August 2, 2021 | Week 29 | Volume 1 | Issue 3
Welcome to the Florida Disease Activity Update, from the desk of Dr. Jonathan Day! It is 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. Find an archive of past issues here, including a summary of 2020 arboviral disease activity in Florida.
THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE

August usually marks the beginning of intensive arboviral transmission in Florida. During epidemic years, transmission can continue into November, so arboviral surveillance during the next three months will be a critical part of forecasting potential arboviral transmission hot-spots within the state.

In Florida, four mosquito-borne arboviruses 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). The current transmission risk posed by each of these viruses is reviewed below.
WHAT ARE WE CURRENTLY SEEING?
Eastern Equine Encephalitis (EEEV) Activity
Transmission of EEEV in central Florida and the Florida Panhandle remains the greatest current arboviral threat to humans, equines, and exotic animals in Florida.

Twenty-two (22) new EEEV antibody-positive sentinel chickens were reported during week 29, bringing the total number of chickens testing positive for EEEV antibody in 2021 to 138. Most of the new positive chickens were reported in Walton County.

The number of EEEV-positive horses reported in 2021 remains at 12. All 12 of the positive horses are widely dispersed in North Florida and the eastern Florida Panhandle. We are now in the time of year (June through August) when the transmission of EEEV to horses is historically the highest (see below). The 12 equine cases reported so far this year is well below the mean value of 60 annual cases reported in Florida since 1982.
Likewise, we are in the time of the year when transmission of EEEV to humans is also historically the highest (June through August, see below).
There has been no indication so far this year that there is a high threat of EEEV transmission to humans or equines. There is, however, always a threat of sporadic transmission of EEEV to humans and equines during August and September. This is especially true for central Florida and the Florida Panhandle during the late summer and early autumn of 2021. A review of 2020 EEEV activity is included in the archived summary report.
West Nile Virus (WNV) Activity
The threat of WNV transmission remains low in Florida.

So far during the 2021 Florida arboviral transmission season, seven WNV antibody-positive sentinel chickens have been reported in Citrus (1), Lee (5), and Manatee (1) Counties. The five Lee County WNV antibody-positive sentinel chickens were reported this week, so it will be important to continue monitoring the transmission of WNV in southwest Florida.

The low number of WNV antibody-positive sentinel chickens reported so far in 2021 suggests that there was little amplification of WNV during the Florida avian nesting season (April through June). The next threat of WNV amplification will begin with the fall avian migration that starts in late August. A review of 2020 WNV activity is included in the archived summary report.
Dengue (DENV) Activity
There has been no indication thus far in 2021 of locally-acquired dengue infections in Florida, but five travel-related dengue cases have been reported in Broward (1), Hillsborough (1), Miami-Dade (1), and Palm Beach (2) Counties. The entire state of Florida has been unusually wet since the middle of June, favoring production of Aedes aegypti. If dengue virus is introduced into Florida by an infected traveler in an area with high Ae. aegypti populations, there is risk for the initiation of a local dengue transmission cycle. Locally-acquired cases of DENV have been reported in October-November in South Florida, so it will be important to monitor all of South Florida for locally-acquired cases during the remainder of 2021. A review of 2020 DENV activity is included in the archived summary report.
St. Louis Encephalitis Virus (SLEV) Activity
Transmission of SLEV in Florida remains very low. So far in 2021, only two sentinel chickens (both in Palm Beach County) have tested positive for antibodies to SLEV. Both chickens were infected early in the year and there has been no additional indication of SLEV transmission anywhere in the state. As is the case with WNV, fall migration has the potential of seeding SLEV into Florida from the north, and will provide a large number of birds that can serve as susceptible hosts for a late-season SLEV amplification cycle.  
OPERATIONAL STRATEGIES TO CONSIDER
Vector and arbovirus surveillance remains the most important tool vector control agencies have at their disposal. Monitoring arboviral transmission indices (sentinel chickens, positive equines, positive exotics (i.e., emus), and positive humans) provide indicators of local virus transmission, although sometimes not in a timely manner. Monitoring mosquito populations, and their age structure, provides added information about potential transmission risk. Additional vector control efforts in and around sites where virus transmission is known or suspected provides another potential mechanism to mitigate viral transmission.

The Florida regions with the highest concern for arboviral transmission are those where indicators of EEEV transmission (sentinel chickens and EEE-positive equines) are currently high. These include: The Central Florida Counties around Orange County, all North Florida Counties, and the Panhandle Counties around Walton County.
ABOUT DR. JONATHAN DAY

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.
Acknowledgements: 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 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. 
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