In the Central-Bohemian area affected by the flood of 2002, 497 residents were screened for antibodies against the mosquito-borne viruses Ťahyňa (TAHV),West Nile (WNV), Sindbis (SINV) and Batai (BATV; syn. Čalovo) using the haemagglutination-inhibition (HIT) and plaque-reduction neutralization (PRNT) tests. Blood samples were collected in September 2002 when the mosquito populations showed the maximum density following the flood. Antibodies against TAHV (16.5 % persons in PRNT, 14.9 % in HIT), SINV (1.4 % in HIT) and BATV (1.4 % in HIT, 0.2 % in PRNT) were detected. Although 6.8 % and 1.2 % of the subjects tested reactive with WNV in HIT and PRNT, respectively, the results were interpreted as cross-reactivity with tick-borne encephalitis virus. The seroprevalence of TAHV(both inPRNTand HIT) showed no association with gender (15.8%of males, 16.9 % of females), increased with age (1.4 % of persons younger than 20 years, 11.2 % of persons aged between 20 and 50 years, and 26.2 % of persons older than 50 years were positive), and correlated with the mosquito peri-residential challenge (5.0% residents seropositive in a mosquito-free control zone D – mostly Prague, 14.7 % in a mild-risk zone C, 20.5 % in a moderate-risk zone B, and 28.0 % in the most heavily mosquito-infested risk zone A). The highest TAHV seropositivity rate (> 25 %) was found amongst the inhabitants of the villages Obříství, Kozly, Tuhaň, Chrást, Chlumín and Hostín. Paired blood samples were obtained from 150 of the persons at a 6-month interval: an infection episode with TAHV during or after the flood was clearly evidenced in one person living in Obříství, and less convincing findings of recent TAHV infections were found in other three residents of Chlumín and Obříství (seroconversion and/or significant antibody titres increase detected in HIT only). This serosurvey indicated the existence of an active natural focus of Valtice fever (TAHV infection) stretched along the river Labe nearby Neratovice (Obříství,Chlumín,Tuhaň; Kozly, Tišice, Chrást), and a low TAHV activity area along the lower reaches of the river Vltava between Zlončice and Bukol/Zálezlice.Anincreased population density of mosquitoes after the floodmayhave boosted the incidence of mosquito-borne virus diseases, particularly Valtice fever, in Central Bohemia. An optimum prophylactic strategy to control these diseases would be epidemiological surveillance (including monitoring of both the density of mosquitoes and their rate of infection with viruses in natural foci) on the basis of which antiepidemic measures such as integrated mosquito control can be taken.

        Key words: mosquitoes – Culicidae – Ťahyňa virus – West Nile virus – Sindbis virus – Batai virus – Čalovo virus – flood.