Introduction
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. 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.
What
causes diseases?
The OIE International Aquatic Animal Health Code lists several
infectious diseases of importance and significance to global aquaculture and
aquatic production. They are all caused by pathogens (virus, bacteria and
parasites) and are capable of spreading through the movement of infected host
species. In addition to the OIE listed diseases, there are many diseases of regional
or national interest, which have significant impacts on aquaculture productivity.
Some of these are well-studied and understood, while others are of unknown etiology
or newly emergent. Such diseases can pose equal, if not greater, challenges for
aquaculture development in some regions.
Non-infectious diseases are also common in aquaculture and, although
they generally receive less attention than exotic diseases, can have equally
devastating effects on production over a very short period. Such diseases are
usually caused by ubiquitous opportunistic agents or other biotic and abiotic
conditions. For example, inadequate management, poor water quality, inappropriate
nutrition, aquatic environmental degradation, and exposure to chronic or acute contamination
have all been linked to mass mortalities of a wide range of cultured and wild species.
Significance
The prevention of fish diseases is essential for the
betterment of the fisheries industry, the improvement of farming production,
and the increase in fish resources. Because of the complexity of their
environment, fish are susceptible to viral, bacterial, fungal, and parasitic
infections. These infections will adversely affect growth and development and a
serious infection can be fatal. An outbreak of disease jeopardizes regular
aquaculture and threatens fish yields. Therefore, controlling disease is one of
the most vital tasks in fish culture.
Prevention at the farm level
One major contributor to fish disease
outbreaks is disease pathogen incursions due to international transfers of live
aquaculture animals and other fish products. However, once pathogens enter a
country, much of disease prevention efforts fall on the shoulder of producers.
Good practices at the farm level for disease prevention are generally known and
well documented. The most important and widely used in well-advanced systems
are:
- Exclusion to keep pathogens out of a production unit. In its most advanced form, exclusion involves eliminating water exchange with the surrounding environment through the use of aeration.
- Management. There are other management practices to reduce disease occurrences, including careful control of stocking density, water quality and nutrition that minimizes stress of fish and maintain healthy populations in the culture environment.
- Genetic resistance. Producers can use pathogen-free seed and those selectively bred for genetic resistance. This requires virtually no changes in farm infrastructure. However, experience with shrimp indicates that it takes 5-10 years to develop genetically resistant stocks. Supplementing genetic resistance is the use of vaccines. The Norwegian salmon industry has mostly replaced antibiotics with vaccines.
Prevention at the community level
Moreover,
as with many diseases of terrestrial animals and pests and invasive species,
fish diseases usually do not stay on farms. Once introduced, pathogens
can and do spread to other farms via the water supply, birds and wild fish and
other vectors within the ecosystem. Consequently, producers’ efforts to control
disease tend to be interdependent, that is one farm’s practices affect the
likelihood of disease outbreak, and hence the prevention practices, on another
farm. Because of this interdependence, the aggregate system-level prevention
effort tends to be less than desirable in unregulated systems (Kobayashi and
Melkonyan 2011). As a result, even if the majority of farms practice good
disease management, a single irresponsible farm that introduces a new disease
can result in collective disaster
Conclusion
By addressing these
findings in aquaculture management and regulations, risks for catastrophic
disease outbreak will be reduced, creating a more secure environment for
much-needed new investment in farmed seafood production. With a suite of
private and public sector partners, the World Bank is working to modernize and
improve aquaculture for food security and job creation by tackling the problem
of fish disease at the national regulatory level. Case studies in Chile and
Vietnam have identified key causative factors behind disease outbreaks in
farmed salmon and shrimp and elucidated the ways and means for successful
recovery from disease outbreaks
