June 28, 2023 | Volume 3 | Issue 7 | As of Week 25

A QUESTION FROM OUR READERS

Q. Amelia from Vero Beach, FL, has a follow-up question: “What makes an excellent arboviral amplification host?”

A. A wide variety of vector-borne disease amplification hosts are found in nature. In Florida, the most common amplification hosts for the mosquito-borne encephalitis viruses endemic to the state are wild birds. Excellent amplification hosts fulfill at least three criteria.

First, amplification hosts must be susceptible to infection and circulate enough virus in their blood to infect additional mosquitoes. Some birds, such as American Crows, are highly susceptible to infection and circulate a large amount of the virus for a long period of time. Other species are less susceptible and circulate lower levels of the virus for shorter times.

Second, excellent amplification hosts are abundant and widespread. Species like Mourning Doves, Northern Cardinals, and Blue Jays are consistently abundant and widespread throughout most Florida habitats (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). Less common species, such as the Pileated Woodpecker (Figure 1), have larger home ranges and are less abundant than the avian species that serve as the primary amplification hosts.

Finally, excellent amplification hosts must be readily accessible to mosquitoes. Mourning Doves make very simple nests and hatchlings are easily fed on by mosquitoes from hatching to fledging. Even after they fledge, young birds roost in areas where they are easily found and fed on by vector mosquitoes.

Figure 1. A Pileated Woodpecker, resident of Indian River County, FL ca. 1991.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Florida KBDI map showing surface wetness as of June 26.

Florida has turned blue (Figure 2). 

This level of surface wetness has not supported arboviral amplification in the past, but will support the dispersal of infected mosquitoes in areas where amplification has already taken place. The impact of this dramatic shift in the KBDI for individual disease systems will be discussed in detail below.

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 24, 2023.

Dengue Viruses

Imported cases of dengue continue to be reported in Florida, with a big jump reported during week 25 (Figure 3).

Figure 3. The spatial distribution of imported human dengue cases in Florida as of June 24, 2023. Counties shaded red indicate areas of at least one imported case during the past three weeks.

One hundred travel-associated dengue cases have been reported in 19 Florida counties as of week 25 in 2023. As of week 25 in 2022, 38 travel-associated dengue cases had been reported in Florida. So far, in 2023, we have seen almost three times the travel-related dengue introductions in Florida compared to the same week in 2022. The extremely wet surface conditions in South Florida (Figure 2) favor the production and dispersal of Aedes aegypti. Vector control agencies in the six Florida counties shaded red in Figure 3 should be aware of possible locally-acquired dengue cases resulting from recent travel-related human dengue cases in their jurisdictions.

Malaria

There are now four locally-acquired Plasmodium vivax cases in Sarasota County. Very little is known about these cases. The most important question is the identity of the Anopheles vector.

There have been five locally-acquired malaria cases reported in Texas so far in 2023.

The Florida malaria transmission is likely a focal event. However, the extremely wet conditions in South Florida (Figure 2) may favor the production and dispersal of potential Anopheles vectors and the dispersal of already infected vector mosquitoes.

Eastern Equine Encephalitis Virus



The dry conditions in north central Florida from February through late May 2023 may have favored the amplification of EEEV. The dry conditions ended abruptly on June 19, and the extremely wet conditions (Figure 2) may now favor the dispersal and blood feeding of infected vector mosquitoes. The two areas of concern relative to EEEV transmission are North Central Florida and the western Florida Panhandle (Figure 4).

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of June 24, 2023.

The absence of sentinel chicken surveillance programs in the Florida Panhandle makes it difficult to monitor EEEV transmission in a large part of Florida. Based on what we can see, all of the Florida Panhandle is currently at risk for EEEV transmission in the coming months. 

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 very little evidence of WNV transmission in Florida as of this report. Six WNV antibody-positive sentinel chickens have been reported in Hillsborough (three 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 6/24/23).

The dramatic change in the ground surface wetness throughout all of Florida changes the potential transmission of mosquito-borne viruses and malaria everywhere in the state (Figure 2). The wet conditions will favor the production of Aedes aegypti in areas where travel-associated dengue cases have recently been reported (Figure 3).

Currently, the greatest concern of mosquito-borne disease transmission is EEEV in North Central Florida and the Florida Panhandle (Figure 4). Sentinel chicken surveillance data indicate that there was some amplification of EEEV earlier this year. It remains to be seen how the current surface water conditions will depress or enhance the distribution of mosquitoes already infected with EEEV.

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated cases. Table 2 compares the 2023 data 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 continued amplification and transmission of WNV and SLEV. 

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.