Background
There are now around 12 re-circulation farms, ranging in production capacity from just a few tonnes to several hundred tonnes of fish per annum. Tilapia is considered an ideal species for aquaculture due to its hardy nature, fast growth, tolerance of poor water quality and disease resistance. Furthermore, the tilapia recirculation sector of the UK fish farming industry potentially ticks many green / sustainability boxes. However, like any emerging sector, there have been teething problems which the industry has to overcome. One such problem is the emergence of a new disease, and this article details the first diagnosis of a bacterial disease caused by Franciscella sp on a tilapia farm in England.
Outbreak and post mortem investigations
The Cefas Fish Health Inspectorate was called out to a tilapia farm that was experiencing persistent low level mortalities in fry (0.5 - 5 g), resulting in cumulative losses of up to 20 per cent. The system was run on a recirculation basis using bead filters and records showed no major water quality concerns. The fry were held at 24 – 27°C and were moved progressively from shallow tanks, to deeper tanks, and then suspended hapas (net enclosures) as they grew. Sick fish were seen flashing within the tanks, whilst some appeared lethargic with exophthalmia (bulging eyes) and fungal patches. Closer examination revealed haemorrhaging around the pectoral fins, pale gills, empty intestines and enlarged gall bladders, kidneys and spleens.
Subsequent in-depth investigations by Cefas showed that, although no obvious bacterial pathogens could be grown on standard bacteriological media, this outbreak was very likely caused by a bacterial pathogen from the genus Francisella. Tissues from affected fish contained granulomas with accumulations of intracellular Gram negative bacteria, and molecular biologists confirmed that the DNA was very similar to that from Francisella sp. isolated from diseased Asian tilapia. This species of Francisella causes severe problems to tilapia farmers in Asia and elsewhere. It should be stated that, although the genus also contains the human pathogen F. turalensis (that causes tauremia), there is no evidence to date suggesting that the fish pathogenic strains pose any risk to humans.
Disease prevention
After confirmation of the diagnosis for Francisella sp., the fish farm concerned opted to carry out a voluntary cull and disinfect the site under the supervision of the Cefas Fish Health Inspectorate. The farm is now working with a single hatchery that has provided evidence that its stock has been tested for the main pathogens affecting tilapia. The site is also developing its own in-house broodstock.
With current knowledge, the FHI would recommend culling and disinfecting sites suffering from Francisella sp. problems and starting again. Phased disinfection might be possible, depending on system design. However, if this is not feasible, communications with Asian colleagues suggest that, although the disease causes problems between 21° and 26°C, fish generally show no mortality between 26° and 29°C.
This outbreak has highlighted the fact that fish farmers need be aware that statutory health certificates only state that fish are coming from areas clear of certain listed notifiable diseases. Therefore, when beginning to farm new species, care needs to be taken when sourcing new suppliers to check if their stock is free of other non-notifiable pathogens. We would also recommend checking whether the supplier has an isolated water source, maintains their own brood-stock and operates sound bio-security policies.
Further work
Epidemiological investigations are continuing at the Cefas Weymouth laboratory to determine the likely origin of the disease, as well as assess risks to the tilapia industry, other farmed fish and wild species. The risks from strains affecting tilapia are not easily quantified, and Cefas is looking to develop in-house culture methods for Francisella sp. to support this research. Within the UK, tilapia are only grown in contained systems with discharges going to sewage or soak-aways and would therefore represent a low risk of any potential spread to the wild. Francisella has been found in cod in archive material taken during routine research cruises off the coast of Britain, which suggests that gadoids have long been exposed.
June 2009
