Singapore’s First Chikungunya Outbreak – Surveillance and Response
Ng Lee Ching (Ph.D),
Head, Environmental Health Institute (EHI),
The National Environment Agency (ENV)
13 June 2008
Chikungunya is a mosquito-borne viral disease caused by an alphavirus of the family Togaviridae. Both Aedes aegypti and Aedes albopictus are vectors found to transmit this disease. The disease is characterized by abrupt onset of high fever, arthralgia, myalgia, headache and sometimes rash, which is similar to that of Dengue. The symptoms generally last 1-10 days. However, arthralgia may last for months or years.
Chikungunya fever has been documented as early as 1824 in India and elsewhere, though the virus was only isolated in 1952 during an outbreak in Tanzania (1). Outbreaks in Asia e.g. Philippines, Malaysia, India, Indonesia (3, 4, 5, 6) and Africa e.g. Congo, Uganda, Senagal (8, 9, 10) have been reported. Major epidemics appear and disappear cyclically.
However, due to an unprecedented outbreak in the Indian Ocean in the beginning of 2005, the disease has more recently grabbed international attention (11, 12, 13). More than 1 million cases have been reported from Comoros, Mayotte, Seychelles, Reunion Island and Mauritius. The huge epidemic potential of the disease is demonstrated by the seroconversion of 35% of the Reunion population of 770,000 (14). The epidemic has since moved to Sri Lanka and India in the beginning of 2006 (5) and Italy in 2007 (15).
SURVEILLANCE IN SINGAPORE
In view of the recent outbreaks, a surveillance system was initiated at the end of 2006. The medical community was apprised by Ministry of Health to look out for cases. At EHI, laboratory diagnosis was established and active laboratory based surveillance was set up. The active surveillance involves a network of general practitioners who are encouraged to consider Chikungunya as a possibility when dengue was suspected and testing dengue negative samples from hospitals and general practitioners (with patients’ consent).
Since the start of the surveillance, about 1800 samples have been tested, of which 10 imported cases were detected at EHI from Dec 2006 to Dec 2007. On 10 Jan 2008, the first local case was detected. The patient had consulted a GP in Little India, which is part of EHI’s Chikungunya surveillance network.
Since the notification of the index case in Singapore, MOH carried out active case finding among the residents and workers in the vicinity and within the Little India enclave. The medical community was alerted through MOH circulars. Clinical and workplaces in the affected vicinity and near by were further alerted through telephone communication. To date, EHI has tested about 2,600 blood samples collected by MOH.
The number of Chikungunya tests requested by GPs and hospitals has increased from 18 in Dec 2007 to 200 each for the month of Jan and Feb 2008. Together, the enhanced surveillance confirmed 12 more local cases. All 13 cases were found in the vicinity of Little India. (Fig 1)
Patients infected were treated and isolated at the Communicable Disease Centre (CDC). Blood test were sent to EHI daily, to test if the virus was still present, before they were discharged.
Complementing the case isolation strategy is an aggressive ground vector control measures which includes mass operations involving 95 field offices, more than 4800 premise inspections for mosquito breeding, indoor misting of insecticides, residual spraying of workers quarters and outdoor thermal fogging with insecticides. Besides operations in Little India, similar measures were taken in places visited by patients.
Other agencies like URA, LTA, PUB, MOM and Singapore Contractors Association Limited were also involved in sprucing up the affected area. Community outreach was also intense with media publicity, and advisories to residents, shop owners and foreign workers in the vicinity.
The last local case detected had fever on the 28 Jan 2008. On 21 Feb 2008, 24 days (2 incubation periods) after the last case, the cluster was declared closed. A key to the successful control was the close interagency coordination and help from the public and private partnership.
Surveillance and research continues
Since the detection of the first case, 5 more imported cases have been detected at EHI. Sequencing of the viruses responsible for the local outbreak, revealed that the viruses are different from the ones circulating in Indonesia and Malaysia in the past, but are very similar to the ones that caused the Indian outbreak in 2006. (accession no EU441882 and EU441883).
Together with similar analysis from other studies in the world, this shows that this group of virus has rapidly spread in the globalize world.
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