August 16, 2021 | Week 31 | Volume 1 | Issue 5
Welcome to the Florida Disease Activity Update, from the desk of Dr. Jonathan Day! It is 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.
A QUESTION FROM READERS

Spencer from Jacksonville asks, "What do you mean when you say that a “virus is endemic?”  
 
A: When a virus becomes endemic, baseline levels of transmission are maintained in a geographic area without additional external input of the pathogen. For example, WNV is endemic in Florida, and during some years, viral transmission is reported in sentinel chickens every month of the year.

In contrast, from June-August of 2014, eleven locally acquired human cases of Chikungunya virus (CHIKV) were reported in Florida, but no locally acquired human CHIKV cases have been reported in Florida since 2014. This suggests that CHIKV was introduced into South Florida in 2014, but is NOT currently endemic in the state.
THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE
The entire state of Florida remains exceptionally wet. Currently, the top eight inches of the soil layer are completely saturated in almost every Florida county (see map where blue indicates surface saturation and red indicates extreme drought). These environmental conditions continue to favor mosquito production and dispersal in general, but they especially favor the production and dispersal of Aedes aegypti and Ae. albopictus. High populations of these mosquitoes will support the transmission of dengue, Chikungunya, or Zika viruses if one or more of these viruses is reintroduced into Florida by a viremic traveler, and if that individual comes into contact with susceptible Aedes vector mosquitoes.
In Florida, four mosquito-borne arboviruses currently account for the greatest human disease risk; dengue viruses (DENV), eastern equine encephalitis virus (EEEV), St. Louis encephalitis virus (SLEV), and West Nile virus (WNV). The current Florida transmission risk for each of these viruses is reviewed below.
WHAT ARE WE CURRENTLY SEEING?
Eastern Equine Encephalitis (EEEV) Activity
There is continued evidence of low-level EEEV transmission in Florida north of Indian River County on the east coast and Hillsborough County on the west coast. 

Five (5) new EEEV antibody-positive sentinel chickens were reported during week 31, bringing the total number of chickens testing positive for EEEV antibody in 2021 to 153. The new positive sentinel chickens were reported in Nassau (2), Orange (1), and Walton (2) Counties (see map). Conditions favorable for the transmission of EEEV apparently remain in place throughout the northern half of the state.
 
The number of EEEV-positive horses reported in Florida during 2021 remains at 12. All 12 of the positive horses are widely dispersed in North Florida and the eastern Florida Panhandle. We are at a time of year (June through August) when the transmission of EEEV to horses and humans is historically the highest, especially in North Florida and the Florida Panhandle.
With only 12 EEE equine cases reported thus far in 2021, all indications are that there is not currently a high threat of extensive EEEV transmission to humans or equines in the state. There is, however, always a threat of sporadic transmission of EEEV to humans and equines during August and September. This will be especially true for Central and North Florida and the Florida Panhandle during the late summer and early autumn of 2021.

Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
West Nile Virus (WNV) Activity
The threat of widespread WNV transmission to equines and humans currently remains low in all of Florida.
 
One (1) new WNV antibody-positive sentinel chicken was reported during Week 31 in Lee County. During the 2021 Florida arboviral transmission season, 11 WNV antibody-positive sentinel chickens have been reported. This is well below the number normally reported in Florida by mid-August. However, the number of new WNV-positive sentinels reported each week seems to be increasing, especially in the southwestern part of the State. It remains important to continue monitoring the transmission of WNV during the next 10 weeks. The next threat of WNV amplification will occur during the fall avian migration that begins in a couple of weeks.

Find a review of 2020 WNV activity in the archived Vol 1, Issue 1 report.
Dengue (DENV) Activity
There has been no indication of locally-acquired dengue infection in Florida during 2021. Five (5) travel-related dengue cases have been reported in Broward (1), Hillsborough (1), Miami-Dade (1), and Palm Beach (2) Counties. This is a historically low number for mid-August and may represent a low number of travelers and visitors entering Florida from dengue-endemic areas.

Wet conditions across Florida do favor the reproduction and local dispersal of Aedes aegypti and Ae. albopictus. If dengue virus is reintroduced into Florida by an infected traveler in an area with high numbers of Ae. aegypti or Ae. albopictus, the risk of local dengue transmission remains a real possibility. Locally-acquired DENV cases have been reported in October and November in South Florida, so it will be important to continue monitoring the introduction of travel-related human dengue cases, especially in Miami-Dade and Monroe Counties. 

Find a review of 2020 DENV activity in the archived Vol 1, Issue 1 report.
St. Louis Encephalitis Virus (SLEV) Activity
Transmission of SLEV in Florida remains extremely low. So far in 2021, only two sentinel chickens (both in Palm Beach County) have tested positive for antibodies to SLEV. Both of the sentinel chickens were infected early in the year and there has been no additional indication of SLEV transmission anywhere in the state. The SLEV may no longer be endemic in Florida and the future transmission of SLEV in Florida may depend on viral reintroduction. As with WNV, fall avian migration has the potential of seeding SLEV into Florida from the north. In addition, the fall migration will provide a large number of susceptible birds that may serve as amplification hosts to support a late-season SLEV transmission cycle.
OPERATIONAL STRATEGIES TO CONSIDER
Vector and arbovirus surveillance remains the most important tool vector control agencies have at their disposal. Monitoring arboviral transmission indices (sentinel chickens, positive equines, positive exotics (i.e., emus), and positive humans) provide indicators of local virus transmission, although sometimes not in a timely manner. Monitoring mosquito populations, and their age structure, provides added information about potential transmission risk. Additional vector control efforts in and around sites where virus transmission is known or suspected provides another potential mechanism to mitigate viral transmission.

The Florida regions with the highest current concern for arboviral transmission are those where indicators of EEEV transmission (sentinel chickens and EEE-positive equines) are currently high. These include: The Central Florida Counties around Orange County, all North Florida Counties, and the Panhandle Counties around Walton County.
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 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 and are posted at: fireweather.fdacs.gov/wx/kbdi_4km.html.
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