3. Preferred Australian Response Options

3.1 Overall Policy for Furunculosis

Furunculosis is an exotic, highly contagious bacterial disease of salmonid fish. The disease has the potential to cause serious economic loss in the farmed salmonid industries in Australia due to the morbidity and mortality expected in these naive populations. The disease also has the potential to cause morbidity and mortality in wild salmonid populations and possibly in native salmoniform species.

The policy of control of an outbreak of furunculosis in Australia depends upon both the nature of the outbreak and the response strategy adopted. The choice of response options will be decided by the chief veterinary officer of the state or territory or the director of fisheries (or both) in which the outbreak occurs. This decision will be made in consultation with the relevant industry sector where major aquaculture industries may be affected. Epidemiological investigation will be used to assist in the decision.

There are currently no government–industry cost-sharing arrangements to cover the costs of control operations or to compensate for destroyed stock. Successful implementation of a control or eradication program may not be possible without first establishing agreement on these costs.

There are three possible response options for an outbreak of furunculosis in Australia:

• option 1—eradication of furunculosis from Australia. This is the preferred option.
• option 2—containment, control and zoning of the pathogen to areas with endemic infection, prevention of further spread and protection of uninfected areas
• option 3—control and mitigation of disease by implementing management practices that decrease the incidence and severity of clinical outbreaks.

Each of these control options involves the use of a combination of strategies, which may include:

• quarantine and movement controls on fish, fish products and items in declared areas to prevent spread of infection
• destructionand safe disposal of all clinically diseased or dead fish as soon as possible, to prevent further bacterial shedding
• decontamination of facilities, products and equipment to eliminate the bacterium on infected premises and to prevent spread of infection
• surveillance to determine the source and extent of infection and to provide proof of freedom from the disease
• zoning to define and maintain infected and disease-free zones
• treatment of infected fish by antibiotic use or vaccination (or both)
• an awareness campaign to encourage cooperation by the industry and the community.

An uncontrolled outbreak of furunculosis would cause short-term and long-term production losses with consequent dislocation and economic losses in the salmonid farming industry and associated production, sales and export industries. It will therefore be necessary to act immediately to control or eradicate the disease.

The chief veterinary officer (CVO) or director of fisheries in the state or territory in which the outbreak occurs (or both) will be responsible for developing an emergency animal disease response plan. This plan will be submitted to the Aquatic Consultative Committee on Emergency Animal Diseases (AqCCEAD), who will provide advice on the technical soundness of the plan and its consistency with AQUAVETPLAN.

CVOs or directors of fisheries (or both) will implement the disease control measures as agreed in the emergency animal disease response plan and in accordance with relevant legislation. They will make decisions on follow-up disease response measures in consultation with AqCCEAD. The detailed response measures adopted will be determined using the principles of control and eradication (see Section 2), epidemiological information about the outbreak and the financial feasibility of the option.

For information on the responsibilities of the other state or territory disease control headquarters and local disease control centres, see the AQUAVETPLAN Control Centres Management.

3.2 Response Options

The circumstances surrounding an outbreak of furunculosis will greatly influence selection of the most suitable response option. Figure 3 details the actions that should occur immediately on suspicion of furunculosis. In this situation, it is critical that measures are taken to contain any potential spread of disease while laboratory tests are done to confirm infection. These measures are clearly identified in Sections 2.2 and 2.3.

It is important to note that suspicion of disease may not always precede confirmation of A. salmonicida subsp. salmonicida. For example, it is possible that the first knowledge of the pathogen’s presence in Australia may be confirmation of the pathogen from a routine health sample taken from a non-salmonid fish (e.g. an imported ornamental fish species while still in quarantine).

If furunculosis or the presence of A. salmonicida subsp. salmonicida is confirmed, refer to Figure 4, which has been developed to help identify the most appropriate response option. This decision tree is flexible, and highly dependent on the specific situations experienced. Although eradication is the preferred option, the decision to choose eradication must be taken carefully as:

• it will involve destruction of fish for which compensation is not assured
• it may be difficult to determine the extent of the infection given the potential for reservoirs of infection in the environment and the difficulties associated with culturing the pathogen from covertly infected fish.

If epidemiological evidence suggests no further spread in a closed system, such as a recirculation aquaculture system or an ornamental fish tank, then responding to the finding by eradication is reasonably straightforward and desirable.

In any system where there is potential contact with wild fish, it is likely that some exposure may have occurred by the time the disease is confirmed. In these circumstances, a decision must be made as to whether it is too late to attempt eradication (i.e. whether the ‘horse’, or in this case ‘fish’, has already bolted).

Figure 3. Flowchart of activities to follow when furunculosis is suspected

The number of fish affected may influence the decision to eradicate stock. For example, if a small salmonid hatchery containing a few broodstock and some fingerlings is affected, the short-term cost associated with destroying these fish will not be as great as the cost associated with destruction of a sea pen of Atlantic salmon near harvest weight. The cost of these fish will be hundreds of thousands of dollars if one pen is affected, and potentially millions of dollars if multiple pens are affected.

3.2.1 Option 1—Eradication

If the response option of eradication is chosen, at each site identified as infected there must be:

• an immediate outbreak investigation, particularly tracing of potential spread onto and out of premises
• zoning to define the declared area and disease-free areas
• quarantine and movement controls or restrictions on fish, fish products, water and any other vectors (including fomites ) located within declared areas to prevent further spread of infection, as discussed in Section 2.2
• removal of untreated discharge water from a semi-closed or closed system; untreated discharge water must not be released into the environment
• destruction of all clinically diseased fish; emergency harvest is not an option
• destruction of all exposed or potentially exposed, clinically normal fish from the water. If there is any doubt as to exposure, fish are to be treated as exposed. Market-size fish may be emergency harvested provided it can be done in a manner that will not pose a risk of further disease spread. Small fish may not be allowed to grow out
• destruction of any other non-salmonid fish that may be involved
• harvesting of unexposed fish that are of market size. Small fish that have not reached market size may be allowed to grow out, provided there is no risk of exposure. If the situation changes and these fish are exposed or potentially exposed, then they must be removed immediately
• ongoing decontamination of facilities, products, equipment, vehicles, boats etc. throughout the eradication process to eliminate the bacterium from infected premises and to prevent spread in declared areas
• education and disease awareness for those involved in, or affected by, the eradication process.

Treatment with antibiotics and vaccination are not options for eradication.

For further discussion on eradication of disease, refer to Section 2.6.1.

3.2.2 Option 2—Containment, control and zoning

If the response option of containment, control and zoning is chosen, measures implemented include those outlined in Section 3.2.1 above, with the following variations:

• Destruction of all exposed or potentially exposed, clinically normal fish from the water is an option, but treatment with antibiotics will achieve reduction in pathogen release and will avoid costly destruction of stock. Market-size fish should be emergency harvested, provided it can be done in a manner that will not pose a risk of further disease spread. Small fish may be allowed to grow out, but should be treated if clinical disease develops.
• Destruction of other cultivated, non-salmonid fish that may be involved, as these fish cannot be released from the infected premises or infected region alive and hence cannot be sold.
• Harvesting of unexposed fish that are of market size. Small fish that have not reached market size may be allowed to grow out. If the situation changes and these fish are exposed or potentially exposed, or develop clinical disease, then they must be monitored and treated if clinical disease develops.

In addition, the following measures may also be taken:

Sites may be fallowed until the disease is contained.

Unexposed fish that may be exposed or potentially exposed can be vaccinated. Vaccination against furunculosis must occur before fish are translocated, and allow sufficient time for the vaccine to increase the immunity of the unexposed fish population to the disease.

In areas where there is uncertainty as to whether infection remains, restocking with sentinel fish stocks may be useful.

For further discussion on containment, control and zoning of disease, refer to Section 2.6.2.

3.3 Option 3—Control and mitigation of disease

If the response option of control and mitigation of disease is chosen, there must be:

• implementation of husbandry, management and hygiene practices that aim to reduce stress on fish and minimise bacterial load to decrease the incidence and severity of furunculosis outbreaks
• vaccination of unexposed fish that may potentially be exposed
• treatment with antibiotics of clinically affected, exposed and potentially exposed fish to reduce the morbidity and mortality associated with outbreaks of disease
• fallowing of sites to decrease the infectious load on the site between batches of fish.

For further discussion on control and mitigation of disease refer to Section 2.6.3.

3.3 Criteria for proof of freedom

Proof of freedom from furunculosis, which may be important for trade, can be demonstrated at the aquaculture establishment, zone and country level. Criteria for proof of freedom at each level are given in the OIE Aquatic animal health code for the various OIE-listed diseases (OIE 2009).

Furunculosis is not an OIE-listed disease. However, the criteria for demonstration of proof of freedom for listed diseases can be used as a guide for Australia to demonstrate proof of freedom from furunculosis, if required.

3.4 Funding and compensation

There is currently no cost-sharing agreement in place between industry and governments for an emergency response to furunculosis. Under this situation, compensation is not assured should fish be destroyed as part of the response strategy.

Further information on the impact of the disease is provided in Section 1.7.

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