Serological responses to two serovar-independent ELISA antigens ofActinobacillus pleuropneumoniaein Australian commercial pig herds

The challenge of detecting herds sub-clinically infected with Actinobacillus pleuropneumoniae

The introduction into a naïve herd of animals sub-clinically infected with Actinobacillus pleuropneumoniae (App) is frequently the cause of clinical pleuropneumonia and the identification of such infected herds is a priority in the control of disease. Different serological tests for App have been developed and a number of these are routinely used. Some are species-specific whereas others identify more specifically the serotype/serogroup involved which requires updated information about important serotypes recovered from diseased pigs in a given area/country. Serotyping methods based on molecular techniques have been developed lately and are ready to be used by most diagnostic laboratories. When non-conclusive serological results are obtained, direct detection of App from tonsils is sometimes attempted. This review addresses different techniques and approaches used to monitor herds sub-clinically infected by this important pathogen.

Evaluation of recombinant Mycoplasma hyopneumoniae P97/P102 paralogs formulated with selected adjuvants as vaccines against mycoplasmal pneumonia in pigs

Pig responses to recombinant subunit vaccines containing fragments of eight multifunctional adhesins of the Mycoplasma hyopneumoniae (Mhp) P97/P102 paralog family formulated with Alhydrogel(®) or Montanide™ Gel01 were compared with a commercial bacterin following experimental challenge. Pigs, vaccinated intramuscularly at 9, 12 and 15 weeks of age with either of the recombinant formulations (n=10 per group) or Suvaxyn(®) M. hyo (n=12), were challenged with Mhp strain Hillcrest at 17 weeks of age. Unvaccinated, challenged pigs (n=12) served as a control group. Coughing was assessed daily. Antigen-specific antibody responses were monitored by ELISA in serum and tracheobronchial lavage fluid (TBLF), while TBLF was also assayed for cytokine responses (ELISA) and bacterial load (qPCR). At slaughter, gross and histopathology of lungs were quantified and damage to epithelial cilia in the porcine trachea was evaluated by scanning electron microscopy. Suvaxyn(®) M. hyo administration induced significant serological responses against Mhp strain 232 whole cell lysates (wcl) and recombinant antigen F3P216, but not against the remaining vaccine subunit antigens. Alhydrogel(®) and Montanide™ Gel01-adjuvanted antigen induced significant antigen-specific IgG responses, with the latter adjuvant eliciting comparable Mhp strain 232 wcl specific IgG responses to Suvaxyn(®) M. hyo. No significant post-vaccination antigen-specific mucosal responses were detected with the recombinant vaccinates. Suvaxyn(®) M. hyo was superior in reducing clinical signs, lung lesion severity and bacterial load but the recombinant formulations offered comparable protection against cilial damage. Lower IL-1β, TNF-α and IL-6 responses after challenge were associated with reduced lung lesion severity in Suvaxyn(®) M. hyo vaccinates, while elevated pathology scores in recombinant vaccinates corresponded to cytokine levels that were similarly elevated as in unvaccinated pigs. This study highlights the need for continued research into protective antigens and vaccination strategies that will prevent Mhp colonisation and establishment of infection.

Evaluation of diagnostic assays for the serological detection of Actinobacillus pleuropneumoniae on samples of known or unknown exposure

Accurate diagnosis of exposure to Actinobacillus pleuropneumoniae is important for maintaining negative farms. In the present study, the ability of a dual-plate complement fixation (CF) assay and 3 commercially available enzyme-linked immunosorbent assays (ELISAs; quad-plate ELISA-1, single-plate ELISA-2, and single-plate ELISA-3) in detecting serological evidence of A. pleuropneumoniae exposure was compared using serum samples of experimentally infected or vaccinated pigs, or field samples from the United States. Forty-two pigs were divided into groups of 2 pigs and were inoculated with 1 of 15 A. pleuropneumoniae strains representing all known serovars of A. pleuropneumoniae, or with Actinobacillus suis, or were vaccinated with a bacterin containing A. pleuropneumoniae serovar 1, 3, 5, or 7. Serum samples collected at the day of inoculation or vaccination and 7, 14, 21, and 28 days later were used to compare the assays. On samples from experimentally infected pigs, the dual-plate CF assay, quad-plate ELISA-1, single-plate ELISA-2, and single-plate ELISA-3 had sensitivities of 0.46, 0.74, 0.13, and 0.13 and specificities of 0.90, 1.0, 1.0, and 1.0, respectively. Vaccinated pigs were identified only by the dual-plate CF assay and the quad-plate ELISA-1. In addition, 90 serum samples with unknown A. pleuropneumoniae exposure collected under field conditions were tested with all assays. The agreement of the 4 assays on field samples was slight to fair. While several assays are available for demonstration of A. pleuropneumoniae exposure, differences in assay targets complicate test choices. Decisions on which assay or combination of assays to use depend on the specific reasons for running the assays.

Evaluation of serovar-independent ELISA antigens of Actinobacillus pleuropneumoniae in pigs, following experimental challenge with A. pleuropneumoniae, Mycoplasma hyopneumoniae and Pasteurella multocida

OBJECTIVE

To compare the sensitivity and specificity of six serological enzyme-linked immunosorbent assays (ELISAs) based on serovar-independent antigens of Actinobacillus pleuropneumoniae (App) and investigate cross-reactivity in disease-free pigs challenged with Mycoplasma hyopneumoniae and Pasteurella multocida.

DESIGN

Five experimental pig trials using direct challenge with App serovars 1, 7 or 15 or direct challenge with M. hyopneumoniae and/or various dose rates of P. multocida.

PROCEDURE

A 39-kDa outer membrane protein antigen and five recombinant antigens from the apxIVA gene of App were evaluated. The latter were derived from the ApxIVA N-terminus (ApxIVA-N, ApxIVA-NP, ApxIVA-NPS) or C-terminus (ApxIVA-C, ApxIVA-CP). Pigs were sampled after challenge and clinical and necropsy findings evaluated.

RESULTS

The 39-kDa ELISA had high sensitivity but lacked specificity, with significantly increased cross-reactivity following P. multocida challenge. ELISAs based on ApxIVA N-terminus antigens were significantly more sensitive than C-terminus antigens for the detection of App-induced disease. Although ApxIVA-N and ApxIVA-NP ELISAs had increased reactivity following P. multocida challenge, they retained high specificity for App-induced disease (90-93%). Affinity purified ApxIVA-NP antigen had marginally better specificity than ApxIVA-N, without reduced sensitivity. Mycoplasma hyopneumoniae did not affect serological cross-reactivity. In disease-free pigs, the specificity of the ApxIVA-NPS ELISA may be adversely affected by nasal carriage of apparently low-virulence App strains.

CONCLUSIONS

ApxIVA-N-based ELISAs can be used for evaluating App status in commercial herds, but some appear limited by high carriage rates of low-virulence App. The 39-kDa antigen is only of merit in exclusion of App disease by negative serology.