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. Vector-borne diseases have a remarkable ability to move around and establish transmission cycles in new habitats. The three terms you ask about address pathogen movement and establishment of transmission cycles.

Locally-acquired cases occur when a local mosquito vector acquires a pathogen by blood feeding on an infected host, completes extrinsic incubation (development of the pathogen to maturity in the vector) with a resulting salivary gland infection, and blood feeds on a susceptible host. For example, 19 cases of dengue have been reported in South Florida so far this year (see below). None of these dengue cases had a history of travel two weeks prior to the onset of symptoms, and all were likely infected by local mosquitoes in South Florida.

Travel-related vector-borne disease infections refer to individuals with a recent travel history outside of the state or territory that they reside in, who are bitten by an infected vector, and then return to their home where they are diagnosed with a vector-borne disease. For example, between 2010 and 2021 there were 812 travel-related malaria cases reported in Florida for an annual average of 68 cases per year. The majority (811 cases) of these infections were not acquired in Florida, with the possible exception of 2010 when a single locally-acquired malaria case was reported in Jacksonville, Florida.

Pathogen introductions occur in a number of ways. For public health purposes, vector-borne disease pathogen introduction or translocations occur when there is evidence that a vector-borne disease was acquired in a geographic location where it normally isn’t found. This can occur through the movement of infected people or infected animals from areas where a pathogen is endemic to areas where it is not normally found. When this happens, the infected person or animal may be bitten by a competent local vector that, upon completion of extrinsic incubation, can infect humans and animals living in the new location. For instance, if a traveler was sick with malaria after traveling internationally, they could infect local Anopheles mosquitoes that may become infective and transmit malaria to other humans living in the area. Pathogen translocation may also occur when infected vector mosquitoes are transported into a new geographic area where they bite and infect susceptible people or animals.

Arboviral transmission and travel-related human cases continue to increase in Florida. Five new locally-acquired dengue cases were reported in Miami-Dade County. In addition, 52 new travel-related dengue cases, one new WNV-positive equine (Sarasota County), and 50 new WNV antibody-positive sentinel chickens were reported in Florida during the past week.

Transmission of EEEV has definitely slowed down in Florida. Historically, the heaviest transmission months for equines and humans have been June, July, and August (Figure 1).

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.

A single SLEV antibody-positive sentinel chicken was reported in Volusia County, bringing the total number of positive sentinel chickens in Florida to five (in five different counties). There is no indication of any active SLEV transmission foci in Florida. At this time, SLEV does not pose a risk to human health in Florida.

Transmission of WNV in Florida continues to increase. Fifty new WNV antibody-positive sentinel chickens were reported during Week 37. 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. A WNV-positive horse that was likely infected in late August was reported in Sarasota County last week. 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.

Fifty-two new travel-related dengue cases were reported in Florida during Week 37. Thirty-six of these were reported in Miami-Dade County (Figure 3). A total of 423 travel-related DENV cases have been reported in Florida thus far in 2022. Of these, 385 (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.

Nineteen locally-acquired dengue infections have been reported in South Florida in 2022. Eighteen are in Miami-Dade County, and one is in Collier County.

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

The number of travel-related dengue cases reported in Florida has increased steadily during the past month with an average of 52 new cases each week for a total of 423 in 2022. This is well above the 12-year average (2010-2021) of 105 travel-related dengue cases reported in Florida annually. The high number of travel-related dengue cases reported so far in 2022 increases the risk of establishing additional dengue transmission foci in South Florida.

The 12-year average of locally-acquired dengue cases in Florida is 16 per year, a number that was surpassed during Week 37. The steady increase in travel-related dengue cases along with the 19 locally-acquired dengue cases suggests the possibility of a significant outbreak of dengue in South Florida during the coming weeks.

Historically, the majority of locally-acquired dengue cases reported in Florida have had onset in August and September, so it is likely that there are a number of locally-acquired dengue infections yet to be reported.

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.

Specific operational strategies will be discussed during the 2022 arboviral transmission season depending on where and when vector-borne disease transmission becomes obvious in Florida.

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.

There is currently an extremely high risk of local dengue transmission in South Florida (Figure 3 and Table 1). The 19 locally-acquired dengue cases and the continued reports of travel-related dengue in South Florida (52 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.

Learn more about our work in protecting public health on clarke.com.