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DENGUE FEVER IN THAILAND: HOW TO SPOT EPIDEMICOUTBREAKS AGAINST THE ENDEMIC BACKGROUND

September 13, 1999

Dengue is a viral disease which is prevalent in a hundred or so tropical countries. Up to 50 million cases have been recorded each year in the world according to WHO. It is in fact induced by four viruses (Dengue serotypes 1, 2, 3 and 4) belonging to the flavivirus family. The mosquito Aedes aegypti is the vector. Two forms of dengue exist. The first is the less severe, producing symptoms of varying intensity (fever, muscular pain, headaches, digestive disorders), whereas the second (haemorrhagic dengue) is extremely serious, provoking death in 2-15% of cases, with haemorrhages and sometimes encephalitis. In South-East Asia, since the late 1950s, haemorrhagic dengue has been one of the main causes of hospitalization. In Thailand, where the disease is endemic, from this last decade 50 000- 100 000 cases are registered per year.
Little is known about the epidemiology of heamorrhagic dengue in the tropical countries. This knowledge gap prompted IRD researchers to study the occurrence of this version of the disease, in conjunction with the Center for Vaccine Development, over a substantial period, 1983 to 1995. The objective was to establish sound indicators for variations of the disease on which to base preventive policies. It was above all a question of developing an ability to distinguish periods of epidemic outbreaks from others where the illness exists in its endemic form yet shows peaks that are simply a seasonal feature.
In order to conduct the investigation, the research team developed a method of calculation that can precisely identify the limits of epidemics independently of seasonal variations. They defined as months of epidemic outbreak those where the number of cases recorded was significantly higher than the seasonal coefficient. This seasonal coefficient corresponds to the average number of cases recorded each month (with the exception of months of epidemics), calculated based on the 13 years studied. They subsequently applied this epidemiological model to data recorded from 1983 to 1995 in the 73 provinces of Thailand.
At the end of this study, it appears that, from 1983 to 1990, there was a series of epidemics that affected nearly all the provinces (in 1984-85, 1987, 1989-90). Between 1991 and 1995 haemorrhagic dengue epidemics hit only 40 of them simultaneously and they occurred principally in the North and North East (in 1991, 1993-94) and in the Southern region.
Also brought into evidence is that the intervals between epidemics last about two years on average. This is not a lapse of time long enough for the non-immune population to be renewed (babies and young children) which could have explained the epidemic surges. It appears therefore that an epidemic is due rather to the successive circulation of different serotypes against which populations had little immune resistance. Another point that emerges is hat such epidemics spring up sometimes despite environmental sonditions unfavourable for the virus, during the dry season or when the first rains arrive. Such a large quantity of virus circulating in humans and mosquitoes, the viral reservoirs, could explain why epidemics recur in environmental conditions which otherwise would not normally encourage them.
These results emphasize the necessity of acquiring data on the different serotypes that circulation when epidemics arise and thus bring a better understanding of the dynamics of dengue. The researchers from IRD and the Research Center for Emerging Viral Diseases have in consequence embarked on a more detailed study of the data recorded in Thailand since 1983.

The epidemics of haemorrhagic dengue from 1983 to 1995 in Thailand


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= epidemic occurring in a province lasting 1 month
The dark areas mark the years and principal regions affected by epidemics.



For Further Information

Contact : Philippe Barbazan et Jean-Paul Gonzalez, Institut de Recherche pour le Développement, Research Center for Emerging Viral Diseases, Center for Vaccine Development, Institute of Sciences and Technology for Development. Mahidol University at Salaya. 25/25 Phutthamonthon 4, Nakhonpathom 73170, Tha'-lande. Tél. et Fax: (662) 441 0189, E-mail: fnpbb@diamond.mahidol.ac.th


Bibliographie
DJ. Gubler and G. Kuno, Dengue and Dengue Haemmorrhagic Fever, 1997. CAB International, New York.
Annual Epidemiological Report, CDC, Ministry of Public Health Thailand.





Institut de Recherche pour le Développement, Paris (IRD)