Dengue in Florida in 2013 Gives No Cause for Comfort
Florida received a rude awakening to the danger of mosquito-borne dengue virus (DENV) when there were 22 dengue cases in Key West in 2009. Then in 2010 Key West had a total of 66 cases. The incidences of cases per population size were among the highest for dengue in the world. The incidence in Key West, population size 20,000, was 110/100,000 and 330/100,000 in 2009 and 2010 respectively.
Although there have been sporadic isolated single cases of dengue in a few different regions of south Florida in 2011, there was no further appearance of the type of transmission that occurred in Key West. Some may have hoped that this signaled that Florida was not at great risk from DENV. And then there was Martin County during the summer of 2013. The dengue outbreak in Martin County, focused in the tiny community of Rio and then in nearby Jensen Beach, demonstrated again that Florida remains at great risk from mosquito-borne DENV. The Martin County epidemic illustrates the types of focal outbreaks other Florida counties might expect to encounter. It is vital that Florida mosquito control and public health learn from each outbreak to make improvements in providing effective control to mitigate future dengue outbreaks.
The 2013 Rio dengue outbreak began on August 14th with the appearance of the first reported case. Onset of symptoms was thought to be August 1. Martin County Mosquito Control (MCMC) under the able leadership of Gene Lemire immediately swung into action. Other dengue cases followed on August 21 and August 22. By August 23 there were 7 cases and it was clear that the initial region where transmission was occurring was the community of Rio.
The MCMC's actions were impressive. All of the MCMC personnel, all seven of them, went into Rio with a vengeance to control the Aedes aegypti in the area. The initial visits to Rio verified that Ae. aegypti were present and in very noticeable numbers. MCMC personnel gave testimony to this by personally experiencing Ae. aegypti attack rates while they visited the area. MCMC's first steps were to attack the situation by: 1) adulticiding by ground, 2) conducting sweeps of the community by visiting every house and business to destroy Ae. aegypti larval habitats, 3) adulticiding with hand-held sprayers for adult Ae. aegypti populations and 4) enlisting residents in participating in keeping their property Ae. aegypti-free and advising everyone to take personal protection precautions.
It would be a mistake to dismiss the 2013 Martin County outbreak as inconsequential. After all there were only 23 cases. Despite the small number of cases, the incidence of dengue, especially in Rio, was enormous. The Rio community consists of about 1,000 people and about 600 housing units. Among the 23 cases there were conservatively eight Rio residents likely certainly to have been infected in Rio, and the remaining cases were in people with residence outside of Rio though investigation indicated the majority of these likely to have been infected while visiting Rio. Though Rio was clearly the focus for transmission, we cannot use any of the Rio non-resident but Rio-related cases to establish the incidence in Rio because we can never know the total at risk population (the denominator of the incidence). The total number of people visiting Rio during this period that is needed for this denominator is unknown. Therefore using only the eight cases in Rio residents per 1,000 Rio population the incidence in Rio is ~800/100,000. Recall the incidence in Key West in 2010 was 330/100,000 population. Little Rio had nearly a 2.5 fold greater incidence, eclipsing Key West's 2010 risk of dengue. Clearly MCMC and the Martin County Department of Health (MCDOH) had a major problem.
MCMC's first sweep of the entire Rio community was completed on August 25. They found that the Ae. aegypti house index in Rio was 0.38. This means that 38% of the houses in Rio had containers with Ae. aegypti larvae! After treating containers and emptying water wherever possible, a second sweep found only 14% and a third sweep weeks after the initial case found an index of 0 percent. The area of sweeps had to be expanded as new cases were confirmed outside Rio in Jensen Beach. Concurrent with the MCMC's efforts MCDOH was also promoting public awareness throughout the region with public service announcements, educational literature, and visits to various local organizations about the danger and the need for public awareness to reduce Ae. aegypti habitats and to take personal protection measures.
The last documented case of dengue in Martin County was reported in September. On Sept. 13 the Florida and Martin County Health Departments conducted a survey in Rio and Jensen Beach collecting bloods from a randomized sample of 400 of the residents to ascertain the extent and distribution of dengue infections during the outbreak. An additional case was identified with onset of symptoms for this case in July. In addition the survey collected information about the behaviors and responses taken during the outbreak by the sampled population. As of this date, the complete results of the survey have not been made available.
Though the results are not available one might predict results similar to the study of the 2009 Key West outbreak where it was determined that roughly 5% of the Key West population had been infected with DENV. Though the sampling strategy for the Rio outbreak has not been reported yet it would be surprising for Rio's and Key West's outbreak to be substantially different. Certainly detecting a real difference between them is difficult considering the typical statistical error variances associated with studies of this nature. What would one predict for the survey? If all 400 people sampled were Rio residents, then about 20 infections should be observed. If the samples were a random collection from Rio and Jensen Beach, the number of prior infections would likely be lower depending on how many from Jensen Beach visited Rio and considering the apparently lower transmission rate that occurred in Jensen Beach. Jensen Beach is a much larger community compared to Rio with roughly 11,000 residents. I would predict that far less than 20 infections will be detected by the serosurvey. We will await the serosurvey report and conclusions.
What does this mean for Florida?
Florida mosquito control and public health professionals must be better prepared to fight dengue! DENV is here to stay in Florida, and we will likely see more focal epidemics like in Key West and Rio. Dengue could appear wherever Ae. aegypti or perhaps Aedes albopictus populations are in sufficient numbers to support transmission. Florida's MCDs and Departments of Health will have their hands full even in these very focal outbreaks with high risk of disease, high incidence, and requiring aggressive mosquito control and public participation in source reduction and personal protection.
One shudders at the prospect of a more widespread dengue outbreak, or Chikungunya outbreak or, God forbid an outbreak of Ae. aegypti supported yellow fever, a true catastrophe. Think of the effort undertaken for little Rio to conduct sweeps of about 600 homes. What would occur with a similar incidence to Rio's in Florida's southeast metropolitan area from West Palm Beach to Miami, population roughly 5,000,000. There could be 40,000 dengue cases! Lee County, population 645,000 with 5000 cases! Thousands more infected and likely more susceptible to severe dengue from another DENV serotype. It is unrealistic to contemplate Martin County MC's sweeps for literally thousands of homes and businesses. It is vital that there is mandatory homeowner and public responsibility for source reduction to keep Ae. aegypti and Ae. albopictus in check.
I contend that we already know what is needed to mitigate Ae. aegypti and Ae. albopictus transmission. Florida mosquito control can only do so much in reducing Ae. aegypti and Ae. albopictus populations. Both the Keys MCD and Martin County MC did reduce these populations with varying degrees of success in mitigating transmission. However, we have also learned that aggressive public action is essential for success. I was sad to hear of reports by MC personnel of homeowners who knowingly failed to clean up Aedes producing containers on their property, similar to the reports in Key West. I learned of homeowners who simply waited for MC personnel to clean up their properties for them. There is a segment of almost every community that willfully is not heeding the information being provided. These people are set on following the maxim of Huxley College President Quincy Wagstaff (Groucho Marx) to his faculty in the movie Horsefeathers, "Whatever it is, I'm against it."
What can be done to improve public participation and responsibility for reducing the vector populations produced on their own properties? Florida's health departments must reconsider their messaging to the public. I do not believe there is any evidence that current messages to the public are having the desired effect. I urge Florida's communities to address whether a citizen has the right to allow the production of mosquito populations that endanger the health and well-being of everyone in the community. For example, restaurants must be cleansed to avoid food poisoning, so surely homes must be cleansed to reduce disease. I urge that this danger is made part of the messaging that will raise a public outcry against those citizens who refuse to clean up their properties. Such an outcry could result in greater code enforcement and even fines against offenders
Florida's mosquito control agencies must identify those high risk areas in their counties with substantial populations of Ae. aegypti or Ae. albopictus. Responsible mosquito control and public health professionals should institute practices now to reduce these populations and to keep them under control. The other alternative is to wait for the first dengue, chikungunya or, heaven forbid yellow fever cases to learn where the vector populations are and before taking actions. Surely we can do better than that. Preventive control must be done using source reduction by the MC agency and through information to the public by mosquito control and the Health Department that successfully elicits public participation to reduce mosquito habitats around homes and businesses as a matter of year round action.
I greatly appreciated the assistance of Gene Lemire, Director of MCMC, for providing details about the timeline and MC action in Martin County.
Walter J. Tabachnick, Ph.D. - Retired
Florida Medical Entomology Laboratory
Department of Entomology and Nematology
University of Florida
Vero Beach, FL