Serological response to Actinobacillus pleuropneumoniae serovar 7 infection in a commercial pig herd

Serological responses to Actinobacillus pleuropneumoniae serovar 7 infection were monitored by enzyme-linked immunosorbent assay in a cohort of 66 pigs between weaning and market. Antibody concentrations were high (63/65 seropositive) at 4 weeks of age but declined to low levels from 8 to 12 weeks. Mean antibody concentrations rose significantly (p less than 0.001) between 12 and 23 weeks. Between 8 and 23 weeks of age, 33 (51.5%) of 64 surviving pigs seroconverted to A pleuropneumoniae serovar 7. Peak antibody concentrations in the seroconverting pigs usually (28/33) occurred at 23 weeks. Seroconversion to A pleuropneumoniae during the grower/finisher phase was not significantly associated (p greater than 0.05) with passive antibody concentrations at 4 weeks of age, lack of vaccination against Mycoplasma hyopneumoniae, or weaning weight. Pleuropneumonic lesions were evident at slaughter in 4 (6.3%) of 64 pigs. A pleuropneumoniae serovar 7 was isolated from 2 of 4 lungs with pleuropneumonia and from another lung with lesions considered typical of enzootic pneumonia.

Humoral antibody response and protective immunity in swine following immunization with the 104-kilodalton hemolysin of Actinobacillus pleuropneumoniae

Five cesarean-derived, colostrum-deprived pigs were given three adjuvant-supplemented subcutaneous and one intravenous injection of the purified 104-kDa hemolysin from serotype 1 Actinobacillus pleuropneumoniae CM-5. Six control animals received phosphate-buffered saline only. Five of six control pigs died within 24 h after challenge. The sixth control pig was moribund and euthanized after 48 h. All six pigs had pleuropneumonia, and A. pleuropneumoniae was isolated from all six lungs. None of the vaccinated pigs died as a result of challenge. After being euthanized, two pigs in this group had no lung lesions but three had chronic pleuropneumonia involving 10, 20, and 40% of the lung tissue. A. pleuropneumoniae was isolated from lung lesions of these three animals but not from the two pigs without lesions. The prechallenge hemolysin-neutralizing antibody titers in the vaccinated pigs were 1:10,900, 1:10,600, 1:4,800, 1:3,900, and 1:3,000, in order of increasing lung involvement. None of the control pigs had neutralizing antibodies. Enzyme-linked immunosorbent assay (ELISA) antibodies to capsule, lipopolysaccharide, and hemolysin were not detected in serum samples collected from the control pigs. In the vaccinated group, prechallenge sera did not contain ELISA antibodies to capsule or lipopolysaccharide. ELISA antibodies to the hemolysin were detected only in the prechallenge and postchallenge serum samples. These results indicate that pigs immunized with the 104-kDa hemolysin of serotype 1 A. pleuropneumoniae are protected against challenge with virulent bacteria. The association between neutralizing antibodies and protection indicates indirectly that the hemolysin is an important virulence factor.

Serodiagnosis of pleuropneumonia using enzyme-linked immunosorbent assay with capsular polysaccharide antigens of Actinobacillus pleuropneumoniae serotypes 1, 2, 5 and 7

Capsular polysaccharide antigens of serotypes 1, 2, 5 and 7 of Actinobacillus pleuropneumoniae were used in enzyme-linked immunosorbent assays (ELISAs) to test sera from experimentally infected and field pigs. Specific reactions were found in sera of experimental pigs with antigens of serotypes 1, 5 and 7 whereas the serotype 2 antigen was cross-reactive. A 1:200 serum dilution was used for testing of 300 sera from 21 swine herds in southern Ontario. Cases of pleuropneumonia had occurred in 11 of these herds, but not in the others. The negative cut-off value was the mean optical density at 405 nm (OD405) + three standard deviations (SD) for 16 negative reference sera. Sera from four pigs naturally infected with Actinobacillus suis were tested and found to react to varying degrees with each of the antigens. Therefore a second cut-off value was determined as the mean OD405 + 2 SD for the A. suis sera. Sera which, in the ELISA produced OD readings above the latter cut-off were considered positive for antibodies to A. pleuropneumoniae; those which were lower than the former cut-off were considered negative. Readings between the two cut-off values may have been due to low positive titers or cross-reactivity, possibly with A. suis, and could not be used to predict pleuropneumonia. Of the pleuropneumonia-free herds, none had positive reactors to serotypes 5 or 7, whereas one and two herds had positive reactors to serotypes 1 and 2, respectively. Of the pleuropneumonia positive herds, six had positive reactors to serotype 1, one to serotype 2, four to serotype 5, and eight to serotype 7.

Capsular polysaccharide antigens for detection of serotype-specific antibodies to Actinobacillus pleuropneumoniae

Capsular polysaccharides (CPS) of serotypes 1, 2, 5 and 7 of Actinobacillus (Haemophilus) pleuropneumoniae were obtained from 18 h culture supernatants by precipitation with hexadecyltrimethyl-ammonium bromide (Cetavlon) followed by extraction with sodium chloride and reprecipitation in ethanol. These crude extracts, and portions purified further by phenol extraction to remove contaminating proteins, were evaluated as antigens for the detection of serotype-specific antibodies to A. pleuropneumoniae in sera from immunized rabbits and swine by an enzyme-linked immunosorbent assay. The crude extracts reacted strongly with homologous antisera, but except for serotype 1 showed considerable cross-reactivity with antisera to other serotypes. Phenol extraction greatly improved the serospecificity of the antigens from serotypes 1, 7 and, to a lesser extent, 5. The serotype 2 CPS antigen showed poor reactivity following phenol extraction, and did not appear as useful for detection of serotype-specific antibodies.

Prevalence of seroreactors to the 104-kilodalton hemolysin of Actinobacillus pleuropneumoniae in swine herds

A rabbit homologous polyclonal antiserum to the 104-kilodalton hemolysin of Actinobacillus pleuropneumoniae serotype 1 strain CM-5 was specifically produced and used in an antigen capture enzyme-linked immunosorbent assay (ELISA) to detect swine serum antibodies to this potentially important virulence factor. Sera from pigs experimentally infected with the most common disease-producing serotypes (serotypes 1, 2, 5, and 7) of A. pleuropneumoniae produced positive results in this ELISA. Of 144 serum samples collected from 10 herds free of pleuropneumonia and 155 serum samples from 11 herds with a history of the disease, 68 (47%) and 148 (95%), respectively, were found positive by the ELISA. In addition, pigs naturally infected with Actinobacillus suis produced antibodies which seroreacted in this ELISA. The results indicated that a high proportion of swine have antibodies seroreactive with the 104-kilodalton hemolysin produced by A. pleuropneumoniae.