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Maps show US counties with Aedes mosquitoes since 1995
Researchers have compiled a list of counties in the United States where the mosquito at the heart of the recent Zika virus outbreak has been documented during the past 21 years.
Data culled from various sources by researchers from the CDC’s Division of Vector-Borne Diseases and Colorado State University showed that 183 counties across the country recorded the presence of Aedes aegypti mosquitoes between January 1995 and March 2016. Many more counties — 1,241 — recorded the presence of A. albopictus, another Aedes mosquito known to transmit the Zika, dengue and chikungunya viruses, which have caused recent outbreaks in the Western Hemisphere, chiefly in Latin American and the Caribbean. The three viruses represent “increasing threats to human health,” the researchers wrote in the Journal of Medical Entomology.
John-Paul Mutebi, PhD, entomologist at the CDC’s Division of Vector-Borne Diseases and co-author of the study, said information in the report will better equip local officials to be on the lookout for new viral diseases and help educate clinicians to be on alert for sick patients and to recognize and report disease cases. Mosquito surveillance and control also will be improved.
“Accurate and up-to-date information for the geographical ranges of A. aegypti and A. albopictus in the United States is urgently needed to guide surveillance and enhance control capacity for these mosquitoes,” Mutebi told Infectious Disease News.
“State and local health departments and mosquito control districts need to know what types of virus-carrying mosquitoes exist in their area.”
Study follows outbreaks
In the Western Hemisphere, outbreaks of Zika, dengue and chikungunya have been limited mostly to Latin America and the Caribbean, with only minor localized outbreaks in the U.S.
Mutebi and colleagues used records from federal and local mosquito surveillance sources, including the CDC’s ArboNET database, and warned that their data “should be viewed as compilations of records based on convenience sampling rather than representing systematic surveys.” They considered the mosquitoes to be “present” in a county in any given calendar year if at least one specimen of any life stage of the mosquito was collected during that year.
Figure 1. Map showing the U.S. counties where the Aedes aegypti mosquito — the primary vector in the current Zika outbreak — was recorded between 1995 and 2016. The number of years with at least one mosquito reported has been marked as yellow (1 year), orange (2 years), and red (3 or more years).
Source: Entomological Society of America
Between the start of 1995 and the end of 1999, A. aegypti was recorded in only 11 counties in Arizona, Florida and Texas. By March 2016, however, that number had increased to 183 counties in 26 states and Washington, D.C., most of them along the southern perimeter of the U.S., but also in the Mid-Atlantic region. The mosquitoes were recorded in four states considered to be geographic outliers: Colorado, Kansas, Michigan and New Hampshire.
“Perhaps the most concerning development for A. aegypti is its establishment in the Southwest, most recently in California in 2013,” Mutebi and colleagues wrote. “The discovery of established populations of A. aegypti in central and Southern California resulted in a substantial, and still ongoing, public health response that has included enhanced human and mosquito surveillance, education, and intensive mosquito control.”
A. albopictus, which was introduced into the U.S. in 1985, was recorded in 370 counties between 1995 and 1999. By March 2016, the mosquito had been recorded in 1,241 counties in 40 states and Washington, D.C., including 514 counties from 34 states where it was collected in at least 3 years. It is most consistently reported in the Southeast, south-central and Mid-Atlantic states and along the southern Ohio River Valley. It is unclear, Mutebi and colleagues said, whether A. albopictus has yet to become established across the full geographic range of territory where it can persist.
“Although the climate in some newly invaded areas is conducive to reproduction and survival of this mosquito, in other areas, harsh winters may prevent survival of overwintering eggs into the spring,” they wrote. “In addition, effective and timely vector control may eliminate highly localized introductions, for example, in tire facilities.”
Increased surveillance likely led to higher numbers
According to Mutebi and colleagues, a substantial increase in the number of counties reporting the two species of Aedes mosquitoes — particularly between 1995 and 2004 — was likely a result of broader surveillance efforts, rather than a geographic spread of the insects.
“Moreover,” they wrote, “these data represent presence rather than abundance of the mosquitoes, and it should not be assumed that the climate across counties in which the mosquitoes are present provides similar potential for population establishment and expansion.”
Recently released maps showing the CDC’s best estimate of the potential range of the Aedes mosquitoes were partly based on the information included in the study, said Candice Hoffmann, CDC spokeswoman.
“These maps will assist states in deciding if they need to do more surveillance and to plan for possible mosquito-borne outbreaks,” Hoffmann said.
Figure 2. Map showing the U.S. counties where the A. albopictus mosquito was recorded between 1995 and 2016.
Source: Entomological Society of America
There have been localized outbreaks of dengue and chikungunya in the U.S., but conditions here — including cooler temperatures, better sanitation, improved housing with screens and air conditioners and better surveillance — mean large outbreaks caused by viruses transmitted by the two mosquitoes “are unlikely to recur in the continental United States unless socioeconomic conditions deteriorate to mimic those seen in previous centuries or if other modes of transmission for these viruses became more widespread,” Mutebi and colleagues concluded. – by Gerard Gallagher
Disclosure: The researchers report no relevant financial disclosures.