Equine Encephalitis Viruses: Eastern Equine, Venezuelan Equine, and Western Equine

Reportable by Providers and Laboratories

About this Reportable Disease

Infectious agent: These three viruses are members of the genus Alphavirus, family Togaviridae.

Description of illness: Systemic infection has an abrupt onset and is characterized by chills, fever, malaise, arthralgia, and myalgia. The illness lasts 1 to 2 weeks, and recovery is complete when there is no central nervous system involvement. In infants, the encephalitic form is characterized by abrupt onset; in older children and adults, encephalitis is manifested after a few days of systemic illness. Signs and symptoms in encephalitic patients are fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, cyanosis, convulsions, and coma. Approximately a third of all people with EEE die from the disease. Death usually occurs 2 to 10 days after onset of symptoms but can occur much later. Of those who recover, many are left with disabling and progressive mental and physical sequelae, which include can range from minimal brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction. Many patients with severe sequelae die within a few years. EEE has been reported in equines in Tennessee but so far not in humans. VEE and WEE have not been reported in Tennessee.

Interactive Disease Data

Laboratory Reporting for this Disease

Arboviral Panel (IgG and IgM Antibody Assay)

TEST NAME	Arboviral Panel
DISEASE(S)/DISORDER(S)	Eastern Equine Encephalitis Western Equine Encephalitis St. Louis Encephalitis California Encephalitis (LaCrosse)
ALTERNATIVE NAME(S)	None
METHODOLOGY	Focus Diagnostics' Arbovirus IFA IgM Focus Diagnostics' Arborvirus IFA IgG
SPECIAL INSTRUCTIONS	Send acute sample at time of collection; do not wait for convalescent sample to be drawn.
ORDERING INFORMATION	Lab Web Portal PTBMIS: Test Order Code: ARBOVRS Test Order Description: Arbovirus Panel

Specimen Requirements

Patient Preparation	Paired Serum Samples: - Acute: drawn at illness onset and - Convalescent: drawn 7 to 14 days later Note: Send acute sample at time of collection; do not wait for convalescent sample to be drawn.
Specimen Collection	Asepticallly collect whole blood in Red Top tube (no anticoaglulant).
Specimen Labeling	Specimen must be labeled with at least two unique patient identifiers and match accompanying test order.
Specimen Processing	Allow sample to clot at room temperature prior to centrifugation. Aseptically transfer serum to a sterile, plastic, screw-capped vial.
Specimen Storage and Preservation	Store serum at 2 - 8°C. If shipping is to be delayed for longer than five (5) days, the specimen should be frozen at (-20)°C or colder.
Specimen Transportation	Ship specimen at 2 - 8°C. If specimen has been frozen, ship on Dry Ice. Specimen must be placed in an individual biohazard bag with absorbent material and should be shipped to the laboratory on the day of collection. All infectious substance shipments must conform to U.S. Department of Transportation (DOT) Hazardous Materials Regulations (HMR 49 C.F.R. Parts 171-180).
Specimen Acceptability and Rejection	The following submissions will not be tested: - Specimens not properly identified with the patient's name and date of birth and matching the submission form. - Convalescent serum that is collected sooner than seven (7) days from the collection date of the acute sample.
Testing Location	Knoxville

Laboratory Reporting

Reporting requirements apply to all laboratories located within Tennessee, as well as laboratories outside of Tennessee that test residents of Tennessee, including laboratories located within healthcare facilities. Healthcare providers and laboratories in the same healthcare facility both have a duty to report. The type of organisms and analytes laboratories must report to TDH for 2026 are indicated, and there are several ways laboratories can report results to TDH.

Manually report results to TDH by faxing or mailing a completed PH-1600 form to your local health department, or faxing to the state health office at (615) 741-3857
Automatically submit results to TDH via electronic laboratory reporting (ELR), which automates the process of sharing data with TDH using interoperability standards.
- See the ELR Onboarding Handbook for details on the onboarding process, checklist, frequently asked questions, business rules, message format, and vocabulary.
- To initiate the ELR onboarding process with TDH, register in the Trading Partner Registration (TPR) system TPR provides documentation for Promoting Interoperability (PI) attestation and milestone letters to document onboarding progress. Contact MU.Health@tn.gov for assistance.
Submit online via NBS. NBS is TDH's reportable disease system. To request an NBS account for reporting Complete this user survey to request an NBS account for reporting.
Blood lead levels can be sent via fax ( (615) 741-3857), entered online, or reported using the instructions at this link.

Information about this Reportable Disease for Healthcare Providers

Clinical Summary

Equine encephalitis viruses—Eastern (EEEV), Western (WEEV), and Venezuelan (VEEV)—are mosquito-borne alphaviruses that can cause an abrupt systemic febrile illness (chills, fever, malaise, muscle and joint aches) and, in some patients, neuroinvasive disease (meningitis/encephalitis).
For Eastern equine encephalitis, most infections are asymptomatic, but severe neurologic disease occurs in a small proportion; CDC notes EEEV incubation is typically 4–10 days and that severe EEE has a high case-fatality with frequent long-term neurologic sequelae among survivors.
For Western equine encephalitis, CDC notes WEEV is mosquito-borne and can range from a febrile illness to severe neurologic disease; no human cases have been reported in the U.S. since 1999, but clinicians should consider it in compatible illness with relevant travel/exposure history and coordinate testing with public health.
Diagnosis for suspected equine encephalitis virus infection is typically coordinated with public health. CDC emphasizes serology (IgM) with confirmatory neutralizing antibody testing for EEEV (and molecular testing in select severely immunocompromised patients), and there is no specific antiviral treatment for EEEV or WEEV—management is supportive (e.g., stabilization, fluids, and management of complications). Tennessee lists equine encephalitis viruses as reportable conditions.

Clinical Links & Info

Review the case definition | CDC

Access the case report form (2018) | TDH

Healthcare Provider Reporting

Healthcare reporting requirements apply to all providers located within Tennessee, as well as providers whose patients reside in Tennessee.

Providers must report cases of all diseases and conditions listed through one of these methods:

• Mail or fax a completed PH-1600 form to your local health department or fax to the state health office at (615) 741-3857

• Send automatically via electronic case reporting (eCR). See this TDH webpage for more information on eCR, register at the Trader Partner Registration website, or contact MU.Health@tn.gov for assistance.

• Submit online via NBS. NBS is TDH's reportable disease system. To request an NBS account for reporting Complete this user survey to request an NBS account for reporting.

• Blood lead levels can be sent via fax ( (615) 741-3857), entered online, or reported using the instructions at this link.

Information about this Reportable Disease for the Public

What It Is

Equine encephalitis viruses—Eastern equine encephalitis virus (EEEV), Venezuelan equine encephalitis virus (VEEV), and Western equine encephalitis virus (WEEV)—are mosquito-borne viruses that can cause illness in people and equines (such as horses). These viruses belong to the alphavirus group and can cause a sudden flu-like illness and, in some cases, inflammation of the brain (encephalitis).

How It Spreads

People become infected mainly through the bite of an infected mosquito. These viruses are not spread through casual person-to-person contact. Reducing mosquito bites (repellent, protective clothing, and limiting exposure when mosquitoes are active) lowers risk.

Signs and Symptoms

Many infections may cause no symptoms. When illness occurs, it can start with fever and other flu-like symptoms; severe disease can include neurologic symptoms such as severe headache, vomiting/diarrhea, seizures, drowsiness, or behavioral changes (especially described for EEE). Because encephalitis can be life-threatening, seek urgent medical care for sudden severe symptoms after mosquito exposure.

Prevention

There are no human vaccines or specific medicines to treat eastern equine encephalitis, so prevention focuses on avoiding mosquito bites and supporting mosquito control efforts. Similar mosquito-bite prevention measures also reduce risk for other mosquito-borne encephalitis viruses.

Reporting and Public Health Response in Tennessee

In Tennessee, equine encephalitis viruses (Eastern, Venezuelan, and Western) are reportable. Reporting helps public health track cases, identify risk, and guide prevention messaging and mosquito control actions.

This Page Last Updated: March 25, 2026 at 8:53 PM