August 2, 2023 | Volume 3 | Issue 11 | As of Week 30

A QUESTION FROM OUR READERS

Q. Caroline from Bloomington, IL asks: “OK. I just read your Newsletter. Do I have this correct; malaria is spread by a mosquito biting an infected human? Do malaria-carrying mosquitoes then have offspring with malaria? Does this happen with any of the other diseases? If so, wouldn’t “small” outbreaks be more alarming?

A. What you are describing is transovarial transmission, also called vertical transmission. This is when a mosquito or tick blood feeds on an infected organism, picks up a pathogen, completes extrinsic incubation, becomes infected (including her ovaries), and passes the pathogen on to her offspring via her eggs. 

Arthropod vectors transmit nematode worms (dog heartworm), protozoa (malaria), bacteria (Lyme disease and Rocky Mountain spotted fever), and viruses (West Nile). Of these, only bacteria in ticks and viruses in mosquitoes are transmitted vertically.

In your area, the La Crosse encephalitis virus (LCEV) is transmitted by Aedes triseriatus (Figure 1). This virus overwinters in the cold and in drought-resistant eggs laid by infected female mosquitoes in the fall. Eggs hatch in the spring, and larvae emerging from infected eggs are infected with LCEV. These larvae go on to produce LCEV-infected adult female mosquitoes that can transmit the virus with their first blood meal.

Vertical transmission is less efficient than horizontal transmission (i.e., infected vertebrates pass a pathogen to blood-feeding mosquitoes that complete extrinsic incubation and can then infect new vertebrates through blood feeding). It is a good thing that vertical transmission is rare and inefficient. Otherwise, you are correct; small outbreaks could quickly get out of control.

Figure 1. Aedes triseriatus by James Gathany, USCDCP.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of July 31st, 2023

Most of Florida remains exceptionally wet except for the Gulf Coast from Pinellas County south to Naples (Figure 2). These saturated surface conditions may favor the transmission of some diseases, such as eastern equine encephalitis and dengue 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 late season amplification that will begin at the end of August as migrating birds start to move south. 

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 July 29, 2023

Dengue Viruses

One new locally-acquired dengue case was reported in Miami-Dade County last week, bringing the total number of locally-acquired cases there to six. Approximate infection dates for these six cases are early January, early March, early June, mid-June (two cases), and late June. It appears that local transmission has been ongoing in Miami-Dade County for all of 2023. Whether local transmission will take off in the next few weeks in Miami-Dade County remains to be seen. 

Eleven 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 introductions remains high in Florida.  

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 29, 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 additional malaria transmission within the transmission zone and it appears that this outbreak may have been contained. 

Eastern Equine Encephalitis Virus



Sentinel chickens positive for antibodies to EEEV continue to be reported in North Florida and the Florida Panhandle (Figure 4). One area of immediate concern regarding the transmission of EEEV in Florida is that 12 new EEEV antibody-positive sentinel chickens were reported in Nassau County last week. This level of EEEV transmission to sentinel chickens indicates that there are a lot of infected mosquitoes in the area and that the transmission risk of EEEV to humans is extremely high. 

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of July 29, 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 single WNV antibody-positive sentinel chicken was reported in Bay County last week. Transmission of WNV remains extremely low in Florida and the most likely place where amplification may occur is along the Gulf Coast (Figure 2).  

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 7/29/23)

Transmission of EEEV continues 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. Of particular concern is the recent spike of EEEV antibody-positive sentinel chickens in Nassau County (Figure 4), indicating the presence of many infected mosquitoes. Public health service announcements, additional targeted vector control, and heightened surveillance should be ongoing in Nassau County.  

The number of travel-associated dengue cases continues to increase in Florida (Figure 3), and the report of a new locally-acquired dengue case in Miami-Dade County indicates that at least one new transmission zone may be established there. Indications are that local transmission may have been ongoing for all of 2023 in Miami-Dade County. The fact that two dengue serotypes (2 and 3) are now circulating there 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 14 Florida counties during the past three weeks (Figure 3). The risk of establishing locally-acquire dengue transmission foci in these counties is high, and increased surveillance efforts should be undertaken in these counties.

The report of a WNV-positive blood donor in Escambia County who was infected in mid-July and a WNV antibody-positive sentinel in Bay County last week indicates that WNV-infected mosquitoes are present and active in the western Florinda Panhandle. However, WNV transmission is extremely low throughout all of Florida (Table 2).

Finally, it appears that the outbreak of human malaria in northern Sarasota County is close to over. Exceptional surveillance and enhanced vector control in the transmission zone may have kept this outbreak to a minimum.  

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, especially in Nassau County, and they appear to be actively transmitting EEEV.  

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.