September 7, 2021 | Week 34 | Volume 1 | Issue 8 |
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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 the public from vector-borne diseases.
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Roxanne from Ft. Collins, Colorado asks: What happened to the 20 year St. Louis encephalitis virus outbreak cycle in Florida?
A: The 20-year cycle of SLEV epidemics and outbreaks in Florida is actually a myth. The first case of human SLE in Florida was reported in Miami in 1952, coincident with an SLE outbreak in Jamaica. My guess is that SLEV first arrived in Florida aboard a migrating bird. Table 1 (below) provides a summary of the 69-year history of SLEV in Florida.
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It is evident from Table 1 that the average time between human outbreaks is 3.4 years. The average number of years between SLE epidemics (1959, 1961, 1962, 1977, and 1990) is 6.4 years. There has not been a case of human SLE reported in Florida since 2014, and one wonders whether Florida residents will ever again be at risk for an SLE outbreak or epidemic. In the interest of full disclosure, I am probably the one who started the 20 year SLE outbreak/epidemic myth.
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THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE |
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Not much has changed this week relative to groundwater saturation. There continues to be a coastal drying trend from the Florida Keys to Miami and north to Ft. Pierce. There is also a central Florida drying trend from Osceola County west to Hernando County. And there is a small dry down west of Jacksonville. The western Panhandle was completely saturated last week by the remnants of Hurricane Ida. These dry downs may favor the late-season amplification of WNV and SLEV in migrant birds, but the risk of a widespread outbreak of WNV or SLEV in Florida is rapidly disappearing. |
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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). Human outbreaks caused by these arboviruses typically occur in early summer (DENV and EEEV) or late summer/early fall (WNV and SLEV), so the window for a large Florida outbreak caused by any one of these viruses is closing. The current Florida transmission risk for each of these viruses is reviewed below. |
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WHAT ARE WE CURRENTLY SEEING? |
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Eastern Equine Encephalitis (EEEV) Activity
The transmission of EEEV is ramping down, which is what we typically see during this time of year. However, there has been a recent delay in the reporting of sentinel chicken results, so we may see a spike of positives next week as the data catch up.
Two (2) new EEEV antibody-positive sentinel chickens were reported during week 34, both in Orange County, bringing the 2021 total number of EEEV antibody-positive sentinel chickens to 168.
One (1) new EEEV-positive horse was reported this week in Santa Rosa County in the western Florida Panhandle. The onset date was 8/1/21, which means that the horse was likely infected during the last week of July. The total number of EEEV-positive horses reported in Florida during 2021 is now 15, well below the annual average (1982 to 2021) of 60 positive horses. The risk of a widespread epizootic in Florida horses for the remainder of 2021 is low, but the sporadic transmission of EEEV to horses remains a possibility.
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The continued sporadic transmission of EEEV in the northern half of Florida indicates that the EEEV transmission season is not over. The area around Walton County remains a region of special concern for potential human EEE cases and additional transmission of EEEV to humans and equines.
Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
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West Nile Virus (WNV) Activity
Twenty-two (22) new WNV antibody-positive sentinel chickens were reported this week. This is not as bad as it looks because these results include two weeks of reporting due to a Flavivirus HIA screening failure on 8/20/21.
However, it is evident that transmission of WNV in Florida is picking up. The new positive sentinel chickens were reported in Bay (4), Lee (13), Nassau (1), Sarasota (1), and Walton (7) Counties. The current distribution of WNV antibody-positive sentinel chickens is indicated by the Figure below.
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Major transmission of WNV is currently focused along the west coast of the Florida Peninsula and in the central Florida Panhandle.
Find a review of 2020 WNV activity in the archived Vol 1, Issue 1 report.
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Dengue (DENV) Activity
The wet conditions in south Florida continue to favor the local production and dispersal of Aedes aegypti and Ae. albopictus, but it is difficult to initiate a transmission cycle in the absence of the pathogen. A widespread and extensive outbreak of human dengue transmission in Florida is unlikely at this time.
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Find a review of 2020 DENV activity in the archived Vol 1, Issue 1 report.
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St. Louis Encephalitis Virus (SLEV) Activity
The transmission of SLEV in Florida remains extremely low. So far in 2021, only two sentinel chickens (both in Palm Beach County in mid-April) have tested positive for antibodies to SLEV. St. Louis encephalitis virus may no longer be endemic in Florida and the future transmission of this virus may be dependent on its reintroduction into the state. As with WNV, fall 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. I saw my first Barn Swallow this week, so fall migration has officially begun for me.
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OPERATIONAL STRATEGIES TO CONSIDER |
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Vector and arbovirus surveillance remains one of the most important tools that vector control agencies currently have at their disposal. Arboviral transmission indices (sentinel chickens, positive equines, positive exotics (i.e., 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 provides added information about potential transmission risk. Additional vector control efforts in and around sites where virus transmission is known or suspected of recently occurring 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, especially those around Walton County.
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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.
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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.
All of the graphics used in this Newsletter were designed and developed by Gregory Ross.
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Clarke has been helping make communities more livable, safe and comfortable since 1946.
Learn more about our work in protecting public health on clarke.com.
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