Biohazards of investigations on the transmissible spongiform encephalopathies

There is considerable current interest in the agents that cause the spongiform encephalopathies: scrapie, transmissible mink encephalopathy, kuru, and Creutzfeldt-Jakob disease (CJD). The unusual properties of these agents, their elusiveness, and their pathogenicity for humans (in the cases of kuru and CJD) make these agents interesting subjects of investigation but also make imperative a consideration of their potential biohazards in the laboratory. In view of both the potential pathogenicity of these agents and the potential hazards of many laboratory procedures, a series of physical containment levels, each of which corresponds to a range of composite risk factors, are suggested. The estimated composite risk factor used is a function of the potential pathogenicity or relative risk factor of the agent and the potential hazard of a laboratory procedure. The lowest risk factors (1 to 2+) correspond to levels of containment similar to those recommended by the Center for Disease Control for class II microorganisms, while the highest risk factors (5 to 8+) correspond to levels similar to those for class III microorganisms. The use of such a biohazard ranking system aids in a rational approach to selection of equipment and procedures.