It continues to be Clarke’s privilege to share Dr. Day's weekly analysis of arbovirus disease activity in Florida with mosquito control professionals across the state. Our shared goal with Dr. Day is to provide timely and actionable information that mosquito control programs can use to make operational decisions and protect public health from vector-borne diseases.

An archive of all past newsletter issues remains available on the Clarke website.

A. There are many definitions of bridge vector in medical entomology. I agree with you and most commonly associate the term with the eastern equine encephalitis virus transmission cycle. Transmission of EEEV in Florida is extremely complex, which may explain why there has never been a significant epidemic of EEE in the state. From November through early spring, EEEV is maintained in Florida's hardwood swamps from the western Panhandle to the central Peninsula. In these swamps, the virus undergoes endemic transmission between Culiseta melanura and overwintering resident birds along with spring and fall migrants. Transmission of EEEV is pretty much restricted to swamp habitats during the winter maintenance phase of the transmission cycle due mainly to temperature and rainfall/drought cycles.

During the warm wet spring months, infected birds and mosquitoes disperse out of the swamps and establish secondary transmission foci involving different mosquito species including Culex nigripalpus, Coquillettidia perturbans, Mansonia species, and spring floodwater Aedes species. These mosquitoes are opportunistic blood feeders that willingly feed on avian and mammalian hosts and are what I consider bridge vectors, bridging EEEV transmission from the hardwood swamps to adjacent upland habitats. During years of exceptional viral amplification, bridge vectors can initiate epizootic cycles involving equines and exotic birds such as emus. Bridge vectors are also responsible for the epidemic transmission of EEEV from infected amplification hosts to humans.

Two West Nile and St. Louis encephalitis transmission cycles occur in Florida. The urban transmission cycle involves Culex quinquefasciatus as the primary endemic, epizootic, and epidemic vector. In the rural transmission cycle, Culex nigripalpus serves as the primary vector. These species blood feed opportunistically on birds and mammals. Therefore, there is no need for bridge vectors in the Florida WNV and SLEV transmission cycles.

Arboviral transmission and travel-related human cases of vector-borne disease continued to increase in Florida during the past week. Three new locally-acquired dengue cases were reported in Miami-Dade County bringing the total to 22. In addition, 41 new travel-related dengue cases, one new human WN case, and 40 new WNV antibody-positive sentinel chickens were reported in Florida last week. There were no reports of EEEV transmission last week, and we seem to be outside of the peak transmission period for EEEV in Florida, but it is important to remember that the risk of EEEV transmission in Florida is never zero.

Four new SLEV antibody-positive sentinel chickens were reported in Citrus, Sarasota (two new positives), and Sumter Counties, bringing the total number of positive sentinel chickens in Florida to nine (Figure 1). These positive sentinel chickens are widely dispersed (Figure 1), suggesting that the virus is being moved around and reintroduced into the state, possibly by migrating birds. At this time, SLEV does not pose a risk to human health in Florida.

Transmission of WNV in Florida continues to increase. A second WN neuroinvasive human case was reported in Volusia County. Forty new WNV antibody-positive sentinel chickens were reported during Week 38. Transmission of WNV continues to be reported throughout the state (Figure 2), but two primary transmission foci continue to be evident, one in the western Panhandle and one in the southwest Peninsula. The heavy transmission of WNV as indicated by sentinel chicken seroconversions along with the reports of equine, human, and WNV-positive mosquito pools all indicate that there remains a significant risk of WNV transmission to humans in Florida. Historically, the majority of humans infected with WNV in Florida have had an onset in August (Figure 3), so we appear to be on the downside of the risk curve for human WN infection.

Forty-one new travel-related dengue cases were reported in Florida during Week 37. Twenty-eight of these were reported in Miami-Dade County (Figure 4). A total of 464 travel-related dengue cases have been reported in Florida thus far in 2022. Of these, 423 (91%) originated in Cuba. All four dengue serotypes have been introduced into Florida from Cuba this year, a fact that may play into the severity of human infections reported in Florida during the coming months.

Twenty-two locally-acquired dengue infections have been reported in South Florida in 2022. 

As of Week 37, 2022, the following mosquito-borne disease transmission events and pathogen introductions have been reported in Florida:

Vector and arboviral surveillance remain some of the most important tools that vector control agencies currently have at their disposal. Arboviral transmission indices (sentinel chickens, positive equines, positive exotics such as emus), positive humans, and positive mosquito pools provide indicators of local virus transmission, although sometimes not in a timely manner. Monitoring mosquito populations and their age structure gives additional information about potential transmission risks. Supplemental vector control efforts in and around sites where virus transmission is known or suspected of recently occurring provide another mechanism to mitigate viral transmission.

There is currently a low risk of EEEV transmission in Florida.

There is currently an extremely low risk of SLEV transmission in Florida.

There is currently a high risk of WNV transmission in Florida in the two transmission foci shown in Figure 2, and especially in southwest Florida in the counties surrounding Lee County.

There is currently an extremely high risk of local dengue transmission in South Florida (Figure 4 and Table 1). The 22 locally-acquired dengue cases and the continued reports of travel-related dengue in South Florida (54 new cases per week) increase the risk of establishing additional dengue transmission foci and increase the risk of a significant outbreak of locally-acquired dengue. As of now, all of South Florida is at risk for a dengue outbreak. Increased Aedes aegypti control in areas surrounding locally-acquired and travel-related dengue cases will help to reduce the number of infected and newly emerged vector mosquitoes.

Source reduction, the emptying, and where possible the destruction, of all water-holding containers remains the most productive control method against the likely dengue vector, Aedes aegypti. Cryptic Ae. aegypti breeding habitats remain a huge problem in places where this species is abundant. The location of these cryptic habitats remains one of the primary challenges for vector control agencies dealing with dengue outbreaks.

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.

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