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Equine Influenza Virus
Christine My the ELISA. Studies in humans found that diagnostic sensitivity improved when infected mucosal cells, in addition to secretions, were included in the sample. The distinct advantage of the ELISA is that results can be available within 15 minutes of setting up the test. Unfortunately, a negative result does not definitively rule out the presence of influenza virus.Despite these shortcomings, in an outbreak situation in which you are sampling multiple horses, the antigen-capture ELISA provides a valuable method of rapidly identifying the presence of virus (ie, sensitivity increases, the more horses you sample). 3. Polymerase Chain Reaction (PCR). PCR detects segments of the viral RNA from the nasopharyngeal swab and is a highly sensitive test that isbecoming more readily available. Reverse transcription-PCR (RT-PCR) techniques have been described using nested PCR with primers from the nucleoprotein gene and a single set of primers from the matrix gene. These techniques were compared by Quinlivan and coworkers13 who found that the more sensitive method was using the single set of primers from the matrix gene. This method was also less prone tocontamination. PCR was found to have a sensitivity equivalent to or higher than virus isolation. Reverse transcription-PCR has been used to sequence the entire virus, allowing the subtype and strain to be determined. This technology may prove useful in surveillance programs in the future when difficulties with virus isolation are encountered. 4. Immuno-PCR. An immuno-PCR technique that combines ELISAwith PCR to detect the nonstructural protein (NS1) and hemagglutinin of equine influenza virus was described by Ozaki and coworkers16 and was found to have a higher sensitivity than RT-PCR. This test is currently not readily available. 5. Paired Serum Antibody Titers. Paired serology has historically been the diagnostic technique of choice and remains very useful, particularly when PCR, ELISA, andvirus isolation are not available. The first of two paired serum samples is collected as early in the disease course as possible (acute phase) and the second serum sample is collected 2 weeks later (convalescent phase). Serum harvested from the acute phase sample should be frozen and submitted at the same time as the convalescent sample. Antibody titers can be measured using either thehemagglutination inhibition (HI) test, virus neutralization (VN) test, or single radial hemolysis (SRH). Confirmation of equine influenza is made when the titer in the convalescent sample is equal to or greater than four times higher than the acute titer (HI and VN tests) or has increased by 50% or 25 mm2 on the SRH test.17 Interpretation may be complicated in horses that already have a high acute phase titer byvirtue of vaccination, prior exposure, or a delay in collection of the acute phase sample. The SRH test is more reproducible and more sensitive than HI tests and is

191 better able to differentiate repeatedly vaccinated horses; however, it is not widely available because it is more labor intensive than the HI test and, therefore, less practical when sample submission rates are low. An ELISA thatdetects antibodies against hemagglutinin has recently been described and may be more useful than the HI test.18 Serological assays have also been developed that detect antibodies developed against the nonstructural protein (NS1).16 NS1 antibodies are not produced following vaccination; therefore, this test would be useful for differentiating vaccinated from nonvaccinated horses.

TreatmentOnce a horse is demonstrating clinical signs of equine influenza (cough, fever, serous nasal discharge), treatment is largely symptomatic. Strict stall rest and adequate nutrition cannot be emphasized enough, to allow the horse a complete and uncomplicated recovery. Recommendations for rehabilitation are 1 week of rest for each day of fever (approximately 30-60 days). Returning to work before this...