August 16, 2023 | Volume 3 | Issue 13 | As of Week 32

A QUESTION FROM OUR READERS

Q. Steve from Lecanto, FL, asks: “I would like to know why we use only female mosquitoes for resistance testing; the products are not ingested, so why would the sex of the mosquito make a difference?”

A. According to Dr. Janet McAllister from the CDC in Ft. Collins, CO, it has to do with the size difference between male and female mosquitoes where females are significantly larger than males.

Dr. McAllister writes, “The male genome is important in contributing to the next generation. Male mosquitoes can be used for resistance testing if mosquito size does not influence assay results. In fact, there are documented cases in which resistance has been sex-linked in males. If males are never assayed, the sex-linked trait will be missed.

In traditional assays that rely on dose/mortality methods, the size of the mosquito significantly impacts the results in the same way that child vs adult drug doses are different. In these cases, even small differences in size between field‐collected mosquitoes and the comparator colony mosquitoes (or between males and females) yield significantly different results.

In the bottle assay (Figure 1), males track the response of females (unless sex-linked, where only males will survive) tested just 15 minutes earlier, so testing males is not an issue. Using males is also not an issue when looking for single-nucleotide polymorphism mutations. Males contribute resistance alleles just like the females and males must also have the mutation for there to be homozygous resistant individuals in the next generation.

Assays like microplates can also be used to test males, but in these tests, males should be compared to other males and not to females. These assays relate the size of the mosquito to the amount of enzyme present.

Historically, rather than bother with these nuances, researchers preferred to test just females because they are targeted for control. However, when getting enough mosquitoes to test for resistance is an issue, throwing away half the collection doesn’t make good sense and males should be tested.”

Figure 1. Set up of a typical bottle bioassay for insecticide resistance testing in mosquitoes. Photograph by Eva Buckner, University of Florida, Florida Medical Entomology Laboratory.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of August 14, 2023.

Dry areas are beginning to appear in Florida. The western Panhandle and the central Gulf Coast continue to report dry areas (Figure 2). The southern Florida Peninsula and most of north Florida continue to be wet. These surface conditions may continue to favor the transmission of diseases, such as eastern equine encephalitis in the Florida Panhandle and dengue in South Florida by supporting vector production and the dispersal of already infected mosquitoes.

The drying conditions in the Florida Panhandle and along the central Gulf Coast may favor the amplification of WNV and SLEV as migrant birds begin to move south in the coming weeks. 

Table 1 summarizes the status of vector-borne disease transmission in Florida so far in 2023.

Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of August 12, 2023

Dengue Viruses

Another locally-acquired dengue case was reported in Miami-Dade County last week bringing the total to 11 (nine in Miami-Dade County and two in Broward County). These infections occurred in January (1 case), March (1), June (3), and July (6), suggesting ongoing dengue transmission throughout 2023. 

Fourteen new travel-associated dengue cases were reported in Florida last week (Figure 3). These cases were widely distributed throughout the state, indicating that the level of travel-associated dengue introductions remains high in Florida.  

All four dengue serotypes are circulating in Florida, possibly increasing the risk of severe dengue infections later in the year. 

Figure 3. The spatial distribution of travel-associated human dengue cases in Florida as of August 12, 2023. The 19 Counties that are shaded red indicate areas of at least one travel associated dengue case reported during the past three weeks.

Malaria

The locally-acquired malaria case count in a small transmission focus in Sarasota County remains at seven. The earliest case was likely infected in mid-April, and the most recent case was likely infected in mid-June.

The outbreak appears to have been contained.

Eastern Equine Encephalitis Virus



Five sentinel chickens tested positive for antibodies to EEEV in Leon (two positive chickens), Orange (1), and St. Johns (2) Counties this week, bringing the total number of EEEV antibody-positive sentinel chickens reported in Florida this year to 103. 

St. Louis Encephalitis Virus

The first SLEV antibody-positive sentinel chicken of 2023 was reported this week in Walton County. 

West Nile Virus

A second human West Nile case was reported this week in Escambia County. This individual was likely bitten by a WNV-infected mosquito in mid-July. Four WNV antibody-positive sentinel chickens were reported last week in Bay (one positive sentinel), St. Johns (2), and Walton (1) Counties bringing the total number of WNV-positive sentinel chickens reported in Florida during 2023 to 17. The current focus of WNV transmission remains in the western Florida Panhandle.

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 8/12/23).

Transmission of EEEV in Florida seems to be slowing, while transmission of WNV in the western Florida Panhandle continues to be reported. 

Travel-associated dengue cases continue to be reported throughout Florida. Nineteen Florida counties have reported travel-associated dengue cases in the past three weeks, increasing the possibility of locally-acquired dengue outbreaks in those counties in the coming weeks (Figure 3)

The transmission of WNV and SLEV is currently well below normal in Florida (Tables 1 and 2). However, a recent human WNV case in Escambia County and continued reports of WNV antibody-positive sentinel chickens indicate infected mosquitoes transmitting WNV in the state. The first SLEV antibody-positive sentinel chicken of the year in Walton County indicates that SLEV is back in the state. 

New travel-associated malaria cases continue to be reported in Florida. Last week, new cases were reported in Lee (one case), Miami-Dade (2), and Sarasota (1) Counties. However, it appears that the outbreak of locally-acquired Plasmodium vivax in northern Sarasota County has been contained.  

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated vector-borne disease in Florida. Table 2 compares the 2023 data with long-term expected values. Both malaria and EEE show above-normal human activity. Transmission of SLEV and WNV is well below normal, but we know that WNV-infected mosquitoes are present and active in the Florida Panhandle. Even though the transmission of EEEV to sentinel chickens and equines is below normal, infected mosquitoes are present in the northern half of Florida, where a risk of transmission remains.  

Jonathan Day, Professor Emeritus of Medical Entomology at the University of Florida and stationed at the Florida Medical Entomology Laboratory in Vero Beach 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.