Serologic profiling of Haemophilus parasuis-vaccinated sows and their litters using a novel oligopeptide permease A enzyme-linked immunosorbent assay reveals unexpected patterns of serological response and maternal antibody transfer.

Haemophilus parasuis is an economically important swine pathogen with 15 recognized serovars. An enzyme-linked immunosorbent assay (ELISA) was developed that detects serum antibodies to the oligopeptide permease A (OppA) polypeptide membrane protein present in the reference strains for 13 of the H. parasuis serovars. Using the OppA-ELISA, H. parasuis serologic profiles were assessed on 2 swine farms, with seroconversion defined as an OppA-ELISA sample-to-positive (S/P) ratio ≥0.5. Ten gilts from each farm were vaccinated for H. parasuis using either a live avirulent culture vaccine (farm 1) or an inactivated autogenous vaccine (farm 2). Seroconversion occurred in 100% of farm 1 gilts and 90% of farm 2 gilts, with a mean S/P ratio (MSPR) of 3.36 and 1.43, respectively. The OppA-ELISA MSPRs were determined for 2 piglets, 1 male and 1 female, randomly selected from 10 first-parity (P1), 10 second-parity (P2), and 10 third-parity (P3) litters farrowed by respective vaccinated gilts on each farm. On both farms, postfarrowing MSPRs and rate of seropositivity were highest in P1 versus P2 and P3 dams. Parity 1 piglets had higher MSPRs and rates of seropositivity versus later parities, with the difference being significant (P < 0.05) on farm 2. Polymerase chain reaction analysis of nasal swabs indicated that 100% of farm 1 piglets and 47-84%, depending on parity, of farm 2 piglets were H. parasuis-colonized at weaning. The results indicated that H. parasuis vaccination of gilts will not maintain serologic responses in the OppA-ELISA over their reproductive lifetimes, and that maternally derived antibodies do not prevent H. parasuis colonization of piglets.