October 4, 2023 | Volume 3 | Issue 19 | As of Week 39

A QUESTION FROM OUR READERS

Q. Dave from Vero Beach, FL, asks: “How did the breeding success of resident wild birds in Florida affect the transmission of West Nile virus in 2023?”

A. It is difficult to say because other factors influence WNV transmission patterns, especially the cycling of rainfall and drought - but wild birds are an incredibly important part of this transmission pattern. The resident wild bird breeding season is over with the exception of some odd-ball species like Mourning Doves that breed all year long (they are, after all, related to pigeons).

I track resident wild bird populations in Indian River County and in 2023, Blue Jays, Northern Cardinals, and Mourning Doves fledged multiple broods during the summer injecting a large number of birds susceptible to WNV infection into the environment.

However, it doesn’t matter how many susceptible amplification hosts are in the environment if the virus isn’t present. During the 2022 arboviral transmission season, there were 492 WNV antibody-positive sentinel chickens reported in Florida. Of the 26 Florida counties with a sentinel chicken arboviral surveillance program, only Indian

River County reported no WNV seroconversions (Figure 1).

Figure 1. Spatial distribution of sentinel chicken seroconversions to WNV in Florida during the 2022 arboviral transmission season.

So far in 2023 the number of WNV seroconversions is about a quarter of that reported during 2022 and Indian River County is still without a reported seroconversion (Figure 2). It doesn’t matter how extensive the avian amplification population is if there is no virus to amplify. 

Figure 2. Spatial distribution of sentinel chicken seroconversions to WNV in Florida during the first nine months of the 2023 arboviral transmission season.

Historically, the West Nile virus deck has been reshuffled each year by fall migrants moving south into the Caribbean Basin, Central America, and South America. It will interesting to see what Figure 2 looks like at the end of 2023.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 3. The Keetch-Byram Drought Index for Florida as of October 2, 2023.

South and Central Florida is extremely wet, North Florida is wet, and the Florida Panhandle is dry (Figure 3). The only place where groundwater conditions are likely to support arboviral transmission is in South Florida, where outbreaks of locally-acquired dengue continue to be reported.

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 30, 2023.

Dengue Viruses

Since our last report, there have been 15 new locally-acquired dengue cases in Miami-Dade County. Twelve were DEN 3, and three were IgM-positive. Infections occurred between mid-August and mid-September. 

Thirty-five new travel-associated dengue cases have been reported in Florida since our last report, bringing the total number of travel-associated dengue cases so far in 2023 to 334, 167 cases above the 13-year average (2010-2022) of 167 (Table 2). These new cases were widely distributed throughout the state (Figure 4).

Figure 4. The spatial distribution of travel-associated human dengue cases in Florida as of September 30, 2023. The 16 counties that are shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

There is some indication that the locally-acquired dengue transmission cycle has peaked. The 2020 outbreak of locally-acquired dengue peaked in June (Figure 5). While overall transmission of locally-acquired dengue (2009-2023) peaks in August (Figure 6). Transmission of locally-acquired dengue so far in 2023 is shown in Figure 7, with an apparent August peak.

Figure 5. The temporal distribution of locally-acquired human dengue cases in Florida during 2020.

Figure 6. The temporal distribution of locally-acquired human dengue cases in Florida from 2009 through September 30, 2023.

Figure 7. The temporal distribution of locally-acquired human dengue cases in Florida as of September 30, 2023.

Malaria

Three new travel-associated human malaria cases have been reported in Broward, Jackson, and Lake Counties since our last update, bringing the total number of travel-associated human malaria cases reported in Florida to 53. Most of these are Plasmodium falciparum which originated in Africa. 

Eastern Equine Encephalitis Virus



No new indications of EEEV transmission have been reported in Florida. 

St. Louis Encephalitis Virus

No new indications of SLEV transmission have been reported in Florida. 

West Nile Virus

Forty-two new WNV antibody-positive sentinel chickens have been reported since our last update. Most activity was reported in northeast Florida and the central Florida Panhandle (Figure 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 9/30/23).

We are past the point in the 2023 Florida arboviral transmission season where there is a threat of a major outbreak of human EEE or WN. Even human dengue transmission seems to be on the downside of the transmission curve (Figure 7).

We are entering the part of the Florida arboviral transmission season where sporadic equine (EEE and WN) and human (DEN, EEE, and WN) cases may be reported. The risk of arboviral transmission in Florida is never zero.

There is still a lot of WNV transmission to sentinel chickens in North Florida and the Florida Panhandle, so vector control and public health agencies in those areas should remain vigilant.

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.