June 1, 2022 | Volume 2 | Issue 4 | As of Week 21
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

Spencer from Merritt Island, FL asks: “I plan to travel to Costa Rica this spring. Are there any vector-borne infections that I need to be aware of?”

A: The two mosquito-borne diseases that you might encounter in Costa Rica are dengue and malaria.

Of the two, dengue should be your primary concern. This is especially true in urban habitats at low elevations. The primary dengue vector, Aedes aegypti, flourishes in these habitats where it is a domestic species found indoors and in gardens surrounding homes and hotels. It is most active during the daytime (with blood-feeding peaks in the early morning and late afternoon). This mosquito blood feeds low on the body, usually around the ankles, and is easy to identify by its bright silver markings on a black background. Currently, there are only two Central American countries reporting an above-average number of human dengue cases: Belize and El Salvador.

Malaria is relatively rare in Costa Rica. Plasmodium falciparum infections have been reported from the Osa Peninsula in the south. Plasmodium vivax has been reported in the north. Because of the limited malaria transmission in the north, the CDC advises travelers to practice mosquito avoidance rather than use antimalarial drugs. Mosquito avoidance includes the use of insect repellents outdoors, wearing protective clothing and sleeping in well-screened air-conditioned buildings. The CDC advises travelers in the Osa Peninsula to take antimalarial drugs including atovaquone-proguanil, chloroquine, doxycycline or mefloquine.

By the way, if you plan to visit tropical rainforests, be aware of possible infection by the human botfly. Human botfly larvae burrow under the skin and feed in the fatty subcutaneous dermal layer. If infected, fourth instar larvae will appear in a breathing hole at the skin surface four to six weeks after your trip.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA DURING 2022

Florida mosquito-borne disease transmission and pathogen introductions have remained low in 2022. For the three locally-transmitted disease pathogens (EEEV, SLEV, and WNV), this is likely due to the rainfall patterns reported in Florida from January through May of 2022. 

The ideal groundwater levels for a heavy EEEV transmission year (1991) and a SLEV epidemic year (1990) are shown below with arrows indicating the beginning of June. 
Modeled Water Table Depth: Epidemic SLE vs. Epizootic EEE
Figure 1. Groundwater profiles for heavy EEEV (1991) and SLEV (1990) transmission years with arrows indicating Week 21.
In Week 21 (the end of May), we are at a point where the ground surface conditions should be wetting to support the transmission of EEEV. The western Florida Panhandle KBDI for May 30, 2022, shows just such a regional wetting event (Figure 2).
Figure 2. The May 30, 2022, Keetch Byram Drought Index for Florida.
For heavy WNV amplification and transmission, we are at a point in the year where there should be a deep drought in areas where WNV transmission will eventually appear. This is the current situation in much of South Florida (Figure 2). Recent focal wetting has been reported in the southcentral Florida Peninsula. This may favor the amplification of WNV. Sentinel chicken seroconversions to WNV will be the first indication of a significant amplification event.
Year to Date Summary of Mosquito-Borne Disease Transmission

As of Week 21 in 2022, the following mosquito-borne disease transmission events and pathogen introductions have been reported:
Table 1. Summary of mosquito-borne disease transmission
and travel-related introductions in Florida as of May 28, 2022
The transmission of EEEV, SLEV, and WNV has been slow to start in 2022 as measured by sentinel chicken seroconversions. An EEEV-positive horse was reported in Columbia County with infection in early May. This slow start to the 2022 arboviral transmission season is likely rainfall-related (see KBDI discussion above). This may change as the rainy season begins, especially in South Florida. So far, most (17 out of 19) of the sentinel chicken seroconversions to EEEV have occurred in the central peninsula around Orange County. This is the transition zone between the very wet North Florida and the very dry South Florida regions. 

Travel-related introductions of DENV (27) and malaria (22) have been reported during the first five months of 2022. All of the travel-related DENV introductions have been in South Florida with 17 reported in Miami Dade County. In 2020 there were 49 travel-related DENV introductions into Florida and 71 locally-acquired DEN cases. By contrast, in 2019 there were 406 travel-related introductions and 16 locally-acquired cases. The take-home message here is that large locally-acquired human DENV outbreaks can occur with only a few travel-related introductions. 
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 arboviral transmission in some parts of Florida. It is important to note that arboviral transmission risk is never zero. The dry conditions reported in the central Florida Peninsula during the early part of 2022 have started to disappear (Figure 2). This may favor the amplification and transmission of WNV, especially along the South Florida Gulf Coast.  

It is also important to continue monitoring travel-related introductions of DENV and malaria. Historically, introductions of DENV into Miami Dade and Monroe Counties have sparked outbreaks of locally-acquired DEN. 
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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