October 18, 2021 | Week 40 | Volume 1 | Issue 14 |
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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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Kim from Kissimmee, Florida asks a question we'll tackle in two parts over this issue and the next. Part 1: "Why are droughts often associated with St. Louis Encephalitis (SLE) and West Nile (WN) epidemics"
A: As discussed last week, drought has frequently been linked to epidemics of SLE and WN. In some cases, the drought preceding the epidemic was severe. A very telling example is the 1961 Florida SLE epidemic. Between October 3 and December 17, 1961, 25 clinical cases and 7 deaths due to SLE infection were reported in Pinellas, Manatee, and Sarasota Counties. That same year, mosquito control efforts in all three counties had been discontinued in August due to extremely dry conditions and the absence of mosquito biting complaints. So, why are early season droughts an important precursor of future SLE or WN epidemics? One very important reason is that drought forces Culex mosquitoes and avian amplification hosts together in limited freshwater habitats where they interact and, if the virus is present in the mosquitoes or in the birds, a focused arboviral amplification is the result. Why Culex mosquitoes? Tune in next week for Part 2 of this question.
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THE WEEK IN REVIEW:
ARBOVIRAL TRANSMISSION IN FLORIDA
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Low-level indicators of active arboviral transmission continued to appear in Florida during Week 40 of 2021. For WNV, two new human cases, one new WNV-positive horse, and five new WNV antibody-positive sentinel chickens were reported. One EEEV antibody-positive sentinel chicken, one SLEV antibody-positive sentinel chicken, and one travel-related human dengue case were also reported. West Nile is the only arbovirus that currently threatens humans in Florida.
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WHAT ARE WE CURRENTLY SEEING? |
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Eastern Equine Encephalitis (EEEV) Activity
A single EEEV antibody-positive sentinel chicken was reported this week in Walton County, bringing the 2021 number of EEEV seroconversions in Florida to 175. This is above the 11 year (2010-2020) average EEEV seroconversion rate of 101 per year, The current distribution of the 2021 sentinel chicken seroconversions to EEEV appears in Figure 1.
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Figure 1. The distribution of sentinel chicken seroconversion to EEEV as of week 40 (October 9, 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
Two new human WN cases were reported this week, both in Collier County, bringing the total number of human WN cases in Florida to four (three in Collier County and one in Miami-Dade County). The onset dates for the two new cases were August 17 and 22, indicating that these individuals were likely infected with WNV during the first two weeks in August.
A second WNV-positive horse was reported in Florida this week in Suwannee County. Symptom onset was October 3, suggesting that the horse was infected during the last week of September.
Five (5) new WNV antibody-positive sentinel chickens were reported this week in Charlotte, Duval, Lee, Sarasota, and Walton Counties. The current distribution of the 2021 sentinel chicken seroconversions to WNV appears in Figure 2.
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Figure 2. The distribution of sentinel chicken seroconversions to WNV as of Week 40 (October 9, 2021).
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The total number of 2021 WNV seroconversions currently stands at 96, well below the 11-year average (2010-2020) seroconversion rate of 408 per year.
Find a review of 2020 WNV activity in the archived Vol 1, Issue 1 report.
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Dengue (DENV) Activity
One new travel-related case of dengue was reported in Miami-Dade County this week bringing the total number of travel-related dengue cases in Florida during 2021 to nine.
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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
One new sentinel chicken seroconversion to SLEV was reported in Walton County this week. This is the first seroconversion since the two reported in Palm Beach County last April. The Walton County seroconversion, along with the Clay County SLEV isolation from a mosquito pool, suggests that SLEV is on the move in Florida, probably in migrating birds. It remains to be seen whether SLEV can become reestablished in Florida prior to the 2022 transmission season.
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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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