Clinical Evaluation & Disease
Diagnosis & Reporting
La Crosse (LAC) virus disease should be considered in any person with a febrile or acute neurologic illness who had recent exposure to mosquitoes, especially during the summer months in areas where virus activity has been reported.
In addition to other more common causes of encephalitis and aseptic meningitis (e.g., herpes simplex virus and enteroviruses), other arboviruses (e.g., West Nile, St. Louis encephalitis, eastern equine encephalitis, and Powassan viruses) should also be considered in the differential etiology of suspected LAC virus disease.
LAC virus disease is a nationally notifiable condition. All cases should be reported to local or state public health authorities in a timely manner. Reporting can assist local, state, and national public health authorities to recognize outbreaks and to implement control measures to reduce future infections.
Clinical Evaluation and Disease
Most LAC virus infections are clinically inapparent, and the vast majority of infections remain undiagnosed. The incubation period for LAC virus disease ranges from 5 to 15 days. Initial symptoms include fever, headache, nausea, vomiting, fatigue, and lethargy. The disease can progress to encephalitis, meningoencephalitis, or aseptic meningitis. Symptoms of encephalitis can include altered mental status, seizures, speech problems (aphasia, dysarthria), paresis or paralysis, movement disorders, and cranial nerve palsies. LAC virus disease is most common in children under the age of 16 years.
In acute LAC virus neuroinvasive disease cases, cerebrospinal fluid (CSF) examination typically shows a mildly elevated white blood cell count and normal glucose; CSF protein is elevated in about one third of cases. The peripheral white blood cell count is usually elevated. Computed tomography (CT) brain scans are usually normal, while electroencephalographic (EEG) abnormalities are more common. EEG results often resemble those seen in cases of herpes simplex encephalitis.
Approximately 1% of LAC virus disease cases are fatal. Neurologic sequelae, including epilepsy, hemiparesis, and cognitive and neurobehavioral abnormalities, have been reported in 6%−15% of all diagnosed cases.