Management Implications:
The reduction of disease in wild mammalian and avian populations by human intervention has been effective in some instances (e.g., rabies in foxes, brucellosis in bison, botulism in ducks), for which active management practices have been employed (Peterson et al. 1991; Grenfell and Dobson 1995; Wobeser 1997).
The ability to control diseases in wild fishes should be possible in concept, if there is an interest in doing so and if the economic and environmental price is acceptable.
Control of disease in wild populations takes several forms, each directed at reducing R0 to less than 1.0, which results in the elimination of the pathogen from the population.
There are four main types of intervention:
Culling:
—This is a form of ‘‘active host control’’ (Grenfell and Dobson 1995). By reducing the density of a stock in a given area, the population can, and must be, reduced to a level less than Nt, which will result in the loss of the pathogen from the population. In aquaculture, splitting populations to achieve lower densities is often carried out to reduce disease.
In severe cases, culling infected stocks is done to remove pathogens from populations. However, although this management technique is effective in reducing or eliminating disease, populations of many stocks—especially endangered stocks—in the wild are usually low, and reducing their numbers further by this method would generally not be a viable management choice.
Vaccination:
‘‘Active disease constraint’’ by the use of vaccines to control diseases of wildlife is prominent, especially with reference to rabies and brucellosis in elk and bison and to phocine distemper (Campbell and Charleton 1988; Peterson et al. 1991; Grenfell 1992; Grenfell et al. 1992).
There are explicit levels of immunization that must be achieved to reduce the susceptible population below the level permissive for disease development. In effect, the proportion of successfully vaccinated hosts must be greater than 1 2 R0 21 to prevent disease. Therefore, the smaller R0 is, the fewer animals must be vaccinated to prevent disease.
For example, 16.7% of the population should be immunized if R0 5 1.2 (just above the threshold of 1.0), 50% if R0 5 2.0, and 80% if R0 5 5.0. Vaccination of fish in the wild would be technically challenging, but possible, perhaps by using vaccine-impregnated caddis flies.
Many of the difficulties associated with immunization of fish in the wild are not operative in aquaculture facilities, and vaccination has been efficacious for a number of fish diseases (see, for example, Ellis 1988). If R0 can be calculated for a particular disease in an aquaculture facility, it should be simpler to ascertain the levels of vaccination necessary to prevent disease with some precision.
This could be of considerable use when designing a vaccine regime. Obviously, the effectiveness of a vaccine program is contingent on an efficacious vaccine that is protective for the duration of fish residence at the facility (to release or market size) and on the cost–benefit ratio for administration of the vaccine.
Chemotherapy:
—‘‘Active disease constraint’’ via chemotherapy operates by decreasing the duration of infectiousness by lowering the period during which the pathogen can be transmitted. It is unlikely that this would ever be a feasible method of reducing disease in wild fish, due to technical and environmental constraints.
Reducing spread to cherished populations:
‘‘Passive acceptance’’ indicates there is no active effort to reduce disease in a population that is nota focus of interest. Rather, exposure of a population of concern to an infected population is prevented.
Physical isolation of captive populations is practiced routinely in aquaculture and is frequently effective in decreasing the dissemination of disease to stocks that are of particular value.
The judicious movement of fish within and external to watersheds that have certain pathogens is even now a common management strategy for reducing the probability of disease in valued populations.
Author:
PAUL W. RENO
