VetoFish - Normes sanitaires

Manual of Diagnostic Tests for Aquatic Animals (2003)

INTRODUCTION
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INTRODUCTION
The clinical signs in fish with the diseases listed in the OIE Aquatic Animal Health Code (referred to hereafter as the Aquatic Code) are not pathognomonic. Moreover, these infections may occur as subclinical infections of asymptomatic pathogen carriers.

The only dependable approach for diagnosis of fish diseases therefore lies in the specific identification of the pathogens using laboratory methods. These methods, which are suitable for the diagnosis of isolated cases of disease as part of national aquatic animal health surveillance/control programmes, form the main contents of this the Manual of Diagnostic Tests for Aquatic Animals (referred to hereafter as the Aquatic Manual).

Such health surveillance programmes aim to determine, from the results provided by standardised laboratory procedures performed with samples collected according to defined rules, the health status of aquatic animal stocks from a particular production site and even a geographical zone or entire country. The satisfactory implementation of such aquatic animal health surveillance/control programmes, requires the existence of both adequate legislation and resources in each country interested in aquatic animal health.

The diagnostic methods presented in this Aquatic Manual are all direct diagnostic methods. Due to the insufficient development of serological methodology, the detection of antibodies to pathogens in fish has not thus far been accepted as a routine method for assessing the health status of fish populations. However, the validation of some serological techniques for diagnosis of certain infections could arise in the near future, rendering the use of serology more widely acceptable for diagnostic purposes. In earlier editions of the Aquatic Manual, the only diagnostic methods described for screening or diagnosis of fish diseases have been based either on isolation of the pathogen followed by its specific identification, or on the demonstration of pathogen-specific antigens using an immunological detection method. However, in recent years, molecular techniques such as the polymerase chain reaction (PCR), DNA probes and in-situ hybridisation have been increasingly developed for these purposes. The experiences of the last decade indicate that the PCR techniques will eventually supersede many of the classical direct methods of infectious agent detection. It is clear that in many laboratories, the PCR is replacing virus isolation or bacteria cultivation for the detection of agents that are difficult or impossible to culture. There are several reasons for this trend, including that virus isolation requires: i) the presence of replicating viruses; ii) expensive cell culture and maintenance facilities; iii) as long as several weeks to complete the diagnosis; and iv) special expertise, which is missing or diminishing today in many laboratories. Although PCR assays were initially expensive and cumbersome to use, they have now become relatively inexpensive, safe and user-friendly tools in diagnostic laboratories. Where a PCR method has been standardised sufficiently for its use for a listed fish disease to become widely and reliably available, it has been added to the more traditional methods in the Aquatic Manual. For the most part, molecular methods for fish diseases are recommended for either direct detection of the pathogen in clinically diseased fish or for the confirmatory identification of a disease agent isolated using the traditional method. With one or two exceptions, molecular techniques are currently not acceptable as screening methods to demonstrate the absence of a specific disease agent in a fish population for the purpose of health certification in connection with international trade of live fish and/or their products. There is a need for more validation of molecular methods for this purpose before they can be recommended in the Aquatic Manual. The principles and methods of validation and quality control of PCR methods for diagnosis of aquatic animal diseases are described in Chapter 1.1.3.

General information on diagnostic techniques for fish diseases is given in Chapter I.1. Mollusc and crustacean diseases differ in some ways from fish diseases. For example, diagnostic methods must be direct because these animals do not produce antibodies to pathogens. With the unavailability of the traditional pathogen isolation methods used for fish diseases, molecular techniques, particularly PCR, have increasingly supplemented the more traditional histological and tissue smear methods described in the Aquatic Manual for the listed mollusc and crustacean diseases, not only for diagnosis of clinical cases but also for screening programmes to demonstrate the absence of the specific disease agent for health certification purposes. General information on diagnostic techniques for mollusc diseases is given in Chapter I.2. and for crustacean diseases in Chapter I.3.

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