October 12, 2021 | Week 39 | Volume 1 | Issue 13 |
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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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Alan from Vero Beach, FL asks: "Will the severe drought and wildfires in the western USA, especially California, affect the transmission of West Nile virus?"
A: Drought yes. Wildfire, probably not. As with SLEV, WNV has two primary transmission cycles: urban and rural. Drought affects both cycles by regulating Culex oviposition behavior. More about that next week. Historically, drought was a major factor driving both urban and rural SLE and WN epidemics including the Florida SLE epidemics in 1959, 1961, 1962, 1977, and 1990, and the Texas epidemics in 1965, 1966, 1975, and 1976. More recently, drought played a critical role in the 2012 Texas WN epidemic. Wildfires clear a lot of vegetation and probably kill a lot of adult mosquitoes. As of October 1 this year, 7,738 wildfires burned 2.4 million acres in California. Even with these impressive numbers, adult mosquitoes have an uncanny ability to find microhabitats where they can escape droughts, hurricanes, tornados, freezes, and probably wildfires.
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THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE |
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We seem to be entering a drying trend everywhere in Florida with the exception of the Panhandle. This, however, will likely be too late in the 2021 transmission season to impact arboviral transmission. However, rainfall patterns during this winter will set the stage for the 2022 transmission season that will begin in earnest in May. The current risk of a widespread and extensive arboviral outbreak in Florida is 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. Three other mosquito-borne diseases have the potential to cause human outbreaks in Florida. These are Chikungunya virus, Zika virus, and malaria. None of these have been active in Florida during the 2021 transmission season. |
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WHAT ARE WE CURRENTLY SEEING? |
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Eastern Equine Encephalitis (EEEV) Activity
Transmission of EEEV in Florida for the 2021 season continues to ramp down. A new clock for the 2022 EEEV transmission season will begin in November. Migrant and resident birds that encounter infected mosquitoes in Florida’s hardwood swamps over the winter will play a key role in determining the intensity of EEEV transmission in 2022.
No new EEEV antibody-positive sentinel chicken were reported during Week 39. The total number of EEEV antibody-positive sentinel chickens reported in Florida during 2021 is 174.
There were no new EEEV-positive horses reported in Florida this week. The total number of EEEV-positive horses reported during 2021 is 16.
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The transmission of EEEV to horses in 2021 was below average with only 16 positive horses reported (the average annual number of EEEV-positive horses reported in Florida is 60). However, the number of sentinel chicken seroconversions to EEEV reported in 2021 was above average (173 vs an annual average of 115). Because of the high number of seroconversions to EEEV in 2021 (indicating a large number of infected mosquitoes), we may continue to see sporadic transmission of EEEV in the northern half of the state. The area around Walton County and all of North Florida remain regions of special concern for potential sporadic human and equine EEEV transmission through the end of 2021.
Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
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West Nile Virus (WNV) Activity
The autumn WNV transmission surge appears to be in progress. Nine (9) new WNV antibody-positive sentinel chickens were reported this week: three in Bay County, one each in Citrus and Putnam Counties, and four in Walton County. Remember that there were 23 new WNV-positive sentinel chickens reported in Walton County last week. All of those chickens were replaced with naïve birds and it will take at least two weeks for the new birds to test positive, assuming that they were infected on their first night of exposure.
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As we enter Week 40, I expect that we may see a continued increase in the number of positive sentinel chickens, and the possibility of WNV transmission to equines and humans may increase. This is especially true for the Florida Panhandle where only three counties (Bay, Leon, and Walton) participate in sentinel chicken surveillance, and where there have been WNV seroconversions during the past couple of weeks. South Florida, where there have been reports of positive sentinel chickens, positive mosquito pools, and human cases, is also at risk for additional human and equine WN cases.
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 are starting to disappear. Populations of Aedes aegypti and Ae. albopictus continue to be high across the state, but the initiation of a local transmission cycle will not be possible in the absence of the virus. The introduction of DENV into Florida through infected travelers remains low. The total number of travel-related dengue cases in Florida during 2021 is eight, well below the annual (2010-2020) average of 103. A widespread and extensive outbreak of human dengue transmission in Florida is unlikely at this time. Locally-acquired human dengue cases reported next spring will be the first indication of a potential dengue outbreak in Florida.
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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
Indicators of SLEV transmission in Florida remain rare. So far in 2021, only two sentinel chickens (both in Palm Beach County in mid-April) have tested positive for antibodies to SLEV and a SLEV-positive mosquito pool was recently reported in Clay County. St. Louis encephalitis virus may no longer be endemic in Florida. Future transmission of this virus may be dependent on the reintroduction of SLEV into the state, probably in migrating birds.
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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 intense WNV transmission to sentinel chickens in South Florida and the Florida Panhandle indicates an increased risk of WNV to equines and humans during the next eight weeks in this region of Florida.
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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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