August 29th 2016
Since August 2016, locally transmitted cases of Zika virus have been reported in Singapore. Singapore is a major travel hub and a gateway to Asia, so this is quite concerning. Zika virus has been documented in several Asian countries since it first emerged in 1951, including Malaysia, India, Indonesia, Thailand, Vietnam and Pakistan, long before to the Brazilian outbreak. It is possible it has been present in Singapore in the past, but not tested for. It has, after all, been documented in neighboring Malaysia in the past, and Aedes Aegypti, which spreads Zika Virus, is present in Singapore. If the virus was present previously, the question is, why is it causing an outbreak only now? Is it because of increased awareness and diagnosis? Alternatively, it is possible it has newly been introduced into Singapore. If that is the case, the infection would have been imported into the country through travel, and once imported, would have become endemic in the local mosquito population. Testing of mosquitoes in Singapore will be important.
How has Asia been affected to date? Since the 2015 epidemic in Brazil, new cases have been detected in Thailand, Laos and Cambodia. The new outbreak of cases in Singapore was detected after a spike in febrile illness in a particular geographic area near a construction site and high rise residential buildings. It appears there is local transmission of Zika virus in this outbreak, as most cases have no travel history, but the initial source of the outbreak is unclear. I have not seen any epidemiologic data to show whether there was a traveler from Brazil or another affected area who brought the infection into Singapore. A detailed epidemiologic study is required to determine the first case, and look at whether they had travelled anywhere. So, the first question is, was Zika virus already present in Singapore, and if not, how and when was it introduced? The second question is, whether the spread is entirely through mosquitoes, or whether there is also some human to human spread. The mosquito which spreads Zika virus is present in Singapore, but it is also documented that the infection can be spread between humans through sexual transmission.
Mosquito control (environmental controls) and prevention of mosquito bites (by people in affected areas) is key, as is protection of pregnant women. Cases who have been infected should be counselled about the risk of sexual transmission. In addition, individuals can take measures to reduce the risk of being bitten – such as using insect repellants and wearing clothing that covers the arms and legs.
There is no available drug or vaccine at this time, although research is underway to develop these. Zika virus is in the same class of viruses as dengue, for which a vaccine has been developed, so it should be possible to develop a vaccine. There is also a promising method of mosquito control using a bacteria, Wolbachia, which makes the mosquito resistant to viruses like Zika. Any strategy to control and reduce breeding of mosquitoes can have an impact. This includes spraying, use of mosquito nets, and reducing breeding sites around residential areas. Aedes Aegypti breeds in pools of stagnant water (such as barrels, tires, pots or other areas of water pooling), so any breeding sites should be removed. A better understanding of other mosquitoes which are capable of transmitting the virus is also needed, to target control strategies effectively, as geographic sites and breeding patterns can very for different mosquito species.
The major concern with Zika virus is the risk of microcephaly, which will only be seen 9 months after the epidemic commenced – so cases may occur from May 2017 onward. More immediately, cases of Guillain- Barre syndrome may occur in people of any age infected with Zika virus. This is a neurological syndrome which manifests with paralysis and weakness, and has been associated with Zika virus infection. Singapore needs to enhance it’s surveillance of GBS and microcephaly. If transmission of Zika virus is established in Singapore, as appears to be the case, the conditions are present for the epidemic to increase, so public health efforts should be focused on mitigating the epidemic through available mosquito control measures and through surveillance, case finding and better understanding of the local transmission patterns. The key is, how much transmission is mosquito borne and how much is human to human. Understanding this will help tailor control strategies. Local transmission in Singapore also raises the threat of more widespread transmission within Asia, either through infection of suitable mosquito species or human to human transmission. In a globalised world, travel and trade mean infections can spread around the world rapidly. High population densities in many Asian cities and the endemicity of mosquito vectors such as Aedes Aegypti throughout Asia, mean that there is an imminent risk to Asia and the surrounding region, including Australia. Singapore is a major travel transit stop from Australia, and awareness, surveillance and testing should be heightened in the region.
Raina MacIntyre is the Director of ISER, a Centre for Research Excellence dedicated to epidemic response.
ISER produces Epiwatch, a rapid outbreak intelligence service
MacIntyre Research Lab
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