The Aedes aegypti mosquito has been found in 28 states since 1995.

With summer heat and humidity bringing out mosquitoes, health officials have been especially concerned about a possible return of the Zika virus.

In Florida where the majority of the Zika outbreaks in the United States occurred, state health officials continue to spray for mosquitoes and closely monitor pregnant women for any sign of the disease, which can cause serious birth defects.

“Zika remains a threat for Floridians, especially pregnant women and their infants after they are born, and women who will become pregnant,” Florida Surgeon General and Secretary of Health, Dr. Celeste Philip, said during a call to health providers in May.

A study from the Centers for Disease Control and Prevention (CDC) published this week revealed another factor that could increase the risk of a Zika outbreak: The mosquitoes that spread the virus have been expanding their habitat in the United States.

The report, an update from one published last year, found that the Aedes aegypti mosquito, which was responsible for most of the spread of the Zika virus, and the Aedes albopictus, which is less connected to the Zika virus but can transmit other tropical diseases like dengue and chikungunya, have spread even farther in the United States than originally thought.

The CDC researchers reported that Ae. aegypti specimens had been found in 28 states and 220 counties from 1995 to December 2016. The Ae. albopictus was found in 1,368 counties in 40 states and the District of Columbia during that time.

This report showed a significant rise in the number of areas where these insects are found. Between 1995 to 1999 the Ae. aegypti was found in just 11 counties and three states (Arizona, Texas, and Florida). During that same time period the Ae. albopictus was found in 370 counties, virtually all in the Southeast region of the country, although it was also identified in Kansas, Texas, and Oklahoma.

Today the Ae. albopictus has been found as far west as California and as far north as Washington and New Hampshire.

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Published in the Journal of Medical Entomology, the updated report revealed that mosquito species were actually even more spread out than initially realized.

After mosquito surveillance increased due to the threat of Zika infection, CDC researchers surveyed local officials about what they found from March through the end of December to see if more specimens were being found.

They discovered officials in an additional 38 counties had found an Ae. aegypti specimen, and officials in a whopping 127 counties found at least one Ae. albopictus specimen. This means compared with the original 2016 report, there were 10 and 21 percent increases in the total number of counties where Ae. albopictus and Ae. aegypti were found, respectively.

The new numbers are likely the result of increased surveillance in 2016, and not due to an actual spike in the mosquito population.

Rebecca Eisen, PhD, research biologist with the CDC’s Division of Vector-Borne Diseases, and co-author of the study, said the study showed how important local surveillance is in understanding mosquito populations.

“We expect that with continued intense surveillance for these species we will undoubtedly produce additional county collection records, especially in areas that are environmentally and climatically suitable for the mosquitoes to survive and reproduce,” she told Healthline in an emailed statement.

Getting this data and understanding it could be vital to help prevent or fight future outbreaks of insect-borne diseases.

“This information will help to target limited public health surveillance resources and help to improve our understanding of how widespread these mosquitoes are,” Eisen said in a statement released earlier this week.

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Dr. Ian Lipkin, director, Center for Infection and Immunity at the Columbia University Mailman School of Public Health, said the report highlights why robust mosquito surveillance is important. He added that due to decreases in funding, mosquito surveillance “has not received the support it should.”

“Surveillance was better 20 years ago than it is today,” Lipkin told Healthline. As a result, he believes the mosquito population is likely undercounted.

“When we’re showing more now, it truly is more,” he said of the mosquito population.

Lipkin added that a warming climate has made large swaths of the United States habitable for these mosquitoes, particularly the Ae. albopictus, which was documented in 40 states.

“It’s really worrisome,” Lipkin said. “We’ve had dengue in Texas and dengue in Florida … we’re going to see risk for this up in the Northeast.”

While cases of the Zika virus have largely dissipated in much of the western hemisphere, Lipkin said the increasing spread of mosquitoes means more people could be at risk for a host of other diseases.

“It’s becoming a national problem,” he said. “The other thing to bear in mind is we don’t know what kind of other infectious disease they may carry.”