Not too long back Dengue epidemics were part and parcel of life in certain parts of the US (1). Florida had its last Dengue epidemic in 1934. Keeping mosquito-borne diseases at bay in the lush, warm, wet sub-tropical Florida climate is a constant tug-of-war and recent history ominously suggests that laxity in stringent, community-wide mosquito control could deliver net advantage to mosquitoes and the diseases they carry. Florida’s climate allows year-around residence of Aedes aegypti, the mosquito species that transmits Dengue and Zika in Central and South America. Thus, there’s no guarantee Dengue won’t make a comeback or that Zika can’t gain a foothold in the state. Exploring chances of Dengue and Zika in Florida helps appraise their capacity to spread into continental US and suggests that holding them at arm’s length is a fragile standoff at the best of times.
The Three Mosquiteers* In Florida’s History: Dengue, Malaria, Yellow Fever
Since at least the 1970s, Florida’s become one of the foremost global tourist destinations but a 2013 article on the history of mosquito-borne diseases in Florida (2) reminds us just how recent that transformation is. For long Florida was sparsely inhabited, its entire population numbering just 34730 in 1830 (3). As recently as the 1950s, Florida was a place to flee from in summer (4), of course only by those who could afford to do so.
Why? Simply, until recently, life in much of Florida was considered so unbearable (5), much of its population stayed concentrated in the north between Alabama and Georgia, the Florida Panhandle, a largely disease- and poverty-stricken area between Jacksonville and Pensacola, known as the ‘malaria belt’, also home to cholera, dengue, diphtheria, hookworm, influenza, pellagra, pertussis, smallpox, tetanus, typhoid, yellow fever. Though largely forgotten today, mosquito-borne diseases in the form of malaria, dengue and yellow fever thus inform a great deal of Florida’s history. Maurice Provost, the first director of what would become the Florida Medical Entomological Laboratory recalled in 1973 (see below from 6),
“My most vivid memories of malaria-control days in Florida are of the morning-after inspections of so many of the humble shacks we sprayed with DDT. The poor housewife often enough would come to me with tears of joy and show me a basketful of dead bedbugs, roaches, and other vermin, and she would exclaim that her family had spent the first night of their lives without annoyance from biting or creeping things”
Florida’s last dengue epidemic was in 1934. Reads like so much ancient history now. Why? Mosquito control (7), a stupendous achievement pretty much taken for granted not just now but already a generation back in 1981 (see below from 8, emphasis mine).
“Florida would not be where it is today were it not for mosquito control. That alone makes a lot of people mad, but they weren’t around when you could not go outside after dark and most of the coastal communities closed down during the summer. Most of the people who fight your programs are newcomers. A newcomer is now somebody who came here after 1970.”
Complacence Belied, After 1934, In 2009 Locally Acquired (autochthonous) Dengue Returned To Florida
While sporadic travel-related Dengue infections remained, Florida didn’t report any locally acquired (autochthonous) Dengue since 1934, a record broken in 2009. Starting in July 2009 and continuing until April of the following year, a total of 28 locally acquired Dengue cases were reported in Key West, FL (See figure below left from 9). Eventually a total of >90 locally acquired Dengue cases were reported in Key West alone (See figure right from 10).
Starting from tiny Key West in 2009 (see left from 11), by 2013, locally acquired Dengue had expanded upward to encompass at least 8 Florida counties (see right from 12).
Local acquisition means complete local mosquito-to-local human-to-local mosquito cycle, the step necessary for outbreaks and eventually epidemics too. Definitely the opposite of welcome news. The silver lining in this dismaying story was that Maimi-Dade County, a port with heavy traffic from Dengue-endemic countries, doesn’t appear to have conditions sufficient to sustain local Dengue transmission. Puzzling similarities in the Counties with the most number of cases, namely Key West-containing Monroe in 2009-2010 and Martin in 2013, were
- Neither is a major port of entry for either aviation or shipping.
- Both had larger numbers of locally acquired cases compared to other Florida counties.
How did Key West and Martin favor local Dengue transmission? Studies suggest factors they have in common are (12, see table below from 11)
- Tendency to keep open-air water receptacles such as bird-baths without frequently changing them.
- Keep windows open >50% of the time.
- Have >50% vegetation on their property.
OTOH, local Dengue transmission was greatly reduced if
- Empty standing water containers were changed weekly.
- Air-Conditioners (A-Cs) were used >50% of the time.
- Mosquito repellents were used routinely.
Situation in Florida is even more precarious given the fact that one study found local Florida Ae. aegypti mosquitoes capable of Vertically transmitted infection of the Dengue strain isolated from the 2009 Key West outbreak (13). When infected with Dengue virus, ~8% of these Florida mosquitoes were found capable of vertically transferring it to their eggs. Thus, Florida’s just barely keeping Dengue at arm’s length and even the slightest laxity could be all it takes for it to gain a stable foot-hold. Threat of Zika just adds to the strain on public health. Not to mention the really scary scenario if Aedes albopictus also became capable of transmitting Dengue and Zika, something it can’t at present. This would be really scary given how much more widespread this mosquito species is all across the continental US and indeed much of the temperate regions of the world.
Trouble is collective memory’s fickle and easily breeds complacence. When persistent, deeply vexing problems such as the perennial scourge of mosquitoes get intensively abated within just a generation, as happened in Florida, it doesn’t take long for collective, generation-spanning amnesia to dictate the conversation. Already back in 1991, public opinion apparently supported the idea that mosquito control officials greatly exaggerate the threat of disease to justify their jobs (4). What’s easily forgotten in such short-sighted political and economic debates is that mosquito (vector) control is inherently resource and personnel intensive and only works with sustained community support and participation (14). To quote Hribar (2, emphasis mine),
‘Is it too expensive to control Aedes aegypti? Equipment, training, pesticides, and people cost money. To do the job right, a lot of time must be devoted to seeking out larval habitats and eliminating them. Adult emergences must be dealt with promptly. The public must cooperate with public health and mosquito control agencies in the fight against Aedes aegypti. Whatever the cost surely it will be less than the hospitalization, medicines, lost wages, and funeral expenses that may be the alternative ‘
Though climate and location render Florida and other states in the Gulf Coast of the United States vulnerable to Dengue and Zika, common-sense, practical measures can do a great deal to minimize and even prevent them from getting established in the US. Apart from aggressive, community-based mosquito (vector) control, air-conditioning and using screens on doors and windows can greatly stem Ae. aegypti‘s capacity to complete Dengue and Zika‘s transmission cycle. Of course, prevention is greatly facilitated by widespread use of centralized air-conditioning and heating systems, something only to be expected in an advanced economy like the US. Given how important tourism is to Florida’s economy, one would hope the state apparatus wouldn’t hesitate to pull out all stops to prevent mosquito-borne Dengue and Zika from taking root in the state.
*: Defined here as mosquito-borne parasitic and viral diseases.
1. Bouri, Nidhi, et al. “Return of epidemic dengue in the United States: implications for the public health practitioner.” Public health reports 127.3 (2012): 259. http://www.ncbi.nlm.nih.gov/pmc/…
2. Hribar, L. G. “Influence and impact of mosquito-borne diseases on the history of Florida, USA.” Life Excit. Biol 1 (2013): 53-68. https://blaypublishers.files.wor…
3. Cody, Scott K. “Florida’s population center migrates through history.” Florida Focus 2.1 (2006): 1-5) http://www.bebr.ufl.edu/sites/de…
4. Mulrennan, J. A. “Benefits of mosquito control.” Mosquito control pesticides: ecological impacts and management alternatives. Conference Proceedings. Scientific Publishers, Inc. Gainesville, Florida, USA. 1991.
5. Gaiser, D. “The importance of mosquito control to tourism in Florida.” Proceedings of the Florida Anti Mosquito Association (1980).
6. Provost, Maurice W. “Environmental Quality and the Control of Biting Flies.” Symposium on Biting-Fly Control and Environmental Quality. 1973.
7. Mulrennan Jr, John Andrew. “Mosquito control-Its impact on the growth and development of Florida.” Insect Potpourri: Adventures in Entomology (1992): 75.
8. Harden, F.W. 1981. You and the environment. Journal of the Florida Anti-Mosquito Association 52:60-61. http://floridamosquito.org/Archi…
9. Trout, A., et al. “Locally Acquired Dengue-Key West, Florida, 2009-2010.” Morbidity and Mortality Weekly Report 59.19 (2010): 577-581. http://www.cdc.gov/mmwr/pdf/wk/m…
10. Rey, Jorge R. “Dengue in Florida (USA).” Insects 5.4 (2014): 991-1000. Dengue in Florida (USA)
11. Radke, Elizabeth G., et al. “Dengue outbreak in key west, Florida, USA, 2009.” Emerg Infect Dis 18.1 (2012): 135-7. http://wwwnc.cdc.gov/eid/article…
12. Teets, Frank D., et al. “Origin of the dengue virus outbreak in Martin County, Florida, USA 2013.” Virology reports 1 (2014): 2-8. https://www.researchgate.net/pro…
13. Buckner, Eva A., Barry W. Alto, and L. Philip Lounibos. “Vertical transmission of Key West dengue-1 virus by Aedes aegypti and Aedes albopictus (Diptera: Culicidae) mosquitoes from Florida.” Journal of medical entomology 50.6 (2013): 1291-1297. https://www.researchgate.net/pro…
14. Parks, Will, and Linda Lloyd. Planning social mobilization and communication for dengue fever prevention and control: a step-by-step guide. World Health Organization, 2004. https://www.researchgate.net/pro…