August 23, 2023 | Volume 3 | Issue 14 | As of Week 33

A QUESTION FROM OUR READERS

Q. Spencer from Merritt Island, FL, asks: “In light of the fact that we just completed earth’s hottest July on record, how does the effect of increasing worldwide temperature affect vector-borne disease transmission cycles?”

A. I believe that the warming of our planet may have a profound effect on worldwide disease transmission cycles.

The time from when a mosquito feeds on an infectious host until that mosquito becomes infective and can transmit the pathogen to additional hosts is known as the extrinsic incubation period (EIP). The EIP is temperature sensitive and, in general, the warmer it is, the faster EIP is completed.

In a 2007 paper published by Dr. Stephanie Richards and colleagues, it was reported that increasing temperature resulted in higher infection and dissemination rates in Culex quinquefasciatus females infected with WNV.

In a 2020 review, Kamiya et al. summarized studies on the EIP of dengue viruses. One study reported a nearly 10-fold decrease in the EIP time of mosquitoes held at 20^oC (68^oF) and 30^oC (86^oF). The EIP for mosquitoes maintained at 30^oC was 6 days, while mosquitoes maintained at 20^oC completed EIP in 56 days.

Just a slight warming may decrease EIPs in all vector-borne disease systems. Disease transmission by mosquitoes is linked to EIP as well as the average lifespan of females. Aedes taeniorhynchus (Figure 1) is an abundant mosquito along the Atlantic and Gulf coasts from New Hampshire to South Texas. In Florida, this species is not an important arboviral vector. This is due, in part, to the short lifespan of this species where females generally live for less than three weeks, too short a time to complete the EIPs of Florida arboviruses. A small increase in average summer temperatures in Florida may shorten the WNV EIP in Ae. taeniorhynchus to a time where this species may become a significant disease vector. 

Figure 1. Aedes taeniorhynchus by Larry Reeves, University of Florida, Florida Medical Entomology Laboratory. This mosquito species is an abundant species in Florida, but not a significant disease vector.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of August 21, 2023.

The Florida Panhandle is beginning to dry while the Florida Peninsula continues to be extremely wet, except for the area around Sarasota (Figure 2). The extremely wet conditions in the southern Florida Peninsula may favor dengue transmission by producing large numbers of Aedes aegypti. Drying conditions in the Florida Panhandle may favor late-season amplification and transmission of West Nile virus. Transmission of eastern equine encephalitis virus in Florida should be nearing its end for the 2023 Florida arboviral transmission season.  

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 19, 2023

Dengue Viruses

Four additional locally-acquired dengue cases were reported in Florida last week. All had infection dates in mid-July. One was in Hardee County, one in Polk County, and two in Miami-Dade County (Figure 3). The Hardee and Polk County cases were the first reported in those counties since 1934, suggesting that Aedes aegypti populations are prevalent throughout South Florida and that introductions of the dengue virus by infected travelers may result in focal outbreaks of locally-acquired dengue.  

Figure 3. The spatial distribution of locally-acquired dengue cases in Florida, 2009-2023.

Twenty new travel-associated dengue cases were reported in Florida last week (Figure 4). These cases were widely distributed throughout the state, indicating that the level of travel-associated dengue introductions remains high in Florida.

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

Figure 4. The spatial distribution of travel-associated human dengue cases in Florida as of August 19, 2023. The 19 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, and the malaria outbreak there appears to have been contained.

Forty travel-associated malaria cases have been reported in 16 Florida counties so far in 2023 (Figure 5).

Figure 5. The spatial distribution of travel-associated human malaria cases in Florida as of August 19, 2023.

Eastern Equine Encephalitis Virus



Four new sentinel chickens tested positive for antibodies to EEEV last week in Alachua, Leon, Martin, and Walton Counties last week, bringing the total number of EEEV antibody-positive sentinel chickens reported in Florida this year to 107.  

St. Louis Encephalitis Virus

Transmission of SLEV in Florida remains virtually non-existent.  

West Nile Virus

Two new human West Nile cases were reported in Florida last week. One was an Escambia County blood donor who later developed West Nile Fever. This individual was likely infected in early August. The second was an asymptomatic blood donor in Bay County who also was likely infected in early August.

Nineteen new WNV antibody-positive sentinel chickens were reported last week in counties throughout Florida. Thirty-six WNV antibody-positive sentinel chickens have been reported in Florida so far in 2023 (Figure 6). This is well below the expected number of positive sentinel chickens (Table 2), but the level of WNV transmission, especially in the Florida Panhandle has increased during the past two weeks. 

Figure 6. The spatial distribution of WNV antibody-positive sentinel chickens in Florida as of August 19, 2023.

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 8/19/23).

Transmission of EEEV in Florida seems to be slowing, while transmission of WNV in the Florida Panhandle continues to increase. 

Travel-associated dengue cases continue to be reported throughout Florida. Nineteen Florida counties have reported travel-associated dengue cases in the past three weeks, increasing the possibility of locally-acquired dengue outbreaks in those counties in the coming weeks (Figure 4).

Locally-acquired dengue cases continue to be reported in Florida. Last week individual locally-acquired dengue cases were reported in Hardee and Polk Counties (Figure 3) for the first time since 1934, suggesting that Aedes aegypti populations are abundant throughout the southern half of the state and that the risk of focal dengue outbreaks is currently high.

The transmission of WNV and SLEV is currently well below normal in Florida (Tables 1 and 2). However, recent human WNV cases associated with blood donors in Escambia and Bay Counties indicate that infected mosquitoes are active in the western Florida Panhandle. The recent increase in WNV antibody-positive sentinel chickens throughout the state indicates that infected mosquitoes are active and possibly increasing in Florida. 

New travel-associated human malaria cases continue to be reported in Florida. Last week two new cases were reported in Hillsborough and Orange County, bringing the total number of travel-associated cases reported in Florida during 2023 to 40.  

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. Even though WNV values are well below normal, it is evident that transmission of WNV is on the move, especially in the Florida Panhandle. The transmission of WNV will need 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.