Elsewhere on DAFF
2. Principles of Control and Eradication
This section provides background information to enable the choice of the most appropriate response option following detection of furunculosis or Aeromonas salmonicida subsp. salmonicida in Australia.
Furunculosis can cause high mortality and morbidity in susceptible salmonid populations. The disease is exotic to Australia, but if introduced it has the potential to have a serious impact on the salmonid farming industry and wild salmonid populations.
Furunculosis is highly contagious. Covert infection is a feature of the disease. Depending on conditions, the pathogen can remain viable for months in the environment. A. salmonicida subsp. salmonicida can also survive in a non-culturable but viable (NCBV) state, which restricts the range of effective measures for monitoring and surveillance. Molecular biology techniques may be able to detect NCBV material if present.
There are essentially three disease response strategies available to minimise the impact of this disease if exclusion strategies were unsuccessful in keeping furunculosis out of Australia:
- Eradication—eradication of furunculosis and A. salmonicida subsp. salmonicida from Australia. This is the preferred response option, which is the highest level of response measure and may also incur the highest cost in the short term. It is acknowledged that eradication may not be possible if the disease is present in wild fish populations. Currently in Australia, there are no formal agreements in place to compensate for stock losses where destruction of fish is carried out as a means of eradicating a disease7.
- Containment, control and zoning—containment of the disease and bacterium to areas with identified infection, prevention of further spread and protection of uninfected areas.
- Control and mitigation of disease—the implementation of management practices that decrease the incidence and severity of clinical outbreaks. In the short term this is the lowest level of response measure and may incur the least cost. However, it may also incur significant cost in the long term through its impact on production.
The basic principles of eradication and other response options are described in the AQUAVETPLAN Enterprise Manual and the AQUAVETPLAN Control Centres Management Manual. Appendix 1 of the AQUAVETPLAN Enterprise Manual lists the state/territory legislation relating to disease control and eradication.
If furunculosis or A. salmonicida subsp. salmonicida is detected, the general principles for any response option include:
- rapid detection and identification of disease and infection
- rapid definition
- of the nature and extent of the outbreak
- of the extent of exposure of wild fish populations and environmental reservoirs
- implementation of response measures - prevention of bacterial spread by controlling stock and water movement, within and between farms
- ensuring good management practices and biosecurity control measures.
Selecting the most appropriate option will depend on:
- the system(s) in which furunculosis is detected (e.g. semi-open system, closed system)
- the location and presence (or absence) of reservoirs of infection. This must acknowledge the limitations of detecting the pathogen in the environment and in salmonids with covert furunculosis, and the potential for non-salmonids to be infected with A. salmonicida subsp. salmonicida short-term costs of the response measure and disruption to production, acknowledging that no formal compensation mechanism currently exists if fish are destroyed
- long-term costs of production with or without the presence of the pathogen
- long-term costs of control should the pathogen become endemic.
These factors are all influenced by whether farmed or wild fish (or both) are affected, as described below.
2.1 Aquatic animal systems
For the purpose of aquatic animal disease control in Australia, four systems are used to describe the methods used to farm aquatic animals and for wild aquatic animals. These systems are open, semi-open, semi-closed and closed. The AQUAVETPLAN Enterprise Manual fully explains each of these systems in the context of generic disease control. The following sections provide a summary of these systems in the context of furunculosis.
2.1.1 Open systems
Fish growing wild in rivers, lakes and the ocean are considered to be in an open system, as there is no control over fish or water movement. In Australia, Victoria, South Australia, New South Wales and Tasmania have wild salmonid populations growing in open systems.
2.1.2 Semi-open, semi-closed and closed systems
The main systems involved in salmonid aquaculture are semi-open and semi-closed systems with a small amount of culture in closed systems for juvenile fish.
Semi-open systems
Semi-open systems are systems where there is control of fish movement but no control of water flow or the aquatic environment (e.g. cage-pen culture in estuaries or lakes). Although there may be control of the farmed aquatic species in these systems, there is no control over wild aquatic species that may be in close contact with the farmed species .
Cages can become damaged, thereby allowing fish to escape into the wild. Wild fish species are able to swim in and out of the cages at any time if the fish are small enough.
Fish and feed wastes can enter the environment directly, as can any pathogen shed by infected fish.
Semi-closed systems
Semi-closed systems are systems where there is control of fish movement and some control of water flow (e.g. raceway culture using water drawn off a river).
These systems are not designed to be self-contained, and so preventing inflow or outflow of water will have adverse effects on the fish. Although control and treatment of discharge water is possible, it is unlikely to be feasible due to the large volumes of water involved.
A potential hazard is the risk of wild fish being able to enter farm waterways, and possibly ponds, via intake water from the rivers. Farmed fish can also escape, and if water continues to discharge pathogens may enter the environment.
Pathogens can potentially be spread by predators in both semi-open and semi-closed systems.
Closed systems
Closed systems are those where there is total control over water and fish (e.g. a recirculation hatchery system). These systems are used to a small degree in salmonid culture for early growing of fingerlings. These systems present the simplest scenario for control of furunculosis.
2.2 Methods to prevent spread and eliminate pathogens
2.2.1 Quarantine and movement controls
- disease in salmonids causing high morbidity and mortality with evidence of extensive haemorrhage in affected fish, with or without furuncles
- suspicion or confirmation (i.e. if the bacterium is isolated from a non-salmonid fish) of A. salmonicida subsp. salmonicida.
Establishment of quarantine areas
When furunculosis is suspected, specified areas should be established (Figure 2; see the AQUAVETPLAN Enterprise Manual, Section A for more details), including:
- declared area— includes the restricted area and control area
- infected premises or area— a clearly defined area, which may be all or part of a premise, lease or waterway in which an emergency aquatic animal disease exists
- restricted area— area around infected premises or area
- control area— a buffer between the restricted area and free areas
- free area— non-infected area (this area is not considered a ‘declared area’ and may include large areas of Australia in which the presence or absence of A. salmonicida subsp. salmonicida remains unassessed).
Note that if furunculosis is detected or suspected in wild fish, there may be limited ability to define the extent of the ‘infected area’ due to difficulty in determining the potential range of movement of wild fish.
Fish farms that are insured against stock losses will generally be able to claim for loss of fish where fish have died from disease, but will not be able to claim for loss of fish where fish have been destroyed as part of a disease control strategy.
Figure 2 - Establishment of specified areas to control furunculosis
When declaring quarantine areas, consider:
- other salmonid farms in the area
- for freshwater hatcheries, other farms within the same watershed or linked by movement of vehicles, equipment and personnel
- for marine operations, other farming operations within a distance in the order of kilometres (based on epidemiological risk factors as outlined in Jarp and Karlsen (1997) and McClure, Hammell & Dohoo (2005), which, though not specifically about furunculosis, provide very useful information regarding risk factors and spread of disease) - the presence of native or introduced fish populations susceptible to furunculosis
- the presence of potential vectors (e.g. wild fish, copepods and bivalves)
- environmental factors, such as the direction and strength of water flow
- live fish transportation between and within freshwater and marine operations (including smolts going out to cages, broodstock and marine cage towing)
- fish harvesting and transportation to processing plants
- discharge of processing plant effluent
- transportation of consumer-ready products
- disposal of dead fish
- disposal of waste products from processing plants
- recreational fishing activities.
Movement controls
The feasibility of restrictions and bans and the extent to which these are able to be enforced will depend on the location of infection, the location and type of enterprises affected and the control response option chosen (i.e. whether the aim is to eradicate the disease agent or to control its spread).
Implementation of bans and restrictions will be a dynamic process. Implementation will be determined by the location and extent of the ability to define the disease outbreak (where clinical furunculosis is present) and the distribution of infected fish and reservoirs (where covert furunculosis is suspected or A. salmonicida subsp. salmonicida is detected in a non-salmonid fish)—or both.
Movement controls include banning:
- the movement of all live fish into or out of declared area(s)
- the release of fish into river systems or marine locations in declared area(s)
- the movement of fish between different river systems, between marine farm locations, and between marine and freshwater farm locations within the declared area(s)
- recreational fishing in the declared area(s)
- or restricting the use and movement of equipment and personnel within and between river systems (or farms) and marine farms within the declared area(s).
2.2.2 Zoning
If furunculosis were to become endemic in specific regions of Australia, a zoning policy specific for the disease and for the bacterium A. salmonicida subsp. salmonicida may be necessary to protect non-infected areas and to prevent further spread of infection. Zones would be based on the distribution of any vectors or reservoirs present or suspected (if appropriate), the geographical and hydrological characteristics of water bodies and landforms, and predictions of the most likely method of spread of infection. Zoning may rely on the identification of biogeographic barriers. A corresponding surveillance and monitoring program for furunculosis would be required to support the zoning policy.
Principles of zoning for infected and non-infected zones in Australia are outlined in the AQUAPLAN zoning policy guidelines. Detailed information on general requirements for surveillance for recognition of freedom from infection is provided in the Aquatic animal health code (OIE 2009; Chapter 4.1).
Zoning for furunculosis could be difficult depending on where it is detected. Covertly infected fish populations can become established and are very difficult to detect. Reservoirs of infection can become established in the environment and in wild fish populations, and are unlikely to be successfully eradicated. Once established in a river system or in a migratory wild fish population, infection is easily spread. These factors make it very difficult to protect furunculosis-free zones.
2.2.3 Tracing
Tracing a disease outbreak is the process of retrospectively determining the method and pattern of disease spread and finding the source of the outbreak. Tracing investigations form part of the outbreak investigation, and are crucial in determining all confirmed and potential locations of the disease, as well as defining restricted and control areas. The information gathered from tracing will assist in shaping the most appropriate response action. A full outbreak investigation, which includes tracing, must be conducted as soon as possible after the detection of furunculosis.
The immediate steps required are to trace-back all contacts with infected fish, premises and sites (to establish the source of the outbreak) and to trace-forward all contacts with infected fish, premises and sites (to establish the current location and potential spread of infection).
The presence or absence of predisposing factors must be examined when determining the duration of the outbreak and estimating the time and source of initial infection. It is possible that a covert infection may have been present for some time before clinical disease becomes apparent. Much information may also come from a detailed investigation regarding what is happening on the affected farm or area.
As a guide, items that must be traced are:
- fish
- farmed fish, including broodstock, fingerlings, smolts, fish for stocking waterways, fish for farm dams etc.
- wild fish, including both salmonid and non-salmonid species (acknowledging that it is not possible to trace individual wild fish) - fish products—eggs, fish for consumption, effluent and waste products from slaughter and processing
- water—input and output
- vehicles—fish transport vehicles, feed trucks, visitors’ cars, boats
- equipment—fish cages, nets, other floating installations, tools and instruments
- personnel—farm workers, sales and feed representatives, tradespeople, veterinarians, scientists, technicians and visitors
- processing facilities—particularly if there is a possibility that infected fish have been harvested and sent for processing.
Neighbouring fish populations
Neighbouring fish farms and processing plants may become or may already be infected depending on a number of factors, including their location, and the severity and duration of infection on the infected premises11.
Maps identifying the location of neighbouring fish farms, processing plants, waterways and hydrographic data are helpful to monitor the potential spread of the pathogen.
The location of susceptible wild fish species and vectors should also be confirmed both upstream and downstream of the infected freshwater site. In the marine environment, knowledge of tidal and current patterns in the affected area is critical. Further sources of infection may be identified if a number of facilities share common water.
For information on the location of farming establishments and wild fish populations at risk of infection, contact the relevant state and territory fisheries or agriculture agency (see the AQUAVETPLAN Enterprise Manual, Appendix 5 for contact details).
2.2.4 Surveillance
Surveillance is necessary to:
- define the extent of the infection
- detect new outbreaks
- establish restricted and control areas to which quarantine and movement restrictions can be applied
- establish infected and non-infected areas or zones for a furunculosis zoning program
- monitor the progress and success of an eradication or control strategy.
At present, there is no recognised surveillance program either in Australia or overseas that is specific for A. salmonicida subsp. salmonicida. Detailed information on general requirements for surveillance for recognition of freedom from infection is provided in the Aquatic animal health code (Chapter 1.4; OIE 2009).
Confirmation of furunculosis in Australia is defined in Section 1.4.1 of this manual. It would be based on observation of clinical signs in salmonids, and then isolation and confirmed identification of A. salmonicida subsp. salmonicida. Diagnosing subclinical infections is difficult, but the stress-inducible furunculosis (SIF) test may be of value.
As also noted in Section 1.4.1, it is possible that the first detection of A. salmonicida subsp. salmonicida in Australia may be from a non-salmonid fish. The detection of A. salmonicida subsp. salmonicida in this way would invoke the same disease response as would detection of clinical or covert furunculosis.
2.2.5 Treatment of infected host species
In an outbreak, treatment of fish can decrease morbidity and mortality and can reduce the shedding of pathogen into the environment.
Farmed fish can be effectively treated for furunculosis using antibiotics, generally administered via the feed. When medicating populations of fish (e.g. salmon in sea cages), preparation and distribution of medicated feed can take a number of days, particularly if the medication is milled into the feed; hence, there is generally a lag period between the decision to treat and feeding the medication to the fish.
Considerations for the use of antibiotics for the treatment of furunculosis include:
- the cost of treatment, including preparation, labour, administration and equipment
- the fact that sick fish do not eat, so in-feed medication will assist in protecting asymptomatic fish, but not fish with clinical disease. The lag period required for preparation and distribution of feed may mean that the number of sick fish will continually increase before the feed arrives at the farm
- that treatment will not clear a farm of infection—covert infections are likely to persist even with treatment
- the necessity to apply withholding periods, so treatment of fish populations close to slaughter may not be feasible
- that multiple drug-resistant strains of A. salmonicida subsp. salmonicida can develop.
In the event of an outbreak it is essential that samples be collected for culture and sensitivity testing using an approved method before the administration of any treatment (Hawke et al. 2006ab). This will ensure that the treatment does not preclude pathogen isolation and culture, and will also ensure that the most appropriate and effective antibiotic treatment is selected.
The decision to treat the fish (or not) will also depend on the circumstances. For example, eradication will be the preferred response option if furunculosis is detected in Australia. If furunculosis is detected in a farmed population of salmonids, the decision may be made to slaughter out this population. However, this may take weeks to months, and during this time there will be considerable shedding of A. salmonicida subsp. salmonicida. Treatment may assist in decreasing this shedding during the slaughter-out process, provided the lag period between deciding to treat and getting medicated feed to fish is kept short.
Antibiotic treatment may be limited to incidences when there is no alternative control method (e.g. use of antibiotics to prevent loss of fish stocks during a severe furunculosis outbreak where large numbers of fish are still clinically healthy).
Minor use permits exist in Australia for treatment of salmonids using oxytetracycline and florfenicol, both of which have been used to treat furunculosis overseas. In particular, florfenicol has clinically been shown to be very effective for this disease overseas (P Hardy-Smith, pers. obs.). The withholding period for oxytetracycline is 1600 degree days13 for initial treatment and 1000 degree days for florfenicol. Further details are available from the Australian Pesticides and Veterinary Medicines Authority (APVMA).
Multiple drug-resistant strains of A. salmonicida subsp. salmonicida have been detected overseas (McIntosh et al. 2008). While drug-resistant A. salmonicida subsp. salmonicida remains in the environment, attempts to eradicate infection through the continued use of antibiotics will be unsuccessful. This is because as antibiotic concentrations decrease in the fish, reinfection can occur (McCarthy 1977).
2.2.6 Treatment of host products and byproducts
Trade regulations, market requirements, food safety standards and potential spread of the pathogen must be considered when determining the treatment and processing, and the destiny of fish products and byproducts.
A. salmonicida subsp. salmonicida can survive in dead fish, even when frozen for up to 50 days (Ferguson 1988). Therefore, there may be a risk of the dissemination of infection to uninfected areas if there is a possibility that the infected frozen product could come in contact with and be transferred to susceptible fish species in those areas.
Furunculosis is not a recognised zoonosis, so infected fish may be consumed after the treatments and appropriate drug withholding times outlined above.
Viable fish eggs may be surface disinfected to prevent mechanical transmission. Surface disinfection of eggs is discussed in Section 1.6.3.
2.2.7 Destruction of hosts
Destruction of fish must be both hygienic and humane as outlined in the AQUAVETPLAN Destruction Manual. There must be no spillage of infectious waste. Increased bacterial shedding may occur if fish are stressed at slaughter, therefore the least stressful methods should be used.
There are many different methods to anaesthetise or destroy fish, all of which have limitations. These methods are discussed in the AQUAVETPLAN Destruction Manual and include using standard harvesting procedures, and chemical anaesthesia and euthanasia.
The most appropriate method of destruction depends on the:
- size and number of fish
- deadline for slaughter—this depends on the pressure of infection and the risk of further spread
- resources available for slaughtering fish—a slaughter-out policy may require crews to work around the clock
- type of system
- destination—human consumption or disposal. Although not a human pathogen, furunculosis can result in unsightly lesions within and on the fish carcase
- slaughter facilities—site, equipment and methods available
- experience and availability of personnel.
Any chemicals used must be approved for that use by the APVMA.
In addition, any chemical that is used directly or indirectly for the control of an animal disease is governed in its use by relevant ‘control of use’ legislation in each state and territory. The relevant state or territory authority (in most cases this is the veterinary registrar within the relevant state department of primary industry or agriculture) should also be consulted for advice before the chemical is used.
2.2.8 Disposal of fish deemed to be infected
In an outbreak of furunculosis, fish will either die from the disease, be harvested for human consumption or be destroyed. Disposal of the fish will depend on the cause of death.
For more details on the disposal of fish, see the AQUAVETPLAN Disposal Manual.
2.2.9 Decontamination
Due to differences in farming enterprises, disinfection protocols may need to be determined on an individual basis involving the farm manager, the state or territory chief veterinary officer and/or the director of fisheries. The protocol should take into consideration the factors outlined in Section 1.6, in particular:
- the source and location of infection
- the type of enterprise (e.g. farm, processing plant, hatchery, grow-out ponds)
- water source
- the construction materials of the buildings and structures on the site
- the design of the site and its proximity to other waterways or buildings
- current disinfection and biosecurity protocols
- workplace safety concerns
- environmental impact of the disinfectant protocol
- legislative requirements (occupational health and safety, environmental protection, chemical use)
- availability of approved, appropriate and effective disinfectants.
Effective decontamination of equipment, materials, tanks and buildings requires thorough cleaning before disinfection.
The ability of A. salmonicida subsp. salmonicida to remain viable for long periods in sediments must be considered.
For more information on decontamination methods, disinfectants and their indications, see the AQUAVETPLAN Decontamination Manual.
2.2.10 Vaccination
Vaccination is used overseas in the management (but not elimination or eradication) of furunculosis. Vaccination is discussed in Section 1.5. In Australia, the use of vaccination in the control of furunculosis would depend upon the choice of response strategy.
Eradication
Vaccination is not appropriate for the eradication of furunculosis because:
- it does not prevent or eliminate covert infections of fish (Hiney, Smith & Bernoth 1997)—such fish are capable of transmitting infection to non-infected populations
- vaccinated fish may interfere with surveillance and monitoring programs, as they may test falsely positive for the bacterium if a diagnostic test that does not differentiate between viable and non-viable bacteria is used
(e.g. polymerase chain reaction techniques).
Containment, control and zoning
Vaccinated fish populations may test falsely positive to immunological or DNA-based tests since such tests are currently unable to distinguish between the presence of the vaccine or a natural infection (Hiney, Smith & Bernoth 1997; Høie et al. 1993). Therefore, vaccination may interfere with surveillance and control programs if such tests are used. However, bacterial culture will distinguish between viable pathogen and killed vaccine agent. Vaccination may be useful for fish in areas outside the infected area to provide an additional layer of protection.
Control and mitigation of disease
If furunculosis became endemic in Australia, vaccination would be a useful management tool to increase population immunity, as it has been in overseas countries.
Although no vaccines for the typical strain of A. salmonicida are available in Australia, a vaccine containing an atypical strain of A. salmonicida is currently available and being used. It is not certain whether this vaccine would confer a level of immunity against the typical strain that causes furunculosis.
A list of commercially available vaccines for the typical strain, which are available overseas, is given in Appendix 2. Details of how to obtain approval for use of these vaccines in Australia are provided in Appendix 3.
2.2.11 Vector control
A potential problem with the control of furunculosis is the control of biological vectors, such as wild fish, birds and aquatic invertebrates (e.g. copepods and molluscs). ‘Cleaner fish’ (mentioned in Section 1.2) are not routinely used in Australia.
Wild fish
Controlling wild fish is impossible in many areas. Attempts can be made to prevent contact between wild fish and farmed fish, but there is limited ability to do this in marine farming operations.
Birds
Sea pens, raceways, open air tanks and ponds may attract birds and so must be covered (e.g. using nets or tank roofs) to prevent access by birds, which can potentially spread disease.
Copepods and molluscs
Attempts can be made to decrease contact between copepods and molluscs, and farmed fish (e.g. by keeping organic buildup on nets to a minimum and removing fouling from boat hulls).
2.3 Environmental considerations
Environmental considerations in the control of furunculosis include the following:
- Discharge of infected or potentially infected effluent into catchment areas or natural waterways will lead to further spread of infection. This could also lead to the establishment of reservoirs of infection in wild fish populations and waterways. If eradication of furunculosis from a farm is chosen as the response option, it is likely that discharge of infected effluent will cease.
- The destruction and disposal of infected carcases and other material may have an impact on the environment. This impact must be minimised to ensure that infection is not spread.
- The use of disinfectants and antibiotics could impact on the environment, especially if used in larger than normal quantities or concentrations as is possible in a disease control situation. Minor use permits for oxytetracycline and florfenicol provide instruction on what local environmental monitoring is required when these antibiotics are used.
2.4 Sentinel animals and restocking measures
Following an outbreak, fish species known to be more susceptible to furunculosis may be restocked as sentinel fish. Brook trout is considered the most susceptible species in fresh water, and Atlantic salmon the most susceptible species in sea water. However, before stocking with sentinel fish, it is important to determine whether or not there are further risks associated with translocating fish between areas.
A. salmonicida subsp. salmonicida can survive in sediments for months, as discussed in Section 1.6.2. Fallowing time required before restocking will need to be assessed on an individual basis. The OIE Aquatic animal health code (OIE 2009; Chapter 4.4) also provides guidance on fallowing in aquaculture. The fallowing period will depend on the number of sites with confirmed diagnoses, the features of the sites (including season) and the extent of the outbreak. If entire sites or leases cannot be fallowed at once, the usefulness of this procedure is reduced.
In Scotland, where furunculosis is endemic, it has been common practice since the late 1970s to completely destock, disinfect and allow salmon-farm sites to fallow. This decreases the infection pressure on the site by removing the main sources of infection and allowing separation of year classes. A fallowing period of up to one month, and cleaning of nets and equipment, allows restocking without risk of re-infection providing other sources of infection are also managed (Munro 1988). This practice does not eliminate infection but appears to be an effective disease control measure where the aetiological agent is endemic.
For eradication, restocked fish must be free of covert or overt infection or disease, confirmed by laboratory testing of an acceptable sample of the restocked population. If areas are declared free of furunculosis, fish introduced into those areas must also be free from infection.
2.5 Public awareness
2.6 Feasibility of control or eradication of furunculosis in Australia
- Eradication—eradication of furunculosis and the pathogen A. salmonicida subsp. salmonicida from Australia.
- Containment, control and zoning—containment of the disease and pathogen to areas with enzootic infection, prevention of further spread and protection of uninfected areas.
- Control and mitigation of disease—the implementation of management practices that decrease the incidence and severity of clinical outbreaks.
2.6.1 Response option 1: Eradication
The eradication option is directed at removing the risk of exposure of unexposed fish populations to the pathogen and preventing further spread of infection.
Eradication may be feasible if initial epidemiological investigations reveal:
- a limited focus or distribution of infection
- an apparent point source or limited point sources
- no apparent involvement of wild fish or reservoirs in the environment
- that containment is economically and technically feasible.
Depending on the level of exposure, different fish populations require different response strategies.
Clinically diseased populations of fish
These fish, along with infectious waste, are the main source of A. salmonicida subsp. salmonicida in the environment. Immediate destruction and disposal of all clinically affected populations of fish and dead fish is essential to the success of an eradication strategy. Emergency harvesting is not an option for these fish, although some of the equipment used for harvesting (e.g. pumps, percussion stunning devices) may be used in the removal of fish.
There is currently no mechanism or government–industry cost-sharing arrangement to compensate farmers for the destruction of fish.
If A. salmonicida subsp. salmonicida is confirmed in a non-salmonid species and the preferred option is eradication, then clinically diseased populations of fish must be destroyed.
Disposal methods should be chosen carefully to ensure there is no contact of the dead fish with waterways or vectors.
Exposed or potentially exposed, clinically normal populations of fish
These fish are safe for human consumption and emergency harvesting may be the quickest option to remove market-size fish in the shortest time. Emergency harvesting of fish involves a risk of further transfer of infection, which may jeopardise the success of an eradication strategy. A final decision about the use of emergency harvesting will need to be made on a case-by-case basis.
Fish that are not at market size must be destroyed. Growing fish until they reach market size is not an option, as these fish could spread infection to wild or feral fish stocks.
For both clinically diseased and exposed (or potentially exposed) populations of fish, strict control measures to prevent further spread of infection during emergency harvesting and destruction must be implemented and followed. This includes:
- disinfection or decontamination of all equipment and personnel involved in harvesting, slaughter and processing
- application of quarantine restrictions and procedures to the infected site, including personnel, equipment and vehicles
- ensuring only on-site processing where possible
- implementation of strict movement and disinfection procedures to the transport of fish to off-site processing plants. These will then become infected sites, and quarantine procedures will apply to these sites
- holding, treatment and safe disposal of slaughter and processing discharge, including holding water and waste offal
- ensuring that the final product will not result in the spread of infection; this may require placing restrictions on which products may be released to the market place (e.g. allowing skinless fillets but not whole fish).
If A. salmonicida subsp. salmonicida is confirmed in a non-salmonid species and the preferred option is eradication, then all exposed or potentially exposed fish must be destroyed and disposed of in a manner that prevents further spread of disease.
Unexposed populations of fish
Where epidemiological evidence suggests that the possibility of exposure of fish populations to disease is very low, young (i.e. pre-market sized) unexposed populations of fish may be allowed to grow out. However, these fish must be closely monitored during the growing-out period. Strict on-farm transportation and processing biosecurity protocols are still important.
Market-sized unexposed fish may be emergency harvested and slaughtered for human consumption.
Destruction of unexposed fish populations located within a declared area or within a destocking area will decrease the chance of spread of infection to fish stocks and prevent propagation of the disease. Compensation is not formally assured if these fish are destroyed.
2.6.2 Response option 2: Containment, control and zoning
This response option is more likely than response option 1 (eradication) if:
- A. salmonicida subsp. salmonicida is confirmed or suspected to be present in wild fish populations
- there are doubts regarding the extent of the infection
- he extent of infection means that the cost associated with the eradication is deemed too high. Compensation is not formally assured for stock that is destroyed.
Containment of disease requires zoning. The feasibility of a zoning program can only be assessed at the time of the outbreak, taking into account the required movement restrictions on fish, people, vehicles, boats and market access for the fish products and byproducts.
Clinically diseased populations of fish
These fish, along with infectious wastes, are considered to be the main source of A. salmonicida subsp. salmonicida in the environment, and constitute the greatest risk for spreading the infection to uninfected zones.
Clinically diseased populations of fish should be destroyed to prevent further spread of disease. Antibiotic treatment for these populations will reduce mortality, but will not eliminate infection and hence is not recommended.
Exposed or potentially exposed, clinically normal populations of fish
A successful zoning program relies on the implementation of movement restrictions on exposed or potentially exposed populations of fish that prevent infection spreading to uninfected zones.
In a declared area, controlled grow out and slaughter may be feasible without further spread of infection, although fish should be treated as if infected.
Antibiotic treatment of a fish population may be considered feasible and appropriate where epidemiological evidence suggests possible exposure to disease. Treatment can be cost-effective and used as a temporary measure to prevent losses of fish until other preventive practices can be implemented. Antibiotic-resistant strains of A. salmonicida subsp. salmonicida have become established in some regions overseas and this must be considered when deciding to treat with antibiotics.
Treatment will also mean that antibiotic residues may be present in the fish if they are destroyed. Fish cannot be harvested for human consumption within the withholding period for the antibiotic used.
Destroying fish is an option for containment, control and zoning, as it is can decrease the infectious load on a site and minimise the spread of infection. There is currently no established mechanism of compensation for fish that are destroyed, hence destroying fish for this purpose must be carefully considered.
Unexposed populations of fish
Control options for unexposed populations of fish are the same as those outlined for eradication (Section 2.6.1). The implementation of a zoning program and associated control measures to maintain uninfected zones would be necessary.
Vaccination of unexposed fish is an option if it is desirable to move unexposed fish into a region where they may be exposed to disease. For example, the translocation of young salmonids from a freshwater hatchery that does not have disease to a marine site where disease is known to be present. In this case, vaccination against furunculosis must occur before fish are translocated, and with sufficient time for the vaccine to increase immunity of the fish to the disease19. Vaccination does not provide total protection, so these fish must be closely monitored for evidence of infection with naturally occurring A. salmonicida subsp. salmonicida. The use of culture methods will allow detection of live A. salmonicida subsp. salmonicida if present in sufficient numbers, but will not be suitable for the detection of covert infection. Strains of bacteria used in commercially available vaccines are killed and will therefore not grow on culture.
2.6.3 Response option 3: Control and mitigation of disease
The principles of control and mitigation of disease are to reduce the impact of furunculosis. This involves implementation of management practices that decrease the incidence, distribution and severity of clinical outbreaks.
The general control measures for containment, control and zoning described in Section 2.6.2 also generally apply in the control and mitigation of disease, except for the measures associated with zoning. In this response option, zoning is not considered and hence neither are the strict restrictions placed on movement of exposed or potentially exposed populations of fish.
All efforts should still be made to prevent the spread of disease and minimise the risk of exposure in naive populations. Likewise, a vaccination strategy to vaccinate unexposed populations may be implemented, particularly where there is a risk of exposure.
If a clinical disease outbreak occurred, treatment with antibiotics is justified and encouraged to reduce overall mortality in affected populations of fish.
2.7 Trade and industry considerations
2.7.1 Export markets
The Australian Quarantine and Inspection Service (AQIS) should be contacted for further information regarding health certification of exports.
2.7.2 Domestic markets
A cautious approach is required for the harvest of exposed or potentially exposed product for the domestic market, due to the potential for further spread of infection. As previously stated, A. salmonicida subsp. salmonicida is not a zoonosis. Therefore, provided normal seafood safety practices are followed, there are no public health risks associated with harvesting infected fish for human consumption. If healthy, potentially infected or infected fish are destined for human consumption, the chief medical officer and health authority of the relevant state or territory should be notified that there are no human health concerns associated with A. salmonicida subsp. salmonicida, and that furunculosis is not a zoonotic disease.
Decisions regarding the release of fish or fish products to the domestic market will depend on the response strategy implemented.
7 Fish farms that are insured against stock losses will generally be able to claim for loss of fish where fish have died from disease, but will not be able to claim for loss of fish where fish have been destroyed as part of a disease control strategy.
11 It is also worthwhile to use the investigation of neighbouring fish farms and processing plants to alert these facilities about the outbreak and provide immediate education on prevention, including implementation of risk management strategies.
13 Degree days’ equals the number of days multiplied by the water temperature, hence 1000 degree days may be 100 days at an average water temperature of 10 ºC or 50 days at an average water temperature of 20 ºC.
19 The time required for immunity to develop in the fish will depend on water temperature. Commercially, at temperatures >10 ºC, a minimum period of two weeks is necessary.
09 Mar 2010