According to a study, spraying insecticides to control sand flies (which spread visceral leishmaniasis (kala-azar)) inside homes in Indian villages reduced their numbers by 27% between 2016 and 2022.

The study, which involved researchers from CARE India and AIIMS Patna, suggests that using the insecticide alpha-cypermethrin in households where cases of visceral leishmaniasis have been reported could reduce the prevalence of this vector-borne disease by 6-40%.

This pesticide is commonly used on crops like cotton, grains and soybeans.

The study was conducted in 900 households in 50 villages in Bihar, Jharkhand and West Bengal, in areas where kala-azar is endemic. It was part of the elimination programme launched in 2015.

The study findings were published in The Lancet Infectious Diseases journal.

Visceral leishmaniasis is caused by the Leishmania parasite, which is transmitted by the bite of the phlebotomine sandfly.

It mainly affects internal organs such as the liver, bone marrow and spleen, causing symptoms such as fever, weight loss and anemia. If left untreated, this disease can be fatal.

According to the World Health Organization (WHO), most cases occur in Brazil, East Africa and India.

The study also said that kala azar contributed 12.5% ​​of the cost of indoor insecticide spraying to control vector-borne diseases in India, second only to malaria, which accounted for 76% of the cost.

However, researchers noted that evidence is limited regarding the cost-effectiveness of indoor spraying in significantly reducing disease cases.

Despite this, 70–80% of India’s budget to eliminate internal leishmaniasis is spent on indoor spraying.

The researchers analyzed three datasets for their study. The first dataset tracked the number of sandflies present in each home, with data collected every two weeks from 2017 to 2022 using light traps provided by the US Centers for Disease Control and Prevention (CDC).

About 2,30,000 sandflies were caught.

The second dataset assessed the quality of indoor insecticide spraying in these households, while the third dataset contained surveillance data on visceral leishmaniasis cases from January 2013 to December 2021, obtained from India’s national case registry, the Kala-azar Management Information System (KAMIS).

The study found that indoor insecticide spraying in 11 villages across four blocks in Bihar led to an overall reduction of 27% in sand fly populations.

The researchers’ model also predicted that three years of continuous indoor spraying could significantly reduce new cases of visceral leishmaniasis and related deaths.

Specifically, a 30% reduction in sandfly numbers due to spraying was estimated to result in a 17% reduction in new cases and a 9% reduction in deaths over the same period.