Dengue haemorrhagic fever

Factors increasing the risk of DHF outbreaks The occurrence of DHF outbreaks is linked to a number of factors, including the density of mosquito vectors, particularly that of Aedes aegypti. The precise population density of Ae. aegypti that is needed to sustain dengue virus transmission epidemically or endemically has yet to be determined, but experience in Singapore in recent years suggests that house indices as low as 2% are sufficient for the epidemic transmission of dengue in areas where there is a low level of immunity in the human population. In many instances, a small number of actively biting female mosquitos has infected an entire household. Virus transmission is, of course, increased by denser human populations. Urbanization in tropical countries has resulted in both a proliferation of Ae. aegypti and an increase in the number of susceptible human hosts. In cities, the movement of viraemic persons is a more important means of transporting dengue viruses than the movement of Ae. aegypti mosquitos. Places where people congregate during the day may be important sites of dengue virus transmission. For example, children at school bitten by infected mosquitos may take the virus home or to other parts of the city. Dengue virus may also spread in settings involving large numbers of people, such as in hospitals where visitors, patients and staff may be bitten by infected Ae. aegypti. Infected persons may carry dengue virus to towns and rural areas from cities where the disease is epidemic or endemic, but the factors affecting dengue virus transmission and maintenance in towns are not well documented. Introduction of dengue virus by the air travel of infected passengers over long distances has repeatedly occurred in the Pacific and Caribbean regions during the past 30 years. A distinct seasonal pattern in DHF outbreaks is evident in most places. In tropical regions where monsoon weather patterns prevail, DHF hospitalization rates increase during the rainy season and decline several months after the cessation of the rains. This decline may be related to a decrease in mosquito biting activity, a decrease in the longevity of female mosquitos, or both, and