Spraying Insecticide Inside Homes Significantly Reduces Sandfly Numbers and Disease Spread in India

Spraying insecticide inside homes in Indian villages to kill sandflies, known to spread visceral leishmaniasis or kala-azar, was estimated to cut down their numbers by 27 per cent from 2016-2022, a study found. Researchers estimated that spraying the insecticide alpha-cypermethrin in homes when an instance of visceral leishmaniasis is detected could potentially lower numbers of the vector-borne disease by 6-40 per cent. The insecticide is used on cotton, cereals, soya beans, among others.

The international study, including researchers from CARE India and All India Institute of Medical Sciences, Patna, looked at 900 homes across 50 Indian villages in Bihar, Jharkhand, and West Bengal, where visceral leishmaniasis is endemic and indoor spraying of the insecticide was encouraged as part of an elimination programme, introduced first in 2015. It was published in The Lancet Infectious Diseases journal.

Impact of Insecticide Spraying

The vector-borne disease is caused by infection with leishmania parasites, spread by the bite of phlebotomine sandflies. Most cases occur in Brazil, east Africa and India, according to the World Health Organization. Visceral leishmaniasis affects ones internal organs, especially liver, bone marrow and spleen, and symptoms can include irregular bouts of fever, weight loss and anaemia. If left untreated, the condition can be fatal.

Kala-azar accounts for 12.5 per cent of the cost of indoor spraying of insecticides for controlling vector-borne diseases, the second highest after malaria 76 per cent, the authors said. However, there is no strong evidence of the cost-effectiveness of spraying insecticides inside homes in slashing disease case numbers, even as indoor spraying takes up 70-80 per cent of Indias budget directed at eliminating visceral leishmaniasis, they said.

Previous research in this regard include only observational surveillance studies and circumstantial evidence, the authors added. For the study, the researchers analysed three datasets. The first contained sandfly numbers present in each home included in the study, data for which was collected every two weeks from 2017-2022 using light traps provided by the US Centers for Disease Control and Prevention CDC. Close to 2.3 lakh sandflies were caught.

Data Analysis and Findings

The second dataset had quality-assurance data of the indoor insecticide spraying from the same homes where the sandflies were caught in the first dataset. The third set comprised surveillance data including visceral leishmaniasis cases every month from January 2013 to December 2021. The data came from the Kala-Azar Management Information System KAMIS, Indias national case registry.

Our analysis suggested that, across the 11 villages in four blocks of Bihar in which indoor spraying was started or stopped during the study period, the activity was associated with an overall reduction in sandfly abundance of 27 per cent, the authors wrote.

Their model also predicted that three years of indoor insecticide spraying, resulting in bringing down sandfly numbers, led to an earlier and stronger reduction in new cases of visceral leishmaniasis and associated deaths.

Our model predicted that a 30 per cent reduction in vector abundance due to indoor spraying of insecticide would translate to a 17 per cent reduction in the total number of new cases and a nine per cent reduction in the number of visceral leishmaniasis-related deaths during those 3 years, the authors wrote.