There is considerable scope for more effective use of DNA-based methods of pathogen detection and disease diagnosis in Asia-Pacific aquaculture. However, implementation of standardised practices that produce reliable, useful and comparable data will require a significant investment in research, training and infrastructure development. Effective implementation will also be assisted by enhanced communication between aquatic animal health practitioners in the region and scientists with expertise in disease diagnosis and pathogen detection.
Although there are some common themes, it is also evident that there are significant differences in the current relevance of DNA-based methods of pathogen detection for the different aquaculture sectors. DNA-based methods are particularly suitable for detection and diagnosis of shrimp and mollusc pathogens because of the absence of an antibody response in invertebrates and lack of suitable cell lines for virus cultivation. In shrimp, the primary pathogens are well known and many DNA-based methods have already been developed. However, in molluscs there is very limited knowledge of pathogens and few diagnostic procedures of any kind are being employed in the Asia-Pacific region. In fish, antibody and culture-based diagnostic methods are available and considered to be robust and effective for routine diagnostic applications. As such, DNA-based methods in fish appear to be most suitable for confirmatory diagnosis and rapid screening of low level or unapparent infections. To achieve maximum impact, it is essential that research and training programs recognise these differences and are tailored to reflect current levels of knowledge and sector-specific needs.
Where DNA-based tests are available and/or suitable, the most significant impediment to effective implementation is the lack of standardised methodologies that are validated for specific applications. There is a need for international agreement on methodologies that have been rigorously evaluated and accredited for specific applications in disease diagnosis and pathogen screening. There is also a need to ensure that tests are performed by trained staff with access to standardised reagents and suitably equipped laboratories.
Because of existing limitations on the reliability and accessibility of the methods, international standards recommended by OIE do not presently include DNA-based methodologies. However, the potentially high sensitivity and specificity and relatively low cost of these tests has resulted in a surprisingly rapid adoption rate in Asia, particularly for shrimp pathogens. Therefore, it is essential that DNA-based tests are assessed on their merits against existing technologies and that programs to achieve improved performance and international standardisation should be developed. It is also essential that these programs should assist and complement the activities of OIE in obtaining internationally agreed test standards.
There are a number of pathogens for which DNA-based test methodologies are published or available commercially. However, in general, further research is required before standardised and validated DNA-based test protocols can be implemented for disease diagnosis and pathogen detection in the major aquaculture sectors in the Asia-Pacific region. Research needs vary for each pathogen depending on the existing knowledge base and state of the technology.
Recommendation Programs of international research cooperation should be developed and coordinated by FAO/NACA. The research should be conducted by managed collaborative networks and provide the information and technology necessary for delivery of suitably specific and validated tests for pathogens of fish, shrimp and molluscs in the Asia-Pacific region. Two research programs are proposed:
Program A: Identification and characterisation of potential pathogens of molluscs, shrimp and fish in the Asia-Pacific region.
This program should focus on improving the knowledge base by identification of new and emerging pathogens (through health screening, epidemiological investigation and subsequent molecular characterisation), relating pathogens in the region to those described elsewhere, and defining the extent of genetic variation between related pathogens in the region. The program should include the following priority projects:
Health screening and pathogen identification in molluscs;
Characterization of WSSV and YHV strain and pathotype variation in prawns;
Characterization of Haplosporidium, Marteilia and Perkinsus spp. infecting molluscs in Asia;
Characterization of VNNV strain variation in grouper and other fish of economic importance;
Characterization of emerging fish diseases including red spot and streptococcal infections.
Program B: Development and validation of DNA-based diagnostic and detection methods for diseases of aquaculture in the Asian region.
This program should draw on information currently available or obtained from Program A to develop suitably specific DNA-based diagnostic methods and to evaluate and validate the methods for disease diagnosis and pathogen screening programs. The research program should include the following priority projects:
Standardisation and validation of group and strain-specific DNA-based detection tests for WSSV and YHV-complex viruses;
Development and validation of species and strain-specific DNA-based detection tests for mycobacteriosis, viral nervous necrosis and epizootic ulcerative syndrome in Asia-Pacific;
Development and validation of DNA-based detection tests for Haplosporidium, Marteilia and Perkinsus spp. in Asia-Pacific.
The implementation of effective DNA-based diagnosis is severely constrained by the availability of scientists and technicians with skills in pathology and molecular diagnostic technologies.
Recommendation. FAO/NACA should develop training programs for staff from key laboratories in the region. Training is required in the following priority areas:
The use of standard histopathological methods for health screening of fish and molluscs.
The use of standard DNA-based methods for pathogen detection including sample collection, application of test protocols and the analysis and interpretation of test results. Initially, training should focus on detection of shrimp pathogens.
There is a need to improve communication links between practitioners and scientists with recognised expertise in disease diagnosis and pathogen detection.
Recommendation. FAO/NACA should establish and maintain sector-based (fish, molluscs, shrimp) communication networks of diagnostic practitioners and internationally recognised experts in aquatic animal health. Activities of the networks should include:
Exchange on information pathogen distribution in the Asia-Pacific region and the availability of diagnostic tests and reagents;
Development of cooperative research projects and training programs;
Development of cooperative programs for test validation and laboratory accreditation.
Lack of standardisation of tests and test protocols is a major impediment to effective implementation of DNA-based methods in the Asia-Pacific region. Standardisation requires international agreement and cooperation in test selection, practitioner training and laboratory accreditation. Improvements in the reproducibility, validity and comparability of data resulting from accreditation will also assist OIE in assessing the suitability of DNA-based methods for detection of listed pathogens.
Recommendation. FAO/NACA should develop a program of accreditation of standard DNA-based tests and laboratories with the required standards of operation and expertise to conduct the tests effectively. The program should be administered by NACA through pathogen-specific reference laboratories with the following functions:
Maintain accredited tests and reagents including reference standards;
Monitor standards and provide technical advise to accredited laboratories;
Provide definitive diagnosis in difficult or unusual cases;
Archive pathogens for future reference.