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1.
A study was carried out in 125 farrow-to-finish pig herds to assess the relationships between pathogens involved in respiratory disorders and to relate these findings to clinical signs of respiratory diseases and pneumonia and pleuritis at slaughter. Clinical examination and sampling were carried out on four different batches in each herd (pigs aged 4, 10, 16 and 22 weeks). Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, swine influenza viruses (SIV), porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) were detected by serological or PCR tests. Pneumonia-like gross lesions and pleuritis were scored at the slaughterhouse. The results indicate that the percentage of pigs PCR-positive for PCV2 at 4, 10 and 16 weeks old was associated with the percentage of pigs PCR-positive for M. hyopneumoniae at these ages. On the other hand, the percentage of pigs with antibodies against PRRSV at 10, 16 and 22 weeks was positively correlated with the percentage of pigs seropositive for M. hyopneumoniae at 22 weeks, with the percentage of pigs with antibodies against SIV H1N1 and SIV H1N2 and the percentage of pigs sero-positive for A. pleuropneumoniae serotype 2. The findings also indicate that, within the five studied pathogens, M. hyopneumoniae, PRRSV and SIV H1N1 are the major pathogens involved in pneumonia-like gross lesions even though PCV2 may play a role. A. pleuropneumoniae serotype 2, in association with PRRSV, is significantly associated with extensive pleuritis. Respiratory diseases could be significantly reduced by implementing measures including appropriate management practices to control these pathogens.  相似文献   

2.
Thirty cohort pigs were followed from birth to slaughter to study epidemiological patterns of porcine pleuropneumonia caused by Actinobacillus pleuropneumoniae. The study was conducted within a larger 380-animal study of vaccination against Mycoplasma hyopneumoniae and A. pleuropneumoniae in a 340-sow farrow-to-finish piggery with 4-month weaning, operating a continuous system of intensive production in the North Island of New Zealand. The cohort pigs were randomly allocated into two equal groups: vaccinated and control. Pigs in the first group were vaccinated at 2 and 4 weeks of age with both M. hyopneumoniae vaccine and A. pleuropneumoniae vaccine at separate vaccination sites. A series of nasal swabs was taken at 4, 8, 10, 11, 12, 14, 16 and 18 weeks of age. Each swab was streaked onto the surface of a selective medium on the farm and the plates were immediately transported to a laboratory and incubated at 37 degrees C for 5 days. After the trial, pigs were slaughtered at an average of 132 days of age, lungs were examined and samples taken for bacteriological culture and isolation. Thirty-five out of 256 samples produced haemolytic colonies which were Gram-negative, V-factor-dependent and positive to the CAMP test. A. pleuropneumoniae was first isolated at 4 weeks of age from one vaccinated pig. This finding suggests that piglets became infected in the farrowing pen and the source of infection might be a carrier sow. The interval-specific cumulative incidence of A. pleuropneumoniae infection reached a maximum of 54% and 40% at 11 weeks of age in the vaccinated and control groups, respectively. Infection status of the litter is considered to be a factor influencing morbidity in infected herds during weaner and grower periods. Our results suggest that simultaneous vaccination with M. hyopneumoniae and A. pleuropneumoniae vaccines at 2 and 4 weeks of age might lessen the prevalence but cannot absolutely prevent A. pleuropneumoniae infection during the weaner or grower-finisher periods.  相似文献   

3.
A longitudinal study was carried out to investigate the diversity and persistence of Mycoplasma hyopneumoniae (M. hyopneumoniae) strains in four infected pig herds. In each herd, 20 pigs were randomly selected and blood and/or bronchoalveolar lavage (BAL) fluid was collected at 6, 10, 14 and 26 weeks of age. In the BAL fluid, quantitative PCR and MLVA (multiple-locus variable number of tandem repeats (VNTR) analysis) testing were performed for detection and typing of M. hyopneumoniae strains, respectively. At 26 weeks of age, the prevalence and severity of lung lesions were recorded at slaughter (minimum 50 pigs belonging to the same batch as the investigated pigs). The percentage of pigs testing positive on qPCR increased from 35% at 6 weeks to 96% at 26 weeks of age. With MLVA testing, positive pigs were found from 14 weeks onwards. Within each herd, only one distinct strain was detected, although clonal variants were identified in two herds. In three of the herds, the strain remained present until slaughter age. The percentage of pigs with Mycoplasma-like lesions ranged from 38% to 98%, and the average pneumonia score ranged from 1.7 to 11.9, respectively. The present field study documented that within a herd, mainly one distinct M. hyopneumoniae strain was present that persisted in the same animals for at least 12 weeks. This implies that the immune response of the animals following infection is not able to rapidly clear the infection from the respiratory tract.  相似文献   

4.
The association between the average daily weight gain (from approximately 4 to 20 weeks of age) and the serological responses to respiratory infections was examined in a longitudinal study including 825 pigs from eight chronically infected herds. Pigs were bled every 4th week (starting from approximately 4 weeks of age), and sera were analyzed for antibodies to Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae serotypes 2, 5-7 and 12.Mixed analysis of covariance analyzed the relationship between the average daily weight gain and a categorical variable defining seroconversion as none, early or late as compared to the median time (estimated across herds) of seroconversion for the particular pathogen. The variables "gender", "weight at an approximate age of 4 weeks" and "time" (defining the exact length of the follow-up period), were included as explanatory variables, and "litter" and "herd" were included as explanatory random variables. The individual pig was the unit of concern.The variable defining time at seroconversion was not significantly associated with the average daily weight gain, when evaluating models across all eight herds. The apparent lack of effect could be because most pigs included in the study were subclinically infected, or because a temporary negative influence of the infections is hidden due to an increased growth in the period following infection.In conclusion, at least in these eight herds, seroresponses to M. hyopneumoniae and A. pleuropneumoniae could not be used to predict the effect of the pathogens on the daily weight gain.  相似文献   

5.
Dual infection of pigs with swine influenza virus (SIV) and Mycoplasma hyopneumoniae was carried out to compare the clinical and pathological effects of dual infection in caesarian derived and colostrums deprived (CDCD) pigs, with that of a single infection with M. hyopneumoniae. In Experiment 1, 40-day-old CDCD pigs were inoculated only with SIV (A/Sw/Hok/2/81, H1N1). The virus was isolated from nasal swabs for 5-6 days. None of these pigs showed clinical signs of infection throughout the experimental period. These results suggested that this strain can infect pigs but is only slightly pathogenic when it is inoculated singly to a CDCD pig. In Experiment 2, 60-day-old CDCD pigs were inoculated with M. hyopneumoniae and then were inoculated with SIV (A/Sw/Hok/2/81) at 1 week (MHYO-7d-SIV-7d group) or 3 weeks (MHYO-21d-SIV-7d group) after M. hyopneumoniae inoculation. Macroscopically, dark red-to-purple lung lesions were observed in all of pigs at 14 or 28 days post-inoculation. Percentages of dark red-to-purple lung lesions in dual infection groups (MHYO-7d-SIV-7d group: 18.7 +/- 4.2%, MHYO-21d-SIV-7d group: 23.0 +/- 8.0%) were significantly (P < 0.05) increased compared to those of each control group in which pigs were inoculated only with M. hyopneumoniae (MHYO-14d group: 4.7 +/- 2.9%, MHYO-28 group: 3.3 +/- 2.4%). Microscopically, bronchial epithelial lesions (epithelial disruption, degeneration, hyperplasia and formation of microabscess) were frequently observed in dark red-to-purple lung lesions of only the dual infection groups. These results demonstrate that the lung lesion of pigs inoculated with M. hyopneumoniae and SIV is more severe than that of pigs inoculated only with M. hyopneumoniae.  相似文献   

6.
Blood samples from 777 pigs, originating from 9 different herds, were collected at slaughter and examined for antibodies to Mycoplasma hyopneumoniae and Actinobacillus (Haemophilus) pleuropneumoniae by the indirect hemagglutination assay (IHA) and the complement fixation (CF) test, respectively. Results were compared to pathological and microbiological findings. Antibodies to M. hyopneumoniae in positive titers of 1/80 or higher were found in 62% of the samples. The relationship between positive IHA titers to M. hyopneumoniae and gross findings indicative of enzootic pneumonia of pigs (EPP), histological findings indicative of EPP, the isolation of M. hyopneumoniae and the demonstration of M. hyopneumoniae by indirect immunofluorescent testing ranged from 64% to 68%. No correlation was noted between positive IHA titers and the isolation of Mycoplasma flocculare. Positive antibody titers to A. pleuropneumoniae of 1/10 or higher were detected in 5% to 85% of the samples from individual herds. Positive titers to A. pleuropneumoniae serotype 2 were found in 71% to 79% of the sampled animals from herds with high frequencies of pneumonic lesions indicative of pleuropneumonia. In herds with low frequencies of pleuropneumonia, positive titers were recorded in from 0 to 4% of the tested pigs. However, no statistical association was found between pleuropneumonia and positive titers to A. pleuropneumoniae serotype 2 in individual animals. Twenty-one per cent of samples with positive CF titers to A. pleuropneumoniae showed antibodies to more than one serotype.  相似文献   

7.
The role of the heat-labile haemolysin of Actinobacillus pleuropneumoniae in acute porcine pleuropneumonia was examined. A virulent strain was compared with an isogenic haemolysin-deficient mutant in experimental infections. The pigs which received the virulent strain showed clinical signs of acute respiratory disease whereas the animals infected with the mutant strain appeared to be less severely affected. At post mortem examination, both groups showed similar acute pulmonary lesions and pleurisy typical of A pleuropneumoniae infection. The bacterial antigen representing the haemolysin was detected in lung lesions infected with the parent strain but not in those infected with the mutant. These results demonstrate that the haemolysin of serotype 2 A pleuropneumoniae is not an essential factor for the production of the lesions of pleuropneumonia in pigs.  相似文献   

8.
In this paper, multilevel logistic models which take into account the multilevel structure of multi-site pig production were used to estimate the variances between pigs produced in Danish multi-site pig production facilities regarding seroconversion to Actinobacillus pleuropneumoniae serotype 2 (Ap2) and Mycoplasma hyopneumoniae (Mh). Based on the estimated variances, three newly described computational methods (model linearisation, simulation and linear modelling) and the standard method (latent-variable approach) were used to estimate the correlations (intra-class correlation components, ICCs) between pigs in the same production unit regarding seroconversion. Substantially different values of ICCs were obtained from the four methods. However, ICCs obtained by the simulation and the model linearisation were quite consistent. Data used for estimation were collected from 1161 pigs from 429 litters reared in 36 batches at six Danish multi-site farms chronically infected with the agents. At the farms, weaning age was 3-4.5 weeks, after which batches of pigs were reared using all-in/all-out management by room. Blood samples were collected shortly before: weaning, transfer from weaning-site to finishing-site, and sending the first pigs in the batch for slaughter (third sampling). Few pigs seroconverted at the weaning-sites, whereas considerable variation in seroconversion was observed at the finishing-sites. Multilevel logistic models (initially including four levels: farm, batch, litter, pig) were used to decompose the variation in seroconversion at the finishing-site. However, there was essentially no clustering at the litter level-leading to the use of three-level models. In the case of Ap2, clustering within batch was so high that the data eventually were reduced to two levels (farm, batch). For seroconversion to Ap2, ICC between pigs within batches was approximately 90%, whereas the ICC between pigs within batches for Mh was approximately 40%. This indicates that the possibility for Mh to spread between pigs within batches is lower than for Ap2. The diversity in seroconversion between batches within the same farm was large for Ap2 (ICC approximately 10%), whereas there was a relative strongly ICC (approximately 50%) between batches for Mh. This indicates that the transmission of Mh is more consistent within a farm, whereas the presence of Ap2 varies between batches within a farm.  相似文献   

9.
Pigs, asymptomatically infected with Actinobacillus pleuropneumoniae in their upper respiratory tract, can transmit the infection. Detection of such animals is indispensable to prevent the intake of the disease in a herd. This study was conducted to evaluate bacteriology, polymerase chain reaction (PCR) and serology for the detection of subclinically infected pigs. Pigs were inoculated onto the tonsils with an A. pleuropneumoniae serotype 9 strain (n=12, group 1) or phosphate buffered saline solution (PBSS) (n=5, group 2). To prevent infection of the lungs, pigs of group 1 were treated three times with sodium ceftiofur as an aerosol. A third group (n=5) was inoculated intranasally with the same strain. All animals were euthanized 30 days post-inoculation (dpi). In pigs of group 1, clinical signs were not observed. A small lung lesion was found in only one pig and A. pleuropneumoniae was isolated from this lesion. The bacterium was not isolated from the lungs of animals that did not develop lung lesions. A. pleuropneumoniae was demonstrated in tonsils of 9/12 animals using bacteriological isolation, whereas it was demonstrated in mixed bacterial cultures from tonsils of all 12 animals by PCR. In non-infected animals (group 2), clinical signs were not observed and A. pleuropneumoniae was not demonstrated in any sample. All intranasally infected animals (group 3) developed disease signs and lung lesions. High antibody titers against ApxI, ApxII and heat-stable antigens were detected in animals that developed lung lesions. Antibody titers against these antigens were low or absent in all other pigs. It was concluded that pigs carrying A. pleuropneumoniae in the upper respiratory tract generally do not show measurable antibodies in serum. Therefore, sensitive methods for the detection of the etiological agent such as PCR are required to identify carrier animals, while serological methods are not suitable.  相似文献   

10.
The efficacy of a subunit vaccine containing the Apx toxins of Actinobacillus pleuropneumoniae and transferrin-binding proteins was determined. Ten pigs were vaccinated twice with the vaccine. Eight control animals were injected twice with a saline solution. Three weeks after the second vaccination, all pigs were endobronchially inoculated with 10(6.5) colony-forming units (CFU) of an A. pleuropneumoniae serotype 9 strain. In the vaccine group, none of the pigs died after inoculation. Only one pig of the control group survived challenge. Surviving pigs were killed at 7 days after challenge. The mean percentage of affected lung tissue was 64% in the control group and 17% in the vaccine group. Actinobacillus pleuropneumoniae was isolated from the lungs of all animals. The mean bacterial titres of the caudal lung lobes were 5.0 x 10(8) CFU/g in the control group and 3.0 x 10(6) CFU/g in the vaccine group. It was concluded that the vaccine induced partial protection against severe challenge.  相似文献   

11.
The objectives of this study were to investigate the interactions between Mycoplasma hyopneumoniae and porcine circovirus type 2 (PCV2) and to establish a model for studying the pathogenesis of and testing intervention strategies for the control of PCV2-associated porcine respiratory disease complex (PRDC). Sixty-seven pigs were randomly assigned to four groups. Group 1 (n=17) pigs served as controls, group 2 (n=17) pigs were inoculated with M. hyopneumoniae, group 3 (n=17) pigs were dual infected with M. hyopneumoniae and PCV2, and group 4 (n=16) pigs were inoculated with PCV2. Pigs were inoculated intratracheally with M. hyopneumoniae at 4 weeks of age followed by intranasal inoculation with PCV2 at 6 weeks of age. Dual-infected pigs had moderate dyspnea, lethargy, and reduced weight gain. The overall severity of macroscopic lung lesions, PCV2-associated microscopic lesions in lung and lymphoid tissues, and the amount of PCV2-antigen associated with these lesions were significantly (P <0.05) higher in dual-infected pigs compared with all other groups. Four of 17 (23.5%) dual-infected pigs had decreased growth rate and severe lymphoid depletion and granulomatous lymphadenitis associated with high amounts of PCV2-antigen consistent with postweaning multisystemic wasting syndrome (PMWS). PCV2-antigen in lung tissue was most often associated with M. hyopneumoniae-induced peribronchial lymphoid hyperplasia, suggesting that this is an important site for PCV2 replication in the lung. This study indicates that M. hyopneumoniae potentiates the severity of PCV2-associated lung and lymphoid lesions, increases the amount and prolongs the presence of PCV2-antigen, and increases the incidence of PMWS in pigs.  相似文献   

12.
Alveolar macrophages were collected at necropsy from pigs inoculated with Mycoplasma hyopneumoniae or Actinobacillus pleuropneumoniae or both and were tested for phagocytic capabilities, using in vitro techniques. Macrophages from noninoculated littermates were used as controls. Alveolar macrophages from pigs inoculated with either M hyopneumoniae or A pleuropneumoniae had significantly (P less than 0.05 to P less than 0.0025) higher phagocytic capacity than that of noninoculated controls. Macrophages from A pleuropneumoniae-inoculated pigs were comparatively more stimulated than were those from M hyopneumoniae-inoculated pigs. Pigs inoculated with M hyopneumoniae and then challenge-exposed with A pleuropneumoniae 2 and 4 weeks later had greatly reduced phagocytosis. Infection with M hyopneumoniae or A pleuropneumoniae caused stimulation of alveolar macrophage functions, and M hyopneumoniae infections may have suppressed phagocytic responses when pigs were challenge-exposed with a secondary pathogen (A pleuropneumoniae). This potential suppression may represent a prediposition of the host by M hyopneumoniae to secondary bacterial infections.  相似文献   

13.
The efficacy of in-feed medication with tylosin for the treatment of enzootic pneumonia was examined in an experimental Mycoplasma hyopneumoniae infection model. One group of 10 conventional M. hyopneumoniae-free pigs was inoculated intratracheally with a highly virulent field isolate of M. hyopneumoniae; a second group of 10 pigs was inoculated in the same way and after 12 days was given tylosin at 100 mg/kg feed for 21 days; a third group of 10 pigs was inoculated with sterile culture medium, and these pigs were not given tylosin. The pigs were examined daily for clinical signs and each pig was given a respiratory disease score. Thirty-three days after they had been infected the pigs were euthanased, the lung lesions were quantified and samples of lung were processed for immunofluorescence testing for M. hyopneumoniae. The mean (sd) respiratory disease and lung lesion scores were significantly higher (P<0.05) in both the infected groups than in the uninfected group. Between 23 and 33 days after infection the mean respiratory disease score of the pigs treated with tylosin was 0.54 (0.22), significantly (P<0.05) lower than that of the infected pigs which were left untreated, 1.54 (0.46); similarly, their average lung lesion score, 1.72 (1.20), was significantly lower than that of the untreated pigs, 5.27 (3.85).  相似文献   

14.
A field trial was conducted to study Mycoplasma hyopneumoniae (Mh) infection dynamics by nested polymerase chain reaction (nPCR) and serology in pigs of a farm affected by enzootic pneumonia (EP). Moreover, correlation of Mh detection at different respiratory tract sites with presence of EP gross and microscopic lung lesions was assessed. These parameters were studied and compared between vaccinated (two doses at 1 and 3 weeks of age versus one dose at 6 weeks of age) and non-vaccinated pigs. Animals were monitored from birth to slaughter by nPCR from nasal swabs and by serology. From 3 to 22 weeks of age, an average of three pigs per treatment and per batch were necropsied (n = 302). The remaining pigs were sent to the slaughter (n = 103). Nasal, bronchial and tonsillar swabs were taken from the necropsied/slaughtered pigs; gross and microscopic EP-suggestive lung lesions were also assessed. Single and double vaccination resulted in earlier seroconversion and higher percentage of Mh seropositive pigs compared to control group. At slaughter, double vaccinated pigs showed lower percentage of EP-compatible gross lung lesions and lower Mh prevalence at upper respiratory tract sites (nasal cavity and tonsil) than control pigs.  相似文献   

15.
Nine strains of mycoplasmas were isolated from the lungs of 5 pigs with clinical signs of naturally acquired enzootic pneumonia. Mycoplasma hyopneumoniae was isolated from the lungs of 1 pig and M. hyorhinis from the lungs of 4. An unidentified mycoplasma, which utilized arginine, grew rapidly in broth culture and produced centred colonies on solid medium, was isolated from the lungs of 4 pigs. The pathogenicity of the isolated strain of M. hyopneumoniae was determined by inoculation of pigs from an enzootic pneumonia-free herd. Enzootic pneumonia was produced in the lungs of all 5 pigs inoculated intranasally and intratracheally with broth cultures of the organism isolatied by limit dilution techniques. Enzootic pneumonia was produced in 3 of 6 pigs inoculated intranasally and intratracheally with M. hyopneumoniae purified by the passage of colonies on agar blocks. M. hyopneumoniae was isolated in pure culture from the lungs of all pigs with induced pneumonic lesions.  相似文献   

16.
The aroA gene from Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 was isolated and sequenced. The gene complemented the aroA mutation in Escherichia coli AB2829. A kanamycin resistance cassette was inserted into the aroA gene and the mutant gene was reintroduced into A. pleuropneumoniae by allelic replacement. Intratracheal infection of susceptible pigs with A. pleuropneumoniae aroA caused no signs of respiratory disease or lung lesions in any of the animals at a dose 10(4) times the dose reliably known to induce acute pleuropneumonia; all animals infected with the unaltered control strain developed acute disease. The aroA mutant was rapidly eliminated from the lungs and tonsil of infected animals. The mutant may represent a safely attenuated strain for use in live bacterial vaccination or the delivery of antigen by the intranasal route. However, the residence time of the mutant in the respiratory tract of the pig may be too short for it to be useful in generating a protective immune response.  相似文献   

17.
This study was designed to determine in six-week old specific pathogen free pigs, the effect of previous experimental exposure to Mycoplasma hyopneumoniae and transmissible gastroenteritis virus on a challenge infection with Actinobacillus pleuropneumoniae. Pigs exposed simultaneously to M. hyopneumoniae and transmissible gastroenteritis virus appeared more resistant to challenge (one week later) with A. pleuropneumoniae. Four pigs out of a group of ten died following the challenge infection, compared to all ten pigs in the control group not submitted to previous infections. Clinical signs and lesions were also less severe in the previously infected group than in the control group. Pigs submitted to a single previous infection with M. hyopneumoniae only appeared to be less resistant to the challenge infection than pigs submitted to the dual previous infection with M. hyopneumoniae and the transmissible gastroenteritis virus. A correlation was found between the resistance of pigs to the challenge infection and their serum gammaglobulin levels.  相似文献   

18.
The objective of this study was to compare the detection rate of bacterial agents in bronchoalveolar lavage fluid (BALF), taken without visual control, to that in affected lung tissue obtained from the same pig at necropsy. BALF and affected lung tissue were examined for Mycoplasma hyopneumoniae using PCR, and standard cultural methods were used for Actinobacillus pleuropneumoniae, Bordetella bronchiseptica, Haemophilus parasuis, Pasteurella multocida and Streptococcus suis. All pigs with a history of respiratory symptoms were submitted as live animals for routine diagnostic examination. In each animal the site of lavage, marked by injecting methylene blue, differed from the site of pneumonic lesions. M. hyopneumoniae was detected more frequently in lung tissue than in BALF in cases with moderate or severe lung lesions. The detection rates of M. hyopneumoniae were higher in the BALF of pigs with mild lesions. Cultural examination of BALF was at least as satisfactory as affected lung tissue for detecting B. bronchiseptica, H. parasuis and P. multocida.  相似文献   

19.
Complement-fixing antibody to Mycoplasma hyopneumoniae in the serums of pigs experimentally infected with enzootic pneumonia was demonstrated by comparing the haemolytic titre of guinea-pig complement titrated in the presence of heated test serum, M. hyopneumoniae antigen and unheated normal pig serum with the titre obtained when the antigen was omitted. The haemolytic titres against sensitised sheep erythrocytes were determined after a fixation period of 16 to 18 hours at 5°C. When serums, collected at intervals of 3 to 7 days, from 43 pigs exposed to pigs experimentally infected with enzootic pneumonia were tested, 4.6 or more complement units were first fixed 14 to 44 (mean 23.4) days after contact began. Serums collected subsequently fixed from 4.6 to more than 31 complement units. This positive reaction usually persisted until the pigs were killed 4 to 35 weeks after contact began. Thirty-three had gross enzootic pneumonia lesions and 9 had lung lesions detected microscopically. Serum antibody was not detected in 73 weaned pigs aged 7 weeks in a pneumonia-free herd but serums from 9 of 15 unweaned piglets aged 9 to 14 days in the same herd, fixed between 3 and 7 complement units.  相似文献   

20.
A prospective study was carried out on three intensive farrow-to-finish farms. The aims were to estimate the incidence of Mycoplasma hyopneumoniae infection, to determine when pigs become infected and the pattern of transmission of infection and to verify the relationship between seroconversion and clinical signs. One batch of pigs per farm was followed from farrowing-to-slaughter. Blood samples were taken at 10, 27, 70, 94, 125 and 147 days of age, from 44, 48 and 44 pigs per farm. Colostrum and blood samples were also taken from the sows. Animals were checked clinically once a week and coughing rates were recorded. Antibodies against M. hyopneumoniae were detected by a blocking ELISA. At 27, 70 and 94 days of age most pigs on the three farms were seronegative, suggesting that no circulation of M. hyopneumoniae occurred during the growing period. Thereafter, a high proportion of pigs seroconverted, indicating that infection occurred soon after the transfer of the animals to the finishing houses. Differences were detected between farms in the incidence of seroconversion. Seropositive pigs were widely distributed among the finishing pens, suggesting that in addition to direct contact, other methods of transmission, such as indirect or airborne transmission, may have been important. Coughing started at around the same time as seroconversion. The results showed that the critical period for the transmission of M. hyopneumoniae is around the beginning of the finishing period, when pigs have low concentrations of antibodies against the agent.  相似文献   

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