January 6, 2022 | Week 52 | Volume 1 | Issue 20
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 the public from vector-borne diseases.
This is the final report on the 2021 Florida arboviral transmission season, reflective of data and activity through December 31, 2021.

A Year in Review for 2021 edition will be published as Volume 2, Issue 1 on March 16, 2022. We expect to begin bi-monthly releases of the Newsletter in the spring, and will continue until transmission rates indicate that weekly Newsletter updates are warranted. Thank you for your readership and continued interest in this Newsletter!
A QUESTION FROM READERS
Bobby from Kissimmee, FL asks: “Under what circumstances would you expect the reintroduction of ZIKAV into Florida to result in a significant human outbreak?” 

A: There are currently four mosquito-borne human pathogens (Chikungunya virus, Dengue viruses, Zika virus, and malaria) that are periodically reintroduced into Florida by infected human travelers. None of these pathogens are currently endemic in Florida. Five things must happen to facilitate an outbreak of locally-transmitted disease in Florida by any of these pathogens.

First, there must be a significant disease outbreak in a part of the world where there is regular travel to and from Florida. This was the case in early 2021 when the only reported travel-related Chikungunya case in Florida originated from Brazil, where there was an ongoing outbreak of CHIKV.

Second, infected travelers with circulating pathogen loads sufficient to infect local mosquitoes must arrive in Florida. 

Third, the appropriate mosquito species (Ae. aegypti and Ae. albopictus for CHIKV, DENV, and ZIKAV or Anopheles spp. for malaria) must feed on infected travelers and acquire the pathogen.

Fourth, the blood-fed mosquitoes must successfully complete a temperature-dependent extrinsic incubation period (usually two to three weeks) and become infective.

Finally, the infective mosquitoes must feed on susceptible human hosts. Of the four pathogens listed above, DENV is the most likely to periodically reestablish in Florida while malaria seems to have the most difficulty reestablishing (more about this in a future Newsletter).
A FINAL REPORT FOR THE 2021
ARBOVIRAL TRANSMISSION SEASON IN FLORIDA
The 2021 arboviral transmission season was average to below-average for the four viruses of concern in Florida. Low-level background transmission of EEEV, SLEV, and WNV continued in Florida through the end of 2021. Local transmission of DENV was not reported in Florida during 2021. Since Week 45 (November 22, 2021) there were additional reports of virus transmission or viral introductions into Florida including:
  • Three new EEEV antibody-positive sentinel chickens and one additional EEE-positive equine.
  • Nine new SLEV antibody-positive sentinel chickens.
  • Fourteen new WNV antibody-positive sentinel chickens.
  • Seven new travel-related cases of DENV.
FINAL SUMMARY FOR THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON
Eastern Equine Encephalitis (EEEV) Transmission
There were no human EEE cases reported in Florida during 2021. One hundred eighty-four (184) EEEV antibody-positive sentinel chickens were reported in Florida during 2021, above the 17-year (2004-2020) annual Florida EEEV seroconversion rate of 130 per year. Twenty (20) EEE-positive equines were reported during 2021, below the 17-year average of 46 per year. 
Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
West Nile Virus (WNV) Transmission
Seven (7) human WN cases were reported in Florida during 2021. Five (5) were neuroinvasive/fever and two (2) were asymptomatic blood donors. Four (4) WN-positive equines were reported in 2021, well below the 20-year average (2001-2020) of 62 positive equines per year. One hundred forty-seven (147) WNV antibody-positive sentinel chickens were reported during 2021, well below the 20-year average of 426 seroconversions per year. Eight (8) WNV-positive mosquito pools were reported in 2021. 
Find a review of 2020 WNV activity in the archived Vol 1, Issue 1 report.
Travel-Related Dengue Activity
Twenty-seven (27) travel-related dengue cases were reported in Florida during 2021, well below the 11-year average (2010-2020) of 112 travel-related dengue cases per year.
Find a review of 2020 DENV activity in the archived Vol 1, Issue 1 report. 
St. Louis Encephalitis Virus (SLEV) Transmission
No human SLE cases were reported in Florida during 2021. Twenty-four (24) SLEV antibody-positive sentinel chickens were reported during 2021, below the 11-year (2010-2020) average of 36 seroconversions per year.
Find a review of 2020 SLEV activity in the archived Vol 1, Issue 1 report.
OPERATIONAL STRATEGIES TO CONSIDER
Vector and arbovirus surveillance remains 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 provides added information about potential transmission risk. Additional vector control efforts in and around sites where virus transmission is known or suspected of recently occurring provides another potential mechanism to mitigate viral transmission.

There is currently little risk of arboviral transmission in Florida, although it is important to note that transmission risk is never zero. It will be important to monitor travel-related dengue cases introduced into South Florida during the next three months. High numbers of travel-related cases may translate to an increased risk of locally-transmitted DENV during the spring of 2022. 
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.
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 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 and are posted at: fireweather.fdacs.gov/wx/kbdi_4km.html.

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