Streptococcus agalactiae is an emerging disease in Australian fish. A challenge model in Qld groper is required to understand the pathogenesis and mode of transmission of S. agalactiae. This information is critical to determine how to manage the spread of disease and identify likely sources of infection. Information generated from this trial will be useful for finfish farmers and marine aquaria to minimise chances of infection, and for state government veterinarians to develop future biosecurity plans for control and eradication of the disease.
There are no diagnostic tests available in Australia to detect S. agalactiae in fish other than bacteriology that takes up to a week for results. Development of rapid and accurate diagnostic tools including Gold standard PCR and in situ hybridisation for S. agalactiae are urgently needed by State and private Veterinary Laboratories to enable rapid diagnosis of S. agalactiae in marine fish and elasmobranchs. Availability of these tools will have flow-on benefits for all States of Australia, allowing increased aquatic animal disease detection and will enhance current and future State surveillance and monitoring of Streptococcosis in fish. Current disease surveillance of fish at aquaculture sites within the GBRMP will be enhanced by the availability of more accurate and rapid diagnostic tools.
Different strains of S. agalactiae have been isolated from Qld gropers, wild marine fish and stingrays in north Queensland by QPIF. There is a need for the development of molecular typing tests to accurately identify different strains of S.agalactiae in order to determine origin (native or exotic) and routes of dissemination of infection, thus enabling AQIS to make informed decisions in relation to the importation of fish. Moreover, commercial vaccine companies can produce effective autogenous vaccines to protect at-risk populations of fish and elasmobranchs, benefiting commercial aquariums, marine aquaculture and research facilities by protecting valuable stock.
Final report
The project demonstrated that S. agalactiae is highly pathogenic to juvenile Queensland grouper, E. lanceolatus and can be spread via infected water, infected food, by injection, or by cohabitation (of S. agalactiae-infected fish with non-infected fish).
This project produced rapid, reliable and accurate diagnostic tools, including PCR and an Immuno-histochemistry (IHC) method to specifically detect S. agalactiae in fish, thus increasing the States’, Territories’, and Australia’s capability for disease testing, surveillance, monitoring and reporting.
Molecular studies showed the S. agalactiae isolates from Queensland grouper were genetically distinct from human, cow, dog, cat or crocodile strains. Molecular studies further showed the Queensland grouper S. agalactiae isolates were most closely related to the grunter, mullet, catfish and stingray strains isolated from north Queensland. All North Queensland fish isolates belonged to strain type ST-261. Importantly, this sequence type has not been associated with human infection anywhere in the world and is substantially different from all terrestrial isolates of GBS.
Knowledge gained from this project will assist in developing biosecurity, health and disease management plans and programs relating to disease control for S. agalactiae in aquaculture facilities, commercial marine aquaria and live reef fish holding facilities. The project has resulted in outcomes that support consumer confidence in the safety of Australian seafood as the S. agalactiae isolates from fish are genetically different from those isolated from mammals and have never caused disease in terrestrial animals to date. Outcomes of this project have assisted in protecting recreational fisheries, through improved knowledge on prevalence and distribution of S. agalactiae within fish species of the Great Barrier Reef Marine Park in Northern Australia.
