September 13, 2023 | Volume 3 | Issue 17 | As of Week 36

A QUESTION FROM OUR READERS

Q. Angela from Winter Park, FL, has a follow-up question: “How does infection with a second dengue serotype place me at higher risk for a severe case of dengue?”

A. As discussed last week, Dengue Hemorrhagic Fever (DHF) and Dengue Shock Syndrome (DSS) are the two most severe and sometimes fatal manifestations of dengue infection.

The most common of these is DHF. During the acute phase of infection, DHF is characterized by the sudden onset of a fever lasting two to seven days with various virus-like symptoms, including rash, joint pain, headache, and sore throat. As the fever subsides and the patient enters the chronic phase of infection, really bad things begin to happen. The characteristic manifestations of DHF appear as plasma leakage (hence the “hemorrhagic” in DHF) from the skin, nose, ears, mouth, and GI tract. Plasma leakage is accompanied by circulatory failure and shock at about the same time that body temperature normalizes.

The secondary-infection hypothesis of DHF and DSS proposes that individuals experiencing infection with a second dengue serotype have a higher risk of developing severe illness. For example, if your first infection was DEN-1 and your second is DEN-3, you run a greater risk of developing DHF or DSS.

A possible explanation for the onset of severe dengue infection is that the preexisting heterologous dengue antibody from the first infection (e.g., DEN-1) recognizes antigens (virus particles) from the second infection (e.g., DEN-3) and forms an antigen-antibody complex (DEN-3 antigen-DEN-1 antibody). The complex binds to leukocyte (especially macrophages) plasma membranes and is absorbed by the macrophage but not neutralized and destroyed. The virus part of the antigen-antibody complex then begins to replicate inside the macrophage.

The previous dengue infection initiates a cellular process known as antibody-dependent enhancement (ADE) that increases the infectivity and replication rate of the dengue virus in the macrophages. ADE is believed to produce viral particles with increased vascular permeability, resulting in hemorrhage and shock in patients experiencing a second dengue infection with a new serotype.

It is important to note that not all patients infected by a second dengue serotype suffer DHF or DSS. A host of secondary factors, including age (young and old) and co-morbidities (cancer or compromised immune systems), may render individuals more susceptible to severe infection. If infected with dengue, it is good for you to know the serotype and also important for you to know what serotypes are currently circulating in areas where you live or visit.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 1. The Keetch-Byram Drought Index for Florida as of September 11, 2023.

The western Florida Panhandle, the Jacksonville area south to Daytona Beach, and the Treasure Coast from Melbourne to Ft. Pierce are showing drying trends. The remainder of the state remains extremely wet (Figure 1).   

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 9, 2023.

Dengue Viruses

There were no new locally-acquired dengue cases reported last week. 

Sixteen new travel-associated dengue cases were reported in Florida last week, bringing the total number of travel-associated dengue cases so far in 2023 to 274, 107 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, with DEN-3 predominating.

Figure 2. The spatial distribution of travel-associated human dengue cases in Florida as of September 9, 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

Three new travel-associated human malaria cases were reported in Florida last week, one each in Broward, Pasco, and St. Lucie Counties. A total of 48 travel-associated malaria cases have been reported in Florida during 2023 (Figure 3).

Figure 3. The spatial distribution of travel-associated human malaria cases in Florida as of September 9, 2023.

Eastern Equine Encephalitis Virus



One new EEEV antibody-positive sentinel chicken was reported in Alachua County last week, bringing the total number of positive chickens to 114, which is below the long-term expected number of sentinel chicken seroconversions of 130 (Table 2).

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens as of September 9, 2023.

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

Three new WNV antibody-positive sentinel chickens were reported in Bay (two positives) and Orange Counties last week, bringing the total number of positive chickens to 72, well below the long-term expected number 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 9/9/23).

Virus transmission and reports of travel-associated human cases (dengue and malaria) appeared to have slowed during the past weeks. Surface wetness/dryness (Figure 1) may favor WNV amplification in areas reporting drying conditions. Surface wetness in South Florida will continue to favor the production of Aedes aegypti and may favor continued dengue transmission.

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

Travel-associated malaria cases continue to be reported in Florida (Figure 3), but it is unlikely that additional transmission foci of locally-acquired malaria transmission will be established this late in the year. 

Sporadic transmission of EEEV to sentinel chickens continues to be reported in Florida (Figure 4). However, we are currently late in the EEEV transmission season (Figure 5), and the risk of a widespread outbreak is decreasing

Figure 5. The temporal distribution of 85 human EEE cases in Florida by month of onset (1955-2023).

Transmission of SLEV has been virtually absent in Florida for the 2023 arboviral transmission season (Tables 1 and 2). It will be interesting to see if the virus reappears during the fall migration season.

West Nile virus transmission to sentinel chickens is way below normal so far in the 2023 arboviral transmission season (Table 2). There is not much time left to establish significant WNV transmission foci, and the situation for the remainder of the year will likely favor sporadic viral 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. As we approach the end of the 2023 arboviral transmission season in Florida, it is unlikely that a major outbreak of EEEV, SLEV, or WNV will occur. Sporadic transmission of these viruses and dengue virus is possible for the remainder of the transmission season. 

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.