An unusual strain of Haemophilus parasuis that fails to react in a species-specific polymerase chain reaction assay.

A total of 30 nasal swabs from pigs preweaned and 11 nasal swabs from sick weaned pigs on a farm in Queensland, Australia, were cultured for the presence of Haemophilus parasuis. Enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) genotyping and indirect hemagglutination and gel diffusion serotyping were performed on the retrieved H. parasuis isolates. A total of 3 genotypes were recognized among the 42 isolates recovered, and 4 representative isolates of each genotype were found to be nontypeable in the Kielstein/Rapp-Gabrielson serotyping scheme. A total of 20 of the 22 isolates of genotype 1 did not amplify in the species-specific conventional PCR number 1 (cPCR1) based on the 16S ribosomal RNA (rRNA) gene but did give the expected PCR amplicon in 2 other species-specific PCR assays, one of which is also based on the 16S rRNA gene. Nine selected isolates representing all genotypes, both positive and negative in the cPCR1, were sequenced, and all showed a 4-base mutation occurring at the forward primer annealing site. The quadruple base pair substitution from GTGG to TGTT near the 3' end of the forward primer sequence may explain the failure of amplification. Diagnostic laboratories should be aware that such failures can occur and should consider having an alternative PCR available to confirm negative results or, alternatively, use phenotypic characteristics for the identification of suspect H. parasuis isolates.