Viability of Actinobacillus pleuropneumoniae in frozen pig lung samples and comparison of different methods of direct diagnosis in fresh samples

Effects of freezing on ability to detect Mycobacterium avium subsp. paratuberculosis from bovine tissues following culture

Mycobacterium avium subsp. paratuberculosis (MAP) is the bacterium that causes Johne's disease in cattle. Although infected cattle can be identified by examining fecal, blood, or milk samples, the gold standard is identification of MAP in tissue samples postmortem. Although tissue samples are commonly frozen, the ability to detect MAP in frozen-thawed tissue samples has apparently not been reported. We therefore determined the ability to detect MAP in tissue samples following freezing. Tissue samples were collected from calves that were either inoculated (IN) 3 mo prior, or contact-exposed (CE) for 3 mo. Following autopsy, tissues were immediately processed for culture, followed by DNA extraction and detection by qPCR. Samples were categorized as positive or negative based on the cycle threshold (Ct) value. The remaining unprocessed tissue samples were frozen at -80°C. After 18 mo, 50 tissue samples designated MAP-positive were thawed and processed for detection of MAP. Four (8%) samples were qPCR-negative, and Ct values of the remaining 46 samples were higher after freezing. Given the small numerical change in Ct values for MAP-positive samples after 18 mo of frozen storage, freezing and thawing may have had some deleterious effects on MAP detection in tissues. Although the decrease in ability to detect MAP-positive samples was minor for IN calves, there may be a greater effect for CE calves that should be considered when freezing tissue samples.

Survival of Actinobacillus pleuropneumoniae outside the pig

Transmission of Actinobacillus pleuropneumoniae is primarily thought to be via direct transfer of mucus from pig to pig. For transfer between farms, the organism may need to persist in the wet or dried state to be carried on an inanimate surface. The survival of A. pleuropneumoniae was investigated under controlled laboratory conditions. In aqueous suspension, survival was improved by the presence of NaCl and mucin; it was prolonged at lower temperature. In dry state, it survived best on a hydrophobic surface either under desiccated conditions or saturated humidity. Detectable viability was maintained for 3-4 days. When frozen, A. pleuropneumoniae survived for more than 17 weeks at -20 °C, but the viability declined to 0.01% during that time. Survival at -70 °C was effective for long term storage. Results obtained from this investigation would be applicable for sampling method, transport techniques, epidemiological study, and biosecurity implementation.

Evaluation of survival of Actinobacillus pleuropneumoniae and Haemophilus parasuis in four liquid media and two swab specimen transport systems

OBJECTIVE

To determine duration and rates of recovery of Actinobacillus pleuropneumoniae and Haemophilus parasuis from 4 liquid media and 2 swab specimen transport systems and compare findings with those of Escherichia coli.

SAMPLE POPULATION

One strain each of A pleuropneumoniae (biovar 1, serotype 1), H parasuis (serovar 5), and E coli (serotype O149:K91:H19).

PROCEDURE

Strains were incubated in brain heart infusion broth supplemented with horse serum and other nutrients or in horse serum alone, with and without nicotinamide-adenine dinucleotide in both instances, for 150 days at 4 degrees C or room temperature (21 degrees C). Similarly, strains were tested in Stuart and Amies transport systems after storage at room temperature for 8 days.

RESULTS

Colony counts greater than those of the initial inoculum were observed after incubation in horse serum for A pleuropneumoniae but not for H parasuis. Overall, incubation at 4 degrees C in the 4 liquid media resulted in longer recovery duration and higher rates than at room temperature. Culture of H parasuis resulted in lower recovery rates and shorter durations of recovery than culture of A pleuropneumoniae, except for culture in horse serum. Haemophilus parasuis survived longer than A pleuropneumoniae in the transport systems, and all organisms survived longer in the Amies system.

CONCLUSIONS AND CLINICAL RELEVANCE

Survival of A pleuropneumoniae and H parasuis indicated that horse serum prolongs survivability, which may result in exposure of more animals during a prolonged period. The Amies system might be a good choice for collection of clinical samples from animals, especially for recovery of H parasuis.

Detection and subtyping of Actinobacillus pleuropneumoniae strains by PCR-RFLP analysis of the tbpA and tbpB genes

A PCR-based procedure for detection and serotype identification of Actinobacillus pleuropneumoniae strains was developed and evaluated. The A. pleuropneumoniae tbpA and tbpB genes were used as targets for amplification of DNA fragments, with a pair of specific primers for each gene. Amplification with tbpA primers rendered a 2.8-kb PCR product from all 12 A. pleuropneumoniae reference strains as well as from Actinobacillus suis strain CCM 5586, while amplification of a 1.9-kb PCR product was observed when testing ten Haemophilus parasuis strains of different serovars. Amplification of the tbpB gene from A. pleuropneumoniae serotypes 1, 6, 8 and 12, and A. suis CCM 5586 rendered an identical 1.8-kb fragment, while from A. pleuropneumoniae serotypes 2, 3, 4, 7, 9, 10 and 11, and H. parasuis strains it produced a 1.7-kb fragment. No PCR amplification product was observed when examining strains of 19 other swine pathogens or closely related species. The minimal detection limit for whole-cell A. pleuropneumoniae templates was between 5-50 and 3 x 10(2)-3 x 10(3) CFU when tbpA and tbpB specific primers, respectively, were used. Restriction fragment length polymorphism (RFLP) analysis of the PCR-generated products rendered different patterns, easily allowing us to discriminate between A. pleuropneumoniae, H. parasuis and A. suis and, more importantly, to distinguish ten RFLP A. pleuropneumoniae groups (the highest discrimination reported so far for a PCR assay with A. pleuropneumoniae), in such a way that the only serotypes with profiles identical to each other were 4 to 11 and 7 to 9. Moreover, the PCR-RFLP analysis was assayed in 36 A. pleuropneumoniae field isolates and in porcine samples (lungs and nasal swabs from experimentally infected animals). In both cases the system proved to be very efficient in A. pleuropneumoniae identification and serotype discrimination.

Evaluation of a saline boiled extract, capsular polysaccharides and long-chain lipopolysaccharides of Actinobacillus pleuropneumoniae serotype 1 as antigens for the serodiagnosis of swine pleuropneumonia

A saline boiled extract (SBE), capsular polysaccharides (CPS) and long-chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 1 have been evaluated in ELISA for the serodiagnosis of swine pleuropneumonia caused by this serotype. Mean optical densities (ODs) obtained with the three antigens using sera from negative herds as well as from animals experimentally and naturally exposed to A. pleuropneumoniae serotypes 1, 9 or 11 were not significantly different. The positive ELISA reaction with anti-serotypes 9 and 11 was unexpected with the CPS, which are supposed to be serotype-specific; LPS, and to a lesser extent proteins, were present in the CPS and appeared to be responsible for this reaction. In addition, sera from animals exposed to a field strain of A. pleuropneumoniae serotype 3 and to Actinobacillus suis presented a significantly lower mean OD (P < 0.001) when LC-LPS were used. Cross-reacting antigens consisted mainly of LPS core-lipid A present in the SBE and CPS. The specificity and the sensitivity of the ELISA were evaluated using three different cut-off values (the OD plus two, three and four times the standard deviation or SD) obtained with 667 negative sera. The diagnostic sensitivity was of 81% with the three antigens and the different thresholds. The diagnostic specificity was of 84, 86 and 88% for the mean plus two, three and four times the SD respectively using the SBE and the CPS, while that obtained with the LC-LPS was of 96, 98 and 99% using the same thresholds. In conclusion, LC-LPS make an easily obtainable antigen and seem to retain the best specificity while minimizing losses of sensitivity.

Evaluation of counterimmunoelectrophoresis for serotyping Actinobacillus pleuropneumoniae isolates and detection of type-specific antigens in lungs of infected pigs

A rapid, simple, and accurate counterimmunoelectrophoresis technique was developed for serotyping cultures of Actinobacillus pleuropneumoniae as well as for detection of their type-specific antigens in the lung tissues of infected pigs. The counterimmunoelectrophoresis test correctly identified all of the reference antigens and more than 99% of 1,200 field isolates of A. pleuropneumoniae representing the 12 established serotypes within 1 h. Counterimmunoelectrophoresis and coagglutination tests did not differ broadly in sensitivity from each other. Both procedures were more rapid and more sensitive than immunodiffusion and indirect hemagglutination tests. A total of 355 lung tissue samples (130 lungs of pigs that died because of acute respiratory problems, 125 lungs of pigs from herds with chronically infected pleuropneumonia, and 100 lungs from apparently healthy pigs at the slaughterhouse) were examined for the presence of A. pleuropneumoniae type-specific antigens by counterimmunoelectrophoresis, coagglutination, and immunodiffusion tests. A. pleuropneumoniae type-specific antigen was found in all 55 samples from which the bacteria had earlier been isolated and in 27 specimens in which they had not been found. Detection of antigen in the lung tissues by coagglutination and counterimmunoelectrophoresis tests was found to be much simpler and much more rapid than conventional culture isolation. Both counterimmunoelectrophoresis and coagglutination tests were found extremely useful in the diagnosis of acute cases of porcine pleuropneumonia. However, these techniques were able to detect only some of the chronically infected carrier pigs.