July 19, 2023 | Volume 3 | Issue 9 | As of Week 28

A QUESTION FROM OUR READERS

Q. Benjamin from Vero Beach, FL asks a follow-up question: “What makes an excellent disease vector?”

A. A wide variety of arthropods serve as disease vectors, including ticks, mites, kissing bugs, black flies, sand flies, tsetse flies, and mosquitoes. Not all arthropods are excellent vectors. For example, there are currently 21 species of Culex mosquitoes reported in Florida. Of these, only two (Culex nigripalpus and Cx. quinquefasciatus) are known to be important vectors of West Nile virus to equines and humans. 

A relatively new mosquito introduction to Florida is Culex coronator (Figure 1). We have no idea whether this species will be an important arboviral vector in the state sometime in the future. 

Excellent vectors fulfill at least three criteria.

First, they must be susceptible to viral infection. Most mosquito species are not susceptible to infection because they have barriers that prevent viruses from penetrating the mid-gut, infecting other organs, and, most importantly, infecting the salivary glands. In those rare species where viruses infect the salivary glands, many never encounter the right hosts for virus transmission.

Second, excellent vectors are abundant and widespread. Species like Cx. nigripalpus and Cx. quinquefasciatus are extremely abundant and widespread in Florida. Species like Cx. coronator are not abundant nor widespread in the state, at least not yet.

Finally, excellent vectors encounter and blood feed on the proper amplification and definitive hosts. For example, Aedes aegypti (Figure 2) is the primary vector of dengue viruses in Florida and throughout most of the world. The preferred host for this species is humans. Humans serve as the amplification hosts for the dengue viruses. Aedes aegypti preferentially feed on humans, acquire dengue virus, complete extrinsic incubation, and transmit dengue virus to additional humans during disease outbreaks.

Figure 1. Culex coronator—The crowner of Sovereigns. Photograph by Dr. Larry Reeves of the UF Florida Medical Entomology Laboratory.

Figure 2. Aedes aegypti—The Yellow Fever mosquito. Photograph by Dr. Larry Reeves of the UF Florida Medical Entomology Laboratory.

READER COMMENTS

Several comments regarding the malaria transmission in Sarasota County that was reviewed in Volume 3, Issue 8 of the Newsletter were received following publication.

Wade Brennan, Manager II of Sarasota County Mosquito Management, wrote to say that the distribution of human malaria cases listed in Issue 8 was incorrect. He pointed out that malaria transmission is in a 5.0 mi² area in North Sarasota County, north of Fruitville Rd. There has been no malaria transmission reported in Venice or Englewood.

Gary Goode, Director of the Palm Beach County Mosquito Control Division, wrote to revisit our experiences during the 2003 human malaria outbreak in Palm Beach County. He remembers that there were no Plasmodium vivax-positive mosquito pools collected during or following the outbreak, mainly because advanced PCR technology for malaria detection was not available at the time. Anopheles quadrimaculatus was high on the list of suspected vectors, but they were rare in the collections made by PBCMCD following the confirmation of eight locally-acquired human malaria infections. Anopheles crucians were collected, but not in great numbers.

Following the announcement of the outbreak, representatives from the FMEL, including Roxanne Connelly, Jonathan Day, and Walter Tabachnick, made a site visit/mosquito collecting trip into the malaria transmission zone. We visited a large homeless camp within the zone and collected mosquito larvae from the bottom of a deep relief canal. All of the larvae were Anopheles crucians. Conditions were very dry, and only small shaded pools were found in the canal. So the connection of An. crucians with the 2003 human malaria outbreak in Palm Beach County is little more than guilt by association.

Finally, Dr. John Edman wrote from Amherst, MA, with the following observations: “One Saturday in March in the 1990s when we were living in Hadley, MA, I was attacked INDOORS in broad DAYLIGHT by a female Anopheles mosquito that I collected and identified as An. crucians. I assumed it was an overwintering female that had recently emerged from “hibernation” and was looking for its first blood meal to initiate a new reproductive season for this outdoor, night‐biting species (as described in the literature). My point being...don’t assume all mosquitoes have read the literature and behave accordingly. In my experience there are a lot more crucians in Florida than quadrimaculatus.”

Gary Goode ends with a similar observation. “While traveling back from Daytona one Thanksgiving, I stopped at a rest area and encountered an Anopheles quadrimaculatus attempting to blood feed in the restroom. The temperature was 47^o F!”

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 3. The Keetch-Byram Drought Index for Florida as of July 17^th, 2023

Florida remains extremely wet (Figure 3). These wet surface conditions may favor transmission of some diseases such as eastern equine encephalitis, dengue, and malaria by supporting vector production and the dispersal of already infected mosquitoes. However, the wet conditions may work against West Nile and St. Louis encephalitis viruses by reducing amplification. 

Table 1 summarizes the current status of vector-borne disease transmission in Florida.

Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of July 15, 2023

Dengue Viruses

A new locally-acquired human dengue case was reported this week in Miami-Dade County, bringing the total number of humans infected in 2023 to three. The individual was likely infected in early June.

The rate of new travel-associated dengue cases reported in Florida continues at a steady pace, although there was a big jump this week (Figure 4).

Figure 4. The spatial distribution of travel-associated human dengue cases in Florida as of July 15, 2023. Counties shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

One hundred and eighteen travel-associated dengue cases have been reported in 20 Florida counties so far in 2023. A large jump of 16 travel-related cases was reported last week. Approximately 57% of the cases reported so far in Florida are serotype 3, and about 65% of the travel-related cases originated in Cuba. The extremely wet conditions throughout all of Peninsular Florida favor the production and dispersal of Aedes aegypti (Figure 2), the primary dengue vector. Vector control agencies in the eight Florida counties shaded red in Figure 4 should be aware of possible locally-acquired dengue outbreaks resulting from recent travel-associated human dengue cases in their jurisdictions.

Malaria

A seventh human Plasmodium vivax case was reported in Sarasota County this week. The individual was likely infected in late June or early July. The case was located within the same transmission zone as the other six. See the Reader Comments above for additional information about the Sarasota transmission zone and the likely mosquito vector. 

Eastern Equine Encephalitis Virus



A human EEE case was reported this week in St. Johns County. The individual was likely infected in early June. Transmission of EEEV to sentinel chickens continues to be reported in North Florida and in the Florida Panhandle (Figure 5). July is the peak transmission month for equine and human EEE transmission. Considering the reporting delay of equine and human cases, we are still in a period of potentially high transmission of EEEV. 

Figure 5. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of July 15, 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 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 7/15/23)

This has been a very unusual year relative to vector-borne disease transmission in Florida. I do not recall ever seeing a year as wet as 2023. The unusually wet conditions throughout Florida currently favor the transmission of EEEV, malaria, and dengue by increasing vector populations and the dispersal of already infected mosquitoes. The saturated conditions work against the transmission of SLEV, and WNV by reducing or eliminating amplification. Dry conditions are currently developing from Hillsborough south to Lee Counties along the Florida Gulf Coast. It remains to be seen whether significant amplification of WNV can occur prior to the beginning of fall migration in late August. 

Currently, the greatest concern of mosquito-borne disease transmission is EEEV in North Central Florida and the Florida Panhandle (Figure 5). Groundwater conditions favoring the amplification of EEEV occurred in this region during the late spring. A steady stream of sentinel chicken EEE seroconversions has been reported in the Florida Panhandle and in North Florida. Obviously, there are infected mosquitoes in these regions of Florida, and the wet conditions are certainly favoring their dispersal. Additional equine and human cases are possible during the coming weeks.

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated disease cases in Florida. Table 2 compares the 2023 data with long-term expected values. Both malaria and EEE are showing above-normal human activity. Transmission of SLEV and WNV is well below normal. Even though the transmission of EEEV to sentinel chickens and equines is below normal, infected mosquitoes are present in the northern half of Florida and they appear to be on the move. 

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.