October 4, 2021 | Week 38 | Volume 1 | Issue 12 |
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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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Betty from Ft. Collins, Colorado asks: Will you explain how antibody tests are used to identify viruses and determine how recently an infection occurred?
A: As discussed last week, viral infection causes a defensive production of antibodies by the infected host. Antibodies can be detected in serum samples and are used to identify viruses and determine how recently the infection occurred. In Florida, hemagglutination inhibition (HAI) assays are used to screen sentinel chicken sera for Flavivirus (SLEV and WNV) and Alphavirus (EEEV and HJV) antibodies. Once a positive sentinel chicken is identified by HAI assay, an enzyme-linked immunosorbent assay (ELISA) is used to identify viral-specific Immunoglobulin M (IgM) antibodies that separate SLEV from WNV, and EEEV from HJV. IgM is the first antibody that appears in response to a viral challenge and indicates a recent infection, but the IgM antibodies are relatively short-lived. The final step in the virus confirmation process is to run a Plaque Reduction Neutralization Test (PRNT), the Gold Standard of viral identification, to confirm the presence of virus-specific neutralizing antibodies.
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THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE |
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Florida remains a vast inland sea and West Nile virus appears to be the only player left in the game as of Week 38. The only two dry-downs left in the state are west of Jacksonville and northeast of Lakeland. The current risk of a widespread and extensive outbreak of EEEV or SLEV 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. 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
Transmission of EEEV in Florida for the 2021 season seems to be coming to an end. The clock for the 2022 EEEV transmission season will begin in November. Migrant and resident birds that encounter mosquitoes in Florida’s hardwood swamps over the winter will play a large role in determining the intensity of EEEV transmission in 2022.
One (1) new EEEV antibody-positive sentinel chicken was reported during Week 38 in St. Johns County, bringing the total number of EEEV antibody-positive sentinel chickens reported in Florida during 2021 to 174.
There was one new EEEV-positive horse reported this week in Bradford County. The onset date for this horse was September 18 and the horse was likely infected in mid-September. The total number of EEEV-positive horses reported during 2021 in now 16.
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The transmission of EEEV to horses in 2021 was below average with only 16 reported cases (the annual average number of positive horses is 60). However, the number of 2021 sentinel chicken seroconversions to EEEV 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 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 surge in WNV transmission seems to be starting. Twenty-six (26) new WNV antibody-positive sentinel chickens were reported this week, one each in Bay, Duval, and St. Lucie Counties, and twenty-three (23) in Walton County. During the past 10 years (2011-2020) there has been a noticeable increase in the number of WNV antibody-positive sentinel chickens reported between Week 36 (early September) and Week 48 (early December) (Figure 1).
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Figure 1. Distribution of WNV antibody-positive sentinel chickens in Florida between Week 36 (early September) and Week 48 (early December) from 2011 through 2020.
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As we enter Week 39, 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 continue to 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 a large number of seroconversions in the past couple of weeks.
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. The introduction of DENV into Florida through infected travelers remains low. A new travel-related case was reported in Miami-Dade County during Week 38, bringing the total number of travel-related dengue cases in Florida during 2021 to 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.
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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. Future transmission of this virus may be dependent on its reintroduction into the state and, as if on cue, a SLEV-positive mosquito pool (Culex quinquefasciatus) was reported in Clay County this week. I will bet the farm (well, my grandparent’s farm) that SLEV moved into North Florida with 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 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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