It continues to be Clarke’s privilege to share Dr. Day's weekly analysis of arbovirus disease activity in Florida with mosquito control professionals across the state. Our shared goal with Dr. Day is to provide timely and actionable information that mosquito control programs can use to make operational decisions and protect public health from vector-borne diseases.

An archive of all past newsletter issues remains available on the Clarke website.

A: Transmission of EEEV in Florida depends on at least three factors. 

First, the virus must be present before it can be amplified and transmitted by vector mosquitos. In many regions of Florida, EEEV may be endemic in freshwater swamps throughout the year. In other parts of the state, the virus may be reintroduced during spring and autumn avian migrations.

Second, susceptible avian amplification hosts, vector mosquitoes, and EEEV must overlap in habitats where viral amplification occurs producing a large number of infected mosquitoes.

Finally, the dispersal of infective mosquitoes out of the initial amplification sites is facilitated by groundwater fluctuations. Wet springs, dry summers, and wet falls are the groundwater sequences that result in high levels of EEEV transmission (see Figure 4 below). Historically, the highest levels of EEEV transmission in Florida have been reported in North Florida and the Panhandle Counties.

There continues to be low-level transmission of EEEV in Florida as measured by sentinel chicken seroconversions and EEEV-positive equines.

Most of the transmission has been reported in a narrow band in Central Florida (Figures 1, 2, and 3). The potential for transmission of WNV in Southwest Florida remains high, but to date, there has been little indication that WNV experienced significant amplification and dispersal during the South Florida spring drought and early summer rainfall events (Figure 2). None of the groundwater conditions reported in Florida to date have tracked the ideal conditions for the amplification, dispersal, and transmission of EEEV or WNV during the first six months of 2022 (Figure 4).

Based on recent rainfall patterns, South Florida is at the highest risk of WNV transmission in the coming weeks.  

Vector and arboviral surveillance remain some of the most important tools that vector control agencies currently have at their disposal. Arboviral transmission indices (sentinel chickens, positive equines, positive exotics such as emus), positive humans, and positive mosquito pools) provide indicators of local virus transmission, although sometimes not in a timely manner. Monitoring mosquito populations and their age structure gives additional information about potential transmission risks. Supplemental vector control efforts in and around sites where virus transmission is known or suspected of recently occurring provide another mechanism to mitigate viral transmission.

There is currently a moderate risk of arboviral transmission in some parts of Florida. It is important to note that arboviral transmission risk is never zero. The transmission of EEEV in the wet to dry transition zone in Central Florida will continue to be an issue of concern during the coming weeks (Figures 1, 2, 3, and 4). The rapid change from dry to wet groundwater conditions in South Florida (Figure 1) during the past three weeks may favor the transmission of WNV in July and August.   

Jonathan Day, Professor Emeritus of Medical Entomology from the University of Florida, is a national expert on mosquitoes and other blood-feeding arthropods that transmit diseases to humans, domestic animals, and wildlife. In collaboration with other researchers, Dr. Day has developed an effective system for monitoring and predicting epidemics of mosquito-borne diseases.

Acknowledgments: This analysis would not be possible without the tireless efforts of multiple agencies across Florida. At least 27 Florida agencies collect serum samples from sentinel chickens each week and mail them to the Florida Department of Health Tampa Branch Laboratory for analysis, compilation and reporting. Data are summarized by researchers at the Florida Department of Health in Tallahassee and reported weekly as the Florida Arbovirus Surveillance Report.

Contributors to this summary and full report include: Andrea Morrison, PhD, MSPH, Rebecca Zimler, PhD, MPH, and Danielle Stanek, DVM, Florida Department of Health, Bureau of Epidemiology; Lea Heberlein-Larson, DrPH; Alexis LaCrue, PhD, MS; Maribel Castaneda, and Valerie Mock, BS, Florida Department of Health Bureau of Public Health Laboratories, and Carina Blackmore, DVM, PhD, FDOH Division of Disease Control and Health Protection. And, Dr. Rachel Lacey, Florida Department of Agriculture and Consumer Services, Animal Disease Diagnostic Laboratory in Kissimmee, FL.

Daily updates of the Keetch-Byram Drought Index (KBDI) are produced by the Florida Department of Agriculture and Consumer Services, Forest Service.

All of the graphics used in issues of this Newsletter are designed and developed by Gregory Ross.

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