Endobronchial inoculation of various doses of Haemophilus (Actinobacillus) pleuropneumoniae in pigs

Twelve-week-old specific-pathogen-free pigs were inoculated deep in the bronchi with Haemophilus (Actinobacillus) pleuropneumoniae strain 13261 in doses ranging from 8 x 10(1) to 9 x 10(7) colony-forming units (CFU). Pigs that survived infection were euthanatized and examined 48 hours after inoculation. The relationship between dose and severity of disease was evaluated clinically and the weight of pneumonic lesions was compared. The relationship between infection dose and weight of pneumonic lesions proved to be unimodal and not linear. Inoculation of 10(4) CFU of strain 13261 resulted in severe pneumonic lesions and mortality of 29%. In contrast, death was not observed after inoculation with 10(6) CFU of strain 13261 and pneumonic lesions were less severe (P less than 0.05). An infective dose of 10(3) CFU induced pneumonic lesions that tended (not statistically significant) to be less severe than those induced by a dose of 10(4) CFU. The peak fever response in all infected pigs was observed from 6 to 12 hours after inoculation. Leukocytosis developed within 12 hours after inoculation, because of an increase of neutrophilic granulocytes. Thereafter, WBC count decreased owing to lymphopenia. Serum iron concentration decreased 80% after inoculation, and zinc concentration decreased 54%.     

Effect of vaccinating lambs against pneumonic pasteurellosis under New Zealand field conditions on their weight gain and pneumonic lung lesions at slaughter

In a field trial, 9174 lambs from seven commercial sheep flocks with a history of subclinical pneumonia were either vaccinated with Ovipast Plus (Intervet) or given a placebo by systematic random allocation; they were vaccinated twice at an interval of four to six weeks and grazed on pasture in the same paddocks. They were weighed at the first vaccination, 11 and 23 weeks later, and one to three days before they were slaughtered. The extent of the pneumonic lesions in their lungs was scored visually postmortem. A subset of pneumonic lung samples was examined bacteriologically and histopathologically. There were no statistically significant differences between the pneumonic lesions at slaughter or the mean average daily weight gains of the vaccinated and placebo-treated lambs between 11 and 23 weeks or between first vaccination and slaughter. The vaccinated lambs had a lower mean daily gain between first vaccination and 11 weeks. The extent of pneumonic lesions at slaughter was negatively correlated with the mean daily gain between first vaccination and slaughter. There were no significant differences between the frequency of isolation of Mannheimia (Pasteurella) haemolytica and Pasteurella trehalosi or the histopathological classification of disease between pneumonic lung samples from the placebo-treated and vaccinated lambs.     

Host determinants of pneumonia in slaughter weight swine

A prospective cohort study was undertaken in a farrow-to-farrow swine herd to describe patterns of pneumonia, and to identify host risk factors associated with the extent of pneumonic lesions in 2 weight groups of slaughter swine. The risk of coughing and pneumonic lesions increased with increasing age of pigs within the herd (P less than 0.0001). The age-specific prevalence of pneumonic lesions was low (2.7%) in pigs less than 16 weeks old at slaughter, but increased rapidly when pigs were between 16 and 22 weeks old (8.6 to 67.9%). After 22 weeks, the prevalence remained relatively constant at about 80%. Associations between possible risk factors and pneumonia were investigated by use of multiple-regression models. Age at weaning (less than 24 days) and birth weight (less than 1 kg) exerted small, but significant (P less than 0.002), effects on the extent of pneumonic lesions in pigs slaughtered at 30 to 50 kg live weight. For pigs slaughtered at 90 to 110 kg, pneumonic lesions were more extensive (P = 0.007) in pigs sired by Yorkshire boars than pigs sired by non-Yorkshire sires (Duroc, Hampshire, Chester White, or American Spotted). Other host factor variables including weaning weight and clinical diseases (atrophic rhinitis, diarrhea, and arthritis) were not associated with pneumonia extent in either weight group. Higher pneumonia percentages were also associated with reduced growth rates in the grower/finisher phase. Pigs sired by Yorkshire boars grew significantly (P less than 0.0001) more slowly from entry into shed 2 (mean, 38 kg) until about the time of exit (mean, 92 kg) than pigs sired by other breeds (747 g/d and 795 g/d, respectively).     

Studies on the pathogenesis and interspecies transmission of respiratory syncytial virus isolated from sheep

Inoculation of lambs with an ovine isolate of respiratory syncytial virus (RSV) by a combined intranasal and intratracheal route resulted in mild respiratory tract illness, with respiratory tract lesions. Lung lesions were characterized by bronchitis and bronchiolitis, hyperplasia of bronchial and bronchiolar epithelium, peribronchiolar and perivascular accumulations of lymphocytes, alveolar septal thickening, and collapse. Respiratory syncytial virus was recovered from the respiratory tract of inoculated lambs, and RSV antigen was demonstrated by immunoperoxidase staining of bronchiolar and alveolar epithelial cell in pneumonic lesions of lambs euthanatized on post-inoculation days 5 and 6. Other primary respiratory tract pathogens were not isolated. Clinical signs of respiratory tract illness or respiratory tract lesions did not develop in the in-contact control lamb. Inoculation of the ovine RSV isolate into calves and deer fawns resulted in infection in both species, and at necropsy, pneumonic lesions were present. A mild to moderate respiratory tract illness developed in the calves, but clinical disease was not seen in the fawns. Lung lesions in fawns were similar to those seen in lambs; lesions in calves were characterized by collapse, scattered areas of parenchymal necrosis, and bronchiolitis. Respiratory syncytial virus was reisolated from the lower respiratory tract of inoculated calves and fawns, and immunoperoxidase-positive epithelial cells were seen in pneumonic lesions. Other primary respiratory pathogens were not detected. Respiratory syncytial virus infection was not demonstrable in control animals that were in contact with inoculated animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Experimental bovine respiratory tract disease with Haemophilus somnus

Eight calves were inoculated into the bronchus with H. somnus. Thirteen calves were inoculated with bovine respiratory syncytial virus (BRSV) and 8 days later with H. somnus. All calves developed necrotizing, suppurative, lobular bronchopneumonia and pleuritis. Clinical signs of disease and pneumonic lesions were significantly more severe in calves that were sequentially inoculated with BRSV followed by H. somnus. Pneumonic lesions in the inoculated calves were similar to those described for naturally occurring H. somnus-associated respiratory tract disease. Control calves inoculated with BRSV alone or sham-inoculated with medium did not develop clinical signs of respiratory tract disease. The BRSV-inoculated control calves developed minimal pneumonic lesions.     

Immunoperoxidase evaluation of the relationship between necrotic lesions and causative bacteria in lungs of calves with naturally acquired pneumonia

An immunoperoxidase technique was used to study the relationship between the necrotic lesions and causative bacteria found in lungs of 53 calves that had naturally acquired pneumonia. Four types of necrotic lesions were identified on the basis of morphologic characteristics as follows: type 1 had coagulation necrosis surrounded by a dense zone of numerous degenerated leukocytes; type 2 was similar to type 1, but the central area of the lesions was severely affected, had no alveolar architecture remaining, and was surrounded by a thin, sparse layer of degenerated leukocytes; type 3 had small swirling accumulation of degenerated leukocytes; and type 4 had necropurulent lesions resembling abscesses. By use of the immunoperoxidase technique, Pasteurella haemolytica serovar 1 antigen was confirmed to be associated with the necrotic lesions in many cases of type 1 and in some cases of types 2 and 3. Although some lesions were induced by other bacteria (Haemophilus somnus or Actinomyces pyogenes), the pneumonic lesions associated with P haemolytica could be differentiated from other pneumonic lesions in calves by use of the immunoperoxidase technique.     

The effect of Pasteurella haemolytica A2 infection on phagocytosis efficiency of caprine broncho-alveolar macrophages

An experiment was designed to study the in vivo effect of Pasteurella haemolytica A2 infection on the phagocytosis activity of caprine broncho-alveolar macrophages and the extent of pneumonic lesions. Twelve healthy local Kacang goats, about 7 months of age, were divided into two groups of six. Goats in group 1 were inoculated intratracheally with 4 ml inoculum containing 2.8 x 10(9) colony-forming units (CFU)/ml of Staphylococcus aureus. Goats in group 2 were inoculated intratracheally with 4 ml of inoculum containing 9.5 x 10(8) CFU/ml of Pasteurella haemolytica A2 isolated earlier from pneumonic lungs of goat. At intervals of 3 and 7 days post-challenge five goats from each group were killed and the lungs were washed with sterile phosphate-buffered saline. Smears were prepared from the lung washing fluid and the number of macrophages with phagocytic activity was determined. At day 3 post-infection, goats of both groups showed a similar pattern of pneumonic lesion. The lung washing fluid of goats in group 2 was found to contain numerous neutrophils and macrophages. Goats in group 2 showed significantly (P < 0.05) higher extent of lung lesions than group 1. Similarly, the average extent of lung lesions was significantly (P < 0.05) more severe in group 2 at day 7 post-infection. The lung washing fluid contained mostly macrophages. The phagocytic activity following S. aureus infection was more efficient and significantly (P < 0.01) higher compared with infection by P. haemolytica A2. There were weak correlations between the extent of pneumonic lesion and the phagocytic activity. Thus, goats with poor phagocytic activity were likely to develop more extensive lung lesions.     

Histopathology of experimental plague in cats

Formalin-fixed paraffin-embedded archival tissues of seven adult cats of both sexes that died after being experimentally infected with Yersinia pestis were examined light microscopically to characterize the lesions. The cats were exposed in two groups using two routes of infection: ingestion of Y. pestis-infected rodent or a subcutaneous injection of Y. pestis to simulate a flea bite. Immunohistochemistry was performed on tissues from all organ systems from a representative cat from each group to determine the distribution of Y. pestis bacilli during infection. In all seven cats, bubonic plague lesions were seen. The lesions of pneumonic plague were present in two cats. Septicemic plague was confirmed in all seven cats by bacteriologic culture. Aggregations of bacteria were seen in lymphoid tissue in all cats and in lung tissues from the two cats with pneumonic plague. The most consistent histologic finding was necrosuppurative inflammation in the lymph nodes. Invariably, Y. pestis bacteria were present in large numbers at affected sites. Orally infected cats had more numerous lesions in the lymph nodes of the head and neck regions. These experimentally induced cases of feline plague document that cats are unique among carnivores in exhibiting bubonic, pneumonic, and septicemic plague following exposure to Y. pestis. The lesions of the orally infected cats were consistent with those previously described for naturally occurring Y. pestis infections in cats and corroborate the contention that cats most commonly contract plague by eating Y. pestis-infected rodents and not via flea bite. The histopathology of Y. pestis disease in these cats is comparable to that described for human plague.     

Pneumonic lesions in lambs in New Zealand: patterns of prevalence and effects on production

AIM: To describe temporal and spatial patterns of the prevalence of pneumonia in lambs in three regions of New Zealand, and determine effects on liveweight gain. METHODS: A longitudinal study was conducted in 1999- 2000 on 14 commercial sheep farms, five from the Southland region of the South Island, and five from the King Country and four from the Northland regions of the North Island. Pneumonia was known to be a problem in lambs on all farms selected. On each farm, 400 lambs were randomly selected at weaning and allocated at random to one of two groups (Groups I and II). Lambs in both groups were weighed at 4-weekly intervals. The extent of pneumonic lesions in lungs was scored visually at slaughter in 40 randomly selected lambs after each weighing (Group I). Lambs in Group II were selected for slaughter by the farm manager on a commercial basis of liveweight and body condition, then assessed for pneumonic lesions. RESULTS: Prevalence of extensive pneumonic lesions (>/=10% lung surface area affected) increased from December to March from 1.4 to 10.1% lambs in Southland (n=1,917), 1.2 to 25.7% in the King Country (n=1,831), and 4.7 to 21.4% in Northland (n=1,485). Overall, 42.3% of lambs in both groups had pneumonic lesions (n=5,233) and in 15.1% of these, >/=5% of the lung surface area was affected. Daily weight gain in the month prior to slaughter was reduced in lambs for which >/=20% lung surface area was affected in Group I (p<0.05) but not in Group II. CONCLUSIONS: The association between pneumonic lesions and reduced weight gain was highly significant in lambs from Group I, but not from Group II. Thus, pneumonia had a negative impact on weight gain, and selection by farmers of lambs for slaughter counteracted this association in lambs from Group II. CLINICAL RELEVANCE: Pneumonia occurs commonly as a subclinical disease in lambs in New Zealand. It has no public health significance and is not recorded at slaughter plants nor reported to farmer clients. Effects of the disease on daily weight gain of lambs is considerable and costly for lamb producers.     

Concurrent infection of lambs with parainfluenza virus type 3 and Pasieureiia haemolytica

When conventionally reared lambs were infected with Pasteurella haemolytica 6 days after exposure to parainfluenza virus type 3, 79% of the animals developed severe pneumonic lesions. Uninoculated lambs or those receiving virus or bacteria alone had significantly lower levels of pneumonia (25%, 21% and 12% respectively). Because 25% of the uninoculated lambs had severe pneumonic lesions it could not be determined whether the combined infection had actually caused the pneumonia or merely aggravated existing lesions. However, it was apparent that neither agent alone was capable of increasing the prevalence or severity of pneumonia in the flock.     

The importance of interleukin-8 as a neutrophil chemoattractant in the lungs of cattle with pneumonic pasteurellosis.

Interleukin-8 (IL-8), an in vitro and in vivo neutrophil chemoattractant, is expressed at high levels in the lesions observed in bovine pneumonic pasteurellosis. Because of the role of neutrophils in the pathogenesis of pneumonic pasteurellosis, we investigated the relative importance of IL-8 as a neutrophil chemoattractant in this disease. Bronchoalveolar lavage (BAL) fluid was harvested from calves experimentally infected with bovine herpesvirus-1 and challenged with Mannheimia haemolytica. Neutrophil chemotactic activity was measured in pneumonic BAL fluid samples treated with a neutralizing monoclonal antibody to ovine IL-8, and compared to the activity in samples treated with an isotype-matched control antibody. Bronchoalveolar lavage fluid was analyzed at a dilution which induced a half-maximal response, and the concentrations of antibody were optimized in a preliminary experiment. Following incubation of replicate samples of diluted pneumonic bovine BAL fluid with 70 microg/mL of IL-8-neutralizing antibody or control antibody, the neutrophil chemotactic activities of the samples were determined using an in vitro microchemotaxis assay. Overall, pretreatment of BAL fluid samples with neutralizing anti-IL-8 antibody reduced neutrophil chemotactic activity by 15% to 60%, compared to pretreatment with control antibody. This effect was highly significant (P < 0.001), and was present in 5 of 5 samples. These data indicate that IL-8 is an important neutrophil chemoattractant in calves with pneumonic pasteurellosis, but that mediators with actions redundant to those of IL-8 must also be present in the lesions.     

Varying effects of infections with Mycoplasma hyopneumoniae on the weight gain recorded in three different multisource fattening pig herds

Pigs in three specialized fattening herds were studied with respect to the effect of infection with Mycoplasma hyopneumoniae on weight gain. Individual pigs were weighed four times at 4-week intervals during the fattening period and their daily weight gain over the rearing period was calculated. A blood sample was collected on each weighing occasion and analysed for the presence of antibodies to M. hyopneumoniae. The lungs of the principals were inspected at slaughter and the extent of pneumonic lesions was registered by a specially developed technique that has been proven to warrant a high degree of repeatability. No serum antibodies to M. hyopneumoniae were detected in one of the herds, and no pneumonic lesions were recorded at slaughter in that herd. In the other two herds, the prevalence of pigs with serum antibodies to M. hyopneumoniae increased from 6 to 54% and from 31 to 81%, respectively, during the fattening period. The prevalence of pneumonic lesions at slaughter in these herds was higher the later the pigs seroconverted. On the other hand, the extension of the lung lesions tended to be higher among pigs that seroconverted early during the rearing period. Infections with M. hyopneumoniae acquired early during the rearing, presumably strengthened by secondary infections and environmental errors, was found to decrease the daily weight gain of the pigs. However, even non-complicated M. hyopneumoniae infections acquired late in the fattening period were associated with reduced daily weight gain. That growth reduction was estimated to be at least 60 g (about 6%) after adjusting for herd, pen, initial weight and sex.     

Examination of the role of Mycoplasma bovis in bovine pneumonia and a mathematical model for its evaluation

The authors screened 34 large cattle herds for the presence of Mycoplasma bovis infection by examining slaughtered cattle for macroscopic lung lesions, by culturing M. bovis from lung lesions and at the same time by testing sera for the presence of antibodies against M. bovis. Among the 595 cattle examined, 33.9% had pneumonic lesions, mycoplasmas were isolated from 59.9% of pneumonic lung samples, and 10.9% of sera from those animals contained antibodies to M. bovis. In 25.2% of the cases M. bovis was isolated from lungs with no macroscopic lesions. The proportion of seropositive herds was 64.7%. The average seropositivity rate of individuals was 11.3% but in certain herds it exceeded 50%. A probability model was developed for examining the relationship among the occurrence of pneumonia, the isolation of M. bovis from the lungs and the presence of M. bovis specific antibodies in sera.     

Bovine pneumonic pasteurellosis: experimental induction in vaccinated and nonvaccinated calves.

A study was undertaken to investigate the effects of the immune response induced by combined aerosol and parenteral vaccination on the lung lesions induced in calves by Pasteurella haemolytica AI. Twenty-four calves, twelve of which had been vaccinated with killed P. haemolytica by aerosol and subcutaneous injection in Freund's complete and incomplete adjuvant were challenged by intratracheal inoculation of live P. haemolytica. Serological response to vaccination was not marked but was best measured by the whole cell agglutination test or by indirect bacterial agglutination rather than by the passive haemagglutination test. Titres of vaccinates were positively correlated with the degree of pneumonic change following challenge while in nonvaccinated controls, titres were negatively correlated with lung lesions. These findings suggest the occurrence of an immunologically mediated hypersensitivity pneumonitis in the lungs of vaccinates and point to the potential efficacy of live bacterial aerosols for stimulation of protective immunity in pneumonic pasteurellosis.     

Naturally occurring Mycoplasma bovis-associated pneumonia and polyarthritis in feedlot beef calves.

Mycoplasma bovis is perceived as an emerging cause of mortality in feedlot beef cattle. This study examined the lesions and infectious agents in naturally occurring M. bovis-associated bronchopneumonia and arthritis and the relationship of this condition with bovine viral diarrhea virus (BVDV) infection. Standardized pathologic, immunohistochemical, and microbiologic investigations were conducted on 99 calves that died or were euthanized within 60 days after arrival in 72 feedlots. Cranioventral bronchopneumonia with multiple foci of caseous necrosis was identified in 54 of 99 calves, including 30 with concurrent fibrinosuppurative bronchopneumonia typical of pneumonic pasteurellosis. Mycoplasma bovis was consistently identified in these lesions by culture and immunohistochemistry, but also commonly in healthy lungs and those with pneumonia of other causes. Focal lesions of coagulation necrosis, typical of pneumonic pasteurellosis, were often infected with both Mannheimia haemolytica and M. bovis. Arthritis was present in 25 of 54 (46%) calves with M. bovis pneumonia, and all calves with arthritis had pneumonia. BVDV infection was more common in calves with lesions of bacterial pneumonia than in those dying of other causes, but BVDV infection was not more common in calves with caseonecrotic bronchopneumonia than those with fibrinosuppurative bronchopneumonia. Retrospective analysis identified cases of M. bovis pneumonia in the early 1980s that had milder lesions than the current cases. The findings suggest that, in at least some calves, M. bovis induces caseonecrotic bronchopneumonia within the lesions of pneumonic pasteurellosis.     

Experimental model of swine pneumonic pasteurellosis using crude Actinobacillus pleuropneumoniae cytotoxin and Pasteurella multocida given endobronchially.

This study was designed to develop and characterize a swine pneumonic pasteurellosis model by concurrent introduction of Pasteurella multocida type A and Actinobacillus pleuropneumoniae crude cytotoxin. After a series of preliminary experiments, a combination of 4 x 10(9) P. multocida and 4,000 toxic units of A. pleuropneumoniae crude cytotoxin was determined to produce optimal results. A total of 48 pigs were divided into four groups of 12 pigs each. The control group received buffered saline only. Four pigs from each group were randomly selected for necropsy 3, 7 and 14 days postinoculation (PI). Inoculation of pigs with P. multocida and A. pleuropneumoniae cytotoxin (group 1) resulted in moderate to severe pneumonia. Pasteurella multocida was isolated from pneumonic lesions, grossly normal lung, and bronchial lymph nodes of all group 1 pigs throughout the 14 day experimental period. Pathological changes typical of field cases of swine pneumonic pasteurellosis were produced. Pigs inoculated with P. multocida alone (group 2) had pneumonic lesions and P. multocida was reisolated from lungs at three days PI. Pasteurella multocida was not isolated from these pigs at 7 and 14 days PI, except for one pig in which an abscess developed in the thorax. Pulmonary lesions induced by A. pleuropneumoniae crude cytotoxin alone (group 3) were transient and resolved by seven days PI. Group 1 pigs had significantly greater lung lesion volumes than group 2 and 3 pigs at 3, 7 and 14 days PI. Statistical analysis indicated a significant interactive effect of P. multocida and A. pleuropneumoniae cytotoxin on the development of lung lesion volumes at 7 and 14 days PI (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pneumonia in pigs induced by intranasal challenge exposure with pseudorabies virus and Pasteurella multocida

The interaction between pseudorabies virus (PRV) and Pasteurella multocida was investigated to determine whether single or combined infections result in pneumonia in 6- to 7-week-old pigs. The effect of the PRV-P multocida challenge exposure on feed consumption, rate of gain, and extent of pneumonic lesions appeared dependent on the PRV dose; however, pneumonic lesions were of bacterial pneumonia. Pigs inoculated with a virulent strain of PRV plus P multocida developed severe pneumonia, whereas pigs given PRV only did not develop pneumonia. Modified-live PRV vaccine had no effect on the occurrence of pneumonia. Average daily gain was most depressed in pigs given the highest dose of virulent PRV plus P multocida.     

Immunohistologic study of pulmonary and lymphatic tissues from gnotobiotic pigs inoculated with ara-T-resistant strain of pseudorabies virus

Ara-T-resistant strain of pseudorabies virus (PRV) was inoculated intranasally into six 2-week-old gnotobiotic pigs. Five inoculated pigs were sneezing and coughing. In pigs 1 to 4 killed on postinoculation days (PID) 3, 5, 7, and 9, respectively, PRV antigen was detected in respiratory epithelial cells, and pigs had severe pneumonitis. In pigs 5 and 6 killed on PID 11 and 13, respectively, PRV antigen was localized in macrophages in alveoli and necrotizing nodules. Immunoglobulin-containing cells (IgG, IgM, and IgA) were detected first in pneumonic lesions in pig 4 killed on PID 9. Detection of immunoglobulin-containing cells was coincident with pulmonary inflammation and regeneration of pneumonic lesions. The number of IgG-containing cells was greater than that of IgM- and IgA-containing cells. Corresponding to transient viral multiplication, IgG-, IgM-, and IgA-containing cells were demonstrated first in lymphatic tissues in pig 1 killed on PID 3 and their number was 5 to 10 times more than those in control pigs 7 and 8. Seemingly, PRV replication in lymphatic tissues stimulated the proliferative response of specific immunoglobulin-producing cells, and the appearance of immunoglobulin-containing cells in the lungs was associated with clearance of PRV and regeneration of pneumonic lesions.     

The Occurrence of Mycoplasmas and Bacteria in Lungs from Sheep in Southern Norway

In order to study the occurrence of mycoplasmas among Norwegian sheep, lungs from a great number of different herds were collected at 3 abattoirs in Southern Norway. The presence of fermenting mycoplasmas and bacteria was examined in both normal and pneumonic lungs to determine whether recovery of these agents could be related to pneumonic changes. Pneumonic lungs demonstrated lesions typical of the condition described as subacute or chronic pneumonia.Mycoplasma ovipneumoniae was found in 87 % of the 126 pneumonic lungs and in 6 % of the 83 normal lungs. Bacteria, mostly Pasteurella haemolytica, were less frequently encountered in the pneumonic lungs, and usually in combination with M. ovipneumoniae. It is concluded that M. ovipneumoniae is widespread among sheep in Southern Norway and can be considered to have etiological significance in subacute or chronic pneumonia, whereas bacteria probably occur mainly as secondary invaders. Changes resulting from moderate invasion by lungworm were found in about half of the lungs, but just as frequently in normal as in pneumonic lungs, and accordingly did not appear to contribute to the pneumonia investigated.     

Immunoperoxidase evaluation of pneumonic lesions induced by Pasteurella multocida in calves

To evaluate the relationship between pneumonic lesions and distribution of bacteria, lungs from calves inoculated with Pasteurella multocida were examined histologically by use of immunoperoxidase technique. Pneumonic lesions fundamentally consisted of broncho-pneumonia with fibrinopurulent pleuritis. The lesions were confirmed to be associated with inoculated P multocida, using the immunoperoxidase technique. The P multocida antigen was detected not only in the bacterial clusters in the lesions, but also in the cytoplasm of infiltrating neutrophils and macrophages. Further, immunoelectron microscopy indicated that the inoculated bacteria generally were phagocytosed and digested by neutrophils.