Disease risk evaluation

Victoria Alday-Sanz, DVM, M.S., Ph.D.

Effective tool for minimizing aquaculture losses

The importation of live aquatic animals must be carefully controlled. Apparently healthy animals can be carriers of pathogens that pass on to populations of different susceptibility.

Diseases are a constant if not altogether routine aspect of livestock rearing, and as such we often find them in aquaculture. Reducing their presence or impact is part of a risk management strategy that would take into account the present health status of the animals in the facility and neighboring area, and the technical level of the farm staff. The legislative framework of the country and region, their diagnostic capacity and access to health management expertise, and the biosecurity plan for each facility must also be considered.

Table 1 provides a scoring system for these parameters in an attempt to quantify the relative importance of each.

Alday-Sanz, Disease risk evaluation tool. Table 1

1Present health status of the facilityPresence and type of primary pathogens.
Possibility of treatment or vaccination.
Diseases caused by opportunistic pathogens imply poor health management.
2Disease records of the last two yearsPresence and type of primary pathogens.
Possibility of treatment or vaccination.
Diseases caused by opportunistic pathogens imply poor health management.
3Capabilities of on-farm staffCapacity to spot potential health problems.
Field diagnostic kits on site.
4Disease status of neighboring farms, areaCheck World Animal Health (OIE) and scientific literature.0-3
Legislative Framework
5National legislation on disease prevention
and control
To prevent the introduction of pathogens into a country or their spread.0-10
If not:
5.1Restriction on the importation of live aquatic animalsBan of importation from countries of lower sanitary status or request
of health certificates.
5.2Regulation of the effluents and waste from aquatic animal processing plantsAvoid the release of pathogens into the natural environment and posterior infection of farmed stocks.0-1
5.3National reporting and isolation of infected facilitiesEarly warning system, isolation of infected facilities and national contingency plans.0-3
5.4Existence of a national diagnostic reference laboratoryAs support and control for the quality of private laboratories.
Updated on exotic and emerging diseases.
6Regional harmonizationAmong neighboring countries to prevent pathogen introduction.0-1
7Facility under certification programAlthough mostly focused on environmental, organic and social issues, they include good aquaculture practices.0-3
Diagnostic Capacity
8Histology-based methodsLow-sensitive technique for disease outbreak diagnosis and detection
of new pathologies.
9Polymerase chain reaction (molecular
Highly sensitive technique for known pathogens detection, surveillance and screening of broodstock and offspring.0-4
10Evaluation of diagnostic qualityOperation under international standards, participation in ring tests, academic level of staff, regular equipment calibration.0-2
11External biosecurityPresence of physical barriers, restriction of incoming vehicles and visitors, water filtration.0-3
12Surveillance of wild animalsSub-clinical infection can result in the transmission of pathogens to farmed animals.0-2
13Introduction of live aquatic animals
into the facility
Sources of animals (eggs, broodstock or larvae) and health certificates need to be evaluated.0-2
14Internal biosecurityHealth management program with a contingency plan in place.0-4
15Regular surveillance of farmed animalsDetect presence and prevalence of pathogens to forecast possible
Total Score0-60
Table 1. Disease risk evaluation tool. The higher the score, the lower the risk of disease outbreak and spread in the facility.

Health Status

Obviously, the health status of the animals at the time of purchasing or renewing an insurance policy is of great importance, but the disease records of the previous two years are still more important. The expression of diseases due to pathogens can be seasonal. Some pathogens, such as white spot syndrome virus in shrimp, can cause outbreaks during cold weather. During warm weather, pathogens like Streptococcus can manifest in different species of fish.

Pathogens can be classified as primary or secondary. Primary pathogens cause diseases by themselves, while the latter, so-called “opportunistic” pathogens need an underlying situation such as poor environmental conditions or a primary pathogen to cause disease.

If monitoring does not detect primary pathogens within two years, they may be considered absent or eradicated. Diseases caused by opportunistic pathogens require investigation of the primary cause, which often is related to poor health management of the animals. Once conditions are corrected, disease problems disappear.

Not all pathogens have the same impacts, so efforts to combat each of them should not be made to the same extent. Pathogens vary in terms of severity of the symptoms, including mortality and means of transmission (particularly vertical transmission), possibility of exclusion, treatment or vaccination. Therefore, the detection of each specific pathogen in a facility should be evaluated in terms of risk to production.

Regional legislation, harmonization

Well-developed national legislation may prevent the introduction of pathogens into a country or their spread within the country. The European Union Directive 2006/88/EC that was enforced in 2008 provides the most complete legislative frame for the protection of aquaculture production. Other countries are already working on the harmonization of their laws using the European directive as a reference.

It should always be taken into account that, while having suitable legislation is important, its implementation is crucial for success. The degree of implementation of legislation varies greatly among countries, and particularly in developing countries – where most of the aquaculture production takes place – the resources for implementation can be deficient.

Effective disease prevention and control legislation needs to take four major issues into account:

  • Restrictions must control the importation of live aquatic animals, as the movement of live animals is the most efficient way of introducing pathogens. Apparently healthy animals can be carriers of pathogens that can be passed on to populations of different susceptibility.
  • Effluents and waste from aquatic animal processing plants need to be regulated to avoid the release of pathogens into the environment and later reinfection of farmed stocks.
  • Disease outbreaks considered relevant for the industry must be reported, and the infected facilities must be isolated to prevent further spread of the pathogen and minimize losses. The earlier this is done, the easier it is to control the spread of disease.
  • A national reference diagnostic laboratory can support private labs on the quality of their methodology and results. The national lab should be regularly updated on exotic and emerging diseases to facilitate quicker response.

Having knowledge of the disease status of a country or region is also relevant due to the continuity of the aquatic bodies, as pathogens do not recognize private properties or boundaries. Often, it is only a matter of time until the disease problem of a neighboring facility or country becomes ours, as well. The risk of disease transfer depends on the type of pathogen and its mode of transmission.

Certification programs

A number of certification schemes address production standards and product quality. The production-oriented programs – Best Aquaculture Practices, GlobalGAP or World Wildlife Fund – deal mostly with sustainability and environmental and social impacts. Quality product programs such as those of Naturland or the International Foundation for Organic Agriculture deal mostly with organic labels, while gourmet products are identified by programs like Label Rouge in France and the United Kingdom.

It is now widely recognized that the implementation of best management practices in farming can bring significant improvements in production, reductions in the outbreaks of disease due to secondary pathogens and increased product quality. But none of these certification schemes specifically covers the area of disease prevention, so complying with the programs does not insure reduction or minimization of losses due to diseases caused by primary pathogens, the ones responsible for catastrophic losses.

Diagnostic capacity

The diagnostic capacity of a farming facility – or its access to it – is of crucial importance for a speedy response to a disease outbreak. Test techniques vary on sensitivity and specificity, and their application should depend of the purpose of the test.

In the case of disease outbreak, low-sensitivity testing such as histology should be used to confirm that lesions are caused by a particular pathogen. In the case of screening or detection of pathogens in the absence of clinical signs, a highly sensitive method such as polymerase chain reaction is necessary.

Evaluation of the quality of diagnostic laboratories and procedures is highly relevant. Operating standards such as EN ISO/IEC, EN 45002 or EN 45003 facilitate this task, as well as the participation in ring tests.


Biosecurity broadly describes the process of managing biological risks in a holistic manner. Identification of risks can be done from two perspectives – bioexclusion or risk coming from the outside and biocontainment risks that spread within a facility. The principles of biosecurity can be applied to different production species and intensive or extensive systems, but the implementation of biosecurity needs to be defined for each of these cases.

The main activities within external biosecurity involve controls on the introduction of live aquatic animals into the facility, whether they are eggs, broodstock or larvae. This is the most efficient pathway for pathogens, so introductions should only be carried out after the acceptance of a reliable health certificate. Surveillance of the health status of wild animals from the surrounding areas can provide information on future risks faced by the facility. The effectiveness of physical barriers, restriction and disinfection of incoming vehicles, water filtration or recirculation, and use of crab and bird fences needs to be evaluated.

The main activities within internal biosecurity involve the active surveillance of farmed animals to determine their health status and possibly detect emerging disease problems at different developmental stages. The compartmentalization of production units, restriction of staff movement and routine disinfection are examples of the control of pathogens within facilities.

As part of the biosecurity plan, each facility needs a contingency plan. The objective of a contingency plan is to recover in the minimum time, at the minimum cost and with minimum disruption after a disease outbreak. Short recovery and effective control are linked to rapid initial response and rapid, effective implementation of biosecurity measures.

(Editor’s Note: This article was originally published in the May/June 2011 print edition of the Global Aquaculture Advocate.)

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