Diagnosis of VEE virus infection in equids requires the demonstration of specific antibodies in paired serum samples collected in the acute and convalescent phases
o PRN antibodies appear within 5–7 days after infection
o CF antibodies within 6–9 days after infection
o HI antibodies within 6–7 days after infection
Vaccination history must be taken into account when interpreting any of the VEE serological test results
o in horses not recently vaccinated with an attenuated live virus strain, demonstration of VEE-specific serum IgM antibodies in a single serum sample supports recent virus exposure.
Any diagnosis of VEE in an individual that is based on seroconversion in the absence of an epizootic should be made with care. Although enzootic subtypes and variants are non-pathogenic for equids, infection will stimulate antibody production to epizootic VEE virus variants