Friday, January 30, 2009

Dengue and Chikungunya in Singapore - endemicity and ecdemicity

Aedes albopictus and Aedes aegypti

The other thing that caught my eye when I was browsing through the Ministry Of Health Weekly Infectious Disease Bulletins, was the incidence of the two similar viral infectious diseases, Dengue and Chikungunya. They kinda jogged my memory with respect to a recent report that Malaysia was grappling with the worst dengue outbreak ever with about 14 deaths and 5062 cases in the first month of 2009. Malaysia had 49,335 cases with 112 deaths in 2008 and 48,846 cases with 98 deaths the previous year.

Singapore, according to the MOH IDB, seems to have the dengue infections under control over this rainy season. Other than a brief spike over the first week of 2009, the number of reported cases have generally hovered pretty much a levels lower than the mid 2007 cluster.

Interestingly the Chikungunya infections have gone up, and is now hovering at an incidence of about 40-60/week. These two viruses produce a similar kind of illness although chikungunya infections tends to be less likely to cause death. They also probably share the same mosquito vectors, principally Aedes aegypti and Aedes albopictus as well as the same ecological niches. The rise in chikungunya infections relative to dengue is probably related to the fact that chikungunya is a relative newcomer compared to dengue.

Dengue is an infection that is regarded as endemic to Singapore, whereas chikugunya is generally regarded as being ecdemic. The supposed ecdemicity of chikungunya implies that the presence of the disease within the community cannot be sustained without being introduced from external sources. The Ministry of Health data does however, suggest that the chikungunya is fast establishing itself as an endemic problem. Most of the cases are now transmitted locally and are not dependent on imported cases.

Given the similarity in vectors and transmissability between dengue and chikungunya, I don't think there is any doubt chikungunya will establish itself firmly within the community, as it has in many other parts of Asia, and it will only be a matter of time before it will be declared to be endemic in Singapore.

Does it bother me? Not really, because it is a relatively mild disease compared dengue. If we can control the mosquito population in Singapore, we should be able to control both fairly well.

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