Potential association of dengue hemorrhagic fever incidence and remote senses land surface temperature, Thailand, 1998.

A pilot study was designed to analyze a potential association between dengue hemorrhagic fever (DHF) incidence and, temperature computed by satellite. DHF is a mosquito transmitted disease, and water vapor and humidity are known to have a positive effect on mosquito life by increasing survival time and shortening the development cycle. Among other available satellite data, Land Surface Temperature (LST) was chosen as an indicator that combined radiated earth temperature and atmospheric water vapor concentration. Monthly DHF incidence was recorded by province during the 1998 epidemic and obtained as a weekly combined report available from the National Ministry of Public Health. Conversely, LST was calculated using remotely sensed data obtained from thermal infrared sensors of NOAA satellites and computed on a provincial scale. Out of nine selected study provinces, five (58.3%) exhibited an LST with a significant positive correlation with rainfall (p < 0.05). In four out of nineteen surveyed provinces (21.3%), LST showed a significant positive correlation with DHF incidence (p < 0.05). Positive association between LST and DHF incidence was significantly correlated in 75% of the cases during non-epidemic months, while no correlation was found during epidemic months. Non-climatic factors are supposed to be at the origin of this discrepancy between seasonality in climate (LST) and DHF incidence during epidemics.

Silent transmission of virus during a Dengue epidemic, Nakhon Pathom Province, Thailand 2001.

In the year 2001 a large dengue fever (DF)/dengue hemorrhagic fever (DHF) outbreak occurred in Nakhon Pathom Province, Thailand. Three thousand one hundred twelve cases of DHF were reported, an attack rate of 393 per 100,000 population. The Nakhon Pathom Provincial Health Office immediately carried out a control action according to WHO recommendations. Active serological surveys and viral RNA isolation were carried out to detect silent transmission of dengue virus in 329 healthy volunteers in Nakhon Pathom Province subdistricts where the dengue epidemic had the highest rate of infection of 2.5 per 1000. Eight point eight percent of these volunteers had a serum sample positive for DF/DHF virus IgM antibody. The highest prevalence occurred in the 15 to 40 year old group. In two instances viral RNA was detected by PCR and dengue serotype 3 was subsequently identified. The data support the hypothesis of subclinical infection with dengue virus. This high frequency of virus circulation combined with a high population density, urbanization and increasing breeding sites for mosquitoes, needs to be taken into account in the evaluation of viral transmission during and after epidemics. This underlines the importance of community-based control in informing people of their involvement in virus transmission and the importance of personal protection.

First isolation of Japanese encephalitis from Culex quinquefasciatus in Thailand.

Isolation of Japanese encephalitis (JE) virus using C6/36 cell and immunofluorescence virus antigen detection techniques was attempted from female mosquitoes collected with CDC gravid traps in Samut Songkhram Province in the central region and in Phuket Province in southern Thailand, in 2003. One thousand and eighty female mosquitoes including 6 species of the Culicidae family (Culex quinquefasciatus, Cx. gelidus, Cx. tritaeniorhynchus, Cx. whitmorei, Cx. vishnui complex, Cx. s.g. culiciomyia) (pooled by specific specimen), were processed for virus isolation. Two pools of Cx. quinquefasciatus yielded a JE virus isolation. This represents the first report of JE virus isolation from Cx. quinquefasciatus in Thailand.

Seroepidemiological survey among schoolchildren during the 2000-2001 dengue outbreak of Ratchaburi Province, Thailand.

From August 2000 to 2001, a dengue outbreak occurred in Mueang district, Ratchaburi Province, Thailand. About 800 cases of dengue infection were reported, and among them, 49.5% were clinically diagnosed as dengue hemorrhagic fever according to the WHO criteria. During the outbreak, the incidence rate of dengue infection in Hin Gong subdistrict was 2.9 per 1,000 population. A seroepidemiological survey was conducted among primary schoolchildren from July 2000 to June 2001, to monitor dengue transmission. In a baseline survey, 283 children were surveyed for dengue antibody and 71% were IgG seropositive. In June 2001, the rate of dengue infection showed an increase of 8.8% with 8.0% among immune children and 10.3% among naive schoolchildren. Among 283 schoolchildren, 90 were followed up 3 times, in September and December 2000, and June 2001. An increase in the rate of seroconversion was observed in the period September to December 2000, while the peak dengue outbreaks in the dry season occurred in February 2001. Serosurveys among schoolchildren appear to be early warning system, and can be advantageous in early dengue control actions, in order to break the chain of transmission before an impending epidemic.