It continues to be Clarke’s privilege to share Dr. Day's weekly analysis of arbovirus disease activity in Florida with mosquito control professionals across the state. Our shared goal with Dr. Day is to provide timely and actionable information that mosquito control programs can use to make operational decisions and protect public health from vector-borne diseases.

An archive of all past newsletter issues remains available on the Clarke website.

A:* The human botfly (Dermatobia hominis) is one of the most fascinating insects on earth.

It ranges from the southern half of Central America through the northern half of South America. Its larvae parasitize humans along with a wide range of other animals including non-human primates. Larvae burrow under the skin and feed in the fatty subcutaneous dermal layer.

The adult female botfly is about the size of a bumblebee and is a very loud flyer. This presents her with a problem; how does she approach a host to lay eggs without provoking a defensive, and possibly fatal, reaction? Dermatobia hominis has solved this problem in two amazing ways.

First, gravid botflies ambush one of more than 40 mosquito species (as well as muscoid flies and even ticks), pin them to the ground, lay three to six eggs on their thorax, and send them on their way unharmed. Then, some of these egg-bearing mosquitoes in search of a blood meal approach a vertebrate host. The mosquito usually lands around the head and begins to probe, initiating the second amazing part of this life cycle.

Stimulated by host body heat, the botfly eggs hatch, and a fully developed first instar larva drops onto the host and burrows under the skin. From there, larval development takes about eight weeks. The mosquito messenger part of this remarkable life cycle has been captured photographically by Dr. Larry Reeves of the UF Florida Medical Entomology Laboratory. In the image below, four botfly eggs can be seen attached to a female Psorophora spp. One of the botfly eggs is in the process of hatching!

There have been profound changes in Florida groundwater levels during the past two weeks, due in part to Tropical Storm Alex that crossed South Florida on June 4^th. The Keetch-Byram Drought Index (KBDI) for May 20^th, 2022 shows dry conditions in most of South Florida. The transition zone between dry South Florida and wet North Florida is marked by the red oval. Within this transition zone, we expect to see mid-summer transmission of EEEV.

Currently, most of the EEEV transmission reported in Florida is in the wet-to-dry transition zone around Orange County (Figure 2). Based on recent rainfall patterns, South Florida is the region at the highest risk of WNV transmission in the coming weeks. 

Vector and arboviral surveillance remain some of the most important tools that vector control agencies currently have at their disposal. Arboviral transmission indices (sentinel chickens, positive equines, positive exotics such as emus), positive humans, and positive mosquito pools) provide indicators of local virus transmission, although sometimes not in a timely manner. Monitoring mosquito populations and their age structure gives additional information about potential transmission risks. Supplemental vector control efforts in and around sites where virus transmission is known or suspected of recently occurring provide another mechanism to mitigate viral transmission.

There is currently a moderate risk of arboviral transmission in some parts of Florida. It is important to note that arboviral transmission risk is never zero. The transmission of EEEV in the wet-dry transition zone in Central Florida will continue to be an issue of concern during the coming weeks (Figures 2, 3, and 4). The rapid change from dry to wet groundwater conditions in South Florida (Figures 3 and 4) may favor the transmission of WNV in the coming weeks.  

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.

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