October 11, 2023 | Volume 3 | Issue 20 | As of Week 40

A QUESTION FROM OUR READERS

Q. Dagne from Pembroke Pines, FL asks: “How has surface wetness in South Florida enhanced dengue transmission during the 2023 arboviral transmission season?”

A. The primary mosquito vector of human dengue in Florida is Aedes aegypti (Figure 1). Aedes albopictus (Figure 2) is a possible secondary vector of dengue in the state. Both of these important mosquito species deposit their eggs in natural (e.g., tree holes and bromeliads) and artificial (e.g., flower pots, discarded containers, and toys) containers. Many of these are small and prone to complete drying during periods of drought. The unprecedented surface wetting seen in South Florida during the late spring, summer, and early autumn of 2023 (Figure 3) has resulted in the constant flooding of these mosquito oviposition sites and the consistent production of mosquito populations.

The steady influx of travel-associated dengue cases into Florida, especially in South Florida, during 2023 has provided a continuous source of dengue-infected humans (Figure 4) who, when exposed to local Aedes mosquitoes, have initiated focal outbreaks of human dengue transmission (Figure 5).

The wet South Florida surface conditions have continued throughout most of the 2023 arboviral transmission season and as we approach the end of the Florida wet season and the beginning of the dry season it will be interesting to continue to monitor the surface wetness and determine whether there will be an end of the transmission of locally-acquired dengue in South Florida this year. 

Figure 1. Aedes aegypti, the primary vector of human dengue in Florida. Photograph by Dr. Larry Reeves, University of Florida, Florida Medical Entomology Laboratory in Vero Beach.

Figure 2. Aedes albopictus, a possible secondary vector of human dengue in Florida. Photograph by Dr. Larry Reeves, University of Florida, Florida Medical Entomology Laboratory in Vero Beach.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 3. The Keetch-Byram Drought Index for Florida as of October 9, 2023.

South and Central Florida remain extremely wet, North Florida is wet, and most of the Florida Panhandle is dry (Figure 3). The only place where groundwater conditions are likely to support arboviral transmission is South Florida, where outbreaks of locally-acquired dengue continue to be reported.

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 7, 2023.

Dengue Viruses

Seventeen new travel-associated dengue cases were reported in Florida last week (Figure 4). Recent travel-associated dengue cases have been reported throughout the state. The counties shaded red in Figure 4 are areas of at least one travel-associated case reported in the past three weeks. 

Fifteen new locally-acquired dengue cases were reported in Florida last week (Figure 5). These infections occurred between mid-August and mid-September, and most were the DEN-3 serotype. 

Figure 4. The spatial distribution of travel-associated human dengue cases in Florida as of October 7, 2023. The 13 counties that are shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

Figure 5. The spatial distribution of locally-acquired human dengue cases in Florida as of October 7, 2023.

There is a slight reporting delay of new dengue cases and it is currently not clear that the outbreak of locally-acquired dengue has peaked in South Florida (Figure 6).

Figure 6. The temporal distribution of locally-acquired human dengue cases in Florida as of October 7, 2023.

Malaria

Two new travel-associated human malaria cases were reported last week in Broward and Palm Beach Counties bringing the total number of travel-associated human malaria cases reported in Florida to 54. 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 in Volusia County last week bringing the 2023 total to two (Volusia and Walton Counties).

West Nile Virus

One new human case of WNV was reported in Okaloosa County last week. The individual was likely infected in late August.

One new WNV-positive equine was reported in Palm Beach County last week, bringing the total number of equine WNV cases reported in Florida during 2023 to two (Pasco and Palm Beach Counties). The most recent horse was likely infected in late September.

Six new WNV antibody-positive sentinel chickens were reported in Florida last week in Duval, Palm Beach, Seminole, Volusia, and Walton (2) Counties.

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 10/7/23).

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.

Transmission and introduction of dengue into Florida remains fluid and dynamic. With the current groundwater conditions in South Florida, it is unclear when dengue transmission will be interrupted (Figure 6).

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. Transmission of WNV in the Florida Panhandle remains a threat, along with dengue transmission throughout South Florida.

It is important to remember that the risk of arboviral transmission in Florida is never zero.

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.