IDT Biologika GmbH: Remarks on the interpretation of test results

Representative isolates of the different subtypes were used here for the analysis of influenza A virus-specific antibodies via the hemagglutination inhibition test. These are tested regularly for their validity for the subtypes occurring in the field.

Seroconversion with influenza normally begins after the 7th day after infection, with a peak to be expected on the 14th day after infection. Therefore, these test results can only be interpreted for the herd status 7 to 14 days before the collection of the serum samples. Should clinical signs of respiratory disease have occurred in the herd shortly before the sampling, we recommend that additional samples be taken at a later time.

Very high antibody titers (=320 to 2560) in the majority of animals of an age group are suggestive of a relatively recent infection of this group.
Low antibody titers (20 to 80) are suggestive of previous infections or infection with subtypes that have a lower virulence or that cross-react (especially in the case of pandemic subtypes).

In general, antibodies will disappear three to four months after infection; only after re-occurring infections with the same subtype does the level stay the same.
If only a low titer of antibodies is detected in just a few animals of a herd, this may be an unspecific test reaction or indicate that these individual animals had contact to influenza prior to their inclusion in the herd. It is also possible that these are residual maternal antibodies of younger animals.
Gilts born to sows that have been vaccinated repeatedly in general do not (or do not clearly) sero-convert after initial vaccination. Challenge studies have however shown that these animals are nonetheless protected from clinical signs.

If the sampled pigs have had several contacts with different subtypes of influenza A virus, cross-reactions to the pandemic subtypes in the test may occur. For this reason, positive antibody titers of the pandemic subtypes must be interpreted with caution. Results are certain only if antibodies are found solely against the pandemic subtypes, or if the titers of the pandemic subtypes are much higher than those against the other subtypes of the test. In the latter case, this may not occur with all sera from a group of animals.