July 26, 2023 | Volume 3 | Issue 10 | As of Week 29

A QUESTION FROM OUR READERS

Q. Shirley from Reading, MA, asks: “Why do mosquito bites itch?”

A. As a female mosquito begins to blood feed (Figure 1), she uses her mouthparts to make a small incision in the skin through which she inserts her proboscis, the feeding tube. The first thing she does is introduce a small amount of saliva into the wound. If the mosquito is infected with a pathogen, it is carried in the saliva and introduced into the circulatory system of the organism on which she is feeding. Mosquito-transmitted pathogens include viruses (like West Nile virus), protozoans (like malaria), or nematode worms (like dog heartworm). The saliva also contains antigens that your immune system sees as foreign, and an immediate immune reaction is mounted at the site of the bite, including swelling, itching, and a raised area around the bite.

Most people are desensitized to the bites of specific mosquito species over time, and the reaction to individual bites is minimized. There are approximately 3,600 mosquito species worldwide, so if you travel and encounter the bite of a new species, you will likely have a localized reaction. You may also react to the bite if you have gone many years without being bitten by a local mosquito species.

Some people are sensitized to mosquito bites and continue to have increasingly bad reactions to them. Young children tend to react violently to mosquito bites because their immune systems have not been desensitized to the bites. Fortunately, they usually outgrow this immune reaction.

Figure 1. Aedes aegypti—The Yellow Fever mosquito, preparing to take a blood meal. Photograph by Dr. Larry Reeves of the UF Florida Medical Entomology Laboratory.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of July 24^th, 2023

Most of Florida remains extremely wet except the coastal region around Sarasota County (Figure 2). These wet surface conditions may favor the 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 22, 2023

Dengue Viruses

Two new locally-acquired dengue cases were reported in Miami-Dade County last week, bringing the total number of locally-acquired cases there to five. These individuals were likely infected in late June and indicate that there is now at least one focus of ongoing dengue transmission in Miami-Dade County.

Eighteen new travel-associated dengue cases were reported in Florida last week (Figure 3). These cases were widely distributed throughout the state and indicate that the level of travel-associated dengue remains extremely high.

A note of concern is that two dengue serotypes (2 and 3) are now reported as locally-acquired cases in Florida. This increases the risk of severe dengue infections later in the year. Most of the travel-associated cases have originated in Cuba, but some cases have also been reported from Brazil and Peru. 

Figure 3. The spatial distribution of travel-associated human dengue cases in Florida as of July 22, 2023. Counties 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.

There has been no recent indication of malaria transmission within the transmission zone. 

Eastern Equine Encephalitis Virus



Sentinel chickens positive for antibodies to EEEV continue to be reported in North Florida and in the Florida Panhandle (Figure 4).

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of July 22, 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

A WNV-positive blood donor was reported in Escambia County this week. This individual was likely infected in mid-July, which indicates that mosquitoes infected with WNV are currently active in the 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 7/22/23)

Low levels of EEEV transmission continue to be reported in North Florida and in the Florida Panhandle (Figure 4), where it remains extremely wet (Figure 2). The risk of EEEV transmission to humans and equines in these regions of Florida will remain high for the next month.

The number of travel-associated dengue cases continues to increase in Florida (Figure 3), and the report of new locally-acquired dengue cases in Miami-Dade County indicates that at least one new transmission zone may have been recently established there. The fact that two dengue serotypes (2 and 3) are now circulating in Miami-Dade County is concerning because this increases the risk of severe dengue inflictions later in the year. Another concerning factor is that travel-associated dengue cases have been reported in 12 Florida counties during the past three weeks. The risk of the establishment of locally-acquire dengue transmission foci in these counties is high.

The report of a WNV-positive blood donor in Escambia County who was infected in mid-July indicates that WNV-infected mosquitoes are present and active in the western Florinda Panhandle.

Finally, it appears that malaria transmission in northern Sarasota County has slowed. It is possible that Plasmodium vivax-infected mosquitoes are still active in the area, so residents and visitors should continue to practice mosquito avoidance for the next couple of weeks. 

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 are showing 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, and they appear to be active.  

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.