September 6, 2023 | Volume 3 | Issue 16 | As of Week 35

A QUESTION FROM OUR READERS

Q. Angela from Winter Park, FL, asks: “Am I immune to dengue because of an infection I had 33 years ago?”

A. This is a good news, bad news situation.

First, the good news. There are at least four dengue virus serotypes, designated DEN-1 through DEN-4. Infection with any one of these dengue viruses provides lifelong immunity to that serotype. You likely continue to have immunity to the serotype you were infected with more than three decades ago when you likely suffered from classic dengue fever. Symptoms include sudden onset of fever, a crushing frontal headache, body aches, a rash, and severe joint pain. There is no cross-protective immunity between serotypes.

This brings us to the bad news side of the equation. Dengue Hemorrhagic Fever (DHF) and Dengue Shock Syndrome (DSS) are two severe and sometimes deadly manifestations of dengue infection. At least two theories have been proposed to explain these dangerous infections.

The first is the secondary-infection or immune enhancement hypothesis that proposes individuals experiencing a second dengue infection with a dengue serotype different from that causing the first infection have a higher risk of developing DHF or DSS. For example, if your initial infection was DEN-1 and your second is DEN-3, your risk of severe illness (DHF or DSS) is high.

The second hypothesis suggests that dengue viruses constantly mutate as they pass between humans and mosquitoes. Some dengue strains may have a greater potential for phenotypic expression of genetic mutations, resulting in increased viral replication, infection potential, and severity of infection.

Regardless of the mechanism, all four dengue serotypes are currently circulating in Florida, and the risk of severe dengue infection in Florida residents who suffered a previous dengue infection, even 33 years ago, is high if they are infected with a new serotype. Of the 244 travel-associated dengue cases reported in Florida so far in 2023, four have been classified as either DHF or DSS.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 1. The Keetch-Byram Drought Index for Florida as of September 5, 2023.

The western Florida Panhandle remains extremely dry. With the exception of the Florida east coast from Jacksonville south to Ft. Pierce, the Florida Peninsula is currently saturated due in part to rainfall from Hurricane Idalia (Figure 1). Arboviral surveillance reporting was light this week due to the hurricane and the Labor Day holiday. We should see a bump in the numbers next week.  

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 September 2, 2023.

Dengue Viruses

Three additional locally-acquired dengue cases with infection in late July were reported last week in Miami-Dade (two cases) and Broward Counties, bringing the total number of locally-acquired dengue cases in Florida to 19 (14 in Miami-Dade, three in Broward, and one each in Polk and Hardee Counties). 

Fourteen new travel-associated dengue cases were reported in Florida last week, bringing the total number of travel-associated dengue cases reported in Florida so far in 2023 to 258, nearly 100 cases above the 13-year average (2010-2022) (Table 2). These new cases were widely distributed throughout the state (Figure 2).

All four dengue serotypes have been reported as travel-associated cases in Florida so far in 2023. Three dengue serotypes (DEN-1, DEN-2, and DEN-3) have been reported as locally-acquired dengue cases in 2023.

Figure 2. The spatial distribution of travel-associated human dengue cases in Florida as of September 2, 2023. The nine counties that are shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

Malaria

Three new travel-associated human malaria cases were reported in Florida last week, one each in Hillsborough, Manatee, and Polk Counties. A total of 45 travel-associated malaria cases have been reported in Florida during 2023. 

Eastern Equine Encephalitis Virus



Three new EEEV antibody-positive sentinel chickens were reported in Leon, Nassau and St. Johns Counties last week, bringing the total number of positive chickens to 113, which is below the long-term expected value of 130 (Table 2).  

St. Louis Encephalitis Virus

Transmission of SLEV in Florida remains virtually non-existent. We will see how this changes now that migrant birds are beginning to arrive from the north.  

West Nile Virus

Five new WNV antibody-positive sentinel chickens were reported in Bay (two positives) and Walton (three positives) Counties last week, bringing the total number of positive chickens to 69, well below the 419 expected seroconversions (Table 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 9/2/23).

Last week was unusual relative to arboviral surveillance in Florida for two reasons: Hurricane Idalia and the Labor Day holiday. The number of positives reported during Week 35 was likely depressed and may rebound next week.

The dramatic change in surface wetness in North Florida due to Hurricane Idalia (Figure 1) may influence arboviral transmission in the coming weeks, especially for EEEV and WNV.

Travel-associated dengue cases continue to be reported throughout Florida (Figure 2). Nine Florida counties have reported travel-associated dengue cases during the past three weeks, increasing the possibility of locally acquired dengue outbreaks in those counties. 

Locally acquired dengue cases continue to be reported in South Florida, and the risk of widespread focal outbreaks remains high. Aedes aegypti populations remain abundant throughout the Florida Peninsula, and the wet conditions in South Florida (Figure 1) will continue to support dengue transmission.

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated vector-borne disease in Florida. Table 2 compares the current 2023 data with long-term expected values. Even though WNV values are well below normal, it is evident that transmission of WNV is on the increase and transmission risk is currently extremely high in the northern half of the state and needs to be carefully monitored during the coming weeks.

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.