September 27, 2021 | Week 37 | Volume 1 | Issue 11 |
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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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Miranda from Chicago asks: What do you mean when you say, “a sentinel chicken has seroconverted to West Nile virus?”
A: When a sentinel chicken that has never been exposed to WNV is bitten by an infected mosquito, particles of virus (virions) are introduced into the chicken’s circulatory system through the mosquito bite. The chicken’s immune system immediately responds by producing antibodies that fight the WN virions. Antibody levels increase over time and in about two weeks antibody becomes detectible in a serum sample. At this point, the chicken is said to have seroconverted to WNV. We can tell from the antibody-positive serum sample that the chicken was bitten by an infected mosquito. More about specific types of antibodies next week. BTW, a huge advantage of sentinel chickens is that the virions do not replicate in the chicken’s circulatory system, so sentinel chickens cannot infect mosquitoes and do not increase the number of infected mosquitoes in the environment.
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THE 2021 FLORIDA ARBOVIRAL TRANSMISSION SEASON TO DATE |
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Week 37 was very quiet relative to arboviral transmission in Florida, and it was an extremely wet week relative to rainfall throughout the state. Almost all of the dry-downs reported in previous weeks have disappeared with the exception of the coastal drying trend from the Florida Keys to Miami and north to Ft. Pierce and a small dry-down west of Jacksonville. The current risk of a widespread and extensive outbreak of WNV 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.
Two (2) new EEEV antibody-positive sentinel chicken were reported during Week 37, one in Brevard County and one in Walton County, bringing the total number of EEEV antibody-positive sentinel chickens reported in Florida during 2021 to 173.
There were no new EEEV-positive horses reported in Florida during Week 37 and the number of EEEV-positive horses reported during 2021 remains at 15.
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The transmission of EEEV to horses in 2021 was below average with only 15 reported cases (the annual average number of positive horses is 60). The number of 2021 sentinel chicken seroconversions to EEEV was above average (173 vs an annual average of 115) (see Table 1 below). Because of the high number of seroconversions to EEEV in 2021, 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 transmission of WNV to sentinel chickens in Florida remains below average through Week 37. Six (6) new WNV antibody-positive sentinel chickens were reported this week, two in Bay County, one in Nassau County, and three in Lee County. This brings the total number of positive sentinel chickens to 56 for the 2021 transmission season, a number that is way below the average 12-year seroconversion rate of 413 positive sentinels per year (see Table 1). But we still have more than three months left in the 2021 transmission season and October and November have been high transmission months during recent years.
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There were no new WNV-positive horses and no new human WN cases reported this week. It is evident from Table 1 that a high number of human WN cases is often accompanied by a high number of WN-positive sentinel chickens (i.e., 2012, 2018, and 2020).
Even though there has been an increase in recent indications of WNV transmission in Florida (sentinel chickens, one horse, one human, and positive mosquito pools in Collier County), the below-average number of sentinel chicken WN seroconversions (59) suggest that the risk of a widespread outbreak of WN in humans and equines is currently low. However, in recent years WNV transmission has increased dramatically in October and November, so areas around Lee County in the Florida Peninsula and Walton County in the Florida Panhandle should remain vigilant.
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. As of Week 37, seven travel-related dengue cases have been reported, 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 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. 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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