July 12, 2023 | Volume 3 | Issue 8 | As of Week 27

A QUESTION FROM OUR READERS

Q. Benjamin from Vero Beach, FL asks: “What is a disease vector?”

A. Many diseases are transmitted by arthropods (Ancient Greek for joint-footed). For example, Rocky Mountain spotted fever is transmitted by ticks, plague is transmitted by fleas, and river blindness is transmitted by blackflies. Mosquitoes transmit many diseases to humans, domestic animals, and wildlife. The arthropods that transmit these diseases are referred to as disease vectors.

To serve as a disease vector, a susceptible arthropod must first acquire a pathogen (virus, filarial worm (dog heartworm), or protozoan (malaria parasite)) by feeding on an infected host. Once acquired, the pathogen must infect the vector in a process known as extrinsic incubation, which may take up to several weeks. Extrinsic incubation ends when the vector’s salivary glands are infected. Once the vector has a salivary gland infection, it can transmit the pathogen with every subsequent blood meal.

Some disease transmission cycles are relatively simple. For example, Anopheles quadrimaculatus (Figure 1) was the major vector of human malaria in Florida up until 1949. Female mosquitoes must acquire malaria parasites from an infected human, complete an infection cycle, attain a salivary gland infection, feed on a susceptible human, and transmit the malaria parasite.

Some disease transmission cycles are more complicated because they involve a separate amplification host cycle (see answers to Reader Questions in Volume 3, Issue 6 and Volume 3, Issue 7). Culex nigripalpus (Figure 2) is an important vector of St. Louis encephalitis virus (SLEV) in Florida. Female mosquitoes first acquire the SLEV by feeding on an infective amplification host such as a Blue Jay. The mosquitoes must then complete extrinsic incubation and attain a salivary gland infection. Once the salivary glands are infected with SLEV, the infected mosquitoes will transmit SLEV every time they probe or blood feed, be it on another bird or a human.

Figure 1. Anopheles quadrimaculatus, the epidemic human malaria vector in Florida until 1949. Photograph by Larry Reeves from the Florida Medical Entomology Laboratory.

Figure 2. Culex nigripalpus, an important vector of St. Louis encephalitis virus in Florida sugar feeding on a mango flower. Photograph by Larry Reeves from the Florida Medical Entomology Laboratory.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Florida remains extremely wet. The wet surface conditions may favor transmission of some diseases such as eastern equine encephalitis and malaria but may work against others such as West Nile and St. Louis encephalitis. 

By far, the most interesting disease transmission event reported so far in 2023 has been the locally-acquired human malaria in Sarasota County (see below). Transmission of the encephalitis viruses (EEEV, HJV, SLEV, and WNV) has been slow to start in the case of HJV, SLEV, and WNV and struggled in the case of EEEV.

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 8, 2023

Dengue Viruses

The rate of new travel-associated dengue cases reported in Florida has slowed (Figure 3). 

Figure 3. The spatial distribution of imported human dengue cases in Florida as of July 8, 2023. Counties shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks.

One hundred and two travel-associated dengue cases have been reported in 19 Florida counties as of week 27 in 2023. As of week 27 in 2022, 43 travel-associated dengue cases had been reported in Florida. So far, in 2023, we have more than doubled the number of travel-associated dengue cases reported in 2022 but we have yet to see a significant number of locally-acquired dengue cases in the state. However, the extremely wet conditions in all of South Florida favors the production and dispersal of Aedes aegypti, the primary dengue vector. Vector control agencies in the four Florida counties shaded red in Figure 3 should be aware of possible locally-acquired dengue cases resulting from recent travel-associated human dengue cases in their jurisdictions.

Malaria

Six locally-acquired human cases of Plasmodium vivax have been reported from a focal transmission zone in Sarasota County. From published reports, this is what is known about the outbreak.

All six of the infections have been caused by Plasmodium vivax, one of the five known species of human malaria. Plasmodium vivax is a protozoan and is the most common and widely distributed human malaria parasite. It causes recurring infections in humans. It is less virulent than Plasmodium falciparum, but can cause severe disease and death in infected humans.

The transmission zone is relatively limited. Sarasota County Mosquito Management indicated that elevated mosquito control efforts were taking place near the coast in Venice and Englewood, between University Boulevard and 17th Street east of Interstate 75, and north Sarasota between Bahia Vista Street and Fruitville Road. The malaria transmission focus appears to be approximately a 5 mi² area. Most of the infections appear to have occurred in early June.

The most interesting revelation in the past week is that three pools of Anopheles crucians have tested positive at the CDC in Atlanta for Plasmodium vivax. Anopheles crucians was the suspected vector in the 2003 Palm Beach outbreak of locally-acquired malaria. Anopheles crucians is a complex of at least six species (https://mosquito-taxonomic-inventory.myspecies.info/sites/mosquito-taxonomic-inventory.info/files/Wilkerson%20et%20al%202004.pdf) and it will be interesting to learn whether one of these species specializes on human blood feeding.

There is a final question about how the six malaria cases contacted infected mosquitoes. Female Anopheles blood feed late at night and are rarely found in air-conditioned buildings. All of the malaria cases needed to have significant late-night exposure to infected female mosquitoes. Professionals like emergency workers, security guards, police, and firefighters are all exposed to mosquitoes late at night and are at increased risk for vector-borne disease infections, including St. Louis encephalitis, West Nile, and malaria. Public health warnings should include all individuals with late-night mosquito exposure risk. 

The Florida malaria transmission event appears to be highly focal. However, the extremely wet conditions in South Florida may favor the continued production and dispersal of Anopheles crucians vector mosquitoes along with the dispersal of already infected vector mosquitoes.

Eastern Equine Encephalitis Virus



Transmission of EEEV to sentinel chickens continues to be reported in North Florida and in the Florida Panhandle (Figure 4). July is the peak transmission month for equine and human EEE transmission, so we are nearing the end of the EEEV transmission threat in the state. 

Figure 4. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida as of July 8, 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 7/8/23)

The unusually wet conditions throughout Florida currently favor the transmission of malaria and dengue by increasing vector populations and mosquito dispersal. The saturated groundwater conditions work against the transmission of EEEV, HJV, SLEV, and WNV by dampening the amplification of these viruses.

Currently, the greatest concern of mosquito-borne disease transmission is EEEV in North Central Florida and the Florida Panhandle (Figure 4), but this threat is rapidly diminishing. Sentinel chicken surveillance data indicate that there was some amplification of EEEV earlier this year but 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 disease system with above normal activity is malaria. Transmission of all of the viruses is well below normal. Transmission of SLEV and WNV is virtually non-existent in Florida. Travel-associated dengue cases continue to be introduced into Florida at a rate surpassing that which was observed in 2022, a year of exceptional locally-acquired dengue transmission in Florida. 

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.