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McGill Laboratory Biosafety Manual - Second Edition, 1997
 


3 Classification of Pathogens

3.1 Conventional Pathogens

Criteria for classification of infectious agents are outlined in the document "Laboratory Biosafety Guidelines", published jointly by Health Canada's Laboratory Centre for Disease Control and the Medical Research Council of Canada (refer to Section 12, "Suggested Reading"). Essentially, microbiological pathogens are classified according to their impact upon the individuals who manipulate them, upon their colleagues, and upon the surrounding community. Agents which pose little or no risk are assigned to Risk Group 1, while those with the greatest hazardous potential are in Risk Group 4. Risk assessment is based upon several factors, including: Before setting up experiments involving new biohazards, consideration should also be given to conditions under which the infectious agent is used. For example, manipulation of large volumes and high concentrations of an infectious microorganism in culture media presents a greater risk than smearing the same pathogen on a slide. Work involving release of microbial aerosols, passage in animals and infection of arthropod vectors also increase the hazard. In these cases, pathogens should be handled as if they were in the next highest risk group: i.e., if the experimental procedure is likely to generate large amounts of aerosolized Risk Group 2 agent, the physical and operational requirements applicable to Risk Group 3 agents should be observed.

An outline of the characteristics of agents in each Risk Group is presented in Table 1. A comprehensive listing of the pathogens in each Risk Group can be found in "Laboratory Biosafety Guidelines".
 
Table 1 - Risks and characteristics associated with pathogens from Risk Groups 1 to 4, and recommended containment level and class of biological safety cabinet
Risk Group Risk Assessment Characteristics Examples Containment Level Biological Safety Cabinet
1
  • low individual
  • low community
  • unlikely to cause disease in animals or humans
  • Lactobacillus spp.,Bacillus subtilis, Naegleria gruberi, Micrococcus spp., E. coli K12 1 Not required
    2
  • moderate individual
  • low community
  • rarely cause serious human or animal disease
  • effective prevention and treatment available
  • limited risk of spreading
  • Hepatitis B virus, Toxoplasma spp., HIV (non-cultured), Ascaris, Salmonella typhimurium 2 Class I or Class II
    3
  • high individual
  • low community
  • may cause serious disease in humans or animals
  • effective prevention and treatment available
  • unlikely to be spread by casual contact
  • Lassa fever virus, Hantavirus, Yersinia pestis, Histoplasma capsulatum, Bacillus anthracis, cultured isolates of HIV* 3 Class I or Class II
    4
  • high individual
  • high community
  • likely to cause very serious disease in humans or animals,
  • readily transmitted from one individual to another, or between animals and humans
  • preventative vaccines or effective treatment not available
  • Marburg virus, Ebola virus, Crimean-Congo hemorrhagic fever virus, Herpesvirus simiae 4 (Class I or II plus positive pressure suits) or Class III

    *Containment Level 2 facilities and Level 3 operational requirements can be used for procedures involving viral isolation and identification. For production activities, use Level 3 physical and operational requirements.

    3.2 Unconventional Pathogens ("Slow Viruses")

    Some progressive neurological diseases are caused by unconventional "slow viruses". Among the slow viruses, prions (proteinaceous infectious particles) have been associated with transmissible degenerative diseases of the central nervous system in humans (Creutzfeldt-Jacob, kuru) and animals (transmissible encephalopathy of mink and scrapie in sheep and goats). These unconventional viruses are resistant to destruction by chemical (10% formalin, glutaraldeyhye, 70% ethanol, iodine) and physical (UV light, ionizing radiation, boiling) procedures. While there have been no documented cases of laboratory-acquired infections, the following precautions should be observed when handling neurological material from infected or potentially infected humans and animals:

    3.3 Genetically Engineered Organisms

    The term "biotechnology" describes a variety of techniques for manipulation of cells; biotechnology has long been used for purposes such as selective breeding of animals and food production (bread, yogurt, beer).

    More recently, in vitro incorporation of segments of genetic material from one cell into another ("recombinant DNA technology") has resulted in altered organisms which can manufacture products such as vaccines, hormones, interferons and enzymes. Genetically engineered organisms are used for treatment of waste and spills, and plants can be made resistant to cold, disease, pests and drought.

    However, biotechnology carries with it the potential for harm. A genetically altered organism may be directly pathogenic or toxic or, if released into the environment, might crowd out beneficial organisms, transfer undesirable genetic traits to wild species or mutate into a pathogenic form.

    The risks associated with recombinant DNA technology are to be assessed by the investigator when submitting the "Application of Use Biohazardous Materials" form to the McGill Biohazards Committee (refer to Section 1.2.1, "The Biohazards Committee"). Such assessments should be based upon the:

    When assessing the risk of, and containment level required by, a genetic engineering protocol, the following approach is recommended: if the components of a genetic manipulation are not hazardous, then the altered organism is unlikely to present a risk, and no restrictions are needed. However, if one of the components is potentially hazardous, a risk level appropriate for the known hazard is assigned and modified as required. Subsequent modifications depend on factors such as:

    3.4 Tissue Cultures

    Cell cultures derived from humans or animals known to be infected with a pathogen, as well as cultures known or suspected to contain infectious microorganisms (e.g., herpesvirus or EBV-transformed cultures) should be assigned to the risk group appropriate for the suspected or known pathogen and handled using the relevant containment level and work practices. Risk groups and containment levels for specific pathogens can be obtained from the federal "Laboratory Biosafety Guidelines" cited in Section 3.1, "Conventional Pathogens" above.

    In addition, mammalian cell cultures may carry unsuspected oncogenic, allergenic or infectious particles. It is impractical, if not impossible, to screen such cultures for all potentially harmful microorganisms: even well-characterized lines with a history of safe use can become contaminated by adventitious, possibly infectious, microorganisms. For this reason, it is prudent to treat all mammalian cultures as moderate risk agents (i.e., risk group 2) and to use containment level 2 facilities and work practices whenever working with them.


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