October 25, 2021 | Week 41 | Volume 1 | Issue 15
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.
A QUESTION FROM READERS
This week we're answering Part 2 of last week's question about West Nile Virus transmission following periods of drought, posed by Kim from Kissimmee, Florida. “Why are Culex mosquitoes the primary vectors of WNV around the world?”
 
A: When an arbovirus is acquired by a blood-feeding mosquito from an infected host, the virus must complete an extrinsic incubation period (EIP). During the EIP, virtually all of the mosquito’s tissues, including her salivary glands, are infected. The EIP is temperature-sensitive (high temperatures result in more rapid completion of the EIP) and is completed one to three weeks after the infective blood meal. After completion of the EIP, the mosquito can transmit the virus to additional hosts for the remainder of her life.

In terms of mosquito species, this is where the story gets more interesting. The burden of getting a mosquito in the genus Aedes or Psorophora from one generation to the next falls on the egg. These mosquitoes lay drought-resistant eggs that can survive for years before hatching. Females in these genera lay eggs as quickly as they can after digesting a blood meal, and the adult females have short lives, usually less than three weeks, which is typically not long enough to complete the EIP and infect additional hosts. Alternatively, the burden of getting Culex mosquitoes from one generation to the next falls on the female. She lays eggs on the surface of the water and these eggs hatch immediately. Gravid Culex females will wait for weeks to choose an appropriate larval habitat. This delay in oviposition gives infected mosquitoes plenty of time to complete the EIP, and in turn, transmit the virus.
THE WEEK IN REVIEW:
ARBOVIRAL TRANSMISSION IN FLORIDA
Some low-level indicators of active arboviral transmission continued to appear in Florida during Week 41 of 2021. For EEEV, one new sentinel chicken seroconversion was reported. For WNV, 13 new sentinel chicken seroconversions were reported. There were no new indications of SLEV or DENV transmission in Florida during Week 41. West Nile is the only arbovirus that currently threatens humans in Florida.
WHAT ARE WE CURRENTLY SEEING?
Eastern Equine Encephalitis (EEEV) Activity
A single EEEV antibody-positive sentinel chicken was reported this week in Brevard County, bringing the number of EEEV seroconversions in Florida during 2021 to 176, which is above the 11-year (2010-2020) average seroconversion rate of 101 per year. However, the risk of a significant Florida EEE outbreak in humans or equines is currently low.
Find a review of 2020 EEEV activity in the archived Vol 1, Issue 1 report.
West Nile Virus (WNV) Activity
Thirteen (13) new WNV antibody-positive sentinel chickens were reported this week in Charlotte (1), Hillsborough (1), Lee (1), Manatee (1), St. Johns (3), and Walton (6) Counties, bringing the total number of WNV antibody-positive sentinel chickens reported in Florida during 2021 to 109. Based on sentinel chicken seroconversion data, the Florida Panhandle remains an area of concern for potential human WNV transmission. However, the historical peak for human WNV infections in Florida is July, August, and September. The current distribution of the 2021 sentinel chicken seroconversions to WNV appears in Figure 1.

Figure 1. The distribution of sentinel chicken seroconversions to
WNV as of Week 41 (October 16, 2021).
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.
Dengue (DENV) Activity
There was no change in transmission of DENVs in Florida during week 41 of 2021.
Find a review of 2020 DENV activity in the archived Vol 1, Issue 1 report. 
St. Louis Encephalitis Virus (SLEV) Activity
There was no change in transmission of SLEV in Florida during week 41 of 2021.
OPERATIONAL STRATEGIES TO CONSIDER
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.   
ABOUT DR. JONATHAN DAY

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.
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.
Presented by
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.