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The Belgian Regional decrees classify human and animal pathogens into three classes of risk and plant pathogens into two classes of risk. The class of risk given to a wild biological species must be regarded as representative of the theoretical maximum risk incurred by humans, animals, plants or the environment.
The classification of a of micro-organism takes into account the risk for health, for the community, and - in the case of the animal and the plant - the possible economic impact of the disease.
The main criteria for the classification are:
- importance of the disease or severity of the infection;
- infectivity (virulence of the strain, infective dose, mode of transmission, natural route of infection);
- host range of the micro-organism and spectrum of specificity of target-species;
- biological stability;
- potential of survival and dissemination in the community or the environment;
- availability and effectiveness of prophylactic or therapeutic measures (such as vaccination or antisera, antibiotics, chemotherapeutic agents, taking into consideration the possibility of emergence of resistant strains).
The classification of the biological risk for the plant includes three additional criteria:
- the prevalence of the micro-organism in the Belgian environment;
- the presence of target-species around the installation or the site of waste disposal;
- the 'exotic' character of the micro-organism.
The class of risk is based on the potential effect of a micro-organism on a healthy human, animal or plant.
For instance, in case of human pathogens, the determination of the class of risk does not account in which an individual may have increased susceptibility to such a micro-organism, e.g., preexisting diseases, medications, compromised immunity, pregnancy or breast feeding (which may increase exposure of infants to some agents).
In that respect, opportunistic pathogens (that cause disease only in individuals who are compromised in either their innate or humoral immune defences) are included in the class of risk 1. However, the most representative of them are listed in the regional classifications.
When the pathogenicity of a viral, bacterial, fungal or parasitic strain is attenuated (by spontaneous emergence, selection or because of the use of the techniques), the user can justify a reduction of the biological class of risk of this strain compared to the class of risk assigned to the parent strain.