Malaria-carrying mosquitoes shift homes from forests to rice fields

The mention of malaria often conjures the image of mosquitoes buzzing in and around homes. But the mosquitoes that spread malaria in northeast India, once associated with forests, may now be finding newer homes in rice fields. A recent study done in the northeastern state of Meghalaya suggests that some of the mosquito species in the genus Anopheles, which are responsible for spreading malaria, are changing composition in the region. This may be linked to deforestation, increased rice cultivation and widespread use of mosquito nets.

India had an estimated 3,389,000 malaria cases in 2022, making it the country with the highest number of cases in South and Southeast Asia. There were a total of 249 million cases recorded worldwide and 608,000 people died from the disease in 85 countries last year, according to an estimate by the World Health Organisation (WHO).

“The initial reason for doing the study in Meghalaya was that the malaria rate has traditionally been extremely high in northeast India,” says Catherine Walton, co-author of the study and senior lecturer at School of Natural Sciences, University of Manchester. There were 237 deaths in the state from the disease in 2007. This number dropped to 40 in 2015 and to three in 2021.

In 2016, India launched a National Framework for Malaria Elimination Programme (NFMEP), which aims to eliminate the disease in a phased manner by 2030. It also aims to keep areas where malaria transmission has been interrupted, free of disease by preventing its re-introduction. The state government of Meghalaya, through its Health & Family Welfare Department, is responsible for implementing NFMEP by early detection and prompt treatment, identifying high risk areas and interrupting malaria transmission by spraying insecticide and distributing insecticide treated bednets.

Malaria is caused by the Plasmodium parasite which spreads when an infected female Anopheles mosquito bites a human. A vector is a living organism carrying a disease-causing agent, which in the case of malaria, is the Anopheles mosquito. “There are more than 400 Anopheles species and they look extremely similar to each other. We think of Anopheles species as having the potential to transmit malaria but it’s only a very small number of those species that are responsible for malaria transmission,” says Walton, who has been studying mosquitoes in Southeast Asia and northeast India for many years.

Genetic diversity of malaria vectors in Meghalaya

The study titled Characterisation of Anopheles species composition and genetic diversity in Meghalaya, northeast India, using molecular identification tools was published in the journal Infection, Genetics and Evolution in February 2023 and co-authored by scientists at the University of Manchester and Indian Institute of Public Health (IIPH), Shillong. This study is a part of a larger set of studies done on malaria focusing on the occurrence and distribution of the disease, Anopheles mosquito species in the state and people’s behaviours associated with the disease.

“Finding out what all vector species are present in a region helps in characterising and quantifying the risk,” says Rajiv Sarkar, assistant professor at IIPH, Shillong. The studies were conducted under the Center for the Study of Complex Malaria in India (CSCMi), a part of the International Center of Excellence for Malaria Research (ICEMR), an international network of research centres located in countries where malaria is endemic.

There are more than 24 Anopheles mosquito species in Meghalaya. Anopheles baimaii and Anopheles minimus, found in forest habitats, have been regarded as the two most important malaria vectors in the northeast. The study found that Anopheles baimaii and Anopheles minimus mosquitoes were rare whereas four other species were abundant and found in rice fields. The results suggest that rice fields might be creating conditions for larger presence of Anopheles maculatus and Anopheles pseudowillmori, which could be spreading malaria either independently or together with Anopheles baimaii and/or Anopheles minimus.

When the scientists planned the study, the rate of malaria in Meghalaya was high but it decreased dramatically when they began their research. This is why they changed the focus of the study to understand if primary vectors were going down, what were the other vectors playing a role in transmitting the disease, says Upasana Shyamsunder Singh, co-author of the study, who undertook this research as a part of her Ph.D. thesis. She is currently a postdoctoral scholar at Vanderbilt University in Tennessee, U.S.

The research work began in 2018 starting with training the staff at IIPH, Shillong to set up the study. The study team collected mosquitoes from three parts of Meghalaya between 2019-2022 – Nonglang in West Khasi Hills and Barato and Nartiang in West Jaintia Hills. This process was also disrupted for some months due to the COVID-19 pandemic. There were 1,389 adult mosquito collections from people’s homes and 144 larval collections from in and around villages.

In this entomological study, after the samples were collected, the first step was morphological identification. Morphological identification involves examining mosquitoes through a microscope and has been traditionally used to identify various species. This was followed by molecular identification, which used the DNA barcoding method. This method is used to identify and discover species using short, standardised segments of DNA.

Every species has its unique barcode. In this study, DNA barcoding involved three steps. First, the DNA was extracted from each individual Anopheles mosquito. Then, this DNA was amplified with polymerase chain reaction (PCR). After that, the DNA sequencing and analysis was done. After collection, the samples were shipped to Indian Council of Medical Research – National Institute for Research in Tribal Health (ICMR-NIRTH), Jabalpur. A lab at the institute processed the specimens using DNA isolation, PCR and sequencing. After this step, Singh analysed the sequences remotely at the University of Manchester.

The study identified 19 species from 2,575 Anopheles specimens (both adult and larvae) collected from two districts. Ten of these species were breeding in rice fields, says Singh. While the scientists did not do a full ecological larval survey to determine how different species were using potential breeding sites, they found Anopheles maculatus, Anopheles pseudowillmori and Anopheles jeyporiensis in large numbers, breeding in rice fields. “You can imagine that rice fields are covering quite a lot of land in the region in Meghalaya, so they are potentially quite important breeding grounds for producing large numbers of mosquitoes,” says Walton.

Malaria, an environmental disease

Rice is a major crop in Meghalaya and the state has over 100,000 hectares under rice cultivation. Scientists have been studying mosquito breeding and malaria transmission in rice fields in India for over two decades now, with studies done in Madhya Pradesh and Gujarat. India is the second largest producer of rice globally. It has 47 million hectares of area under rice cultivation. Rice growing is associated with an increased risk of malaria, according to a Lancet review published in 2022.

According to Walton and Singh, the study’s limitations include not being able to test what were the vector species in the study areas spreading malaria and not being able to collect specimens in ecological breeding habitats other than people’s homes in villages. S.R. Hajong, professor of zoology at North Eastern Hill University (NEHU), who was not involved in the research, says the study is “interesting”. He pointed out the sample collections were “confined to two districts of Meghalaya”. The findings of this study cannot be applied to the entire state because the state of deforestation, land use patterns and climatic conditions vary between different districts in the state, he says. In order to implement better vector control measures, further studies should focus on mosquito biology to find out the breeding pattern, life cycle and lifespan of mosquito species in the state, he says.

“The National Center for Vector Borne Diseases Control (NCVBDC) have to keep up their surveillance and they have to be aware of issues of identification and understand which mosquito species are involved because that could potentially affect malaria control efforts and how they are implemented,” says Walton. She also emphasises the need for a holistic approach and cross-sector talk between government departments and experts in public health, agriculture and science. “Malaria is an environmental disease and we should be thinking about how we use the environment in a sustainable way for food production to keep people healthy and for biodiversity,” she says.

Banner image: A recent study suggests that rice fields in Meghalaya might be creating conditions ideal for the spread of malaria. Photo by Pragyan Bezbaruah/Pexels.