Some perspective on Zika virus
In recent weeks, there has been a great deal of publicity about infection with Zika virus in the Western Hemisphere, especially because of the likely association of infection during pregnancy with microcephaly, a birth defect in which a baby’s brain does not develop normally.
The risk of disease in Florida in persons of all ages remains very low, except in travelers to countries with ongoing outbreaks.
The disease caused by Zika virus is generally quite mild in children and adults, and 80 percent of infections have no symptoms. As Zika virus is currently circulating in Puerto Rico, federal and Puerto Rican public health authorities will (unfortunately) soon have an opportunity to observe and study the effects of Zika virus infection during pregnancy.
There are three mosquito-borne viruses with similar epidemiologic characteristics currently circulating in Central and South America and in the Caribbean: dengue, chikungunya, and Zika. Chikungunya and Zika viruses are new to the Western Hemisphere. All three can be spread by the bite of the same vector species of mosquito: Aedes aegypti is best, but Aedes albopictus also is a competent vector. Both species live throughout Florida. Malaria can also be spread by mosquitoes that live in Florida. Dengue and malaria used to be endemic throughout much of the state before World War II, and yellow fever, with a similar epidemiology, caused periodic epidemics.
Because there is so much travel by Florida residents to countries all over the hemisphere, much of it to visit family and friends, it is not rare for travelers to return with dengue, chikungunya, malaria, and now Zika virus infection. The vast majority of such returned travelers are epidemiologic dead-ends -- there are no secondary cases.
Several times a year we have documentation of dengue acquired in Florida: a single case in a person who lives near someone who has returned from a trip, is ill with dengue, and has infected their household's mosquitoes. There have also been isolated cases of chikungunya acquired in Florida the same way. It is plausible that this will also happen with Zika virus. Local Zika virus transmission has occurred in Puerto Rico but has not yet been documented in Florida.
In 2009, a focus of dengue became established in older parts of Key West, where there are lots of mosquito-breeding environments in people's yards and where people most often depend on shady porches and open screen-less windows and doors to keep cool. There were 66 reported symptomatic human cases in 2010, but the outbreak died out by 2012 after an intensive mosquito control effort. In 2013, a similar focus became established in Martin County, in two neighborhoods of older homes. There were fewer than 25 symptomatic cases, and the outbreak died out after vigorous mosquito control efforts. In 2012, Florida had 120 reported cases of dengue, almost all imported, and there were 160 more in 2013.
There have (so far) been no ongoing foci of chikungunya or Zika virus infections in Florida, or of malaria. We have the right vector mosquitoes, but the vast majority of homes in Florida have screens and air conditioning and this has been important in preventing local transmission. During the times of year when mosquitoes are most active — approximately May to October — most people spend most of their time in air-conditioned spaces and simply are not at very high risk for mosquito bites.
There are reports of transmission of Zika virus infection between spouses (from a returned traveler to a spouse who did not travel), which have been attributed to sexual transmission. This is not the most common route of transmission.
Based on our experience with dengue, chikungunya and malaria, it is quite unlikely that Zika virus will become naturalized in Florida and show ongoing transmission. We will, however, continue to see cases in returning travelers, each of whom could potentially be the source for local mosquito-borne diseases spread in their neighborhood.
The CDC has issued guidance for pregnant women who are considering travel to Central America, the Caribbean, and South America (http://www.cdc.gov/zika/pregnancy/). Zika virus infection is a reportable disease in Florida, by doctors and laboratories. This enables the Florida Department of Health to take preventive action around recognized cases, in coordination with local public health and mosquito control authorities.
You can do yourself and your neighbors a great service by inspecting your property and emptying out potential mosquito-breeding sites. These may include flower pots, tires, kiddie pools, and house-gutters that do not drain properly. Even very small collections of standing water can serve as breeding sites. Leon County has a mosquito control program, but it’s up to us as residents to reduce mosquito-breeding sites around our homes as much as we can. We can also wear long sleeves and long pants, and apply effective insect repellents when we or our children have unavoidable mosquito exposure. Such measures will also serve to reduce the risk of infection with summertime mosquito-borne viruses such as West Nile Virus or Saint Louis Encephalitis Virus, which are long-established in Florida.
Richard S. Hopkins, MD, MSPH, is a retired Florida State epidemiologist.
