June 14, 2023 | Volume 3 | Issue 6 | As of Week 23

A QUESTION FROM OUR READERS

Q. Amelia from Vero Beach, FL, asks: “What is arboviral amplification?”

A. Mosquito-borne viruses are maintained in nature by cycling between vector mosquitoes and vertebrate hosts. Many viruses, such as West Nile and eastern equine encephalitis, cycle between mosquitoes and wild birds. Others, such as La Crosse virus, cycle between mosquitoes and small rodents such as chipmunks. Occasionally, environmental conditions conspire to favor the interaction of mosquitoes and susceptible vertebrate hosts in a way that results in large numbers of infected vertebrate hosts and vector mosquitoes (Avian Serology in a St. Louis Encephalitis Epicenter Before, During, and After a Widespread Epidemic in South Florida, USA | Journal of Medical Entomology | Oxford Academic). This process is referred to as arboviral amplification and is depicted in the cartoon below (Figure 1).

Efficient arboviral amplification is a cascade of viral transmission between susceptible vertebrate hosts (the Common Grackles in Figure 1) and competent mosquito vectors. In Figure 1, birds circulating enough virus to infect mosquitoes are colored red, birds susceptible to a new viral infection are colored black, and birds that are immune to infection due to recovery from a previous infection are colored blue. Likewise, mosquitoes susceptible to infection are colored black and infected mosquitoes capable of transmitting virus to a vertebrate host are colored red. It does not take many feeding cycles to end up with a large number of infected mosquitoes (column 5) and birds (column 6). This level of arboviral amplification results in widespread epidemics of human disease.

Figure 1. Arboviral amplification between avian hosts and vector mosquitoes.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Arboviral transmission continues at a slow pace in Florida. Very little transmission has been reported in the state during the first half of 2023 (Table 1). Recently, most of the Florida Peninsula has become extremely wet (Figure 2). Drying trends continue along the Florida Gulf Coast, the Western Panhandle, and a band in Central North Florida. How these rapidly-changing groundwater conditions will impact arboviral transmission in Florida remains to be seen. 

Figure 2. The Florida KBDI map showing surface wetness/dryness for June 12, 2023.

Imported cases of dengue continue to be reported at a slow rate in Florida. A rare case of locally-acquired malaria (Plasmodium vivax) in South Florida resulted in a Medical Advisory in Sarasota and Manatee counties. Public Health and Vector Control officials responded to a single malaria case in a Sarasota County resident who spent an excessive amount of time outdoors.  

Table 1 summarizes the current status of arboviral transmission in Florida.

Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of June 10, 2023

Recently imported dengue cases increase the risk of locally-acquired dengue transmission, and vector control personnel in the five Florida counties shaded red in Figure 2 should be alert to the possibility of locally-acquired dengue transmission in the coming weeks. Two locally-acquired dengue cases have been reported in Miami-Dade County so far in 2023. 

Dengue Viruses

Imported cases of dengue continue to be reported in Florida (Figure 3).

Figure 3. The spatial distribution of imported human dengue cases in Florida as of mid-June, 2023. Counties shaded red indicate sites of at least one imported case during the past three weeks.

Eighty-four travel-associated dengue cases have been reported in 17 Florida Counties as of Week 23 in 2023. As of Week 23 in 2022, 31 travel-associated dengue cases had been reported in Florida, meaning we are at more than double the pace of travel-associated cases so far in 2023.

Malaria

Twenty-two travel-associated malaria cases have been reported in ten Florida Counties so far in 2023, along with a single locally-acquired malaria case (see above). Locally-acquired malaria cases are extremely rare in Florida, and focal outbreaks of malaria transmission are even less common.

Eastern Equine Encephalitis Virus



North Central Florida remains the most active transmission foci for EEEV so far this year, although active transmission is picking up in the western Florida Panhandle (Figure 4), where a recent drying trend may support the transmission of EEEV in the coming six weeks (Figure 2). 

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of mid-June, 2023.

St. Louis Encephalitis Virus

There has been no evidence of SLEV transmission in Florida so far in 2023. It is possible that the SLEV has become extinct in Florida and will only reappear when re-introduced by fall migrants.

West Nile Virus

There has been little evidence of WNV transmission in Florida as of this report. Five WNV antibody-positive sentinel chickens have been reported in Hillsborough (two positives), Orange, Palm Beach, and Walton Counties thus far in 2023. One WNV-positive equine was reported in Hernando County in early January.

A summary of observed and expected numbers of travel-associated and locally-transmitted mosquito-borne diseases to date appears in Table 2.

Table 2. Summary of expected and observed mosquito-borne disease infections in Florida reported during 2023 (as of 6/10/23)

As indicated above, mosquito-borne disease transmission has had a very slow start in 2023, but the bulk of the transmission season is still in front of us. Recent rainfall patterns (Figure 2) continue to favor the transmission of EEEV in North Florida, and more recently in the western Florida Panhandle (Figure 4). July, August, and September are historically the heaviest arboviral transmission months, so watch this space.

Tables 1 and 2 summarize our current status of arboviral transmission and travel-associated cases. Table 2 compares our current standing with long-term expected values. The only genuinely active mosquito-borne virus is EEEV. West Nile virus is virtually non-existent in Florida and SLEV appears to be gone. It is currently too wet in most of the Florida Peninsula to support the transmission of WNV and SLEV. We must carefully watch rainfall conditions throughout Florida during the coming months.

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, compilation 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.

All of the graphics used in issues of this Newsletter are designed and developed by Gregory Ross.

The Newsletter is edited and distributed by Linda McDonagh.