Longitudinal assessment of two Mycoplasma hyopneumoniae enzyme-linked immunosorbent assays in challenged and contact-exposed pigs

Improving Mycoplasma hyopneumoniae diagnostic capabilities by harnessing the infection dynamics

Mycoplasma hyopneumoniae remains one of the most problematic bacterial pathogens for pig production. Despite an abundance of observational and laboratory testing capabilities for this organism, diagnostic interpretation of test results can be challenging and ambiguous. This is partly explained by the chronic nature of M. hyopneumoniae infection and its tropism for lower respiratory tract epithelium, which affects diagnostic sensitivities associated with sampling location and stage of infection. A thorough knowledge of the available tools for routine M. hyopneumoniae diagnostic testing, together with a detailed understanding of infection dynamics, are essential for optimizing sampling strategies and providing confidence in the diagnostic process. This study reviewed known information on sampling and diagnostic tools for M. hyopneumoniae and summarized literature reports of the dynamics of key infection outcomes, including clinical signs, lung lesions, pathogen detection, and humoral immune responses. Such knowledge could facilitate better understanding of the performance of different diagnostic approaches at various stages of infection.

Development of an indirect ELISA for detection of anti-Mycoplasma hyopneumoniae IgG in naturally infected pathogen-induced convalescent sera

BACKGROUND

Immunization of pigs with an inactivated Mycoplasma hyopneumoniae vaccine (bacterin) generates hyperimmune serum that contains high concentrations of anti-M. hyopneumoniae IgG. Commercially available IgG-ELISA kits cannot distinguish between anti-M. hyopneumoniae IgG in inactivated bacterin-induced hyperimmune sera and convalescent sera resulting from natural M. hyopneumoniae infection. Establishment of an ELISA to detect anti-M. hyopneumoniae IgG in convalescent sera will facilitate the evaluation of the M. hyopneumoniae status of pig farms.

RESULTS

In this study, we expressed and purified recombinant Mhp366-N protein, which contains an epitope recognized by M. hyopneumoniae convalescent sera but not hyperimmune sera, for use as a coating antigen. For the M. hyopneumoniae convalescent serum IgG-ELISA, the optimal antigen concentration, blocking buffer, blocking time, dilution of serum, incubation time with serum, secondary antibody dilution, secondary antibody incubation time and colorimetric reaction time were 0.25 µg/mL, 2.5 % skim milk, 1 h, 1:500, 0.5 h, 1:10,000, 1 h and 15 min, respectively. Validation of the M. hyopneumoniae convalescent serum IgG-ELISA showed a cut-off value of 0.323, the intra-assay CV ranged from 3.27 to 7.26 %, the inter-assay CV ranged from 3.46 to 5.93 %, and the assay was able to differentiate convalescent sera from antibodies to 7 other porcine respiratory pathogens. The convalescent serum IgG-ELISA detected no anti-M. hyopneumoniae IgG in hyperimmune serum samples while a commercial IgG-ELISA identified 95/145 of these sera as positive. The accuracy of the M. hyopneumoniae convalescent serum IgG-ELISA was comparable to the sIgA-ELISA but better than the commercial IgG-ELISA.

CONCLUSIONS

The convalescent serum IgG-ELISA is a reproducible, sensitive, and specific indirect ELISA to detect anti-M. hyopneumoniae IgG in naturally infected pathogen-induced convalescent sera. This ELISA could be used to carry out large scale surveillance of M. hyopneumoniae infection in pig farms regardless of vaccination status.

Detection and phylogenetic analysis of Mycoplasma hyopneumoniae from Tibetan pigs in western China

Enzootic pneumonia (EP), often caused by Mycoplasma hyopneumoniae, occurs in Tibetan pigs between October and December in Western China. The aim of this study was to determine the prevalence of M. hyopneumoniae in Tibetan pig herds and also the prevalence of infection. M. hyopneumoniae was detected using enzyme-linked immunosorbent assay, polymerase chain reaction (PCR), and 16S rRNA sequencing. Twenty-nine inflammatory gross-lesions were observed in 155 lungs of slaughtered pigs. Invasion of focal lymphocytes was confirmed by paraffin sectioning and hematoxylin-eosin staining of lung sections. Bronchoalveolar lavage fluid (BALF) from slaughtered Tibetan pigs and nasal swabs from others were assayed using PCR. The prevalence of M. hyopneumoniae in Tibetan pig herds (via ELISA) was 20.48% (93/454) in 3 provinces (Sichuan, Tibet autonomous region, and Qinghai) between October and December of 2014. The difference in prevalence among animals in six different growing stages was statistically significant (P < 0.05). Anti-M. hyopneumoniae antibody was detected in breeding sows (45.83%; 22/48) and piglets (50%; 3/6). PCR and gel electrophoresis of BALF showed that 6.45% (10/155) of pigs were positive for M. hyopneumoniae. The presence of M. hyopneumoniae in serum was higher in piglets and breeding sows than in any other group. In conclusion, the results of this study showed that M. hyopneumoniae is prevalent among Tibetan pigs between October and December in Western China. To the authors' knowledge, this is the first investigation of M. hyopneumoniae prevalence in Tibetan pigs of Western China using serological tests, PCR, and 16S rRNA sequencing.

Development of a blocking ELISA for detection of Mycoplasma hyopneumoniae infection based on a monoclonal antibody against protein P65

Mycoplasma hyopneumoniae causes porcine enzootic pneumonia, an economically important disease of swine. A more sensitive and reliable method for detection of serum antibodies is needed for epidemiological investigations and to evaluate the effect of immunization. We expressed the M. hyopneumoniae protein P65 in Escherichia coli and produced a monoclonal antibody (mAb) that bound specifically to recombinant P65. Using this mAb, a blocking enzyme linked immunosorbent assay (ELISA) was developed. The blocking ELISA had similar specificity to and sensitivity with the commercial ELISA produced by IDEXX. Thus, this blocking ELISA is a useful test for serological confirmation of M. hyopneumoniae infection.

Antibody responses of swine following infection with Mycoplasma hyopneumoniae, M. hyorhinis, M. hyosynoviae and M. flocculare

Several mycoplasma species possessing a range of virulence have been described in swine. The most commonly described are Mycoplasma hyopneumoniae, Mycoplasma hyorhinis, Mycoplasma hyosynoviae, and Mycoplasma flocculare. They are ubiquitious in many pig producing areas of the world, and except for M. hyopneumoniae, commercial antibody-based assays are lacking for most of these. Antibody cross-reactivity among these four mycoplasma species is not well characterized. Recently, the use of pen-based oral fluids for herd surveillance is of increasing interest. Thus, this study sought to measure pig antibody responses and the level of cross-reactivity in serum and pen-based oral fluids after challenge with four species of swine mycoplasmas. Four groups of four mycoplasma-free growing pigs were separately inoculated with the different mycoplasma species. Pen-based oral fluids and serum samples were collected weekly until necropsy. Species-specific Tween 20 ELISAs were used to measure antibody responses along with four other commercial M. hyopneumoniae ELISAs. Animals from all groups seroconverted to the challenge species of mycoplasma and no evidence of cross-contamination was observed. A delayed antibody response was seen with all but M. hyorhinis-infected pigs. Cross-reactive IgG responses were detected in M. hyopneumoniae- and M. flocculare-infected animals by the M. hyorhinis Tween 20 ELISA, while sera from M. hyosynoviae and M. flocculare-infected pigs were positive in one commercial assay. In pen-based oral fluids, specific anti-M. hyopneumoniae IgA responses were detected earlier after infection than serum IgG responses. In summary, while some antibody-based assays may have the potential for false positives, evidence of this was observed in the current study.