Even as several parts of India are in the grip of severe dengue outbreaks, a team of researchers has found clues about why the mosquito-borne disease might be spreading through the country faster than before. The answer lies in increased temperatures, according to a study published in the journal, Science Advances.

A rise in temperature results in a shorter “extrinsic incubation period” or EIP, which is the time required for the virus to develop in the mosquito, according to the study by scientists from Hyderabad, Guwahati and Liverpool. A shorter incubation period leads to higher transmission rates of dengue infection in a community, the authors said.

India’s mean temperature rose by more than 0.5 degree celsius between 1960 and 2009 as a result of several environmental factors, including climate change. The authors studied the temperatures in different seasons and found that an increase in temperature led to a shorter incubation period for the virus.

The dengue virus is transmitted mainly by the Aedes aegypti mosquito

. Dr Srinivasa Rao Mutheneni, scientist at the CSIR-Indian Institute of Chemical Technology and lead author of the study, said that simply put, extrinsic incubation period is the time from when the mosquito acquires the virus to the time that it is infectious enough to transmit the dengue virus to a person when it bites.

The researchers studied five dengue-endemic states in different climatic zones: Punjab, Haryana, Rajasthan, Gujarat and Kerala. The study looked at the correlation between mean temperatures and EIP. The study also assessed the effect of rainfall on dengue burden.

Lancet study

As it turns out, an analysis of the health impacts of climate change published by The Lancet earlier this week cited climate change as a potential factor for the rise in dengue cases. According to another study published in The Lancet, the number of dengue cases has doubled every decade since 1990, with India being no exception. Between 2010 and 2016, India has recorded a 356% increase in the number of dengue cases, according to the National Vector Borne Disease Control Programme.

Rao and his team developed an equation to find the extrinsic incubation period of mosquitoes. They found that the states where the temperature was higher, the extrinsic period was low. “This means that the mosquitoes were turning infectious in a very short period of time of getting infected with the dengue virus,” said Rao. This, he said, triggered a very rapid transmission of dengue in the community.

The study holds important insights into the epidemiology of the dengue infection. In India, according to the data by the National Vector Borne Disease Control Programme, Kerala, Tamil Nadu, and Karnataka reported the highest burden of dengue cases this year. Rao said that coastal states including Kerala showed a very low extrinsic incubation period which could be a result for the high transmission of the cases. “If there is a very high [longer] extrinsic incubation period, the infected mosquito may not survive till it becomes infectious,” said Rao.

However, Rao explained, temperatures above 33 degree Celsius are not conducive to mosquito breeding, which can limit transmission of the dengue virus. He also said that intermittent rains also play a role in determining the temperatures. “You may think that rains would lead to a drop in temperatures but the drop is not enough to affect the extrinsic incubation period,” he said.

Shrivastav of the CSIR-Indian Institute of Chemical Technology, who published a paper on the dengue virus infection in Mumbai, said that the role of the environment in infectious diseases cannot be ignored. “There is definitely an impact of moisture on the mosquito, its egg laying habits, life-cycle,” he said. “We need to study more to understand the effects on the virulence of the dengue virus.” Virulence refers to the ability of the pathogen to cause the disease. The greater the virulence, the greater the pathogen’s ability to cause the disease.

Confusion over counting

The findings may help predict dengue outbreaks in various parts of the country. Currently, it is uncertain whether the higher number of cases across India is due to better reporting of dengue infections or increased transmission of the virus – or both. Many public health activists believe that dengue cases are actually being under reported and that there are many more cases than is being reflected by government data. That seems to be the case in West Bengal, for instance. Amidst panic about a surge in dengue cases and deaths over the past month, the state health machinery has been underplaying the outbreak by not mentioning dengue as a cause of death in several instances, news reports have alleged.

Moreover, the National Vector Borne Disease Control Programme counts only those dengue cases confirmed by performing an enzyme-linked immunosorbent assay or ELISA test. But it ignoes dengue positive results thrown up by another cheaper and faster diagnostic test. “The NS-1 antigen testing which is cheaper and [for which] the patient gets the results within a day is more popular [and used more],” said Dr Om Shrivastav, infectious disease specialist at Saifee Hospital in Mumbai. Because the National Vector Borne Disease Control Programme does not consider NS-1 antigen testing as a confirmatory test for dengue, hundreds of patients do not get counted as dengue positive cases.

The study on the link between temperatures and the extrinsic incubation period, meanwhile, could help develop preventive measures in regions of the country and among communities that are susceptible to dengue infections.“This insight helps us to identify states with low extrinsic incubation period and focus mosquito control activities there,” said Rao. “In India, we have different climatic zones for which we required tailor-made broadcasting models instead of one model for the entire country.”