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Transcript
Fisheries Research Services How New Diseases Emerge diseases that rapidly kill their hosts can be supported, leading to higher rates of mortality. Disease Sources Existing diseases may become more serious as the host population increases or transmission becomes easier. When there are more cases of disease, mortality increases even if the probability of mortality per case does not change. Stress on the host fish may increase the risk of mortality per case; increased mortality may be adaptive if it increases transmission. Examples of existing diseases Introduction include the sea lice parasite on salmon. In recent years new diseases have been emerging in the marine environment, and particularly in the burgeoning Exotic diseases may be imported as a result of movement aquaculture industry. Examples include infectious of stocks, product, or international shipping. Exotic pancreatic necrosis (IPN) and infectious salmon anaemia diseases may be imported into farmed or wild populations. (ISA) in salmon, and viral haemorrhagic septicaemia (VHS) Exotic diseases may be excessively virulent in naïve in turbot. populations and evolve into less virulent forms with time. Disease Spread An example of an exotic disease is the parasite Gyrodactylus salaris on Atlantic salmon The use of epidemiological theory has been a powerful tool for the development of disease control strategies. A New diseases can also evolve with increasing virulence simple result from such epidemiological theory is the when there is natural selection because of a trade-off population threshold for disease transmission in a single between virulence and transmission. Existing diseases population. This is demonstrated in the equation below. may evolve into more lethal forms. Competition between diseases or disease strains may increase the selection Threshold = Loss of infected host for virulence. An example of a new disease is infectious Transmission of infection to new hosts pancreatic necrosis in salmon smolts in marine waters. The population threshold of disease susceptible fish Control depends upon the relative rates at which infections are Diseases can be controlled by treating or culling infected destroyed when host fish die, or become immune, and fish, interrupting transmission or vaccinating to reduce the rate at which viruses are transmitted to new hosts. the available population. Controls that interrupt transmission select for pathogens that do not kill their Aquaculture has increased both fish populations, and the hosts rapidly. However, excessive reliance on treatment movements of pathogens on transported fish, fish of pathogens can lead to evolution of resistant forms and products, equipment and shipping. Disease transmission this may include virulent forms that spread the disease may be easier among confined or stressed populations before controls are in place. Medications should be used of fish. The equation shows that with increased host in association with fallowing in an intregrated strategy population and easier disease transmission, more virulent which reduces the likelihood of resistance developing. Fisheries Research Services is an agency of the Scottish Executive FRS Marine Laboratory PO Box 101 375 Victoria Road tel +44 (0)1224 876544 fax +44 (0)1224 295511 [email protected] http://www.frs-scotland.gov.uk Aberdeen AB11 9DB UK