Download How New Diseases Emerge

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.
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