Clothes-lines outside bathrooms could have been used by rodents to enter flats Rats could have been the main cause of the Sars outbreak at Amoy Gardens, which killed 42 residents and infected 329, according to a Hong Kong epidemiologist. In an article published in The Lancet, Britain's authoritative medical journal, Stephen Ng Kam-cheung, special lecturer in epidemiology at Columbia University, said earlier hypotheses, including the so-called 'chimney effect', could not satisfactorily account for the distribution of cases and timing of the outbreak. Most cases happened on the upper floors and units 7 and 8 in the middle of the building. 'Roof rats prefer to forage for food above ground in elevated areas. They are also territorial and habitual, and tend to follow the same pathways between their nest and food sources and make return visits time after time,' he said, saying this explained the cluster of cases in units 7 and 8. Clothes-lines installed outside the bathrooms of each unit provided convenient bridges for the rats as they travelled up and down the building, Dr Ng wrote. 'The infection could have passed from rats to man either by rats entering houses and leaving infectious material in bathrooms and kitchens, or by contamination of clothing on clothes-lines,' he said. Although rats and cockroaches have previously been linked to the Amoy Gardens outbreak, other factors were regarded as the primary cause. However, a microbiologist and health officials yesterday expressed reservations about Dr Ng's hypothesis. Reports by both the government and the World Health Organisation have blamed a combination of environmental and health events to the unusual clusters of the infections at the housing estates in Kowloon Bay, especially those in Block E. The factors include the drying up of a U-shaped water trap at the bathrooms and the 'chimney effect' theory, by which contaminated droplets seeped through a cracked sewage pipe on the fourth floor of Block E into a light well, and were blown upwards into the flats. In the article, Dr Ng suggested the outbreak could have been started on March 14 by a rat that was infected after entering an apartment in Block E visited by the man who brought the disease to the building. That rat would have become contagious on around March 19 as the incubation period in rats infected by other forms of coronavirus lasts about two to five days. Dr Ng said it was likely that the first infected rat passed the virus to other rats and spread it to other flats. The infected rats' secretions would have contained large amounts of the virus and were highly contagious, he said. Dr Ng said the rat vector hypothesis was supported by circumstantial evidence, including virologists' suspicions that the Sars coronavirus originated from animals and jumped species to infect man. Also, viral remnants were detected in four of eight samples of rat droppings found beside Amoy Gardens. But he said more experiments had to be conducted on rats before drawing a final conclusion. John Tam Siu-lun, a professor of microbiology at Chinese University, said while rats were a possibility, tests would show if they were Sars-infected.