August 9, 2021 | Week 30 | Volume 1 | Issue 4
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

Q: What is the difference between these two abbreviations: EEE and EEEV?
 
A: One (EEE) refers to a disease that is caused by a specific virus (EEEV). The same is true for WN and WNV and for SLE and SLEV.
THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE

June, July, and the first week of August were exceptionally wet throughout all of Florida. Currently, the top eight inches of the soil layer are completely saturated in every Florida county. These environmental conditions 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 vector mosquitoes.

In Florida, four mosquito-borne arboviruses 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
Eastern equine encephalitis virus (EEEV) currently remains the greatest arboviral threat to humans, equines, and exotic animals in central Florida and in the Florida Panhandle. 
 
Ten (10) new EEEV antibody-positive sentinel chickens were reported during week 30, bringing the total number of chickens testing positive for EEEV antibody in 2021 to 148. The ten (10) Week 30 positive sentinel chickens were widely dispersed throughout Central Florida, North Florida, and the Florida Panhandle, indicating that conditions favorable for the transmission of EEEV remain in place throughout most of the state.
 
The number of EEEV-positive horses reported in 2021 remains at 12. All 12 of the positive horses are widely dispersed in North Florida and the eastern Florida Panhandle. We are now at a time of year (June through August) when the transmission of EEEV to horses and humans is historically the highest.

There has never been a large human outbreak of EEE in Florida. In fact, the highest annual number of human EEE cases reported in the state is five. There were five human EEE cases reported in 1978, 1980, 1991, and 2005, and these cases were usually widely dispersed in three or more Florida counties. In general, high numbers of human EEE cases were usually reported during years when there were also high numbers of equine cases (see below).
Since 1955, there have been 12 years with three or more human EEE cases reported in Florida. Florida equine infections have been recorded and reported since 1982. In general, high human transmission years have mirrored high equine transmission years.
 
With only 12 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 is especially true for central 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 WNV transmission to equines and humans remains low in Florida.
 
Three (3) new WNV antibody-positive sentinel chickens were reported during Week 30, all in Lee County. So far during the 2021 Florida arboviral transmission season, 10 WNV antibody-positive sentinel chickens have been reported. This is well below what is normally reported in Florida by the first week in August. However, the number of positive sentinels reported each week seems to be picking up and it remains important to continue monitoring the transmission of WNV, especially in southwest Florida. The next threat of WNV amplification will begin during the fall avian migration that starts in late August.

Find a review of 2020 WNV activity in the archived Vol 1, Issue 1 report.
Dengue (DENV) Activity
There has been no indication thus far in 2021 of locally-acquired dengue infections in Florida, but five travel-related dengue cases have been reported in Broward (1), Hillsborough (1), Miami-Dade (1), and Palm Beach (2) Counties. The entire state of Florida has been unusually wet since the middle of June, favoring the reproduction and local dispersal of Aedes aegypti and Ae. albopictus. If dengue virus is introduced into Florida by an infected traveler in an area with high Ae. aegypti or Ae. albopictus populations, there is risk for local dengue transmission. Locally-acquired cases of DENV have been reported in October-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, for resulting locally-acquired cases in those areas during the remainder of 2021.

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 chickens were infected early in the year and there has been no additional indication of SLEV transmission anywhere in the state. As with WNV, fall migration has the potential of seeding SLEV into Florida from the north, and will provide a large number of susceptible birds that may serve as amplification hosts for 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.
Acknowledgements: 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. 
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