BACKGROUND: St. Louis encephalitis virus (SLEV) was the most common mosquito-transmitted human pathogen in Florida prior to the introduction of West Nile virus (WNV) in 2001. Since then, SLEV activity in Florida has decreased dramatically. Surveillance for SLEV activity in Florida includes mandatory reporting of human cases as well as serologic testing of sentinel chickens. St. Louis encephalitis (SLE) and WNV illness have similar clinical presentations, and significant antibody cross-reaction can occur on immunologic tests. In addition, clinical testing capacity for SLEV at commercial laboratories has been limited since 2014. This can pose a challenge to health care providers as well as local and state public health agencies.
METHODS: Data were collected by the Florida Department of Health (DOH) Vectorborne Disease Surveillance Program from county health departments and local mosquito control programs. Laboratory testing was performed at Florida DOH Bureau of Public Health Laboratories (BPHL) per national guidelines.
RESULTS: Two cases of SLE were identified in Duval County in August 2014. Cases had symptom onset within two days of each other. The first case was originally reported as a possible WNV illness case due to a positive WNV IgM antibody titer. No SLEV testing was ordered. Confirmatory testing at BPHL indicated that it was actually an SLE infection. The second case was positive for SLEV IgM antibodies and negative for WNV IgM antibodies at both laboratories. One sentinel chicken was positive for SLEV in Duval County the following week. Statewide, 2014 had the highest number of sentinel chicken SLEV seroconversions reported since the introduction of WNV. Increasing numbers of sentinel chicken SLEV seroconversions reported since 2011 (65), 2012 (85), 2013 (96), and 2014 (107). In 2015, only nine seroconversions were reported and no human cases were identified.
CONCLUSIONS: The two cases of SLE reported in 2014 were the first reported since 2003. The timing of symptom onset was similar to the few previous human cases in north Florida, occurring earlier than the historical transmission peak seen in central Florida. A case of WNV illness was also reported in Duval County with the same onset week as these cases. The BPHL identified one of the SLEV infections, highlighting the importance of public health laboratory confirmatory testing, especially since SLEV testing is no longer available commercially. The increasing number of sentinel chickens testing positive for antibodies to SLEV suggests the potential for possible resurgence. This activity would go undetected without sentinel flock surveillance.