October 18, 2023 | Volume 3 | Issue 21 | As of Week 41

A QUESTION FROM OUR READERS

Q. Dagne from Pembroke Pines, FL, has a follow-up request: “I would be very interested to learn more about Aedes aegypti migration strategies as well.”

A. In last week's edition, we discussed the importance of Aedes aegypti and Ae. albopictus in transmitting locally-acquired dengue in Florida. Throughout the 2023 Florida arboviral transmission season, rainfall in South Florida has favored the continuous production of populations of both species (Figure 3), increasing the probability of establishing locally-acquired dengue transmission foci initiated by the entry of travel-associated human dengue cases into Aedes-rich habitats.

Your question about mosquito mitigation is really important. Interrupting and preventing the establishment of locally-acquired dengue transmission foci is the first line of defense for dengue control in Florida. The control of immature mosquitoes is probably the easiest method of mitigation. Many larval habitats are easy to identify and destroy. Bird baths, flooded flower pots, and buckets can be identified and dumped. However, Ae. aegypti is a master at finding and utilizing cryptic larval habitats. Underground cisterns, water-holding plants, shells, and tree holes are all difficult to locate and drain and are frequently used as oviposition sites. The two toys shown in Figure 1 hold more than two gallons of water and are almost impossible to drain (Figure 2). Both of the toys provide a non-stop source of adult mosquitoes.

Because of cryptic larval habitats, controlling Aedes mosquitoes is difficult, especially during wet years when virtually all of the larval habitats remain flooded. This makes a rapid response to travel-associated dengue cases critically important. I’m sure that all vector control programs, especially in South Florida, are aware of this and that there is an immediate response to all travel-associated dengue cases. Hopefully, an exact address will be known, and an immediate inspection will take place. Landing counts, overnight trapping, and identifying obvious larval habitats are all done immediately. If vector mosquitoes are found, adulticide treatments with hand-held foggers around the premises for two or three days will target adult vector mosquitoes and, hopefully, control the older infected females.

We are now at 68 locally-acquired dengue cases and 366 travel-associated cases in Florida, well above the long-term averages (Table 2). Considering the saturated groundwater conditions reported in 2023 (Figure 3), it is not surprising that we have had so much local dengue transmission. The question is, has dengue become endemic in South Florida? 

Figure 1. Two toys that between them hold more than two gallons of water and constantly produce adult Aedes mosquitoes.

Figure 2. It is almost impossible to drain all of the water from the toys.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 3. The Keetch-Byram Drought Index for Florida as of October 16, 2023.

South, Central, and North Florida remain extremely wet. Even the Florida Panhandle has reported a wetting trend during the past week (Figure 3). These wet conditions continue to favor the establishment of local dengue transmission foci in the southern half of the Florida Peninsula.  

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 14, 2023.

Dengue Viruses

Fifteen 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 11 counties shaded red in Figure 4 are areas of at least one travel-associated dengue case reported in the past three weeks. 

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

Figure 4. The spatial distribution of travel-associated human dengue cases in Florida as of October 14, 2023. The 11 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 14, 2023.

It appears that the 2023 transmission of locally-acquired dengue in South Florida has not yet peaked (Figure 6). The unusually wet and hot conditions reported so far in 2023 may have shifted dengue transmission to later in the year. 

Figure 6. The temporal distribution of locally-acquired human dengue cases in Florida by month of onset as of October 14, 2023.

Malaria

Four new travel-associated human malaria cases were reported last week in Miami-Dade (2 cases), Polk, and Seminole Counties, bringing the total number of travel-associated human malaria cases reported in Florida to 58. Most of these are Plasmodium falciparum originating in Africa. 

Eastern Equine Encephalitis Virus



Two new EEEV antibody-positive sentinel chickens were reported in Orange and Walton Counties last week bringing the total number of positive sentinel chickens reported in 2023 to 121, close to the annual number of expected seroconversions (Table 2).

An EEE-positive equine was reported last week in Levy County bringing the total number of positive equines reported in Florida to 12. The Levy County horse was likely infected in early October. 

St. Louis Encephalitis Virus

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

West Nile Virus

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

Twenty-four new WNV antibody-positive sentinel chickens were reported in Florida last week, bringing the total number of positive chickens to 154 (Figure 7). 

Figure 7. The spatial distribution of WNV antibody-positive sentinel chickens in Florida as of October 14, 2023.

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 10/14/23).

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, it is obvious that sporadic transmission of EEEV and WNV will continue to be reported throughout the state. The risk of WNV transmission remains high in North Florida and in the Florida Panhandle (Figure 7).

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 3, 4, 5, and 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, and it remains unclear when the Florida dry season will begin and when temperatures will begin to moderate.

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