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Case Study: Amoy Gardens Outbreak
Towards the end of March 2003, an outbreak of SARS occurred among residents of Amoy Gardens, Kowloon Bay, Hong Kong. As of April 15 2003, there were a total of 321 cases of SARS in Amoy Gardens.
There was an obvious concentration of cases in block E, accounting for 41% of the cumulative total. Block C (15%), block B (13%) and block D (13%) recorded the second, third and fourth highest incidence of SARS infections. The rest of the cases (18%) were scattered in 11 other blocks.
In this case study, we will look at how SARS managed to infect so many people in just one place, and how this could have been prevented.
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On March 14 2003, a 33-year-old man who lived in Shenzhen developed SARS symptoms. The man visited his brother in Amoy Gardens regularly and had chronic renal disease, which was being treated at the Prince of Wales Hospital in Hong Kong. However, on March 14 and March 19 2003, the man visited his brother who owned a flat in block E of Amoy Gardens. He was having diarrhea at that time and he used the toilet there. His brother, sister-in-law and two nurses who attended to him at the Prince of Wales Hospital subsequently developed SARS.
The outbreak reached its peak on 24 March 2003 and declined steadily afterwards. A questionnaire survey conducted by the Hong Kong Department of Health found that in addition to the common symptoms associated with SARS (i.e. fever, chills and headaches), another notable feature with the patients was diarrhea, which was present in some 66% of the cases. The survey also showed that only 4% had history of contact with SARS patients and 8% had visited the Mainland (China) between 17 and 23 of March 2003.
Hence, how did so many people contract SARS from just one man?
To find that answer, we have to understand the environmental conditions in Amoy Gardens.
In Amoy Gardens, each block has eight vertical soil stacks, each collecting waste matter from the same unit of all floors and connected with the water closets, the basins, the bathtubs and the bathroom floor drains. Each of the sanitary fixtures is fitted with a U-shaped water trap (U-trap) to prevent foul smell and insects in the soil pipe from entering the toilets. For this preventive mechanism to function properly, the water traps must be sufficiently filled with water.
Interviews with the Amoy Gardens residents revealed frequent complaints about foul smell in the toilets, which suggest that the U-trap arrangement might not be functioning properly in some units. Since the basins and the bathtubs were frequently used, their U-traps should be charged with water and should have been functioning properly. However, as most households had the habit of cleaning the bathroom floor by mopping instead of flushing it with water, the U-traps connected to most floor drains were likely to be dry and would not have been functioning properly.
In tests carried out in one of the units in block E, it was show that when the exhaust fan in the bathroom was switched on, there was a reflux of air from the soil pipe into the bathroom through the floor drain. It is hence possible that the reflux could have contained droplets of contaminated sewage present in the soil pipe, be dispersed into the bathroom, and be extracted by the bathroom exhaust fan into the light well between adjacent units. The contaminated droplets could then enter other units through open windows.
In addition, tests detected leakage from a sewer vent pipe 2 at the 4/F of block E, and was confirmed by a large visible crack. The cracked sewer vent pipe could have emitted droplets carrying contaminated sewage into the light well every time a toilet was flushed.
1 Amoy Gardens building design
There were signs of cockroach infestation at Amoy Gardens, in particular around the car park, food premises and drain openings. Rodent activities were also detected around refuse collection chambers, the car park and food premises.
Recent laboratory studies have shown that many SARS patients excrete coronavirus in their stools, where it could survive for longer periods than on ordinary surfaces. As many as 2/3 of the patients in this Amoy Garden SARS outbreak were also having diarrhea, contributing to a significant virus load being discharged in the sewerage in block E.
It is probable that the index patient initially infected a relatively small group of residents within block E and subsequently to the rest of the residents in that block through the sewage system, person-to-person contact and the use of shared communal facilities such as lifts and staircases. These residents subsequently transmitted the disease to others both within and outside block E through person-to-person contact and environmental contamination.
The bathroom floor drains with dried-up U-traps provided a pathway through which residents came into contact with small droplets containing viruses from the contaminated sewage. These droplets entered the bathroom floor drain through negative pressure generated by exhaust fans when the bathroom was being used with the door closed. Water vapour generated during a shower and the moist conditions of the bathroom could also have facilitated the formation of water droplets. The chance of exposure was increased given that the bathrooms in apartment units of Amoy Gardens were generally small in size (about 3.5 square metres). Contaminated droplets could then have deposited virus on various surfaces, such as floor mats, towels, toiletries and other bathroom equipment.
Transmission of the disease by airborne, waterborne route and infected dust aerosols has been examined but these were not supported by the epidemiological picture and laboratory results.
The Amoy Gardens outbreak clearly shows that environmental factors can play major roles in the transmission of the SARS virus. Just a sewage system itself can lead to an outbreak.
For that reason, SARS prevention lies not only in personal hygiene, but also in good buildings designs and the cleanliness of the environment.
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