October 25, 2023 | Volume 3 | Issue 22 | As of Week 42

A QUESTION FROM OUR READERS

Q. Dagne from Pembroke Pines, FL, has a final question: “Has the record heat we experienced this summer influenced the transmission of locally-acquired dengue in South Florida?”  

A. Possibly. See the answer to A Question from Our Readers, Volume 3, Issue 14.

In 2023, South Florida has been both warmer and wetter than normal (Figure 4) and both of these factors, along with a large number of travel-associated human dengue cases, may be contributing to the unusual number of locally-acquired dengue case in Florida (Tables 1 and 2).

Whatever the cause, locally-acquired dengue is clearly an issue of great importance in South Florida. The temporal distribution of human cases appears to have shifted to later in the year (Figure 1) compared with the dengue transmission reported in 2020 (Figure 2).

Transmission of locally-acquired human dengue in South Florida between 2009 and 2023 has a peak in August and September (Figure 3). In addition, low level transmission of locally-acquired dengue appears to be present throughout the year (Figure 3).

I think that the unusually high level of locally-acquired dengue in Florida this year is due to a combination of factors. High temperatures, consistently wet ground water conditions, and an unusually high number of travel-associated dengue cases have all contributed to transmission of dengue in 2023. The trend appears to be an ever increasing level of dengue transmission in South Florida and a possible outcome, if these conditions persist, is endemic dengue transmission in Florida. 

Figure 1. The temporal distribution of locally-acquired human dengue transmission in South Florida during the 2023 arboviral transmission season.

Figure 2. The temporal distribution of locally-acquired human dengue transmission in South Florida during the 2020 arboviral transmission season.

Figure 3. The temporal distribution of locally-acquired human dengue transmission in South Florida from 2009 through October 21, 2023.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Most of Florida remains wet, but drying trends associated with the Florida dry season (November-May) are beginning to appear (Figure 4). Drying trends in the western Florida Panhandle, Taylor County, the Tampa Bay area, and Broward County have appeared during the last week.  

Figure 4. The Keetch-Byram Drought Index for Florida as of October 23, 2023.

Table 1 summarizes the status of vector-borne disease transmission in Florida so far in 2023.

Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of October 21, 2023.

Dengue Viruses

Eighteen new travel-associated dengue cases were reported in Florida last week (Figure 5). Recent travel-associated dengue cases have been reported throughout the state. The 10 counties shaded red in Figure 5 are areas where at least one travel-associated dengue case was reported during the past three weeks. 

Ten new locally-acquired dengue cases were reported in Florida last week, all in Miami-Dade County. All of these individuals were infected during mid-September. 

Figure 5. The spatial distribution of travel-associated human dengue cases in Florida as of October 21, 2023. The 10 counties that are shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

Malaria

Seven new travel-associated human malaria cases were reported last week in Brevard, Hillsborough (two cases), Miami-Dade, Polk, and Seminole (two cases) Counties, bringing the total number of travel-associated human malaria cases reported in Florida to 66. Most of these are Plasmodium falciparum originating in Africa.  

Eastern Equine Encephalitis Virus

No new indications of EEEV transmission were reported in Florida last week. 

St. Louis Encephalitis Virus

One new SLEV antibody-positive sentinel chicken was reported last week in Lee County bringing the total number of positive chickens to three.

West Nile Virus

One new human case of WNV was reported in Walton County last week. The individual was an asymptomatic blood donor that was likely infected in mid-September.

Twenty-five new WNV antibody-positive sentinel chickens were reported in Florida last week, bringing the total number of positive chickens to 179, well below the expected annual average of 417 (Table 2).  

A summary of observed and expected numbers of travel-associated and locally-transmitted mosquito-borne viruses and diseases to date appears in Table 2.

Table 2. Summary of expected and observed mosquito-borne disease infections in Florida reported during 2023 (as of 10/21/23).

We are past the point in the 2023 Florida arboviral transmission season where there is a threat of a major outbreak of human or equine EEE or WN. However, sporadic transmission of EEEV and WNV will continue to be reported throughout the state, especially if temperatures remain unseasonably warm. The risk of WNV transmission remains high in North Florida and in the Florida Panhandle.

The introduction and transmission of dengue into Florida remains fluid and dynamic. With the current groundwater conditions in South Florida, it is unclear when the transmission of dengue will be interrupted (Figures 4 and 5, Tables 1 and 2).

Sporadic equine (EEE and WNV) and human (DEN, EEE, and WNV) arboviral transmission will likely continue to be reported in Florida for the remainder of 2023. Recent drying trends throughout the state (Figure 4) seem to be the first indication that the Florida dry season has begun.  

It is important to remember that the risk of arboviral transmission in Florida is never zero, especially during these times of unseasonably warm and wet conditions. We may be entering a period of year-round arboviral transmission in the state.

Jonathan Day, Professor Emeritus of Medical Entomology at the University of Florida and stationed at the Florida Medical Entomology Laboratory in Vero Beach 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.

The Newsletter is edited and distributed by Linda McDonagh.