Studies on Viral Disinfection: An Evaluation of Moist Heat Disinfection for HBV by Using A₀ Concept Defined in ISO 15883-Washer-Disinfectors

Abstract

International Organization for Standardization (ISO) 15883 for washer-disinfectors has introduced the A₀ concept to allow comparison of the lethality of moist heat processes. The A₀ value is the equivalent disinfection time in seconds at 80 °C calculated on the basis of microbial killing kinetics when the disinfection temperature is over 65 °C. Hepatitis B virus (HBV), transmissible only to humans and chimpanzees, is an important heat-resistant, blood-borne pathogen. Therefore, it is mandatory to disinfect HBV thoroughly in the washer-disinfectors employed for surgical instruments. Additionally, it has become extremely difficult to use chimpanzees as experimental models or to perform human volunteer studies. Therefore, it is considered worthwhile to re-evaluate the reported data on the moist heat disinfection of HBV using the A₀ value. In the voluntary active immunization to humans in 1973, HBV serum (infectivity titer: 10^6.5 CID₅₀/mL) underwent moist heat disinfection at 98 °C for 1 min in a flask over an electric burner (conservatively estimated A₀ value: 3786). Then, 0.1 mL was inoculated to each of 29 volunteers. No one revealed evidence of infection clinically or in the laboratory tests available at the time. In 1979, a more sensitive test appeared and revealed three sub-clinically infected volunteers. In the 1980s, there were two chimpanzee experimental models using HBV serum (infectivity titer: 10⁵ CID₅₀/mL). In one model, the serum underwent moist heat disinfection at 98 °C for 2 min in a thermostat bath (conservatively estimated A₀ value: 7571). One milliliter was inoculated to each of two chimpanzees, and both of them revealed no evidence of infection. In another model, the serum underwent moist heat disinfection using two conditions in a thermostat bath, respectively: at 103 °C for 90 s (A₀ value: 24865) and at 65 °C for 10 h (A₀ value: 1138). Ten milliliters of each sample were mixed. Then, the mixture was inoculated to each of two chimpanzees. Both of them revealed no evidence of infection. In the human volunteer study, the serum infectivity titer was more than 30 times (10^1.5 times) higher than that used in the two chimpanzee experimental models. Moreover, the serum was heated in the flask over an electric burner, which is considered less reliable than the thermostat baths to realize uniform heat distribution. It is assumed that these factors were predisposed to the result that a conservatively estimated A₀ value of 3786 failed to inactivate the HBV serum of 10^6.5 CID₅₀/mL. In the two chimpanzee models, it was suggested that A₀ value not less than 1138 was able to inactivate the HBV serum of 10⁵ CID₅₀/mL.