Stephen Ng from Columbia University School of Public Health, New York, USA, questions how one person (the identified source of the outbreak) could have infected so many people from two visits to Amoy Gardens, an apartment block with no communal facilities. A common source of food or water contamination has not been identified. Airborne transmission is thought to have been unlikely by the WHO team sent to investigate the outbreak.
He suggests that the infection could have been passed from rat to man either by rats entering households and leaving infectious material in bathrooms and kitchens, or by contamination of clothing on clothes-lines. The first infected rats could also have spread the virus to other rats in block E and to other blocks, starting an epidemic among rats, and providing the common source for the epidemic in people.
Stephen Ng comments: "The rat vector hypothesis is a strong possibility that needs to be further explored. Epidemiological case-control studies could be undertaken to identify behavioural risk factors and possible mechanisms for rat-to-man infections. For example, if rat contamination occurs at night, people using kitchen and bathroom facilities early in the morning, when cooking breakfast, taking showers, and so on, will be at increased risk. Housewives will be affected more than husbands working away from home. Small children who crawl on the floor will also be at higher risk."