Outbreak of melioidosis in a highly urbanized area

Abstract

In 2022, an outbreak of melioidosis occurred at a highly urbanized district (the SSP district) in Hong Kong. There was a 5-fold increase in cases compared to the average number of cases in the preceding years, with the SSP district accounting for 71.1% of the cases. A total of 568 environmental specimens were collected, including 290 environmental samples and 38 air samples from the freshwater service reservoirs (FWSRs), 16 water samples, and 224 environmental samples from the households of infected cases. Burkholderia pseudomallei was cultured from 24 (10.7%) soil samples obtained from the lawn overlying the FWSRs supplying the SSP district or from the surroundings of the air vents used for pressure equalization in the FWSRs. These culture isolates from soil have the same genotype ST-1996 as the patients' isolates. As B. pseudomallei DNA was also detected from the swabs collected from the internal roofs of the FWSRs and inside the water tap at the home of an infected case, the chlorine concentration at the upstream treatment plant outlet was increased from 1 to 1.2 ppm as a precautionary measure. The number and proportion of newly diagnosed melioidosis cases from the SSP district significantly reduced from 32 (71.1%) in 2022 to 6 (35.3%) in 2023 (p = 0.018).

Keywords: Burkholderia pseudomallei; Melioidosis; outbreak; water; water chlorination.

Figure 1.

Summary of melioidosis cases in Hong Kong, 2014–2023. (A) Yearly number of melioidosis cases. A new episode of melioidosis is defined as Burkholderia pseudomallei culture-positive clinical disease occurring ≥ 18 months following initial diagnosis. (B) Geographical distribution and aggregated number of melioidosis cases in Hong Kong. CW – Central & Western; Ea – Eastern; Is – Islands; KC – Kowloon City; KTs – Kwai Tsing; KTo – Kwun Tong; No – North; SK – Sai Kung; ST – Sha Tin; SSP – Sham Shui Po; So – Southern; TP – Tai Po; TM – Tuen Mun; TW – Tsuen Wan; WC – Wan Chai; WTS – Wong Tai Sin; YTM – Yau Tsim Mong; YL – Yuen Long.

← Figure 2.

Phylogenetic analysis of patient and environmental isolates. (A) Single nucleotide polymorphism (SNP) phylogenetic analysis of clinical and environmental B. pseudomallei isolates in Hong Kong between 2022 and 2023. The B. pseudomallei FDAARGOS_594 strain (GCA_003798365.1) (MLST type 158) was used as the outgroup of the alignment. (B) Core SNP genome phylogenetic analysis of the ST-1996 major outbreak cluster. The RF22001008 sequence was used as the reference for the alignment. The colours of the labels indicate the source of sampling: red labels represent clinical strains isolated from patients residing in SSP district in 2022, green labels represent environmental samples collected in SSP district in 2022, and black labels represent either clinical strains isolated from patients residing outside SSP district, samples collected later in 2023, or reference sequences from the NCBI GenBank. Epidemiological information of the isolates, including the location of sampling (CW – Central & Western district, FWSR A – Fresh Water Service Reservoir A in SSP district, FWSR C – Fresh Water Service Reservoir C in SSP district, Ea – Eastern, Is – Islands district, KTo – Kwun Tong district, KTs – Kwai Tsing district, SK – Sai Kung district, ST – Sha Tin district, SSP – Sham Shui Po district, WTS – Wong Tai Sin district, YTM – Yau Tsim Mong district), and sampling year, was mentioned in brackets.

Figure 3.

Monthly number of melioidosis cases and public health measures in Hong Kong, 2022–2023.

Figure 4.

Summary of weather data from Hong Kong, 2014–2023. (A) Daily mean temperature by month. Data represent the monthly mean ± standard deviation (SD). (B) Daily mean relative humidity by month. Data represent the monthly mean ± SD. (C) Daily mean rainfall by month. Data represent the monthly mean + SD. (D) Tropical cyclone warnings of the intensity of tropical storms or above over the years. Each colour represents a different tropical cyclone. Tropical cyclones are classified in accordance with the World Meteorological Organisation's recommendation by their maximum sustained wind speeds near the centre: tropical storm (TS): 63–87 km/h; severe tropical storm (STS): 88–117 km/h; typhoon (T): 118–149 km/h; severe typhoon (ST): 150–184 km/h; super typhoon (SuperT): 185 km/h or above.