Principles applying to all types of animals are listed first, followed by those which are more specific to fish, crustaceans and molluscs.
Both wild and farmed species are susceptible to infectious pathogens of disease concern.
Risk analysis is the appropriate process to guide the selection of diseases that warrant establishment and maintenance of zones.
In the process of establishing a zone free of a disease or pathogen of significance, there is no valid reason for requiring the testing of a species for which there is scientific evidence to demonstrate that the species is refractory to infection by the pathogen(s) to be tested for.
There is no valid reason to test species in environments that fall outside of the physiological tolerance range or epidemiological transmission range of the pathogen being tested for.
An appropriate sample size must be applied to demonstrate presence or absence of an infectious pathogen in a population (e.g. Cameron 2002).
Some tests available for surveillance (e.g. biomolecular assays[4]) are only indicative of the presence of the particular pathogen and should be applied in conjunction with tests that visually demonstrate presence of the particular pathogen (e.g. bioassay, histology and culture techniques).
Sampling procedures for OIE listed diseases and other pathogens of significance may be lethal to the host, therefore, any surveillance scheme must take into account lethal-sampling limitations, e.g. for limited numbers of susceptible stock, such as valuable broodstock, which present minimal disease transfer opportunities. Alternative, non-lethal, sampling methods may need to be used in these circumstances.
Many pathogens of concern are known to be carried by clinically healthy hosts.
Detection of viral pathogens in clinically normal specimens often requires the use of Level III technology (i.e. tissue culture, electron microscopy, PCR or other biomolecular-based diagnostic tools).
There are no scientifically reliable methodologies to rid a carrier animal population of viral or other directly transmitted pathogens.
Due to the low prevalence of certain significant diseases in healthy wild populations, the diversity of species in an open-environments, the lack of scientific knowledge on the susceptibility of most non-cultured species, and the difficulty in accurately establishing the health status of wild populations, untested populations should be considered suspect for carrying pathogens of significance unless environmental conditions or host susceptibility prove otherwise.
Prevalence of infection in both wild and cultured animal populations is likely to be highly variable, ranging from one infected individual in several thousand animals to all individuals in a population being infected.
Gametes obtained from broodstock infected with a viral disease should be suspect for the pathogen in question. Egg disinfection procedures greatly reduce the risk of vertical transmission of some viral pathogens; however, infectious viruses which are present inside the egg can persist post-disinfection.
While disinfection of fertilized eggs can reduce the risk of infection, Level III technology is often required to demonstrate freedom from infection.
Immunization against some diseases of salmonids exist, however, levels of protection vary.
Many significant viral diseases have a broad host species range in crustaceans. All crustaceans should be regarded as potential carriers unless clearly demonstrated to be refractory to infection, or environmental conditions are not conducive to pathogen transmission/virulence.
Many viral pathogens of crustaceans are transmitted vertically through contamination of spawning fluids.
Gametes from infected broodstock should be presumed to be infected unless scientific evidence has established otherwise. While disinfection of fertilized eggs may reduce the risk of infection, Level III technology is often required to demonstrate freedom from infection.
No effective vaccines are currently available for enhancing tolerance of, or resistance against, significant diseases of crustaceans.
No cell-lines are currently available for isolation and characterization of intra-cellular infectious agents (viruses, some bacteria) of crustaceans.
Apparently healthy shrimp may harbour one or a number of pathogens which cannot be identified due to the absence of pathology and/or insufficiently sensitive detection tests.
No vaccines are currently available for enhancing tolerance of, or resistance to significant diseases of molluscs.
No cell-lines are currently available for isolation and characterization of intra-cellular infectious agents (viruses, some bacteria).
Apparently healthy molluscs may harbour pathogens which cannot be detected or identified due to the absence of pathology and/or insufficiently sensitive/specific detection tests.
Molluscs often provide substrate surfaces for a variety of micro- and macroscopic fouling organisms that may be factors in disease transmission, but are rarely included in routine disease surveillance methods that concentrate on soft-tissue pathology.
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[4] PCR - polymerase chain
reaction based molecular analyses of DNA and RNA. |
