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Chikungunya Virus in Florida: Lessons from Italy, 2007

At one time Florida experienced widespread yellow fever and dengue epidemics. Both diseases are transmitted by the yellow fever mosquito Aedes aegypti. Aedes albopictus, the Asian tiger mosquito, is another important vector of dengue and yellow fever. This 6-legged tiger was introduced into Florida in 1986 and has since spread throughout the state. The yellow fever and Asian tiger mosquitoes are also the main vectors for chikungunya virus (CHIKV), the Alphavirus responsible for widespread epidemics of chikungunya fever in Asia and Africa in 2006- 2007. So what chance does a virus like chikungunya have to gain a foot-hold in Florida, a state where, at one time, yellow fever and dengue ran wild?

Perhaps the most accurate scenario of the potential invasion of Florida by CHIKV already occurred in Italy during the summer and autumn of 2007. In early August, local health authorities in the Province of Ravenna detected an unusually high number of cases of febrile illness in two small neighboring towns with a total population of 3,767 located 6 km from the Adriatic coast. The two villages were separated by a slow moving stagnant river and lock system that produced large numbers of mosquitoes, including Ae. albopictus.

By the end of August, serological testing on suspected human cases confirmed the diagnosis of CHIKV. In addition, CHIKV was isolated by polymerase chain reaction (PCR) from pools of Ae. albopictus. Aedes albopictus was introduced into the Veneto Region of Italy in 1992 through the importation of used tires from Atlanta, Georgia, USA. Aedes albopictus was first identified in the two epicenter villages in 2006, the year before the chikungunya epidemic.

Conditions around the houses in these villages were optimal for vector blood feeding, reproduction, and dispersal. Houses were typically one story and were surrounded by small gardens with many flowers, plants and, most importantly, numerous flower pots. Open sewer systems containing stagnant water were evident just below street level.

The index case of the epidemic is presumed to be a resident of the Italian Provence containing the two epicenter towns. This individual traveled to the chikungunya-active Kerala State in India during June of 2007. He presented with two episodes of fever in late June, 2007. While ill he visited his cousin in one of the affected towns. The cousin, the second case in this outbreak, became ill on July 4th. These two cases were first identified as possible chikungunya infections to epidemiologists and vector control specialists in late August 2007. This reporting delay of eight weeks was crucial to later difficulties associated with control of the outbreak.

An active human disease surveillance system was set up for the entire Italian Provence on August 29, 2007. Control measures included adulticiding and larviciding on public and private land within 100 meters of the residence of all confirmed and suspected chikungunya cases. As of September 21, 2007, 292 chikungunya cases were identified within the transmission zone. Most of the cases continued to be reported from residents and visitors in the initial village epicenters. However, by the end of August, cases were reported with no known exposure in the two villages. This indicated that local transmission in adjacent areas was possibly fueled by dispersing infective Ae. albopictus females.

In spite of intense and prolonged vector control efforts the transmission of CHIKV continued into the third week of September, indicating the difficulty of managing arboviral epidemics once large numbers of adult mosquitoes become infective within a transmission zone. By the end of September the number of new cases began to diminish and the epidemic burned itself out with the onset of cold weather in mid October.

For Florida to realize a CHIKV epidemic we will likely need to have at least three preexisting conditions in place:
1. First, focal areas within the state where Ae. albopictus or Ae. aegypti populations are routinely “off scale.”
2. Second, introduction of CHIKV into one of these areas via an infected traveler or an already infected mosquito.
3. And third, a susceptible local human population with an unusually high level of day-today Ae. albopictus or Ae. aegypti exposure within the introduction zone. I believe that we can reduce the risk of an introduced chikungunya epidemic in Florida by addressing any one of these three conditions through effective Ae. albopictus and Ae. aegypti vector-control programs, a sensitive human disease surveillance system, and excellent communication and coordination between human and public health professionals and mosquito control.

Jonathan F. Day, Ph.D. - Professor of Medical Entomology
Florida Medical Entomology Laboratory
Department of Entomology and Nematology, University of Florida-IFAS, Vero Beach, FL