Determination of hemagglutination-inhibition titers to influenza A virus in porcine sera by use of an enzyme-linked immunosorbent assay

An ELISA-based method to estimate hemagglutination-inhibition (HI) titer was developed. Subtype specificity was obtained by using purified H1 and H3 hemagglutinin antigens. Using the linear relation that exists between ELISA and HI methods, regression lines for H1N1- and H3N2-monospecific porcine antisera were constructed. Approximation of actual HI titer could be obtained from insertion of ELISA values into the appropriate regression line. The HI estimations were within 50% of the actual measured HI value 84% of the time. In young pigs that had suckled immune sows, use of this ELISA revealed estimated HI titer > 320 at 2 and 4 weeks of age. After a typical farm outbreak of influenza A/swine (H1N1), estimated HI titers remain high for 4 to 6 months. Sub-type-specific estimation of the distribution frequency of positive influenza A (H1N1 or H3N2) results for sera from swine in regional herds indicated that 31.3 and 7.4% of the swine tested were positive (HI > 41) for H1N1 and H3N2, respectively. From these observations, we conclude that in many circumstances, an ELISA-based HI estimation method could be used as a substitute for the HI test.