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Understanding How Dengue Can Be Introduced Into Key West

Why should Florida mosquito control and public health professionals prepare for the potential introduction of dengue virus (DENV) into Key West? After all, there have been no more human cases or evidence for transmission of DENV in Key West since the epidemic of 2010 that resulted in 66 cases with the very high incidence of 330 for every 100,000 persons.

Figure 1 is a map from the U.S. Centers of Disease Control web site showing the locations where dengue has been transmitted in the Caribbean basin in 2013 as of July 10, 2013.

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 Dengue3_1 FIGURE 1. Municipalities
reporting human cases of
dengue Jan 1, 2013 – July 10,
2013. (From U.S. Centers for
Disease Control, http://www.healthmap.org/
dengue/

The southern U. S., home to DENV vectors, Aedes aegypti and Aedes albopictus, is directly adjacent to this dengue infested region! The Key West dengue epidemics of 2009 and 2010 demonstrated Key West's susceptibility to DENV when it successfully invades. Further, Key West likely now has about 4,000 (that's 1 in 5) of its residents immune to dengue serotype 1, due to previous infection with DENV-1 during the previous epidemics. These people are at great risk for more severe dengue should they become infected with one of the other 3 DENV serotypes. A recent article reported that infection with one DENV serotype can provide some cross-protection against the other serotypes for as long as two years (Reich et al. 2013. Interactions between serotypes of dengue highlight epidemiological impact of cross-immunity. J. Royal Soc. Interface 10: 20130414. http://dx.doi.org/10.1098/rsif.2013.0414). Perhaps this has contributed to the absence of DENV in Key West since 2010. However, this cross-protection has likely waned and citizens of Key West are now surely at greater risk for severe dengue if other serotypes are introduced!

How was DENV introduced into Key West? Key West has a largely tourism-based economy with hundreds of thousands of visitors from throughout the world. The majority of visitors are from elsewhere in the U. S., Canada and Europe. The Key West Chamber of Commerce web site reports that visitors from Brazil have increased over the past several years. Brazil has had several dengue epidemics making it possible for an infected visitor from Brazil to introduce the virus.

One might assume that the substantial traffic from cruise lines that arrive in Key West could be the source of infected visitors introducing DENV from the Caribbean Basin. The Key West Chamber of Commerce reported about 813,000 cruise line passengers debarking at Key West as part of their cruise in 2012. Figure 2 is a graph that shows cruise ship passengers and crew arriving between April - September of 2013. The large numbers of persons arriving on cruise ships is a major reason that many consider cruise ship traffic as a primary source for dengue introductions.

I was intrigued by the potential for cruise line passengers to introduce DENV and investigated which other ports of call served the ships that stop in Key West. To my surprise, it became clear that most of the other ports of call were not in major dengue endemic areas. For example, the Grand Caymans and the Bahamas are major ports of call for cruise lines that also stop in Key West. However, Key West is actually the first port of call in the majority of cruise line itineraries that stop in dengue endemic locations. Hence, it is impossible for a cruise passenger arriving in Key West from Miami, Ft. Lauderdale or New Orleans to have become infected in the Bahamas or Cayman Islands since they can only be exposed to DENV after they have already left Key West. I do not believe that cruise ship passengers are a major source for introducing DENV into Key West.

So what are some other possibilities? Of course, substantial numbers of travelers also arrive at the Key West International Airport. The Chamber of Commerce reported about 367,000 arrivals for 2012. However, the majority of visitors are likely from elsewhere in the U.S., Canada and Europe, all places without dengue. This does not rule out the possibility of a dengue-infected traveler arriving by plane, but it is a less likely source for the same reasons as cruise ship passengers.

Are there any other likely sources of DENV into Key West? Certainly arrivals by air from dengue endemic regions occur in other Florida airports and in other states. Any of these travelers that then travel to Key West pose a risk of introducing DENV. Though infected cruise ship passengers are unlikely to pose a major threat, the crews of these ships are another matter. I checked the numbers of the crew for several ships arriving in Key West. On average these ships have 1,000-1,200 crew members. From April – Sept 2012 Key West's tourism board listed 106 ships arriving in Key West, a total of about 106,000 – 127,000 crew members, using my estimates. Although Key West may be their first port of call, only a few days earlier at least some of the crew members were in ports in dengue endemic areas on a prior cruise. How unfortunate for Key West that these crew members would arrive in Key West several days after being infected with DENV, at a time when they are likely viremic and likely able to infect Key West Ae. aegypti. How many debark in Key West? How many were recently in a dengue endemic location?

 Dengue3_2 FIGURE 2. Cruise Line Passengers in Key West FL Apr.-Sept. 2013. (From Key West Chamber of Commerce)

What must Key West do to protect its citizens and visitors from a dengue outbreak? Vigilance, more research, and an action plan are essential.

  1. Key West Health Department should have a list of the cruise ships with the ports of call in dengue endemic areas.
  2. Advise all cruise lines with listed ships to monitor the health of crew members to detect crew with febrile symptoms with dengue.
  3. Restrict crew members with febrile symptoms to the ship when in Key West.
  4. Detect the first case of dengue in Key West.
  5. Determine the serotype of DENV in the infected case quickly.
  6. Advise the public immediately of the danger from DENV particularly from DENV 2, 3 or 4.
  7. Implement a plan to effectively and efficiently reduce Key West Ae. aegypti populations:
    1. Aggressive mosquito control.
    2. Aggressive programs by the Department of Health to reduce human mosquito contact.
    3. Aggressive local government policies to ensure the public acts to reduce Ae. aegypti habitats around homes and businesses.

Those who ignore the warning signs should look at Figure 1. The high prevalence of dengue in our southern neighbor countries is an imminent threat.

wjt3Walter J. Tabachnick, Ph.D.
Florida Medical Entomology Laboratory
Professor, Department of Entomology and Nematology
University of Florida
Vero Beach, FL