A new report published in the Journal of Medical Entomology found that there has been a substantial increase in the number of US counties reporting the presence of mosquitoes capable of spreading disease. During the recent Zika virus outbreak in 2016, enhanced surveillance for Aedes Aegypti and Aedes Albopictus mosquitoes across the United States allowed researchers to update “previously presented county-level collection record maps for these mosquitoes.” Zika virus is primarily vectored by the Ae. Aegypti mosquito; however, Ae. Albopictus is also believed to be a competent vector. These are the same mosquitoes that transmit dengue and chikungunya.
The authors reported a “21 percent increase in the number of counties with reported presence” of Ae. Aegypti, for a total of “220 counties in 28 states and the District of Columbia.” Southern California, Arizona, Texas, Louisiana and Florida had the most “widespread county-level distribution,” while more northern states such as Washington and Idaho reported no collection of the mosquito during surveillance efforts. However, the authors note that, while this mosquito thrives in warm, tropical weather, “most regions in the United States experience summer temperatures that are conducive to Ae. Aegypti development and activity for at least some period of time.” The authors also reported a 10 percent increase in the number of counties reporting the presence of Ae. Albopictus, for a total of “1,368 counties in 40 states and the District of Columbia.”
Since January 2015, there have been 5,296 symptomatic Zika virus cases reported to CDC in the US (excluding territories). A majority of these cases (5,024) were diagnosed in travelers returning from areas with local transmission, such as the Caribbean. However, 224 cases were acquired through presumed local mosquito-borne transmission, with two US states—Florida and Texas—reporting local transmission. Zika is not the first arbovirus to recently circulate within the US: local cases of dengue virus and chikungunya virus have also been reported, primarily in the states of Florida, Texas and Hawaii. However, Zika virus has been an unprecedented public health emergency in the United States because, while it is primarily transmitted through the bite of an infected mosquito, it can also be transmitted sexually. Additionally, while eighty percent of people infected with Zika virus will have no or mild symptoms, it can lead to catastrophic birth defects in the fetus of a mother infected with the virus while pregnant.
These latest findings on the increasing geographic distribution of Zika virus vectors underscores the importance of maintaining strong mosquito surveillance capabilities in the US, to ensure that areas with the potential for local transmission (ie, those areas with competent Zika virus vectors) are identified, and to help inform those states that should have more comprehensive testing procedures that are not based solely on travel histories. However, many health departments lack mosquito surveillance and laboratory testing capacity due to dwindling federal funding. After the emergence of West Nile Virus (WNV) in 1999, “Congress appropriated annual funding to support WNV surveillance activities,” and by 2004, “nearly all states and assessed local health departments had well-developed animal, mosquito, and human surveillance systems to monitor WNV activity and anticipate outbreaks.” However, a follow-up assessment in 2012—after a 61% decrease in funding—found that “many health departments had decreased mosquito surveillance and laboratory capacity.”
Our current funding strategy, which focuses on episodic threats rather than investing in building and maintaining vector surveillance capacities within the US, almost ensures that vector-borne diseases will continue to threaten the public’s health. The growing geographic distribution of Aedes mosquitoes puts millions of people at risk for diseases such as Zika, and enhanced surveillance capabilities are needed to ensure that these diseases will be detected before a large outbreak occurs. Additionally, since these mosquitoes serve as a vector for a multitude of diseases, strong surveillance capabilities will not only increase our understanding of the distribution—or potential distribution—of one disease, but of many.