A unique facility to test the infectivity of human-generated airborne infections

Abstract

Tuberculosis (TB), one of the world's greatest killers, is predominantly spread by the airborne route. Drug-resistant M. tuberculosis has emerged as a global public health threat despite effective drugs and disease control strategies. Little is known about M. tuberculosis transmission and the efficacy of necessary environmental (engineering) interventions for infection control; particularly in light of the global HIV/Aids epidemic. This thesis covers the development, validation and calibration of the unique Airborne Infection Research (AIR) facility (apparatus) that utilises a biological model to sample airborne M. tuberculosis by transporting infectious air from patient wards to animal exposure chambers housing guinea pigs. This capability, hitherto a universal limitation due to the unique characteristics of the tubercle bacili, will now allow a collaboration of researchers horn around the world to undertake scientific studies to answer fundamental questions about the infectiousness of drug-resistant M. tuberculosis and the efficacy of various engineering interventions to minimise the spread of airborne disease. These experiments will provide the scientific blue-prints for design of safer health care facilities and the development of improved building and construction standards. The AIR facility, recently completed as pan of this study, was the culmination of a five year research project by a collaborative research team From the SA Medical Research Council, the Council for Scientific and Industrial Research, the Centers for Disease Control, Atlanta, USA; and Harvard University, Boston, USA; made possible with initial finding provided by the US Agency for International Development (USAID) and private sector donors which included the South African National Tuberculosis Association (SANTA). The author, as the engineering research member of the collaborating research team, was responsible for all architectural engineering aspects of the research behind the design, development, operation and, in part, the bioaerosol sampling techniques; and had to develop an in depth appreciation and understanding of M tuberculosis generation, risk and control in order to anticipate what was needed from the apparatus to support the various research projects that are to be undertaken. The various engineering interventions necessary to curtail transmission of infection, such as ultraviolet germicidal irradiation (UVGI) and other electro/mechanical interventions can now be tested and evaluated. The facility, as an apparatus, is capable of supporting the experiments intended for the study of these interventions in that the effects of varying ventilation rates and environmental conditions, such as temperature and humidity on the transmission dynamics of aerosolized infectious particles, are possible. The thesis discusses the hypothesis, aims, results and conclusions of the apparatus development, validation and calibration experiments of the unique state-of-the-art facility. The effectiveness and airtightness (leakage factor) of the air distribution from the wards, the transporting capacity of gram-positive and negative aerosolized bacteria and the efficacy of the in-line UVGI units to the animal infection chambers were conclusively proven via validation experiments. The results presented indicate that from the validated operational parameters of the apparatus the losses were less than 5% for non-biological substances and less than 12% for endospores (Serratia marcescens). No significant losses were noted across the transfer axial fan. A 100% efficacy was achieved across the in-he ultraviolet germicidal irradiation units as no Serratia marcescens were detected in the animal room. The calibration experiment, conducted to calibrate the exposure apparatus of the AIR facility in meeting its purpose to effectively transfer infectious airborne particles from patient wards (clinical unit) to the animal exposure chambers, concluded from the rate of guinea pig infections observed that the AIR facility is a highly effective way to quantify the infectiousness of TB patients. The high rate of observed infections among the guinea pig infections proves conclusively that the AIR facility will serve its purpose to effectively evaluate infectiousness of the ward air and to test the efficacy of engineering interventions to minimize the spread of the disease. The AIR facility now provides unique opportunity to evaluate the efficacy of novel engineering interventions for infection control, particularly in light of the global HIV/Aids epidemic. Future studies that are planned are also discussed.