UF study tracks dengue and West Nile viruses in South Florida mosquitoes

The Aedes aegypti mosquito species resting on someone's skin.
The Aedes aegypti mosquito species, shown here obtaining a blood meal, is a vector for the dengue virus. (iStock)

Imagine your phone pings, and an alert notifies you that mosquitoes in your neighborhood recently tested positive for West Nile virus or dengue virus — or both.

Might you spritz some insect repellant before gardening? Choose to stay indoors at your neighbor’s weekend barbecue?

Such a day may not be far off thanks to new research published March 19 in The Lancet Regional Health – Americas by University of Florida researchers. The research uses nearly real-time molecular test results of trapped wild mosquitoes to strategically guide local mosquito control efforts. The new work is coauthored by investigators from the CDC Southeastern Center of Excellence in Vector Borne Diseases.

The approach could be used to craft public health messaging too, via push notifications or an online dashboard to convey local conditions, says the study’s first author, Heather Coatsworth, who is a research assistant professor in the UF College of Veterinary Medicine, department of infectious diseases and immunology.

West Nile virus, dengue virus, and SARS-CoV-2

In 2020, Miami-Dade County experienced a trifecta of viral outbreaks. In addition to the transmission of SARS-CoV-2, 86 people were also locally infected with West Nile virus and 71 with dengue virus.

Dengue virus and West Nile virus are both arboviruses that are transmitted to humans by mosquitoes. West Nile virus also circulates between wild birds, mosquitoes, and humans. West Nile virus has been present in Florida, and especially southern Florida, for at least 50 years.

Dengue virus cases have been climbing for the past few years in Florida, with most cases concentrated in South Florida. Some are imported via tourists and travelers, but cases of local transmission have also been climbing. It’s been in the Florida Keys for at least the past five years. Visitors to Florida from places where dengue is endemic, such as the Caribbean and Central and South America, may also unwittingly bring the virus with them. Dengue virus has four virus serotypes, numbered 1-4. Each serotype differs genetically one to the other.

The study analyzed mosquitoes trapped in Miami-Dade County between May and December 2020. Coatsworth said the study had a few novel findings, including detecting dengue virus serotypes 2 and 4 in mosquitoes, despite a lack of human cases reported to be infected with these serotypes in the study area.

The prevailing school of thought is that the dengue virus is largely maintained in mosquito populations by also cycling through humans. Because the study timeframe took place during the COVID-19 pandemic, the study authors suggest that perhaps dengue virus serotypes 2 and 4 infections went unreported. Novel research by investigators from the Southeastern Center of Excellence in Vector Borne Diseases last year documented evidence for the vertical transmission of dengue virus in Florida from female mosquitoes to their eggs, without cycling through a human host. However, this is still considered uncommon, Coatsworth said.

Because dengue virus serotype 1 was also reported in people during this time, it raises the possibility for people developing severe forms of dengue—which is also called break bone fever—due to antibody-dependent enhancement reactions when people experience a subsequent infection with a different dengue virus serotype. This risk of severe disease elevates the need to continually engage in mosquito surveillance in hotspot areas, Coatsworth said.

Mosquito traps and molecular tests

The study relied on a gridded network of 309 mosquito trap sites that were implemented in the Miami-Dade County area during the Zika outbreak in 2016. Researchers collected mosquitoes from these traps and brought them to the University of Florida where they underwent testing to look for the presence or absence of West Nile virus and dengue virus

The researchers wanted to explore the possibility of using near real-time molecular testing to inform and guide local responses to mosquito control. Molecular testing refers to laboratory techniques that detect the presence of signature RNA specific to each virus.

The research team screened 7,668 mosquitoes in grouped “pools” and found four mosquito pools infected with dengue virus serotype 2, nine pools infected with dengue virus serotype 4, and nine pools infected with West Nile virus. This represented a positivity rate of close to 3% for the female mosquitoes tested.

All of the dengue-carrying mosquitoes were found in urban areas. But mosquitoes carrying West Nile virus were found in agricultural and recreational areas where there was no prior history of transmission. This showed that the network of traps needs to be expanded in order to detect the actual scope of West Nile circulation, particularly in the southern part of Miami-Dade County.

The presence of dengue virus serotypes 2 and 4 in mosquitoes, without corresponding known cases where people in these areas were also infected with these serotypes, indicates that dengue may be becoming more endemic in Florida than commonly understood. It’s also possible that people were in fact infected with these serotypes during the study period, but that never received a correct diagnosis and instead self-isolated due to COVID-19-like symptoms such as fever, fatigue and muscle pain, Coatsworth said.

Culex and Aedes mosquito species are primarily responsible for spreading dengue virus and West Nile virus. Aedes aegypti was the primary species of concern in carrying dengue in the study, although Aedes albopictus has been found to be similarly suited to carrying the virus in different work in Indian River County.

The team created maps that can be used to inform local mosquito control districts of where new traps need to be placed.

“The existing role of the mosquito control districts in this work cannot be overstated,” said Kaci McCoy, Center Program Coordinator for the Southeastern Center of Excellence in Vector Borne Diseases. “The Miami-Dade County mosquito control district is a great one that provides a high level of service. Like other vector control districts in Florida, MDMCD takes an integrated pest management approach and uses surveillance data to guide treatments.”

With enough sustained funding, real-time molecular testing could help inform the districts where to best allocate limited resources for the best disease control. The Florida Department of Health currently compiles reported data and issues it as weekly mosquito-borne disease surveillance reports, but this often reflects local conditions from weeks to several months past.

The study’s senior author, Rhoel Dinglasan, is a professor of infectious diseases in the UF College of Veterinary Medicine, department of infectious diseases and immunology, director of the Southeastern Center of Excellence in Vector Borne Diseases, and an EPI faculty member.

By: DeLene Beeland