Risk of dengue transmission is measured using number of female mosquitoes

Index developed by Brazilian researchers, which considers full-grown Aedes females, is also being tested to measure risk of Zika and chikungunya transmission.

By Peter Moon  |  Agência FAPESP – A new index that measures the risk of dengue transmission in a city or region in terms of the level of infestation by adult females of the mosquito Aedes aegypti has been described by Brazilian researchers in the journal Acta Tropica. 

The methodology was developed by Maisa Carla Pereira Parra and collaborators under the supervision of Maurício Lacerda Nogueira, a professor at the São José do Rio Preto Medical School (FAMERP), and Francisco Chiaravalloti-Neto, a professor at the University of São Paulo’s Public Health School (FSP-USP), both in São Paulo State, Brazil.

The study was part of the Thematic Project “Epidemiological study of dengue (serotypes 1-4) in a cohort of São José do Rio Preto, São Paulo, Brazil, during 2014-2018”, supported by FAPESP.

According to the researchers, the new method is more practical and reliable than the Breteau Index, which is used by many public health services. In dengue surveillance in Brazil, this index measures the ratio of the number of A. aegypti larvae detected to the number of homes inspected by health workers. 

“The Breteau Index calculated for São José do Rio Preto at the start of 2018 was the highest ever,” Nogueira said. “It was even higher than for 2013, when the region had the worst dengue epidemic in its history, with 18,000 cases. Yet, only 44 cases have been reported so far this year. In other words, at least in our region, the Breteau Index is no longer fit for its purpose as far as dengue prevention is concerned.”

The explanation for the disparity, he went on, may have to do with the immunity acquired by the region’s inhabitants during recent epidemics.

“The trouble with the Breteau Index is that it measures the number of larvae, which live in water during this stage of the mosquito’s life cycle,” Chiaravalloti-Neto explained. “Only adult females transmit the virus. They do so after mating, when they feed on human blood to produce and lay eggs.”

This fact about dengue transmission inspired the researchers to develop a new indicator based solely on the number of adult females. “We concluded that it would be more reliable and easier to calculate. The rationale is that the more adult females there are in the environment, the more people will be infected,” Nogueira said.

Methodology

To calculate the Breteau Index, health workers have to visit all homes in the surveillance area, inspect every water tank, puddle, vase and so on, count the larvae, and add them all up. This procedure has to be repeated regularly and requires a large workforce and budget.

To calculate the new index, 56 special traps were installed in various parts of São José do Rio Preto, releasing an odor similar to that of human skin to attract adult females in search of blood. Once inside the trap, they are unable to leave and eventually die.

“The traps were spaced between 200 and 400 meters apart, corresponding to about half the female’s maximum flight radius or less. They were collected the next day, and the number of adult females was recorded,” Nogueira said.

The experiment was performed twice per week, and data collection covered 62 homes per week for the period between the 36th week of 2012 and the 19th week of 2013.

“At the end of the fieldwork, we had data from more than 1,500 traps. We also collected females to find out by molecular analysis whether they were positive or negative for dengue,” Chiaravalloti-Neto said.

The entomological index was based on the number of adult females of A. aegypti captured per 100 homes in the vicinity of each trap during one week. 

“The number of females collected from a single trap may be small but serves as a sample to calculate the size of the infestation in the neighborhood,” Chiaravalloti-Neto said.

The group mapped the São José do Rio Preto region using the index to plot the number of females per neighborhood during the 52 weeks of the year. Altogether, the traps caught 6,024 insects comprising 1,333 mosquitos of the genus Aedes (536 males and 797 females) and 4,691 of the genus Culex (3,325 males and 1,366 females).

The A. aegypti specimens were pooled in 893 tubes that were tested for the presence of dengue virus. Serotype 4 (DENV-4) was found in 25 mosquitos (2.8% of the tubes), of which 19 were females and six were males. 

“We created databases for traps and dengue cases,” Nogueira said. “Trap data included addresses, installation dates, collection dates, number of specimens, species of mosquito, and molecular analysis results. Dengue case data included reported addresses, date of symptom onset, type, and lab analysis results.”

Lastly, the result of the entomological index was compared with the epidemiological surveillance data for dengue cases in the city between the 36th week of 2012 and the 19th week of 2013.

“We obtained all the notified cases and modeled them to see if the number of cases correlated with a larger or smaller number of females,” Chiaravalloti-Neto said.

More than 2,500 suspected dengue cases were reported in the area studied. Of these, 1,137 were registered as confirmed, and 820 cases tested positive for dengue (72.1%), while 317 matched clinical epidemiological criteria (27.9%). The researchers used 1,450 cases that tested negative for dengue as controls. 

According to Chiaravalloti-Neto, the result of the modeling exercise could not have been better. “When the number of females increased, the risk of dengue incidence was found to increase correspondingly,” he said.

“Our entomological index correlated positively with dengue incidence, especially during the intervals in which vector control measures were applied less intensively,” Nogueira said.

The researchers are now repeating the experiment in a different neighborhood with the aim of validating the results of the first field survey and demonstrating that the new method is indeed a reliable alternative to the Breteau Index.

“The ongoing study is broader than the first. Besides dengue, we’re testing the new method to measure the risk of Zika and chikungunya transmission. But it will be some time until the results are out,” Nogueira said.

The article “Using adult Aedes aegypti females to predict areas at risk for dengue transmission: A spatial case-control study” by Maisa Carla Pereira Parra, Eliane Aparecida Fávaro, Margareth Regina Dibo, Adriano Mondini, Álvaro Eduardo Eiras, Erna Geessien Kroon, Mauro Martins Teixeira, Mauricio Lacerda Nogueira and Francisco Chiaravalloti-Neto can be retrieved from sciencedirect.com/science/article/pii/S0001706X17312020?via%3Dihub.