Encephalitis (Arboviral)

Table of Contents

Disease Information

Overview

Potential Bioterrorism Agent: Category B

Includes: St. Louis Encephalitis (SLE), Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), and LaCrosse Encephalitis (LAC), West Nile Virus (WNV)

Responsibilities

  • Hospital: Report by IDSS, facsimile, mail, or phone
  • Hospital Infection Preventionist: Follow-up required
  • Lab: Report by IDSS, facsimile, mail, or phone
  • Physician: Report by facsimile, mail, or phone

Local Public Health Agency (LPHA): Follow-up Required

Iowa HHS
Disease Reporting Hotline: (800) 362-2736
Secure Fax: (515) 281-5698

A. Etiologic Agent

There are hundreds of viruses worldwide that are spread through insects or other arthropods. More than 30 of these arboviruses have been identified as human pathogens in the Western Hemisphere. In Iowa, four mosquito-borne arboviruses that cause encephalitis in humans have been identified: LaCrosse encephalitis (LAC), St. Louis encephalitis (SLE), Western Equine encephalitis (WEE), and West Nile virus (WNV).

B. Clinical Description 

Encephalitis is an inflammation of the brain. Arboviral infection may result in an acute febrile illness of variable severity and rate of progression associated with neurologic symptoms ranging from headache to aseptic meningitis (inflammation of the linings of the brain and spinal cord) to encephalitis. Many arboviral infections are asymptomatic. Arboviral encephalitis cannot be distinguished clinically from many other causes of encephalitis. Manifestations can include headache, confusion, lethargy, nausea, altered consciousness, vomiting, fever, cranial nerve palsies, paresis (muscular weakness) or paralysis, sensory deficits, altered reflexes, tremors, convulsions, abnormal movements, coma of varying degree, and, in some cases, death. Case-fatality rates range from less than 1% - 60%.

LAC encephalitis initially presents as a nonspecific summertime illness with fever, headache, nausea, vomiting and lethargy. Severe disease occurs most commonly in children under the age of 16 and is characterized by seizures, coma, paralysis, and a variety of neurological sequelae after recovery. Death from LAC encephalitis occurs in less than 1% of clinical cases. In many clinical settings, pediatric cases presenting with central nervous system (CNS) involvement are routinely screened for herpes or enteroviral etiologies. Since there is no specific treatment for LAC encephalitis, physicians often do not request the tests required to specifically identify LAC virus, and the cases are reported as aseptic meningitis or viral encephalitis of unknown etiology.

Less than 1% of SLE viral infections are clinically apparent and the vast majority of infections remain undiagnosed. Illness ranges in severity from a simple febrile headache to meningoencephalitis, with an overall case-fatality ratio of 5% - 15 %. The disease is generally milder in children than in adults, but in those children who do have disease, there is a high rate of encephalitis. The elderly are at highest risk for severe disease and death.

WNV can infect a wide range of vertebrates; in humans it usually produces either asymptomatic infection or mild febrile disease, but can cause severe and fatal infection in a small percentage of patients. With WNV infections, mild infections are common and include fever, headache, and body aches, often with a skin rash and swollen lymph glands. More severe infections are often associated with high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions, paralysis, and rarely, death. Case-fatality rates for WNV range from 3% - 15% of cases with clinical encephalitis. For more information on WNV see the WNV section in this manual.

Most arboviral infections are asymptomatic. Clinical disease ranges from mild febrile illness to severe encephalitis. For the purposes of surveillance and reporting, based on their clinical presentation, arboviral disease cases are often categorized into two primary groups: neuroinvasive disease and non-neuroinvasive disease.

Neuroinvasive disease

Many arboviruses cause neuroinvasive disease such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP). These illnesses are usually characterized by the acute onset of fever with stiff neck, altered mental status, seizures, limb weakness, cerebrospinal fluid (CSF) pleocytosis, or abnormal neuroimaging. AFP may result from anterior ("polio") myelitis, peripheral neuritis, or post-infectious peripheral demyelinating neuropathy (i.e., Guillain-Barré syndrome). Less common neurological manifestations, such as cranial nerve palsies, also occur.

Non-neuroinvasive disease

Most arboviruses are capable of causing an acute systemic febrile illness (e.g., West Nile fever) that may include headache, myalgias, arthralgias, rash, or gastrointestinal symptoms. Rarely, myocarditis, pancreatitis, hepatitis, or ocular manifestations such as chorioretinitis and iridocyclitis can occur.

C. Reservoirs

LAC virus is a zoonotic pathogen cycled between the daytime-biting treehole mosquito, Aedes triseriatus, and vertebrate amplifier hosts (chipmunks, tree squirrels) in deciduous forest habitats. The virus is maintained over the winter by transovarial transmission in mosquito eggs. If the female mosquito is infected, she may lay eggs that carry the virus, and the adults coming from those eggs may be able to transmit the virus to chipmunks and humans.

During the summer season, SLE virus is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and Culex mosquitoes. In Florida, the principal vector is Cx. nigripalpus, in the Midwest, Cx. pipiens and Cx. p. quinquefasciatus and in the western United States, Cx. tarsalis and members of the Cx. pipiens complex.

Birds carry WNV.  The virus usually stays in birds and the mosquitoes that feed on them. Rarely, other kinds of mosquitoes that also bite people and horses pick up the viruses. Humans and horses are generally considered dead-end hosts.  WNV is transmitted principally by Culex species mosquitoes, but also can be transmitted by Aedes, Anopheles, and other species.

D. Modes of Transmission

The bite of an infected mosquito transmits the four types of arboviral encephalitis found in Iowa.  WNV can also be transmitted by blood transfusion.  See the WNV chapter for further information.

E. Incubation period

Incubation period for LaCrosse (LAC), Eastern Equine Encephalitis (EEE), Saint Louis Encephalitis (SLE) and West Nile Encephalitis (WNV) is 5 - 15 days.  

F. Period of Communicability or Infectious Period

The arboviral encephalitides are not communicable from person-to-person.

G. Epidemiology

LaCrosse (LAC) encephalitis was discovered in La Crosse, Wisconsin in 1963. Since then, the virus has been identified in several Midwestern and Mid-Atlantic states. During an average year, 80-100 cases of LAC encephalitis are reported in the U.S. to the Centers for Disease Control and Prevention (CDC).     

Western equine encephalitis (WEE) was first isolated in the United States in 1930. In 1941, a U.S. WEE epidemic involved 300,000 horses and 3,340 humans. Since then, occasional smaller epidemics have occurred.     

St. Louis encephalitis (SLE) is the leading cause of epidemic flaviviral encephalitis in the U.S. SLE is the most common mosquito-transmitted human pathogen in the U.S. While periodic SLE epidemics have occurred only in the midwest and southeast, SLE virus is distributed throughout the lower 48 states. Since 1964, case numbers have fluctuated widely and there have been more than 4,000 confirmed cases of SLE with an average of 100 cases per year (range 2 - 1,967).

WNV was first isolated in the West Nile Province of Uganda in 1937. The first recorded epidemics occurred in Israel during 1951-1954 and in 1957. Epidemics have been reported in Europe in the Rhone delta of France in 1962 and in Romania in 1996. The largest recorded epidemic occurred in South Africa in 1974.

An outbreak of arboviral encephalitis in New York City and neighboring counties in New York state in late August and September 1999 was confirmed as caused by West Nile virus based on the identification of virus in human, avian, and mosquito samples.  By the end of October 1999, WNV had been confirmed in multiple native species of birds from New York City and areas within a 200-mile radius. WNV has also been found to cause encephalitis in horses.  By 2006, WNV had spread to 48 states in the United States.  The first human case of WNV occurred in Iowa in 2002: a total of 37 human cases were reported in 2006.  There were 31 cases of WNV reported in Iowa in 2012.

Over the last 10 years Iowa has averaged 2 cases of arboviral encephalitis other than West Nile per year.

H. Bioterrorism Potential

Category B Agent.  Eastern Equine encephalitis (EEE) virus, is recognized as a category B bioterrorism agent by the CDC because it is moderately easy to disseminate, results in moderate morbidity rates and relatively high mortality rates; and requires specific enhancements of CDC's diagnostic capacity and enhanced disease surveillance.

I. Additional Information

The Council of State and Territorial Epidemiologists (CSTE) surveillance case definitions for arboviral encephalitis can be found at: https://ndc.services.cdc.gov/conditions/encephalitis-arboviral/

CSTE case definitions should not affect the investigation or reporting of a case that fulfills the criteria in this chapter. (CSTE case definitions are used by the state health department and the CDC to maintain uniform standards for national reporting.)

Fact Sheets and Forms

Comment

Because closely related arboviruses exhibit serologic cross-reactivity, positive results of serologic tests using antigens from a single arbovirus can be misleading. In some circumstances (e.g., in areas where two or more closely related arboviruses occur, or in imported arboviral disease cases), it may be epidemiologically important to attempt to pinpoint the infecting virus by conducting cross-neutralization tests using an appropriate battery of closely related viruses. This is essential, for example, in determining that antibodies detected against St. Louis encephalitis virus are not the result of an infection with West Nile (or dengue) virus, or vice versa, in areas where both of these viruses occur. Because dengue fever and West Nile fever can be clinically indistinguishable, the importance of a recent travel history and appropriate serologic testing cannot be overemphasized. In some persons, West Nile virus-specific serum IgM antibody can wane slowly and be detectable for more than one year following infection. Therefore, in areas where West Nile virus has circulated in the recent past, the co-existence of West Nile virus-specific IgM antibody and illness in a given case may be coincidental and unrelated. In those areas, the testing of serially collected serum specimens assumes added importance.

The seasonality of arboviral transmission is variable and depends on the geographic location of exposure, the specific cycles of viral transmission, and local climatic conditions. Reporting should be etiology-specific (see below; the six diseases printed in bold are nationally reportable to CDC):

  • St. Louis encephalitis virus disease
  • West Nile virus disease
  • Powassan virus disease
  • Eastern equine encephalitis virus disease
  • Western equine encephalitis virus disease
  • California serogroup virus disease (includes infections with the following viruses: California encephalitis, Jamestown Canyon, Keystone, La Crosse, snowshoe hare, and trivittatus)

Note: Due to the continued risk of unintentional or intentional introduction of exotic arboviruses into the United States (e.g., Venezuelan equine encephalitis virus), or the reemergence of indigenous epidemic arboviruses (e.g., St. Louis encephalitis and western equine encephalitis viruses), physicians and local public health officials should maintain a high index of clinical suspicion for cases of potential exotic or unusual arboviral etiology, and consider early consultation with arboviral disease experts at state health departments and CDC.

References

American Academy of Pediatrics. 2006 Red Book: Report of the Committee on Infectious Diseases, 27th Edition. Illinois, American Academy of Pediatrics, 2003.

CDC Website. Information on Arboviral Encephalitis. Available at < www.cdc.gov/ncidod/dvbid/arbor/index.htm >

CDC Website. West Nile Virus. Available at < www.cdc.gov/ncidod/dvbid/westnile/index.htm  >.

Heymann, D. L, ed. Control of Communicable Diseases Manual, 20th Edition. Washington, DC, American Public Health Association, 2015.

Evans, A. Viral Infections of Humans: Epidemiology and Control, Second Edition. New York City, Plenum Medical Book Company, 1984.

Moellering, R. Infectious Disease Clinics of North America: Animal- Associated Human Infections. Philadelphia, W.B. Saunders Co., 1991.

Additional Resources

Additional information regarding EEE, pesticide use, occupational exposures and other topics may be obtained using the following websites:

Centers for Disease Control and Prevention www.cdc.gov/EasternEquineEncephalitis/index.html

Centers for Disease Control and Prevention www.cdc.gov/ncidod/dvbid/westnile/index.htm

Centers for Disease Control and Prevention www.cdc.gov/lac/

Centers for Disease Control and Prevention www.cdc.gov/sle/

Centers for Disease Control and Prevention www.cdc.gov/ncidod/dvbid/arbor/weefact.htm