This email contains highlights from the 2021 disease introduction and transmission activity in Florida. A full-length, more detailed report is available for download here.

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 issues remains available here.

A: There are five factors that I monitor and use as indicators of potential real-time arboviral transmission risk in Florida.

First, the spatial and temporal disease transmission patterns and introductions in Florida during the previous three months.

Second, the daily surface moisture levels on a 4 km² resolution throughout Florida as measured by the Florida Department of Agriculture and Consumer Services Keetch-Byram Drought Index (KBDI).

Third, vector mosquito abundance and age structure as inferred by the KBDI data.

Fourth, avian amplification host abundance and reproductive success as measured by line transects in Indian River County and as inferred by KBDI data for the rest of Florida.

Finally, indicators of real-time arthropod disease transmission as reported in the Florida Department of Health, Florida Arbovirus Surveillance Weekly Report.

This is a brief answer to Dennis’ question. For a complete answer to his question, please download the attached Volume 2, Issue 1 pdf; a more inclusive issue of The Year in Review: Vector-borne Disease Transmission in Florida during 2021.

In Florida, four arboviruses currently account for the greatest human disease risk; dengue viruses (DENV), eastern equine encephalitis virus (EEEV), St. Louis encephalitis virus (SLEV), and West Nile virus (WNV). Three additional pathogens are periodically introduced into Florida and sometimes result in locally-acquired human infections: Chikungunya virus (CHIKV), Zika virus (ZIKAV), and malaria.

Vector-borne disease transmission and pathogen introductions into Florida during 2021 were below average. A brief summary of mosquito-borne disease transmission and pathogen introductions into Florida during 2021 appears below.

Eastern equine encephalitis virus was the most active mosquito-borne pathogen in Florida during 2021. While there were no reported human EEE cases in the state during the year, there were 185 sentential chicken seroconversions to EEEV reported in 21 Florida counties. The 12-year (2010-2021) mean EEEV annual seroconversion rate for Florida sentinel chickens is 116 per year, so the 2021 seroconversion rate was above normal. There were 21 EEE-positive equines reported in 16 Florida Counties during 2021, well below the 18-year (2004-2021) average of 45 equine cases per year.

West Nile virus transmission was below normal in Florida during 2021. There were seven (7) human WN cases reported in four Florida Counties during 2021, compared with a 21-year (2001-2021) annual average of 23 cases per year. One hundred forty-seven (147) sentinel chicken seroconversions to WNV were reported in 19 Florida Counties compared with a 12-year (2010-2021) annual average of 421 WNV seroconversions per year. There were eight (8) WNV-positive mosquito pools reported in two Florida Counties during 2021.

There were no locally-acquired dengue cases reported in Florida during 2021 compared with a 12-year (2010-2021) average of 16 locally-acquired cases per year. There were 37 travel-related dengue cases reported in 11 Florida Counties in 2021 compared with a 12-year average of 105 travel-related cases per year.

St. Louis encephalitis virus transmission was below normal in Florida during 2021 when there were no human cases reported. Twenty-four (24) sentinel chicken seroconversions to SLEV were reported in 12 Florida Counties compared with a 12-year (2010-2021) annual average of 34 SLEV seroconversions per year. There was one (1) SLEV-positive mosquito pool reported in Florida during 2021.

There were no locally-acquired chikungunya cases reported in Florida during 2021 compared with a 12-year (2010-2021) average of one (1) locally-acquired case per year. There was one (1) travel-related chikungunya case reported in Florida in 2021 compared with a 12-year average of 552 travel-related cases per year.

There were no locally-acquired nor travel-related Zika cases reported in Florida during 2021 compared with a 7-year average of 206 travel-related and 40 locally-acquired human cases.

There were no locally-acquired malaria cases reported in Florida during 2021. There were 49 travel-related malaria cases reported in 17 Florida Counties during 2021 compared with a 12-year (2010-2021) average of 810 travel-related cases per year.

Vector and arboviral surveillance remain one of the most important tools that vector control agencies currently have at their disposal. Arboviral transmission indices (sentinel chickens, positive equines, positive exotics (i.e., 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 provide added information about potential transmission risk. Additional vector control efforts in and around sites where virus transmission is known or suspected of recently occurring provide another potential mechanism to mitigate viral transmission.

Specific operational strategies will be discussed during the 2022 arboviral transmission season depending on where and when vector-borne disease transmission becomes evident in the state of Florida.

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.

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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