Differential Diagnosis: Patients with VEE lacking neurological symptoms may be difficult to distinguish from patients with other illnesses such as influenza, dengue fever, prodromal Legionnaire's disease, or measles. Patients with neurological symptoms should suggest the diagnosis. More common bacterial and viral as well as fungal and parasitic causes of meningitis and encephalitis should be considered in patients with neurological symptoms. Rickettsial, ehrlichial, and leptospiral illnesses should be considered in patients with headache and fever accompanied by neutropenia, thrombocytopenia, or elevated liver function tests. Other potential bioterrorist agents that may be associated with flu-like prodromes (Bacillus anthracis, Yersinia pestis, Coxiella burnetii, Ebola, and smallpox) need to be considered. Other potential bioterrorist agents that cause flu-like illnesses (Bacillus anthracis, Yersinia pestis, Coxiella burnetii) need to be considered. More common causes of meningitis and encephalitis should be considered in patients with neurological symptoms.
Diagnostic Tests: Prior to the onset of encephalopathy, VEE may be diagnosed by virus isolation from blood (collected without anticoagulant), CSF, or throat swab (up to 5 days); serology (on either serum or CSF); and PCR. IgM in a single serum sample (taken 5-7 days after onset) provides rapid presumptive diagnosis. However, this is only in persons without prior known exposure to VEE complex viruses. Diagnosis also can be confirmed either by antigen-capture enzyme-linked immunosorbent assay (ELISA) or reverse transcriptase polymerase chain reaction (RT-PCR) using a single serum sample or CSF taken early in the febrile, viremic phase. Viremia is brief and terminates as soon as antibodies develop.
Specimen Submission: All specimens must be triple-contained in an approved shipping container and have biohazard labels. Specimens for viral culture must be shipped overnight on ice (+4°C), or frozen on dry ice (–80°C) if delays are anticipated; serology specimens may be shipped at room temperature. Culture is time-consuming and must be performed in BSL3 facilities. The receiving laboratory must be alerted prior to transport by calling (800) 252-8239 (“press 1”). Newly available diagnostic tests may be discussed at that time Specimens must be accompanied by a Specimen Submission Form (G-1A) and submitted to the Texas Department of State Health Services Laboratory, 1100 West 49th Street, Austin, TX 78756.
Additional Tests: Leukopenia and lymphopenia are common. Thrombocytopenia may occur. Elevated serum glutamic-oxaloacetic transaminase (SGOT) levels are common. CSF may be under increased pressure in cases with encephalitis and could contain up to 1000 white cells/mm3 (predominantly mononuclear cells) and exhibit mildly elevated protein concentration.