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.

Q. Jorge from Vero Beach, FL asks: “How do hurricanes affect vector-borne disease transmission cycles?”

A. Hurricanes have two effects on vector-borne disease transmission cycles: immediate and long-range impacts. This week we will discuss the immediate impacts of hurricanes on vector-borne disease transmission cycles.  

Hurricanes disrupt avian and mosquito populations in the immediate track of the hurricane. High winds and heavy rain disperse birds, adult mosquitoes and larval mosquitoes. The September 30th rainfall associated with Hurricane Ian is shown below in Figure 1. Flooding associated with hurricanes may disperse mosquito larvae into lagoons, bays, and even into the ocean. Lower rainfall amounts on the fringe of the hurricane may produce large broods of nuisance floodwater mosquitoes. The disruption and dispersal of avian and adult mosquito populations fragment any disease amplification and transmission foci that were established before the hurricane. In 2022, Hurricane Ian may have disrupted the heavy transmission of WNV reported along the southwest Florida coast.

From 1959 through 1962 more than 300 cases of St. Louis encephalitis were reported in the Tampa Bay area of Florida (Figure 2). A notable feature of these SLE epidemic years was the absence of cases in 1960, a year when the powerful Hurricane Donna tracked up the west coast of Florida (Figure 3).

Hurricanes have immediate impacts on active vector-borne disease transmission cycles. They also have long-term impacts on these cycles, something we will discuss next week.

Hurricane Ian severely disrupted arboviral transmission along the southwest coast of Florida and north through the central peninsula in late September (see this week’s Question From Our Readers). Arboviral surveillance programs throughout South Florida have been impacted and it remains to be seen whether WNV transmission will rebound in the coming weeks. Four new locally-acquired dengue cases were reported in Miami-Dade County, bringing the total number of locally-acquired dengue cases to 27. Thirty-eight new travel-related dengue cases were reported in South Florida. There were no reports of EEEV, SLEV, or WNV transmission in Florida during the past two weeks, due in part to the impacts of Hurricane Ian. The risk of transmission for all three of these viruses in South Florida is currently low, but it is important to remember that arboviral transmission risk in Florida is never zero.

Thirty-eight new travel-related dengue cases were reported in Florida during Week 40. Twenty-six of these were reported in Miami-Dade County (Figure 4). A total of 512 travel-related dengue cases have been reported in Florida thus far in 2022. Of these, 467 (91%) 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.

Twenty-seven locally-acquired dengue infections have been reported in South Florida in 2022.

As of Week 40, 2022, the following mosquito-borne disease transmission events and pathogen introductions have been reported in Florida:

The number of travel-related dengue cases reported in Florida has increased during the past month with an average of 35 new cases each week for a total of 512 in 2022. This is well above the 12-year average (2010-2021) of 105 travel-related dengue cases reported in Florida annually. However, it is evident from Table 1 that high numbers of travel-related dengue cases are not a prerequisite for large outbreaks of locally-acquired dengue (compare years 2019 and 2020 in Table 1). The extent of the 2022 dengue outbreak in South Florida remains to be determined, but it is unlikely that it will reach the level observed in 2010 or 2020. 

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.

There is currently a low risk of EEEV transmission in Florida.

There is currently an extremely low risk of SLEV transmission in Florida.

The current risk of WNV transmission in South Florida has been impacted by Hurricane Ian and it remains to be seen whether WNV transmission will rebound in South Florida during the coming weeks. Transmission of WNV in the Panhandle was not impacted by the hurricane and the risk of WNV transmission there remains high. 

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.

Learn more about our work in protecting public health on clarke.com.