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Trained technicians are important for disease diagnosis
Trained technicians are important for disease diagnosis
FAO/18011/P.Jenkin

Issue

When a disease breaks out at epidemic levels in an aquaculture sector, it frequently constitutes an emergency. Such emergencies can happen at all levels; farm, local area, national, or regional. The most important consideration for avoiding such emergencies is proper advance planning and preparedness.

Possible solutions

Advance planning to deal with serious disease outbreaks can significantly reduce the social and economic impacts that result. In addition, prompt action based on solid contingency planning can reduce the consequent spread of disease agents. Using a fire outbreak as an analogy, economic losses will be lower if the fire is detected quickly and fire fighters arrive at the site promptly, and with the resources necessary to stop the fire. The speed of their arrival on the scene depends on the efficiency of the reporting system, the speed of their response and the availability of fire fighting equipment. The effectiveness of the fire-fighting team depends on their training and their experience under different conditions. Preparation and response to aquaculture disease can be looked at in a similar way. To minimise losses, it is necessary to have good surveillance, accurate disease diagnosis, efficient reporting systems and well-trained specialists who know how to deal with a variety of potential disease emergencies. The needed response may be the same at government/institutional level and at the farm level, but the extent and manner of action may differ between the two.

It is difficult to predict when a new disease might appear, or how much of an impact it is likely to have on a particular aquatic resource. Known diseases with established etiological information are more predictable and thus more easily dealt with. Some disease outbreaks are small and/or localised, and so are more easily controlled. Others may be large, spread rapidly and be difficult to manage. Disease outbreaks are influenced by different factors, including weather, geographic isolation, and transmission dynamics. All these factors can affect the ability of personnel to respond to, contain and tackle a disease outbreak. Private companies should be included in the overall planning process of how disease outbreaks should be dealt with, in close partnership with local, state, and central government agencies. Strategies can be developed based on the information available, including: what diseases are known to occur within an area, and how frequently; conditions which are known to predispose the animals (or plants) to disease; the proximity to other farms and areas where the disease may occur; and, knowledge of any extreme weather conditions that might occur in the area at different times of year.

A trained team must investigate problems as quickly as possible
A trained team must investigate problems as quickly as possible
FAO/FIRI/F.Marttin

Contingency plans should be designed to prepare for the kind of disease outbreak that is "most likely" to occur in a particular area or facility. On rare occasions a new disease occurs, or the impact of a disease is greater than anticipated. To prepare for these unusual but significant incidents, contingency plans must also include "worst-case" scenarios, such as for a highly infectious disease that may spread rapidly and cause heavy mortalities.

One difficulty with "new" disease situations is deciding at which point it is serious enough to warrant classification as an emergency. Actual diagnosis of the disease may be of limited value in the decision making process, since the origin and cause of the problem may be unknown. Samples should be sent for analysis to obtain pathology information, but the time required for laboratory processing is often too long for the results to be of immediate assistance to a farmer suffering acute or severe mortalities of his stock. This is particularly so when the analysis requires identification of a new pathogen and investigation of its modes of transmission. However, firm diagnosis of the disease is not always necessary for decisions to be made on interim control measures. Many diseases have been first described in the past only on the basis of their gross pathology or characteristic features. These descriptions allow farmers or extension staff to make a presumptive diagnosis with a clear, consistent case definition and reach an early a decision on the best disease control measures. Laboratory results may then reinforce or refute the presumptive diagnosis, increasing the level of diagnostic certainty and permitting the refinement of effective control strategies.

Action taken

A carefully designed contingency plan will describe the major actions to be undertaken when a disease occurs. To optimize efficacy and minimize the spread of a disease, these actions should take place immediately following detection or reporting of the outbreak and should include:

  • notification of all staff, individuals, private companies or government agencies that are responsible for the disease control effort;
  • notification of all those likely to be affected by the disease or the proposed control measures;
  • mobilizing trained personnel to the site quickly;
  • determination of the extent of the disease, its nature, speed of transmission, and likelihood of spread to neighboring farms, sites or the local environment;
  • taking measures to prevent any further entry of the disease agent onto a site or into a population;
  • confining the outbreak to a limited area;
  • eradicating the disease where possible (usually this is only feasible in land-based facilities or isolated ponds - there have been few cases of successful eradication of a disease agent from open-water populations);
  • rapid removal of moribund or dead animals from the water, followed by sterile or landfill disposal;
  • follow-up surveillance and monitoring after the disease outbreak is brought under control.
White Spot Virus Syndrome in shrimpPenaeus monodonCourtesy of Donal Lightner
White Spot Virus Syndrome in shrimpPenaeus monodonCourtesy of Donal Lightner

Outlook

At the farm level, the complexity, but by no means the efficacy of a contingency plan, depends upon the size and scale of the operations. Small farms can usually manage with basic plans such as gross surveillance and monitoring, due to their straightforward organizational structure (owner-operated, owner and manager, owner and technician) where everyone has clear roles and responsibilities and good lines of communication exist. Larger farms often have a more complex division of responsibility and thus need more detailed contingency plans. The ability of farms and organisations to co-operate in dealing with disease emergencies depends partly upon the awareness of the short and long term benefits of good planning. Cooperation is best served by a clear assignment of responsibility to the different staff members or to the various representatives of the farms and organizations involved.

 
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