August 31, 2022 | Volume 2 | Issue 13 | As of Week 34
Welcome to the Florida Disease Activity Update from the desk of Dr. Jonathan Day.
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 QUESTION FROM OUR READERS

Q. Benjamin from Vero Beach, FL asks: “How will climate change affect vector-borne disease transmission cycles?”

A. Vector-borne disease outbreaks are driven by environmental factors above and beyond temperature. Cycles of rainfall and drought are the most important drivers of these diseases and it is impossible to predict how rainfall and drought will be affected by global climate change. It seems that on the local level rainfall and drought are being pushed to extremes.

For example, in the United States, extreme drought has recently been reported in the west and southwest, while extreme flooding has been reported in the central and southeast. Extremes, too wet or too dry, do not support vector populations or disease transmission cycles.

One interesting note regarding climate change and disease transmission focuses on St. Louis encephalitis (SLE) in Florida where human cases were first reported in 1952 (Figure 1). Five major epidemics were reported in the state in 1959, 1961, 1962, 1977, and 1990. All five epidemics were preceded by central Florida freezes below 30 °F that lasted at least 48 hours. The last such freeze was in 1989 preceding the 1990 SLE epidemic. There have been no significant freezes or outbreaks of SLE in Florida since 1990. A biological explanation for this observation has been proposed by Day and Shaman (2009) (Severe Winter Freezes Enhance St. Louis Encephalitis Virus Amplification and Epidemic Transmission in Peninsular Florida | Semantic Scholar).
Figure 1. The distribution of human St. Louis encephalitis cases in Florida from 1952 through 1997. Five outbreaks (1959, 1961, 1962, 1977, and 1990) were preceded by major freezes in the central Florida Peninsula.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA DURING 2022

Arboviral transmission and pathogen introductions continued to increase in Florida during the past week. The transmission of locally-acquired dengue in South Florida and new introductions of DENV by infected travelers remain the primary vector-borne disease issue in Florida. In addition, the transmission of West Nile virus continues to increase throughout most of Florida.
 
Low-level transmission of EEEV continues to be reported in Florida. Five new EEEV antibody-positive sentinel chickens were reported last week in Bay, Manatee, Orange (2 new positives), and Walton Counties bringing the total number of positive sentinels to 55, well below the 18-year average (2004-2021) of 133 positive sentinel chickens per year. It remains unlikely that we will see a dramatic increase in the transmission of EEEV in the coming months, although the risk of EEEV transmission in Florida is never zero.
 
There is little indication that there are any active SLEV transmission foci in Florida. I observed a mixed flock of Barn Swallows and Bank Swallows in the airspace above my yard on Sunday (8/28/22). This marks (at least for me) the official beginning of fall migration. The SLEV may be reintroduced into the state by migrating birds and these large flocks will provide an additional source of potential amplification hosts for EEEV, SLEV, and WNV. However, at this time SLEV does not pose a risk to human health in Florida.
 
Transmission of WNV in Florida continued to increase last week. Fourteen new WNV antibody-positive sentinel chickens were reported in Citrus, Lee (4 new positives), Manatee (3), Sarasota (3), St. Johns, Volusia, and Walton Counties. The total number of WNV antibody-positive sentinel chickens reported in Florida during 2022 is now 97. The 21-year average (2001-2021) for sentinel chicken seroconversions in Florida is 413 per year. 
 
Two significant WNV transmission foci seem to be emerging in Florida (Figure 2). One is in the Western Florida Panhandle in the area surrounding Bay County. The second is in Southwest Florida in the area surrounding Lee County. The human case reported in Volusia County indicates that there is currently a significant risk of WNV transmission throughout the Florida Peninsula. 
Figure 2. The distribution of WNV antibody-positive sentinel chickens reported in Florida during 2022 with indicators of high transmission risk zones.
There were 34 travel-related dengue introductions into Florida last week, 23 of which were in Miami-Dade County (Figure 3). A total of 250 travel-related DENV introductions have been reported in Florida thus far in 2022. Of these, 227 (91%) have originated in Cuba. All four dengue serotypes have been introduced into Florida from Cuba this year, a fact that may play into the severity of human infections reported in Florida during the coming months.
 
Eight locally-acquired dengue infections have been reported in South Florida in 2022. Seven are in Miami-Dade County and one is in Collier County. This indicates that there are now multiple dengue transmission foci in South Florida and, most likely, multiple introductions and establishments of DENV in the state. Most of the Miami-Dade cases are DENV3 serotypes while the Collier County serotype is unknown. It remains to be seen how extensive the current dengue outbreak in South Florida will be.
Figure 3. Travel-related dengue introductions in Florida during 2022. Counties colored blue indicate DENV introductions during 2022. Counties colored red indicate the total number of DENV introductions in 2022 and DENV introductions during Week 34.
Year to Date Summary of Mosquito-Borne Disease Transmission

As of Week 34, 2022, the following mosquito-borne disease transmission events and pathogen introductions have been reported in Florida:
Table 1. Summary of mosquito-borne disease transmission and mosquito-borne pathogen introductions into Florida as of August 27, 2022. 
The number of travel-related dengue cases reported in Florida has increased steadily during the past month with an average of 38 new introductions each week for a total of 250 introductions in 2022. 
 
The 12-year average (2010-2021) of travel-related dengue introductions into Florida is 105 per year. The 12-year average of locally-acquired dengue cases in Florida is 16 per year. The steady increase in travel-related dengue introductions into South Florida along with the eight reported locally-acquired dengue cases suggests the possibility of a significant outbreak of dengue in South Florida during the coming weeks. Historically, the majority of locally-acquired dengue cases reported in Florida have had onset in August and September. So far, all eight dengue cases were infected in July, so the worst period for locally-acquired dengue transmission is still ahead of us. 
OPERATIONAL STRATEGIES TO CONSIDER
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.
 
Specific operational strategies will be discussed during the 2022 arboviral transmission season depending on where and when vector-borne disease transmission becomes obvious in Florida.
 
There is currently a low risk of EEEV transmission in Florida.
 
There is currently an extremely low risk of SLEV transmission in Florida. 
 
There is currently a moderate to high risk of WNV transmission in Florida in two transmission foci (Figure 2). One is in the western Panhandle and the other is virtually all of the Florida Peninsula. Arboviral surveillance and reporting during the upcoming weeks will help to determine the current situation relative to the transmission of EEE, SLE, and WN viruses throughout the state.
 
There is currently a very high risk of local dengue transmission in South Florida (Figure 3). The eight locally-acquired dengue cases and the continued influx of travel-related dengue cases into South Florida (38 new cases per week) increases the risk of a significant outbreak of locally-acquired dengue. As of now, all of South Florida is at risk for a dengue outbreak. Increased Aedes aegypti control in areas surrounding locally-acquired and travel-related dengue cases will help to reduce the number of infected and emerging vector mosquitoes. Source reduction, the emptying, and where possible the destruction, of all water-holding containers remains the most productive control method against the likely dengue vector, Aedes aegypti. Cryptic Ae. aegypti breeding habitats remain a huge problem in places where this species is abundant. The location of these cryptic habitats remains one of the primary challenges for vector control agencies dealing with dengue outbreaks. 
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, 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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