Immune response of calves following the inoculation of Mycoplasma dispar and Mycoplasma bovis

A small but significant reduction in the number of Mycoplasma dispar colonising the respiratory tract after intratracheal challenge was observed in gnotobiotic-calves previously inoculated subcutaneously three times with formalin-killed organisms and oil adjuvant. Injection of M. dispar by the intramuscular route, with oil adjuvant, and 2 weeks later by the intratracheal route, without adjuvant, failed to induce immunity to subsequent intratracheal challenge.Following the subcutaneous injection of killed M. dispar, the amount of antibody detected by single radial haemolysis (SRH) increased markedly with increasing age in groups of calves with average ages of 16 to 155 days when first injected. Most calves aged less than 40 days failed to produce an antibody response to a singel injection of M. dispar. With M. bovis a smaller difference was observed between antibody levels generated in calves of different ages; also, calves less than 40 days old produced a detectable SRH antibody response following a single injection of killed M. bovis.IgG1 and IgG2 antibody to M. dispar and M. bovis were measured by ELISA. IgG1 appeared before IgG2 antibody and this was particularly pronounced in younger calves. Also, for both mycoplasmas IgG2 antibody levels were lower in younger than older calves. The IgG1 response to M. dispar was compared in three groups of calves with average ages of 16, 55 and 155 days and was greatest in the oldest and least in the youngest animals. In contrast, the IgG1 response to M. bovis varied little in calves of different ages. It therefore appears that the immune response of young calves to M. dispar is impaired or defective.     

Histopathological findings,phenotyping of inflammatory cells,and expression of markers of nitritative injury in joint tissue samples from calves after vaccination and intraarticular challenge with Mycoplasma bovis strain 1067

Background

The pathogenesis of caseonecrotic lesions developing in lungs and joints of calves infected with Mycoplasma bovis is not clear and attempts to prevent M. bovis-induced disease by vaccines have been largely unsuccessful. In this investigation, joint samples from 4 calves, i.e. 2 vaccinated and 2 non-vaccinated, of a vaccination experiment with intraarticular challenge were examined. The aim was to characterize the histopathological findings, the phenotypes of inflammatory cells, the expression of class II major histocompatibility complex (MHC class II) molecules, and the expression of markers for nitritative stress, i.e. inducible nitric oxide synthase (iNOS) and nitrotyrosine (NT), in synovial membrane samples from these calves. Furthermore, the samples were examined for M. bovis antigens including variable surface protein (Vsp) antigens and M. bovis organisms by cultivation techniques.

Results

The inoculated joints of all 4 calves had caseonecrotic and inflammatory lesions. Necrotic foci were demarcated by phagocytic cells, i.e. macrophages and neutrophilic granulocytes, and by T and B lymphocytes. The presence of M. bovis antigens in necrotic tissue lesions was associated with expression of iNOS and NT by macrophages. Only single macrophages demarcating the necrotic foci were positive for MHC class II. Microbiological results revealed that M. bovis had spread to approximately 27% of the non-inoculated joints. Differences in extent or severity between the lesions in samples from vaccinated and non-vaccinated animals were not seen.

Conclusions

The results suggest that nitritative injury, as in pneumonic lung tissue of M. bovis-infected calves, is involved in the development of caseonecrotic joint lesions. Only single macrophages were positive for MHC class II indicating down-regulation of antigen-presenting mechanisms possibly caused by local production of iNOS and NO by infiltrating macrophages.     

Immunoprophylaxis of experimental Mycoplasma bovis arthritis in calves. Protective efficacy of live organisms and formalinized vaccines

Live Mycoplasma bovis (M. bovis) organisms given subcutaneously or intraperitoneally protected nine of ten calves and eight of nine calves, respectively, from clinical arthritis, while the formalinized vaccine given subcutaneously protected eight of ten calves. In contrast, clinical arthritis was induced in all non-vaccinated calves that were challenged intravenously. The arthritic lesion was more severe in non-vaccinated calves than in the few vaccinated calves that developed clinical arthritis. Unlike formalinized vaccine, live M. bovis culture given subcutaneously provoked a local reaction at the site of injection in most calves in the form of oedematous plaques of about 7–8 cm in diameter. Results suggest that the formalinized vaccine may offer a practical approach to the control of Mycoplasma bovis arthritis in calves.     

Bacteriological, serological, pathological and immunohistochemical studies of Mycoplasma bovis respiratory infection in veal calves and adult cattle at slaughter

Mycoplasma bovis is an important cause of calf pneumonia worldwide. In this study, we examined 140 cattle at slaughter comprising 70 veal calves and 70 beef cattle; 115 animals with pneumonic lesions and 25 without. Lung samples were submitted for bacteriological, histological, and M. bovis-immunohistochemical analyses. Serology for M. bovis was positive in 76% of beef cattle and 100% of veal calves. M. bovis was isolated only from veal calves in 16 out of 64 pneumonic cases. M. bovis was detected by immunohistochemistry in seven bacteriologically positive cases. M. bovis antigen was associated with bronchogenic necrosuppurative or fibrinonecrotizing lesions. Bacteriologically positive and immunohistochemical negative cases were associated with catarrhal bronchointerstitial pneumonia. Results suggest that M. bovis infection may develop into a severe necrosuppurative bronchopneumonia or fibrinonecrotizing pneumonia when associated with a high number of intralesional organisms or, conversely, into a mild catarrhal bronchointerstitial pneumonia when associated with a low number of organisms.     

Increased prevalence of Mycoplasma bovis in the Netherlands

Summary

The epidemiology, therapy, and prevention of M. bovis infections are briefly reviewed In a survey begun in 1982 M. bovis was found frequently in the respiratory of veal calves and beef cattle with respiratory problems. In replacement calves infected with respiratory disease in dairy herds, however, the organism has only been detected since 1986. Respiratory tract specimens collected from calves with respiratory disease were submitted for examination for M. bovis from 1986 to 1991 and originated from 83 herds. Mycoplasma bovis was detected in specimens from 59 of the herds, 20% of which were dairy herds and 80% fattening herds. Arthritis caused by M. bovis was observed in 12 herds until July 1991. Since 1976 when the first mastitis outbreak caused by M. bovis was diagnosed M. bovis has caused 14 more outbreaks. The number of diseased cattle varied from 1 tot 16 per farm, and clinical signs of mastitis varied from mild to severe. In all instances the infection has been eradicated from the herds. Because M. bovis can cause great losses in intensively reared cattle herds, it is advisable to separate purchased veal calves and beef cattle from dairy cattle to prevent further spread of M. bovis.     

Establishment of an antibody avidity test to differentiate vaccinated cattle from those naturally infected with Mycoplasma bovis

Mycoplasma bovis is a major pathogen of bovine respiratory disease (BRD) in China and a live attenuated vaccine has recently been developed. This study aimed to establish an IgG avidity test to differentiate between naturally infected and vaccinated animals. An indirect ELISA (iELISA) was first established in the laboratory to detect antibodies specific to M. bovis using whole cell proteins as coating antigens and serum samples from experimentally infected cattle. The specificity and sensitivity of the iELISA was confirmed using a commercial ELISA kit as a reference standard. Both tests showed substantial agreement as indicated by a κ value of 0.78 (95% confidence interval, CI, 0.62, 0.93), and an overall 92.0% (80/87) agreement between the two tests. Based on the laboratory iELISA, a sodium thiocyanate (NaSCN) competitive iELISA was then developed for the detection of IgG avidity, expressed as relative avidity index (AI).Two-hundred and one experimentally immunised and naturally infected animals were used. These comprised 36 immunised calves, 38 negative control calves, 37 naturally infected calves, 87 calves of unknown status, and an additional three immunised calves that were used for a time trial. By testing true positive and negative antisera from either naturally infected or immunised calves, the AI cut-off value was defined as 70.4%. The diagnostic accuracy of the in-house NaSCN competitive iELISA was determined using serum samples collected from the experimental animals. The IgG avidity test demonstrated 96.0% sensitivity (95% CI 80.5%, 99.3%) and 95.8% specificity (95% CI 79.8%, 99.3%), and was successfully established as a valuable first test for differentiating vaccinated animals from those infected with M. bovis. This test may be a useful tool for clarifying the magnitude of M. bovis infection and in assessing the efficacy of vaccination in exposed animal populations.     

Prevalence of respiratory bacterial pathogens and associated management factors in dairy calves in Taiwan

This study aimed to investigate the prevalence at both farm-level and calf-level and to identify the risk factors of respiratory bacterial pathogens in dairy calves in Taiwan. The status of bovine respiratory disease (BRD) was evaluated by using the Wisconsin scoring system from a total of 400 pre-weaned calves from 32 different farms in Taiwan, then the nasopharyngeal swabs were collected. The prevalence of respiratory pathogens was 84.37% at farm-level and 45.50% at calf-level, and Pasteurella multocida (P. multocida) was the most prevalent pathogen. The presence of Mycoplasma bovis (M. bovis), P. multocida, Mannheimia haemolytica (M. haemolytica) and Histophilus somni (H. somni) were all higher in BRD positive calves than BRD negative calves, but only in H. somni was significant (P<0.001). Then nine farm management risk factors were analyzed by using multivariate logistic regression models to determine the risk factors of respiratory bacterial pathogens (farm and calf-level). In the result at farm-level, only unheated colostrum was significantly associated with pathogen positive farms (Odds Ratio (OR)=11.43). At calf-level, the predominant risk factor for each pathogen, M. bovis, P. multocida, M. haemolytica and H. somni, was late first colostrum feeding (OR=272.82), unheated colostrum (OR=3.41), waste milk feeding (OR=6.59) and high pneumonia treatment cost (OR=2.52), respectively. For effective preventive measures, farmer education on milk and colostrum feeding are urgently warranted.     

Isolation of mycoplasma species from the lower respiratory tract of healthy cattle and cattle with respiratory disease in Belgium

Between 1997 and 2000, a total of 150 healthy cattle and 238 animals with respiratory disease were examined for six Mycoplasma species. Attempts were made to detect Mycoplasma canis, Mycoplasma dispar and Ureaplasma diversum in calves with recurrent disease, and all three of these species were identified in calves with recurrent disease and in healthy lungs. In healthy calves, 84 per cent of bronchoalveolar lavage fluids were mycoplasma free; when cultures were positive, Mycoplasma bovirhinis was the only species isolated. Mycoplasmas were isolated from 78 per cent of animals suffering recurrent respiratory disease and from 65 per cent of acute respiratory cases. Mycoplasma bovis was isolated from bronchoalveolar lavages from 35 per cent of calves suffering recurrent respiratory disease, and from 50 per cent of acute cases, and from 20 per cent of pneumonic cases examined postmortem. M bovis was associated with other Mycoplasma species in 44 per cent of cases. M dispar was also isolated from 45.5 per cent of calves suffering recurrent respiratory disease, often in association with M bovis. M canis was identified for the first time in diseased Belgian cattle. Other mycoplasmas, including Mycoplasma arginini, Mycoplasma alkalescens and U diversum, were isolated less frequently. Associations between mycoplasmas and other pathogens were often observed. Among lungs infected with Pasteurella and/or Mannheimia species, more than 50 per cent were mixed infections with M bovis.     

Changes in plasma fibrinogen levels in experimental Mycoplasma bovis arthritis in calves

Plasma fibrinogen levels were measured as a means of following the course of an intravenous and intraperitoneal challenge of vaccinated and non-vaccinated animals in an experimental Mycoplasma bovis arthritis in calves. Intraperitoneal challenge failed to induce as much elevation of fibrinogen concentration as intravenous challenge in both the vaccinated and non-vaccinated groups.The elevation of fibrinogen levels among the vaccinated calves remained within the normal range of 300–800 mg% throughout, irrespective of the route of challenge. In contrast, the level rose to over 1600 mg% ten days postchallenge in all but one of the non-vaccinated calves that were challenged intravenously. The relatively low plasma fibrinogen levels in non-vaccinated calves that were challenged intraperitoneally correlated with the absence of arthritis in this group. In general, there was an inverse correlation between high fibrinogen levels and protection from M. bovis arthritis.     

Respiratory Pathogens in Québec Dairy Calves and Their Relationship with Clinical Status,Lung Consolidation,and Average Daily Gain

Background

Bovine respiratory disease (BRD) is 1 of the 2 most important causes of morbidity and mortality in dairy calves. Surprisingly, field data are scant concerning the prevalence of respiratory pathogens involved in BRD in preweaned dairy calves, especially in small herds.

Objectives

To identify the main respiratory pathogens isolated from calves in Québec dairy herds with a high incidence of BRD, and to determine if there is an association between the presence of these pathogens and clinical signs of pneumonia, lung consolidation, or average daily gain.

Animals

Cross‐sectional study using a convenience sample of 95 preweaned dairy calves from 11 dairy herds.

Methods

At enrollment, calves were weighed, clinically examined, swabbed (nasal and nasopharyngeal), and lung ultrasonography was performed. One month later, all calves were reweighed.

Results

Twenty‐two calves had clinical BRD and 49 had ultrasonographic evidence of lung consolidation. Pasteurella multocida, Mannheimia haemolytica, and Histophilus somni were isolated in 54, 17, and 12 calves, respectively. Mycoplasma bovis was identified by PCR testing or culture in 19 calves, and 78 calves were found to be positive for Mycoplasma spp. Bovine coronavirus was detected in 38 calves and bovine respiratory syncytial virus in 1. Only the presence of M. bovis was associated with higher odds of clinical signs, lung consolidation, and lower average daily gain.

Conclusions and Clinical Importance

Results suggested that nasopharyngeal carriage of M. bovis was detrimental to health and growth of dairy calves in small herds with a high incidence of BRD.     

Mycoplasma bovis and otitis in dairy calves in the United Kingdom

Signs of severe otitis media in 20% of dairy calves on one farm were associated with Mycoplasma bovis infection, based on isolation from the external ear canal and nares. Affected calves seroconverted to M. bovis and no other significant bacteria were isolated. Infection was considered likely to have originated from cows in the milking herd based on evidence of seroconversion and detection of infection in a milk sample. M. bovis infection should be considered when investigating otitis problems in calves.     

Antimicrobial susceptibility and molecular characterization of macrolide resistance of Mycoplasma bovis isolates from multiple provinces in China

Mycoplasma bovis has spread widely throughout the world via animal movement and has become an important pathogen of bovine respiratory disease. However, the minimum inhibitory concentrations of antimicrobials for Mycoplasma bovis have not been studied in China. The objective of this study was to determine the prevalence and antibiotic resistance of Mycoplasma bovis isolated from young cattle with respiratory infection in China. Mycoplasma bovis was detected in 32/45 bovine respiratory infection outbreaks at beef farms in 8 provinces in China. The isolates were susceptible or had medium sensitivity to ciprofloxacin, enrofloxacin and doxycycline, but were frequently resistant to macrolides (13/32, 41%). An A2058G (Escherichia coli Numbering) mutation located in the rrnA operon in domain V of 23S rRNA was observed in strains that were resistant to macrolides. This single mutations at the rrnA operon in domain V of 23S rRNA may play an important role in the resistance of Mycoplasma bovis strains to macrolides.     

First report of the use of mucosal swabs of the palatine tonsillar crypt for detection of Mycoplasma bovis in naturally infected calves

ABSTRACT

Aims: To compare detection by real-time PCR of DNA from Mycoplasma bovis on mucosal swabs taken from the palatine tonsillar crypt and the mainstem bronchi of clinically asymptomatic calves after slaughter.

Methods: We compared the sensitivity of mucosal swabs taken from two sites: the palatine tonsillar crypt and the mainstem bronchi. Paired samples were taken post-mortem at slaughter from 55 clinically well calves from an infected herd and were tested by real-time PCR for the presence of M. bovis-specific DNA.

Results: Mycoplasma bovis DNA was detected in 51 palatine tonsillar crypt swabs (92.7 (95% CI?=?82.4–98.0)%) and seven mainstem bronchial swabs (12.7 (95% CI?=?5.3–24.5)%). All seven calves with positive mainstem bronchial swabs also had positive palatine tonsillar crypt swabs.

Conclusions: When compared to mucosal swabs of the mainstem bronchi, mucosal swabs of the palatine tonsillar crypt were seven times more sensitive for the post-mortem detection of M. bovis DNA. The viability of detected M. bovis was not assessed, because any cattle carrying viable or non-viable M. bovis DNA were determined to be a potential risk to eradication. Palatine tonsillar crypt mucosa may be a useful anatomical site for real-time PCR detection of M. bovis DNA in naturally infected calves. More work is needed to define the persistence and viability of M. bovis at this anatomical site.

Clinical relevance: The results of this study helped form the basis of surveillance tools used in M. bovis control and eradication efforts. Familiarity with these results may help veterinarians better communicate with their clients about the science behind the eradication efforts.     

The effect of antimicrobial treatment and preventive strategies on bovine respiratory disease and genetic relatedness and antimicrobial resistance of Mycoplasma bovis isolates in a western Canadian feedlot

Feedlot calves (n = 3784) were systematically randomized and allocated in a 2 × 2 factorial study to receive metaphylactic oxytetracycline (OTC) on arrival or no antimicrobial, as well as florfenicol once subcutaneously or twice intramuscularly (48 h apart) if diagnosed with bovine respiratory disease (BRD). Calves of different treatment groups were comingled and followed from placement to re-implantation (~100 days). Animals receiving OTC had a reduced risk of BRD, an increased risk of arthritis, and no significant differences in average daily gain, BRD relapse, overall mortality, or BRD mortality. There were no significant differences between treatment protocols. Deep nasal swabs (n = 233) taken at arrival (n = 122), treatment (n = 77), and swabs from lungs and joints at postmortem (n = 34) were cultured for Mycoplasma bovis from 61 animals ill or dying of chronic pneumonia and arthritis and from 61 healthy calves. There was significant variation in diversity among isolates (n = 51) between study years and different cattle. Metaphylaxis or antimicrobial treatment did not affect the diversity of isolates. Except for tilmicosin, isolates were largely susceptible to tested antimicrobials.     

Comparison of Sampling Procedures for Isolating Pulmonary Mycoplasmas in Cattle

Three sampling procedures were compared to determine the optimal technique for isolating mycoplasmas in cattle with respiratory diseases. The prevalence of mycoplasmas isolated from these animals is also reported. In the first group, bronchoalveolar lavage (BAL) and nasal swab cultures were compared with the corresponding lung cultures from cattle necropsied for fatal respiratory diseases (n = 20). In a second group, nasal swabs were compared with corresponding BAL cultures in living animals with recurrent respiratory pathologies (n = 49). There was complete agreement between the paired BAL and lung cultures. In contrast, nasal cultures were not representative of the mycoplasmas present in the lower respiratory airways. The relative sensitivity and specificity of the nasal swab technique compared to BAL in living animals confirmed that the nasal swab cultures were not predictive of lower respiratory airway pathogens, such as Mycoplasma bovis. BAL is considered to be the best method for isolating M. bovis in cattle with respiratory diseases as it combines reliability and feasibility under field sampling conditions. In the present study, Mycoplasma dispar (43%) and M. bovis (29%) were mainly isolated in mixed infections. This confirms the need to search for mycoplasmas in routine examinations and to take them into account in therapeutic strategies for respiratory diseases in cattle.     

A radial growth precipitation test and its comparison with certain other serological tests for the detection of antibody in bovine sera to Mycoplasma agalactiae subsp. bovis

A radial growth precipitation test is described for measuring antibody to Mycoplasma agalactiae subsp. bovis. The test is quantitative and appears to depend on the production of soluble antigen by growing organisms. When compared with indirect haemagglutination, complement fixation and inhibition of film production, for measuring antibody to M. agalactiae subsp. bovis in bovine sera, it was found to have a sensitivity comparable to that of the complement fixation test.     

A retrospective study of 29 cases of otitis media/interna in dairy calves

Epidemiological data, clinical findings, laboratory data, medical imaging, and outcomes were reviewed in 29 dairy calves with otitis media/interna. Age at admission ranged from 1 to 24 wk. The majority of calves were referred during winter. Clinical signs included drooping ear, ptosis, head tilt, abnormal nystagmus, strabismus, dysphagia, regurgitation, stiff neck, opisthotonos, facial hyperesthesia, and purulent aural discharge. Intranasal endoscopic examination of 5 animals revealed nasopharyngeal collapse in 4. Cerebrospinal fluid (CSF) was abnormal in all of 7 cases. Mycoplasma bovis was cultured from all but 1 case with external ear or tympanic bullae samples (n = 12), and Mycoplasma arginini was cultured from the remaining ear sample. Radiographs of the tympanic bullae were performed in 24 calves, tomodensitometry (CT) in 3 calves and ultrasound in 4 calves. According to medical imaging techniques or necropsy, 69% of the cases were classified as chronic. Mean duration of treatment was 23.3 d. The rate of clinical recovery was 75%.     

Randomized field trial comparing the efficacy of florfenicol and oxytetracycline in a natural outbreak of calf pneumonia using lung reaeration as a cure criterion

BackgroundRespiratory infections are the main indication for antimicrobial use in calves. Optimal treatment duration currently is unknown, but shorter duration would likely decrease selection for antimicrobial resistance.Hypothesis/ObjectivesDetermine differences in cure rate and healing time between animals treated with florfenicol and oxytetracycline in a natural outbreak of respiratory disease using reaeration observed on thoracic ultrasound examination as healing criterion.AnimalsCommercial farm housing 130, 3 to 9 month old Belgian blue beef calves.MethodsRandomized clinical trial during an outbreak of respiratory disease. Metaphylactic treatment was initiated, randomly treating animals with either florfenicol or oxytetracycline. Ultrasonographic follow‐up was done the first day and every other day for a 14‐day period. At the individual animal level, treatment was discontinued when reaeration of the lungs occurred. Differences in cure rate and healing time were determined.ResultsOf the 130 animals studied, 67.7% developed a lung consolidation ≥0.5 cm. The mean ultrasonographic healing time was 2.5 days in the florfenicol group compared to 3.1 days in the oxytetracycline group (P = .04). After single treatment, 80.6% and 60.3% had no consolidations in the florfenicol and oxytetracycline groups, respectively (P = .01). A Mycoplasma bovis strain was genetically and phenotypically determined to be susceptible to both antimicrobials.Conclusions and Clinical ImportanceUltrasonographic lung reaeration shows potential as a cure criterion to rationalize antimicrobial use for outbreaks of pneumonia. In our study, florfenicol resulted in a faster cure and higher reduction in antimicrobial usage than did oxytetracycline.     

Sensitive diagnosis of bovine tuberculosis in a farmed cervid herd with use of an MPB70 protein fluorescence polarization assay

After histopathological examination of a lesion found in a herd member returned a diagnosis of mycobacteriosis, a farmed herd (n = 47) of elk (Cervus elaphus nelsoni) and red deer (C. elaphus elaphus) was investigated for bovine tuberculosis with a battery of antemortem and postmortem diagnostic tests. Every animal was tested with the mid-cervical tuberculin skin test; all 47 had negative results. All of the 16 adult animals and 15 of the 31 calves (approximately 2-years-old) were blood-tested with a lymphocyte stimulation test (LST) and a fluorescence polarization assay (FPA), which detects antibody to the MPB70 protein antigen. At necropsy of the 31 blood-tested animals, tissues were harvested for histopathological examination and culture of mycobacteria. Mycobacterium bovis was isolated from 16 of the 31 animals, and a scotochromogen was also isolated from 1 of the 16 whose tissues yielded M. bovis. Each of these 16 animals, 15 of which were calves, also received a histopathological diagnosis of mycobacteriosis. Other species of mycobacteria, including those belonging to the M. avium and M. terrae complexes, were isolated from an additional 7 animals. The FPA was scored “positive” or “suspect” for 16 animals, 13 (81%) of which were culture-positive for M. bovis. The other 3 animals that were culture-positive for M. bovis had negative FPA results. Of the 3 FPA-positive or FPA-suspect animals that were culture-negative, 2 were suspected to have mycobacteriosis on the basis of the histopathological examination. The 7 animals from which Mycobacterium species other than M. bovis were cultured were all FPA-negative. The only animal with positive LST results was also FPA-positive and culture-positive for M. bovis. The M. bovis isolates had an identical spoligotype pattern, with an octal code of 664073777777600. This is the first report of the isolation and identification of this strain type in Canada.     

Respiratory infections in housed sheep,with particular reference to mycoplasmas

A commercial housed flock with an annual occurrence of pneumonia was investigated. The organism most commonly isolated from the respiratory tract of lambs up to 6 months old was Mycoplasma ovipneumoniae. Mycoplasma arginini, Mycoplasma conjunctivae, Acholeplasma laidlawii, ureaplasmas and Pasteurella haemolytica biotype A were also isolated: viruses were not isolated, but infection with parainfluenza virus type 3 (P13) was indicated by serology. Colostrum derived antibodies to M. ovipneumoniae and M. arginini declined to minimum levels by 50 days. The development of active immunity to mycoplasmas and P13 virus was associated with an increased incidence of clinical respiratory disease. Histopathological examination of the lungs from 34 lambs showed that 15 had lesions of a proliferative exudative (P.E.) pneumonia, a further 11 showed lymphoid hyperplasia sometimes associated with interstitial thickening, and eight showed no significant pathological changes. Isolations of M. ovipneumoniae were highest from animals with P.E. pneumonia, while M. arginini did not appear to be associated with any specific lung changes. P. haemolytica biotype A was isolated from all cases of P.E. pneumonia. M. ovipneumoniae, M. arginini and P. haemolytica were also isolated from the lower respiratory tract of a proportion of 31 ewes examined post-mortem, but P.E. pneumonia was not observed in these animals.