August 10, 2022 | Volume 2 | Issue 10 | As of Week 31
Welcome to the Florida Disease Activity Update from the desk of Dr. Jonathan Day.
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 QUESTION FROM OUR READERS

Q. Carol from Sanford, FL has a follow-up question from Issue 9: “Can mosquitoes transmit the COVID-19 virus?”

A. No. As discussed last week, mosquitoes do not transmit viruses directly from one host to another via contaminated mouthparts. Mosquitoes do transmit viruses (and other pathogens) through biological transmission cycles. More than 200 viruses are transmitted by arthropods. At least 66 viruses are transmitted to humans by mosquitoes. Rabies, COVID-19, the common cold and HIV are not among them.
 
To become infected with a virus, the mosquito must first acquire viral particles (virions) in the blood meal. For respiratory viruses like COVID-19 and the common cold, there are not enough virions circulating in the blood to infect the mosquito. The same is true for neurological viruses such as rabies. For viruses that are transmitted by mosquitoes like West Nile virus (WNV) and St. Louis encephalitis virus (SLEV), there is a very narrow window where enough virus is circulating in an amplification host such as a Northern Cardinal or an American Robin to infect the mosquito. For SLEV, this infection window is only 48 hours long. For WNV, the infection window may be 7 to 10 days. Secondary hosts such as humans and equines seldom circulate enough virus to infect mosquitoes.
 
Once the mosquito has acquired enough virus to cause an infection, there is a relatively long (one to three weeks) period during which the virus replicates in the mosquito’s organs and tissues. When the virus has replicated enough to infect the salivary glands, the mosquito is infective for life and will transmit the virus to a new host every time she takes a blood meal.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA DURING 2022

The transmission of locally-acquired dengue in Miami-Dade County and the introduction of DENV into South Florida by infected travelers continues to be the primary vector-borne disease problem in Florida (please see below).
 
Low-level transmission of EEEV continues to be reported in Florida. Three new EEEV antibody-positive sentinel chickens (Orange (2) and Walton (1) Counties) were reported this week, bringing the total number of positive sentinel chickens in Florida to 43, which is well below the 18-year average (2004-2021) of 133 positive sentinel chickens per year. An EEEV-positive horse with infection in July was reported in Gulf County this week bringing the Florida total of EEEV-positive equines to nine for 2022, also well below the 18-year Florida average of 45 equines per year. It remains unlikely that we will see a surge of EEEV transmission in the coming months.

Transmission of WNV in Florida has increased dramatically in the past two weeks. Fifteen new WNV antibody sentinel chickens were reported last week. Positive sentinel chickens are distributed throughout the state without any clear focus of transmission (Figure 1). It remains to be seen whether a clear WNV transmission focus emerges during the remainder of 2022. 
Figure 1. WNV antibody-positive sentinel chickens reported in Florida during 2022.
There were 37 travel-related dengue introductions into Florida this week, 23 of which were in Miami-Dade County (Figure 2). A third locally-acquired dengue case was reported in Miami-Dade County this week. A total of 137 travel-related DENV introductions have been reported in Florida thus far in 2022. Of these, 116 (85%) have originated in Cuba and 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.

It is evident that local mosquitoes (probably Aedes aegypti) have been infected with DENV and are transmitting the virus to susceptible individuals in South Florida. The full extent of this outbreak remains to be determined. 
Figure 2. Travel-related dengue introductions in Florida during 2022. Counties colored blue indicate DENV introductions in 2022. Counties colored red indicate the total number of DENV introductions in 2022 and DENV introductions during Week 31.
Year to Date Summary of Mosquito-Borne Disease Transmission

As of Week 31, 2022, the following mosquito-borne disease transmission events and pathogen introductions have been reported in Florida:
Table 1. Summary of mosquito-borne disease transmission and introductions in Florida as of August 6, 2022 
The number of travel-related dengue cases introduced into Florida has increased steadily during the past four weeks with an average of 24 new introductions each week. 
 
The 12-year average (2010-2021) of travel-related dengue introductions into Florida is 105 per year. The 12-year average of locally-acquired dengue cases in Florida is 16 per year. The steady increase in travel-related dengue introductions into South Florida and the report of locally-acquired dengue cases in Miami-Dade County suggests the possibility of a significant outbreak of dengue in South Florida during the coming weeks. Historically, most of the locally-acquired dengue cases reported in Florida have had onset in August and September (Figure 3), so the worst period of locally-acquired dengue transmission is still in front of us.
Figure 3. The month of onset for locally-acquired dengue infections in Florida: 2009 to 2021.
OPERATIONAL STRATEGIES TO CONSIDER
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 a low to moderate risk of WNV transmission in Florida. Arboviral surveillance and reporting during the next weeks will help to determine the current situation relative to the transmission of EEE and WN viruses throughout the state.
 
There is currently a high risk of local dengue transmission in South Florida (Figures 2 and 3). The appearance of three locally-acquired dengue cases in Miami-Dade County with onset dates in June and July, along with the continued influx of travel-related dengue cases into South Florida increases the risk of a significant local outbreak. As of now, Miami-Dade County is the most likely spot for such an outbreak. Increased Aedes aegypti control in areas surrounding locally-acquired and travel-related dengue cases will help to reduce the number of infected and emerging 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.  
ABOUT DR. JONATHAN DAY
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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