The Cycle of Disease:
The discussion above described the process of disease in the short term, during the course of a single epizootic in which non disease population fluctuations are discounted.
During longer periods of time, however, there is considerable population fluctuation due to natural mortality from predation and other non-infectious agents, immigration and emigration, and influx from new progeny.
The flux of any natural population is regulated by a carrying capacity characteristic of a given species and environment (Weatherly and Gill 1987).
The presence of disease can alter this equilibrium under certain conditions.
Figure 3 diagrams the interrelationships that occur in a deterministic S–I–R model of disease. Natural increases in the population occur through recruitment of new progeny and immigration; losses occur due to ‘‘natural’’ mortality and emigration.
Disease can markedly affect this intrinsic growth rate.
If the disease is a ‘‘benign’’ one that does not directly kill fish or contribute to the loss of fecundity, there will be no effect on the population, even if the pathogen infects a high proportion of the population.
A disease that has a direct mortality or reduces fecundity can alter the population to a greater or lesser extent.
Because the growth rate of the disease is essentially dependent on b, the rate of conversion from susceptible to infected, the relative magnitudes of b and the intrinsic growth rate of the population will determine the outcome of disease in the long term, as shown in Table 3.
Where b is small, disease will not regulate the final population size nor will the pathogen be retained.
As b increases, however, the pathogen can be retained in the population and, if b is high enough, the pathogen can regulate the population size significantly.
Thus, depending on the contagiousness of the pathogen, the level of infection may be transient, minimal, enzootic, or epizootic.
Again, it should be noted that if a disease causes no specific mortality and does not affect reproduction, it will not affect the population size or growth rate in the long term. In wild fish, disease may affect population size indirectly, for example, by weakening the host and making it more susceptible to predation.
In cultured fish there are also other adverse effects that can be economically important despite a lack of direct deaths.
For example, decreased growth rates at production facilities can cause economic loss, and if market-sized fish have visible lesions or skeletal deformations the market price of the product can be reduced.
Author:
PAUL W. RENO
