September 20, 2021 | Week 36 | Volume 1 | Issue 10 |
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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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Erin from Jacksonville asks: "Why do you refer to St. Louis encephalitis virus, eastern equine encephalitis virus, and West Nile virus as singular, but dengue viruses as plural?"
A: Many viruses exist as a single serotype. This is true for SLEV, EEEV, and WNV, although all three of these viruses have multiple strains with different levels of infectivity and pathogenicity. There are at least four genetically distinct serotypes of DENV, all of which can cause a full spectrum of diseases in humans. In addition, there are at least 47 known strains of DENVs responsible for at least 390 million clinical cases in humans each year. The four dengue serotypes (DEN-1 through DEN-4) cause extreme disease in individuals infected with multiple serotypes and the four serotypes have made the development of a dengue vaccine very challenging.
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THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE |
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The transmission of WNV in Florida started to heat up during Week 36 (see below). Most of Florida remains very wet, especially the Florida Panhandle. There continues to be a coastal drying trend from the Florida Keys to Miami and north to Ft. Pierce. There are also two large dry-downs in Florida, one around Miami and one around Jacksonville, and there is a smaller dry-down reported west of Orlando. 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 currently low.
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In Florida, four mosquito-borne arboviruses currently account for the greatest human disease risk; dengue viruses (DENVs), 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 (DENVs 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 continues to decline throughout Florida, which is what we typically expect during this time of year.
Two (2) new EEEV antibody-positive sentinel chicken were reported during Week 36, one in Nassau County and one in Volusia County, bringing the total number of EEEV antibody-positive sentinel chickens reported in Florida during 2021 to 171.
There were no new EEEV-positive horses reported in Florida during Week 36 and the number of EEEV-positive horses reported during 2021 remains at 15.
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The continued sporadic transmission of EEEV in the northern half of Florida indicates that the EEEV transmission season is not completely over. The areas around Walton County and all of North Florida remain regions of special concern for potential human and equine EEEV transmission.
Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
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West Nile Virus (WNV) Activity
As already noted, Week 36 was an active WNV transmission week. Seven (7) new WNV antibody-positive sentinel chickens were reported this week, all in Walton County, bringing the total number of positive sentinel chickens to 50 for the 2021 transmission season (see map).
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One (1) WNV-positive horse was reported in Lee County this week. The horse was likely infected during the last week in August. In addition, one (1) human WN case was reported in Miami-Dade County. This individual was likely infected in late June.
Collier Mosquito Control District relies, in part, on mosquito pooling for the detection of arboviruses within the County. In July and August, seven WNV-positive mosquito pools were detected at four sampling sites in Collier County. Three were in pools of Culex nigripalpus and four in pools of Cx. quinquefasciatus. It has become increasingly evident during the past five years that Cx. quinquefasciatus is an important vector of WNV in South Florida. This may explain the shift of WNV transmission to later in the fall during recent years in Florida.
The recent sentinel chicken, equine, mosquito pool, and human WNV transmission data indicate that there are two regions of concern for continued transmission of WNV in Florida; one in the Florida Panhandle in areas surrounding Walton County and one in the southern tip of the Florida Peninsula (see map).
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 the initiation of a local transmission cycle will not be possible in the absence of the virus. An additional travel-related case of dengue was reported this week in Hernando County, bringing the total number of travel-related dengue introductions into Florida during 2021 to six (6). 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, the fall avian migration has the potential to seed SLEV into Florida. 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.
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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) continue to be reported. These include: all North Florida Counties and all of the Panhandle Counties, especially those around Walton County. In addition, recent WNV transmission indicates two areas of concern for the potential transmission of WNV to equines and humans; the southern tip of Florida and counties in the central Florida Panhandle.
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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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