July 27, 2022 | Volume 2 | Issue 8 | As of Week 29
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

Sarah from Vero Beach, FL asks: “How does being gravid or freshly blood-fed affect mosquito flight behavior? Do gravid and blood-fed mosquitoes rest more often?”

A*. I think we have all observed how difficult it is for a freshly blood-fed mosquito to exit the feeding site and fly away.

Most mosquito species quickly concentrate and expel excess water in the blood to lighten the load. Many Anopheles species expel water as they blood feed (see the image of Anopheles albimanus below taken by Dr. Larry Reeves of the FMEL). As the weight of the blood is reduced, female mosquitoes fly quite easily.

Gravid mosquitoes have the same potential problem as the added weight of eggs may interfere with their flight abilities, but this added weight does not seem to keep them grounded.

For example, we made aspirator collections at the Lockwood sentinel chicken site in Indian River County in the early 1990s. During droughts, the proportion of gravid Culex nigripalpus increased dramatically, sometimes approaching 80% of the total aspirator collection. We hypnotized that the gravid females remained at the resting site until heavy rains stimulated flight and oviposition. To test this, we collected approximately 250,000 mosquitoes for three successive nights (for a total of 750,000 mosquitoes), marked the mosquitoes in each collection (males and females) with fluorescent powder (a different color for each night), and released them back at the resting site. We expected to collect color-marked gravid females (and possibly males) the following morning. We never collected a single marked mosquito at Lockwood.

Even though gravid females are carrying extra weight, they get up and fly every night just like their host-seeking sisters. Gravid and blood-fed females may or may not rest more often, but they are constantly on the move.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA DURING 2022

Low-level transmission of EEEV to sentinel chickens and equines continued this week in Central Florida (Figure 1). An EEE-positive equine that was likely infected in early July was reported this week in Marion County. This EEEV transmission indicates that mosquitoes infected with EEEV are still present and active in the area of concern highlighted in Figure 1.
Figure 1. The current 2022 transmission of EEEV in Florida as measured by sentinel chicken seroconversions.
The number of WNV antibody-positive sentinel chickens reported in Florida doubled this week to 10 (Figure 2). Transmission of WNV to sentinel chickens is now reported throughout the state and this will bear careful observation during the coming weeks to determine how extensive WNV amplification was and the areas of the state where WNV transmission is most likely to be reported.
Figure 2. The current 2022 transmission of WNV in Florida as measured by sentinel chicken seroconversions.
Year to Date Summary of Mosquito-Borne Disease Transmission

As of Week 29, 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 July 23, 2022 
The number of travel-related DENV cases reported in Florida continues to increase with 11 new introductions reported this week (nine in Miami-Dade County). Seventy-three travel-related introductions of DENV have been reported in Florida so far this year (Figure 3). Most of these introductions have originated in Cuba and all four DENV serotypes have been introduced into Florida from Cuba. A locally-acquired DEN case was reported in Miami-Dade County in June. The continued increase in travel-related DENV cases and the reported locally-acquired DENV case in Miami-Dade County warns of the possibility of a dengue outbreak in South Florida during the coming weeks. Historically, most of the locally-acquired dengue cases reported in Florida have had onset in August (Figure 4). 
Figure 3. The current 2022 summary of travel-related DENV introductions into Florida..
Figure 4. 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 moderate risk of EEEV transmission in the southern part of North Florida, the northern part of Central Florida and the western Florida Panhandle (Figure 1). 
 
There is currently a low risk of WNV transmission throughout the state (Figure 2). 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 moderate risk of local DENV transmission in South Florida (Figures 3 and 4). The appearance of a locally-acquired dengue case in Miami-Dade County in June and the continued influx of travel-related dengue cases into South Florida increases the risk of a 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 DEN cases will help to reduce potentially infected vector mosquitoes. Source reduction, the emptying, and where possible the destruction, of all water-holding containers is particularly important in all of South Florida during the next three months.
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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