Fish Diseases in Aquaculture - By Bruce L. Nicholson, Professor of Biochemistry, Microbiology and Molecular Biology, the University of Maine - Infectious diseases caused by bacteria, viruses and parasites are a primary concern in aquaculture. Indeed, effective control of infectious diseases is one of the most critical elements in successful aquaculture.
Therefore, both the aquaculture industry itself, and mandated governmental fish health management programs, maintain a variety of measures to control and minimize the effect of infectious diseases in culture fish populations in fish farms and aquaculture sites. Nevertheless, diseases can and do occur in these populations. This paper addresses the concern that diseases can spread from aquaculture sites to wild fish with resulting effects on populations of wild fish.
Dynamics of Fish Diseases
The dynamics of the occurrence, severity and spread of diseases within and between fish populations are similar to those associated with diseases in human and terrestrial animal populations. However, one additional component in fish populations is the water environment which may facilitate the spread of disease-causing agents. Diseases are transmitted from one individual to another in one of two ways: vertical transmission or horizontal transmission. In vertical transmission, pathogens are transmitted from one or both parents to offspring through sex cell which for offspring. Horizontal transmission involves the spread of the pathogen from one individual to another through direct contact, air, or water.
The development and severity of a disease following exposure to a pathogen involves a complex web of variables such as: the virulence (ability to cause disease) of the pathogen; the immune, genetic and physiological condition of the host; stress; and population density. Different strains of the same pathogen may vary considerably in their ability to cause disease. That is, some strains can infect a host without causing a clinical disease. Also, individual animals differ in susceptibility to disease. Some individuals may be more resistant to the disease because of their genetic composition or as a result of previous exposure and development of immunity. Conversely, individuals may be more susceptible to a disease because of poor nutritional status, stress or any of a number of other factors. Population density is a particularly important factor in the spread of fish diseases. Diseases can spread more easily within dense populations simply because of increased opportunities for association of infected fish with uninfected fish. The actual development of a disease and the relevant severity of that disease within a fish population is influenced by a complex interaction of these variables associated with the pathogen, the host and the environment.
Diseases in Aquaculture Populations
Infectious diseases pose one of the most significant threats to successful aquaculture. The maintenance of large numbers of fish crowded together in a small area provides an environment conducive for the development and spread of infectious diseases. In this crowded, relatively unnatural environment, fish are stressed and more susceptible to disease. Moreover, the water environment, and limited water flow, facilitates the spread of pathogens within crowded populations.
Control of Diseases in Aquaculture Populations
In their self-interest, the aquaculture industry in Maine and elsewhere in North America and Europe places a high priority on the prevention and control of infectious diseases. Unlike treating human or other animal diseases, few drugs are available for treating diseases in fish. Because of environmental concerns, only two drugs are approved in the United States for use in fish populations. Control of diseases in aquaculture and fish farms relies on a combination of good management practices, use of the few approved and commercially available drugs and vaccines, and prevention of infection.
There are two general categories of fish diseases: diseases caused by pathogens that are indigenous (native) to the local environment and exotic pathogens that normally are not found in that geographical area. Control of diseases caused by indigenous pathogens is attempted through good management practices and the use of approved drugs and/or vaccines. Governmental agencies in the Northeast and elsewhere in the United States and other countries have established fish health management programs that mandate periodic testing of aquaculture populations and certification that these populations are free of specific exotic or high risk pathogens. Importation to the area or movement within the area is restricted to fish and fish eggs that have been certified to be free of these pathogens. It is important to note that such fish must be "free of the pathogen,"not simply without overt disease symptoms. Such disease control programs depend on the availability of sensitive, rapid and cost-effective diagnostic assays. The development of such diagnostic assays is improving rapidly, particularly as a result of current research adapting new developments in molecular biology and biotechnology to fish disease diagnostics.
Diseases in Wild Fish
The fish pathogens found in aquaculture populations also occur in wild fish. Except for "schooling" fish, the population densities of wild fish are comparatively low and the concentration of pathogens from an infected fish is rapidly diluted in the water environment. Although there are a few historical examples of disease outbreaks that significantly reduced certain populations of wild fish species, it generally is thought that diseases do not play a major role in influencing wild fish populations. However, the exact role, if any, of diseases in regulating populations of wild fish is largely unknown. The introduction of an "exotic" pathogen probably poses the biggest threat to wild fish populations.
Disease Implications of Interactions of Aquaculture and Wild Fish Populations
Fish in aquaculture populations, particularly those in ocean net pens, can be exposed to pathogens from wild fish in the area. Because of the conditions of the aquaculture environment, these cultured fish populations may be more at risk of developing disease than the wild fish. There is much scientific documentation of the introduction of pathogens and resulting disease outbreaks into cultured fish populations from naturally infected wild fish. Transmission of diseases also could occur in the opposite direction (i.e., from aquaculture populations to wild populations). In this regard, the greatest concern would be exposure of wild fish to larger than normal concentrations of indigenous pathogens or exotic pathogens from aquaculture sites. However, the fish health management programs mandated in Maine and Maritime Canada significantly reduce the latter threat of introduction of exotic pathogens into wild fish populations. While the spread of diseases from aquaculture populations to wild fish is possible, there is little scientific evidence of such transmissions or, at least, harmful effects of any such transmissions.
Human Health Implications of Fish Diseases
The vast majority of fish diseases are restricted to fish and pose no risk to handlers or consumers.
Source: The University of Maine - January 2000
