September 20, 2023 | Volume 3 | Issue 18 | As of Week 37

A QUESTION FROM OUR READERS

Q. Rob from Newburyport, MA asks: “A friend of mine from Connecticut recently suffered a severe case of Babesiosis. What is this?”

A. Babesiosis is a tick-borne, malaria-like intracellular protozoan disease caused by pathogens in the genus Babesia in the phylum Apicomplexa. It is a eukaryotic microorganism with membrane-bound organelles. There are at least 10 species of Babesia worldwide infecting humans, cattle, dogs, and other animals.

Babesia microti is the most common species infecting humans. This pathogen is found in the Northeast and Upper Midwest of the USA. Babesiosis is not a notifiable disease, and the most recent CDC annual report is for 2020 when 1,827 human cases were reported. New England was a hotspot of transmission, reporting 89% (1,622) of the cases that year with 151 cases reported in Connecticut.

In New England, cases occur inland and on offshore islands, including Nantucket and Martha’s Vineyard (MA), Block Island (RI), and Shelter Island, Fire Island, and eastern Long Island (NY). These islands share a common ecology of scrub habitats that support high deer, rodent, and tick populations and tend to be transmission hotspots for Babesiosis and Lyme Disease.

The primary vector of B. microti is Ixodes scapularis, the blacklegged tick, also known as the deer tick (Figure 1). This tick is classified as a hard tick that takes two years to mature. During the spring of its first year, blood fed adult females drop off deer and other large mammals and lay eggs in the soil. These eggs then hatch into larvae during the early summer. The larvae feed on small rodents, especially white footed mice, infected with Babesia. Infected larvae overwinter in leaf litter, emerge in the spring of the second year, and molt into infected nymphs, the most aggressive stage. Nymphs feed on a wide variety of hosts, including humans. In the fall of the second year, the nymphs molt into adults that attach to large mammals (especially deer), mate, blood feed, and overwinter.

Blacklegged tick nymphs are the primary vectors of B. microti to humans. Nymphs are small, about the size of a poppy seed, and most infected humans do not recall receiving a tick bite.

Many infected people are asymptomatic. Some develop “summer fever” symptoms, including fever, chills, sweats, headache, body aches, appetite loss, nausea, or fatigue. More severe symptoms include hemolytic anemia due to parasites' destruction of red blood cells. Some cases are life-threatening, and no vaccine is available.

Babesiosis is one of the most poorly recognized vector-borne diseases. If you live in a transmission hot zone and are active outdoors, tick checks after each outing are a must, and Babesiosis should be high on the list of possible infections if you suffer from a “summer fever.”

Figure 1. The life stages of Ixodes scapularis. Photograph by Jim Newman, University of Florida, Florida Medical Entomology Laboratory, Vero Beach.

THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

Figure 2. The Keetch-Byram Drought Index for Florida as of September 18, 2023.

The surface wetness trends we have seen in Florida for the past six weeks are continuing. Most of the Florida Panhandle is in a severe drought, the Treasure Coast is drying, and the remainder of the state is quite wet (Figure 2).   

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 September 16, 2023.

Dengue Viruses

There were four new locally-acquired dengue cases reported in Miami-Dade County last week. One was infected in early August and the other three in mid-August. 

Twenty-five new travel-associated dengue cases were reported in Florida last week, bringing the total number of travel-associated dengue cases so far in 2023 to 299, 132 cases above the 13-year average (2010-2022) of 167 (Table 2). These new cases were widely distributed throughout the state (Figure 3).  

All four dengue serotypes have been reported as travel-associated cases in Florida so far in 2023. Three dengue serotypes (DEN-1, DEN-2, and DEN-3) have been reported as locally-acquired dengue cases in 2023, with DEN-3 predominating.

Figure 3. The spatial distribution of travel-associated human dengue cases in Florida as of September 16, 2023. The 10 counties that are shaded red indicate areas of at least one travel-associated dengue case reported during the past three weeks

Malaria

One new travel-associated human malaria case was reported in Florida last week in Broward County. 49 travel-associated malaria cases have been reported in Florida during 2023. Most are Plasmodium falciparum and originated in Africa. 

Eastern Equine Encephalitis Virus



One new EEEV antibody-positive sentinel chicken was reported in Nassau County last week. This brings the total number of positive chickens to 115, which is below the 19-year expected number of sentinel chicken seroconversions of 130 (Table 2).

St. Louis Encephalitis Virus

Transmission of SLEV in Florida remains virtually non-existent. We will see how this changes now that migrant birds are beginning to arrive from the north.  

West Nile Virus

Eleven new WNV antibody-positive sentinel chickens were reported in Nassau, Palm Beach, St. Johns (6 new positives), Volusia, and Walton (2) Counties last week. This brings the total number of positive chickens to 83, well below the 22-year (2001-2022) expected annual seroconversion rate of 417 per year (Table 2). 

A summary of observed and expected numbers of travel-associated and locally-transmitted mosquito-borne viruses and 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 9/16/23).

Virus transmission of EEEV and reports of travel-associated human malaria cases appeared to have slowed during the past weeks. Surface wetness and dryness (Figure 2) may favor WNV amplification in areas reporting drying conditions in the Florida Panhandle and the Treasure Coast. Surface wetness in South Florida will continue to favor the production of Aedes aegypti and may favor continued dengue transmission.

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

Transmission of SLEV has been virtually absent in Florida for the 2023 arboviral transmission season (Tables 1 and 2). It will be interesting to see if the virus reappears during the fall migration season.

West Nile virus transmission to sentinel chickens is way below normal so far in the 2023 arboviral transmission season (Table 2). There is not much time left to establish significant WNV transmission foci, and the situation for the remainder of the year will likely favor sporadic viral transmission.

Tables 1 and 2 summarize our current understanding of arboviral transmission and travel-associated vector-borne disease in Florida. Table 2 compares the current 2023 data with long-term expected values. As we approach the end of the 2023 arboviral transmission season in Florida, it is unlikely that a major outbreak of EEEV, SLEV, or WNV will occur. Sporadic transmission of these viruses along with transmission of dengue viruses is possible for the remainder of the 2023 transmission season. 

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.