May 24, 2023 | Volume 3 | Issue 5 | As of Week 20

A QUESTION FROM OUR READERS

Q. Sarah from Vero Beach, FL, asks: “How do you monitor wild bird abundance in Florida? Is it observational data that can be used by mosquito control?”

A. Yes, it is all observational. We’ve known for decades that the abundance of susceptible wild birds is a prerequisite for large outbreaks of mosquito-borne encephalitis viruses in Florida (Avian Serology in a St. Louis Encephalitis Epicenter Before, During, and After a Widespread Epidemic in South Florida).

We have tracked local wild bird populations in Indian River County since the late 1980s in three ways.

First, during spring and fall migrations, we note the arrival dates of certain key species, such as Barn Swallows and Chimney Swifts, the frequency and duration of migratory waves, and the number of times that migratory waves stall.

Second, during the peak Florida avian nesting season (April-June), we conduct weekly line transect counts along four north-to-south five-mile-long paved routes. Observations are made from a vehicle, and the numbers of individual birds of each species observed are recorded along with observations about where the birds were seen and age when possible.

Finally, daily observations are made at five feeding stations throughout Indian River County. It is easy to observe adult and hatching year bird interactions at the feeding stations and thus quantify the number of newly fledged birds.

We are well into the 2023 Florida avian nesting season. Common Grackles, Blue Jays, Loggerhead Shrikes, Northern Cardinals (Figure 1), and Northern Mockingbirds have all fledged their first brood. 

Figure 1. A male Northern Cardinal resident of Turkey Lake Park in Orlando, Florida circa 1991.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Arboviral transmission continues at a slow pace in Florida. The South Florida avian nesting season is in full swing (see the answer to A Question From Our Readers above).

Rainfall so far this year favors EEEV transmission in North Central Florida (see the answer to A Question From Our Readers in Volume 3, Issue 1 of the Newsletter) and WNV transmission in South Florida (see the answer to A Question From Our Readers in Volume 3, Issue 2 of the Newsletter).  

Imported cases of dengue continue to be reported in Florida, with the majority, 51/74 (69%), in travelers from Cuba. The Dengue 3 serotype continues to predominate, 44/74 (60%), in the travel-related dengue cases reported in Florida. As of week 20 in 2022, 23 travel-related dengue cases were reported in Florida compared with the 74 cases reported so far this year. The imported dengue cases continue to pose a threat of outbreaks of locally-acquired dengue throughout South Florida. Table 1 summarizes the current status of arboviral transmission in Florida.

Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of May 20, 2023.

Dengue Viruses

Imported cases of dengue continue to be reported in Florida (Figure 2).

Figure 2. The spatial distribution of imported human dengue cases in Florida as of mid-May, 2023. Counties shaded red indicate sites of at least one imported case during the past three weeks.

Recently imported dengue cases increase the risk of locally-acquired dengue transmission, and vector control personnel in the five Florida counties shaded red in Figure 2 should be alert to the possibility of locally-acquired dengue transmission in the coming weeks. Two locally-acquired dengue cases have been reported in Miami-Dade County so far in 2023. 

Malaria

Fifteen imported human malaria cases have been reported in nine Florida Counties in 2023. These include single introductions in Duval, Hillsborough, Orange, Osceola, and Sarasota Counties. Two travel-related human malaria cases have been reported in Broward, Leon, and Pinellas Counties, and four have been reported in Miami-Dade County. Locally-acquired human malaria cases appear in Florida during the late summer.

Eastern Equine Encephalitis Virus

North Central Florida is currently the region where transmission of EEEV will most likely occur in the coming months. Nineteen EEEV antibody-positive sentinel chickens (Figure 3) and four EEEV-positive equines (Figure 4) have been reported in Florida in 2023.

Figure 3. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of mid-May, 2023.

Figure 4. The spatial distribution of EEEV-positive equines in Florida as of mid-May, 2023.

St. Louis Encephalitis Virus

There has been no evidence of SLEV transmission in Florida so far in 2023.

West Nile Virus

There has been little evidence of WNV transmission in Florida as of this report. Five WNV antibody-positive sentinel chickens have been reported in Hillsborough (two positives), Orange, Palm Beach, and Walton Counties thus far in 2023. One WNV-positive equine was reported in Hernando County in early January.

It is still too early in the 2023 Florida arboviral transmission season to make meaningful predictions about disease transmission later in the year. 

Imported dengue cases continue to be reported in Florida at a rate surpassing the 2022 introductions as of Week 20, so the possibility of emerging foci of locally-acquired dengue must be closely monitored. Vector control agencies in Broward, Hillsborough, Miami-Dade, Palm Beach, and Volusia Counties should be aware of recently imported dengue cases (Figure 2) and the possibility that these may result in locally-acquired dengue cases in the coming weeks. 

Environmental conditions in North Florida continue to favor EEEV transmission, but there are no indications of an imminent major outbreak. West Nile virus and SLEV transmission is currently low, although the environmental conditions in South Florida favor enhanced mid-summer Flavivirus transmission. 

Jonathan Day, Professor Emeritus of Medical Entomology from the University of Florida, 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.