August 9, 2023 | Volume 3 | Issue 12 | As of Week 31

A QUESTION FROM OUR READERS

Q. Gregg from Palm Bay, FL, asks: “With an increasing number of imported dengue cases in Florida along with reports of locally acquired dengue cases, should we be concerned that dengue viruses will become endemic in Florida? How about malaria?”

A. Dengue—Yes, malaria—probably not.

In fact, dengue may already be endemic in Miami-Dade County, where in 2023, there have been eight locally acquired cases. The approximate infection dates for these cases are early January, early March, early June, mid-June (two cases), late June, and mid-July (two cases). This suggests the possibility of ongoing low-level dengue transmission, or that the viruses are now endemic in Miami-Dade County.

Part of the reason that dengue viruses may become established in South Florida is that dengue is extremely abundant throughout the Caribbean Basin as it is transmitted via a highly efficient vector, Aedes aegypti. This domestic species is most at home in urban environments, blood feeds during the day, and prefers human blood meals. Extensive travel between Florida and the Caribbean Islands, Central America, and South America allows human transport of dengue viruses throughout the region.

Malaria, on the other hand, will have an extremely difficult time becoming reestablished in Florida because the Anopheles vectors (Figure 1) are night-feeding mosquitoes that have been largely excluded from our homes and buildings by window screens and air conditioning. Malaria-infected travelers returning to Florida must have a significant Anopheles exposure to establish a local transmission cycle. For the most part, this is not the case. Exceptions include the homeless, outdoor nighttime workers (emergency workers and security guards), migrant workers, campers, and people who can’t sleep and sit outdoors on their front porches.

Finally, reintroducing human malaria into Florida may not require an infected traveler. A person with a past malaria infection (especially Plasmodium vivax) where the parasite has gone dormant (in the liver) and then recrudesces (usually when the person is under stress) after months or years, may expose that person to Anopheles vectors while parasites are circulating in the blood and initiate an outbreak of locally acquired malaria.

Figure 1. Anopheles crucians by Larry Reeves, University of Florida, Florida Medical Entomology Laboratory. This is the suspected vector in the recent Sarasota County human malaria outbreak.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of August 7, 2023.

Most of the Florida Peninsula remains exceptionally wet, with the exception of the central Gulf Coast from Sarasota County south to Naples. The western and central Florida Panhandle are currently experiencing a drying trend (Figure 2). These surface conditions may continue to favor the transmission of diseases, such as eastern equine encephalitis and dengue, by supporting vector production and the dispersal of already infected mosquitoes.

However, the wet conditions may work against West Nile and St. Louis encephalitis viruses by reducing late-season amplification that will begin at the end of August as migrating birds start to move south. 

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 August 5, 2023

Dengue Viruses

We appear to be off to the races relative to locally acquired dengue transmission. Four new locally acquired cases were reported last week, two in Miami-Dade County and two in Broward County, bringing the total number of locally-acquired cases reported in Florida to 10. Of particular concern are the two new cases in Broward County, suggesting the establishment of a second focus of dengue transmission.

Forty-three new travel-associated dengue cases were reported in Florida last week (Figure 3). These cases were widely distributed throughout the state, indicating that the level of travel-associated dengue introductions remains high in Florida.  

A continued note of concern is that two dengue serotypes (2 and 3) are now reported as locally-acquired cases in Florida. This increases the risk of severe dengue infections later in the year. Most (68%) of the travel-associated cases have originated in Cuba, and most (66%) are dengue serotype 3. 

Figure 3. The spatial distribution of travel-associated human dengue cases in Florida as of August 5, 2023. The 18 Counties that are shaded red indicate areas of at least one travel associated dengue case reported during the past three weeks.

Malaria

The locally-acquired malaria case count in a small transmission focus in Sarasota County remains at seven. The earliest case was likely infected in mid-April, and the most recent case was likely infected in mid-June.

The outbreak appears to have been contained.

Eastern Equine Encephalitis Virus



Two new sentinel chickens tested positive for antibodies to EEEV in Polk and Walton Counties last week bringing the total number of positive chickens to 98. The wet surface conditions throughout most of Florida will continue to favor the dispersal and transmission of EEEV by infected vector mosquitoes (Figure 2). 

St. Louis Encephalitis Virus

There has been no evidence of SLEV transmission in Florida so far in 2023. It is possible that the SLEV has become extinct in Florida and will only reappear when re-introduced by fall migrants.

West Nile Virus

Three new WNV antibody-positive sentinel chickens were reported last week in Walton County, bringing the total number of positive chickens in Florida to 13. Transmission of WNV remains extremely low in Florida, and the most likely place where amplification may occur during the coming weeks is along the Gulf Coast (Figure 2).  

A summary of observed and expected numbers of travel-associated and locally-transmitted mosquito-borne 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 8/5/23).

Transmission of EEEV seems to be winding down in Florida. Historically, most equine and human cases have been reported in June and July (Figures 4 and 5). We seem to be outside of that transmission window, but the record heat and rainfall throughout Florida may shift the curves toward August in 2023.

Figure 4. The monthly distribution of equine EEE cases in Florida from 1982-2023 when a total of 2,481 positive horses were reported in 64 Florida counties.

Figure 5. The monthly distribution of human EEE cases in Florida from 1955-2023 when a total of 84 human EEE cases were reported in 37 Florida counties.

The number of travel-associated dengue cases skyrocketed last week with the report of 43 new infections (Figure 3) along with four new locally acquired cases (two in Miami-Dade County and two in Broward County). The fact that two dengue serotypes (2 and 3) are now circulating in Florida is concerning because this increases the risk of severe dengue inflictions later in the year. Another concerning factor is that travel-associated dengue cases have been reported in 18 Florida counties during the past three weeks (Figure 3). The risk of establishing locally-acquire dengue transmission foci in these counties is high, and increased surveillance efforts should be undertaken in these counties.

The transmission of WNV and SLEV is currently well below normal in Florida (Tables 1 and 2). The next bump in the transmission of these viruses will likely come at the end of August in association with fall migration.

Finally, it appears that the outbreak of human malaria in northern Sarasota County has been contained. Exceptional surveillance and enhanced vector control in the transmission zone may have kept this outbreak focused in a very small geographical area.  

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated vector-borne disease in Florida. Table 2 compares the 2023 data with long-term expected values. Both malaria and EEE show above-normal human activity. Transmission of SLEV and WNV is well below normal, but we know that WNV-infected mosquitoes are present and active in the Florida Panhandle. Even though the transmission of EEEV to sentinel chickens and equines is below normal, infected mosquitoes are present in the northern half of Florida, especially in Nassau County, and they appear to be actively transmitting EEEV.  

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.