Saint Louis Encephalitis found in Port Arthur

Culex mosquito

Jefferson County Mosquito Control District (JCMD) reported results of a positive pool of Saint Louis encephalitis virus (SLEV) in the Port Arthur area.

Kevin Sexton, director of JCMD, said a sample of mosquitoes collected Aug. 5 at Normandy Avenue and 32nd Street in Port Arthur next to Jefferson City Shopping Center and was sent to Austin for testing. JCMD received the results back Aug. 13 showing positive results for SLEV.

According to the Centers for Disease Control (CDC) Saint Louis encephalitis virus (SLEV) is transmitted to humans by the bite of an infected mosquito.  Most persons infected with SLEV have no apparent illness. Initial symptoms of those who become ill include fever, headache, nausea, vomiting, and tiredness. Severe neuroinvasive disease (often involving encephalitis, an inflammation of the brain) occurs more commonly in older adults. In rare cases, long-term disability or death can result. There is no specific treatment for SLEV infection; care is based on symptoms.

"A lot of people can get bit by this and not show any symptoms at all. It can be deadly, but it is a very small percentage that can be affected by it," Sexton said. "It is mainly the elderly and people with compromised immune systems that are susceptible to this."

Sexton said the Culex mosquito, which is known for transmitting the West Nile Virus, is also responsible for transmitting SLEV. This mosquito is most active at dusk and usually goes away at dawn, Sexton said; therefore the same precautions should be taken to protect against SLEV.  Use insect repellent, wear protective clothing, and stay indoors while Culex are most active. 

St. Louis encephalitis virus is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and Culex mosquitoes, the CDC website states. Wild birds are the primary vertebrate hosts. Birds sustain inapparent infections but develop viremias (i.e., virus in their blood) sufficient to infect the mosquito vectors. Birds that are abundant in the urban-suburban environment, such as the house sparrow, pigeon, blue jay, and robin, are principally involved. Humans and domestic mammals can acquire SLEV infection, but are dead-end hosts.

"The mosquito has to bite the infected bird before it bites a human," Sexton said.

Sexton said mosquito control trucks and larvaciders, workers who specifically target mosquito larvae, were dispatched to the affected area Wednesday, Aug. 13, and will continue to treat the area. The King Air, a JCMD airplane that uses a pyrethroid water-based chemical to spray mosquitoes by air will be dispatched Thursday night, Aug. 14, as well, he said. 

"We can load two loads on it and stay up in the air for an hour and a half. It has no odor. You hardly see it when it comes out," Sexton told the Examiner in a 2013 article. "It’s a quick kill and is very effective on the West Nile breeding mosquitoes.”

Less than 1 percent of St. Louis encephalitis virus infections are clinically apparent and the vast majority of infections remain undiagnosed, the CDC website states. The incubation period for SLEV disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days. Onset of illness is usually abrupt, with fever, headache, dizziness, nausea, and malaise. Signs and symptoms intensify over a period of several days to a week. Some patients spontaneously recover after this period; others develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors and unsteadiness. Coma can develop in severe cases. The disease is generally milder in children than in older adults. About 40 percent of children and young adults with SLEV disease develop only fever and headache or aseptic meningitis; almost 90 percent of elderly persons with SLEV disease develop encephalitis. The overall case-fatality ratio is 5 to 15 percent. The risk of fatal disease also increases with age. If you think you or a family member may have SLEV neuroinvasive disease, it is important to consult your healthcare provider for proper diagnosis.

"We haven't heard of any human cases of anything this year in Jefferson County, so this is just kind of a precautionary thing," Sexton said. "People in the area just need to be a little cautious when they go outside."

Most cases of SLEV disease have occurred in eastern and central states; however, an epidemic of SLEV occurred in Dallas in 1966. According to an American Journal of Epidemiology article, a total of 545 suspected cases within Dallas city and county were reported, of which 146 were laboratory-confirmed as SLEV infection. The overall attack rate was 15.2 per 100,000, with a case fatality rate 9.7 percent. During the course of the epidemic, most of the county was sprayed aerially with an ultra-low volume, high concentration malathion mist. The spraying clearly reduced the number and infection rate of vector mosquitoes, the article states.