (ERM) and furunculosis. Parasites with a seasonal occurrence, such as Ichthyophthirius multifiliis (Ich, the cause of white spot) will become more problematic as the period of infection lengthens.
Cyprinids are at the northern limit of their geographic range in the UK and thus are likely to be favoured by increased water temperatures and may be less susceptible to disease. Specifically, the threat of spring viraemia of carp (SVC, disease occurs below 17°C) will decrease. The exception may be koi herpes virus (KHV, disease occurs above 16°C); outbreaks may become more widespread and occur over a longer period.
Major diseases which are exotic to the UK have well defined temperature thresholds for establishment. Increased water temperature lowers the threat of viral haemorrhagic septicaemia (VHS) and infectious haematopoietic necrosis (IHN).
Conversely, the risk of Lactococcus garvieae, epizootic haematopoietic necrosis virus (EHN) and epizootic ulcerative syndrome (EUS) will increase. As freshwater temperatures continue to rise, L. garvieae is likely to advance across Europe, the risk of introduction will increase, and the time period when the bacteria may establish (i.e. when the water temperature is above 15°C) will lengthen.
The occurrence of this disease in southern England at a single location (Bark & McGregor 2001) demonstrates the real risk of introduction and establishment. In the long term, aquaculture will respond to climate change by farming species or strains adapted to higher temperatures.
Since the freshwater ecosystem across much of England and Wales is highly managed, opportunities exist to mitigate the impact of climate change on wild populations. Water quality can be improved by reducing the release of untreated sewage and run-off from agriculture land.
Water could be released into rivers from reservoirs during droughts to mimic normal conditions and allow continuous upstream migration of salmonids, thus avoiding crowding, stress and increased pathogen transmission. The impact of climate change will vary considerably across the country and interventions should be prioritised for locations where disease emergence is most likely.
This will require greater use of geographic information systems and spatial modelling. Biosecurity strategies need to be revised to account for the changing exotic disease threat.