August 24, 2022 | Volume 2 | Issue 12 | As of Week 33
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. Kim from Kissimmee, FL asks: “Why do cases of WNV typically rise rapidly in late summer?

A. From the colonization of North America through the early 20th Century, outbreaks of disease caused by unknown agents were reported during the summer months across the country. These were referred to as “summer fevers” and we now know that the outbreaks were caused, in part, by vector-borne diseases such as malaria, yellow fever, and a variety of mosquito-borne encephalitis viruses. West Nile virus is a summer fever where the number of human cases increases steadily throughout the summer with a July-September peak (Figure 1).

The increase in the number of human WNV cases throughout the summer is due to several factors. First, viral amplification, the cycling of virus between mosquitoes and avian amplification hosts such as Northern Cardinals, is a relatively slow process that begins during the spring avian nesting season and continues through the summer and into the autumn months. During some years, the process is efficient and the number of human cases steadily increases along with amplification throughout the summer. During most years, however, the process is not efficient, and human WNV cases are rare.

A second factor involves rainfall and drought cycles that either enhance or depress amplification cycles. The Florida wet season occurs from May through October. During years when rainfall and drought alternate throughout the summer, viral amplification is enhanced because birds and mosquitoes have intense periods of interaction caused by intense periods of rainfall. During years of extreme drought or extreme rainfall, amplification is depressed because mosquitos and birds have only intermittent interactions.

Finally, increased summer temperature reduces the time required for mosquitoes to become infected with WNV. The extremely high temperatures reported in Florida from July-September reduce the mosquito infection time and increase the number of infected mosquitoes capable of transmitting WNV.
Figure 1. The distribution of human West Nile cases by month of infection in Florida from 2001 through 2022.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA DURING 2022

Arboviral transmission and pathogen introductions into Florida increased dramatically last week. The transmission of locally-acquired dengue in Miami-Dade County and the introduction of DENV into South Florida by infected travelers continues to be the primary vector-borne disease problem in Florida. In addition, the transmission of West Nile virus increased throughout the state last week.
 
Low-level transmission of EEEV continues to be reported in Florida. Three new EEEV antibody-positive sentinel chickens were reported last week (two in Leon County and one in Nassau County) bringing the total number of positive sentinels to 50, which remains well below the 18-year average (2004-2021) of 133 positive sentinel chickens per year. It remains unlikely that we will see a surge in EEEV transmission 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. The virus may be reintroduced into the state during fall migration and pockets of transmission may develop. However, at this time SLEV does not pose a significant risk to human health in Florida.
 
Transmission of WNV in Florida increased dramatically last week. Forty-six new WNV antibody-positive sentinel chickens were reported (Figure 2) along with a single human WN case in Volusia County. Most of the positive sentinel chickens were reported in southwest Florida including Charlotte (8 new positives), Lee (13), and Sarasota (8) Counties. The total number of WNV antibody-positive sentinel chickens reported in Florida during 2022 is now 83. The 21-year average (2001-2021) for sentinel chicken seroconversions in Florida is 413 per year. However, at 50 new seroconversions a week, it will not take long to reach the 21-year average. 

Two significant WNV transmission foci seem to be emerging. 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 44 new travel-related dengue introductions into Florida last week, 34 of which were in Miami-Dade County (Figure 3). A total of 216 travel-related DENV introductions have been reported in Florida thus far in 2022. Of these, 193 (89%) have originated in Cuba and 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.
 
A total of five locally-acquired dengue infections have been reported in Miami-Dade county so far this year. 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 33.
Year to Date Summary of Mosquito-Borne Disease Transmission

As of Week 33, 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 20th, 2022
The number of travel-related dengue cases reported in Florida has increased steadily during the past month with an average of 36 new introductions each week for a total of 216 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 five reported locally-acquired dengue cases in Miami-Dade County 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 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 (Figure 2). 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 five locally-acquired dengue cases reported in Miami-Dade County and the continued influx of travel-related dengue cases into South Florida (36 new cases per week) increases the risk of a significant local outbreak of dengue. As of now, Miami-Dade County is the most likely focus for such an 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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