Resistance of sandflies to DDT in Kala-azar endemic districts of Bihar, India.

Editor – Vector resistance to insecticides has been highlighted by Kasap et al. in their comparative study relating to malaria in Turkey (1). In India, phlebotomine sandflies used to be considered highly susceptible to all insecticides; but indiscriminate use poses a problem of resistance, for example, in Bihar, where spraying with DDT has continued since 1976 (2). Resistance to DDT in Phlebotomus argentipes, a proven vector of Kala-azar in India, was reported for the first time in a village of Samastipur district (3). We therefore undertook a study in seven villages in Patna, Darbhanga, Samastipur, and Vaishali districts to ascertain the present trend of susceptibility of sandflies to DDT spraying in endemic areas of Bihar. Batches of 15–20 bloodfed and active half-gravid female adult sandflies were collected in the early morning by torchlight, with aspirators, and exposed toDDT4% for one hour. Their susceptibility status was determined by the WHO test kit, and mortality was recorded after 24 hours. All the sandflies dead or alive were identified (4). The temperature and humidity were maintained at 24+2 C and 75%, respectively. A village in Patna district, where no DDT spraying had been carried out for 25 years, was selected as a control. Effectiveness of the insecticide was found to be 100% in Patna and Samastipur districts. In Paswantola and Chakkatola villages (Darbhanga district) and Dakshinitola village (Vaishili district), vector mortality was 98.24%, 96.28% and 97.57%, respectively, whereas in another village in Vaishali (Ravidastola) susceptibility was 78.5% in 1998 and 71.42% in 1999. Our observations indicate that the leishmaniasis vector Ph. argentipes has developed resistance to DDT in an area of Bihar, in line with findings reported from Samstipur district (3). Further study is therefore required in endemic areas. n Ram Singh, Assistant Director R.K. Das, Chief Medical Officer S.K. Sharma, Joint Director Kala-azar Unit National Institude of Communicable Diseases, Patna 800 013, India