Functional and metabolic activity of bovine pulmonary lavage cells phagocytically stimulated with pathogenic isolates of Pasteurella haemolytica.

Live Pasteurella haemolytica biotype A, serotype 1 isolates (n = 3) and Escherichia coli K-12, strain W3110, were reacted with bovine pulmonary lavage cell (PLC) suspensions. The comparative effects of the different bacteria on the functional and metabolic activity of alveolar macrophages (AMO) in the PLC suspensions were assessed simultaneously by use of 51Cr release, luminol-dependent chemiluminescence (LDCL), and AMO bactericidal assays. The bovine PLC responded differently to E coli, than to the 3 P haemolytica isolates in each of the 3 experimental test systems; however, responses to each of the P haemolytica isolates were not found to be significantly different. Unopsonized live P haemolytica cells adversely affected the functional and metabolic response of PLC, whereas there was no evidence of a cytotoxic (cytocidal) influence of E coli. A difference in 51Cr release for reaction mixtures containing E coli and P haemolytica was not detected at zero time; however, at each subsequent time, reaction mixtures phagocytically stimulated with P haemolytica had significantly increased amount of 51Cr release (P less than 0.05), compared with those mixtures containing E coli. Bovine AMO in the PLC suspensions were able to effectively kill E coli in vitro, but were unable to prevent survival and subsequent growth of P haemolytica. The luminol-dependent chemiluminescence profiles for reaction mixtures phagocytically stimulated with E coli provided evidence of sustained production of oxygen radicals with antimicrobial capabilities by bovine AMO in the PLC. Production of these highly reactive antimicrobial oxidants appeared initially in cultures containing P haemolytica but, subsequently, their production declined precipitously and ceased altogether.

Rhodococcus (Corynebacterium) equi: bactericidal capacity of neutrophils from neonatal and adult horses.

The capacity of hematogenous polymorphonuclear neutrophilic leukocytes (PMNL) to kill Rhodococcus equi was compared in horses of various ages. A radioisotope bactericidal assay was used to determine the capacity of PMNL to kill R equi. Assays were conducted on PMNL from horses in 3 groups: group I, 13 foals with a mean age of 3.3 days; group II, 10 group-I foals at a mean age of 35.7 days; and group III, adult dams of group-I foals. Bacteria were obtained from the lungs of a foal with R equi pneumonia and opsonized with fresh adult equine serum that contained R equi specific antibody. The mean peak percentage of R equi killed by PMNL was 78.9 for group I, 90.1 for group II, and 87.9 for group III. There was no significant difference (P greater than 0.05) among groups; however, 15% of foals in group I (2 foals) had a mean peak percentage of 30.5 killed, which was significantly (P less than 0.05) lower than the percentage for other foals in group I. The results of our investigation indicated that the capacity of PMNL to kill opsonized R equi is similar in neonatal, young, and adult horses. However, some neonatal foals have a substantially lower capacity to kill R equi, which may be an important factor in the pathogenesis of R equi infections.

Rhodococcus equi: equine neutrophil chemiluminescent and bactericidal responses to opsonizing antibody.

The opsonic capacity of serum containing R. equi-specific antibody was compared with antibody-deficient sera using luminol-dependent chemilumenscence (LDCL) and bactericidal assays. These assays incorporated peripheral blood polymorphonuclear neutrophilic leukocytes (PMNL) exposed to R. equi opsonized with neonatal equine pre-colostral serum (control) or serum from foals with R. equi infections (principal). All sera were complement inactivated at 56 degrees C for 30 min. Bacteria were obtained from the lung of a foal with R. equi pneumonia. Neutrophils were obtained from one adult horse for LDCL and another for bactericidal assays. Chemiluminescence of PMNL exposed to R. equi opsonized with control or principal sera was measured in a liquid scintillation counter. Mean peak LDCL within 1 h was significantly (P less than 0.01) higher with principal sera (2.4 X 10(5) cpm) than with control sera (0.018 X 10(5) cpm). A radioisotope bactericidal assay was used to determine the effect of control or principal sera on PMNL capacity to kill R. equi. Mean peak percent kill of R. equi by PMNL within 2 h was significantly (P less than 0.01) higher with principal sera (95.2%) than with control sera (54.6%). Enzyme-linked immunosorbent assay (ELISA) values for R. equi-specific antibody were determined on all sera. Mean ELISA values were significantly (P less than 0.01) higher for principal sera (71.8) than for controls (0.0). This investigation documents the presence and biological effectiveness of opsonic activity in complement-inactivated sera from foals with R. equi infections and R. equi-specific antibody.

Pneumonic pasteurellosis: examination of typable and untypable Pasteurella haemolytica strains for leukotoxin production, plasmid content, and antimicrobial susceptibility.

Plasmid DNA screening experiments were conducted to determine whether a relationship existed between the presence of plasmids and antibiotic resistance in Pasteurella haemolytica or the capability to produce hemolysin or leukotoxin (cytotoxin). Regardless of plasmid content, all P haemolytica isolates produced characteristic hemolysis on blood agar plates. Similarly, standardized suspensions of living bacteria and sterile concentrated (approx 200:1) culture supernatant from strains representing each of the 15 recognized P haemolytica serotypes and 7 field strains of P haemolytica (biotype A, serotype 1) produced leukotoxin, which was detected by their capability to cause inhibition of the luminol-dependent chemiluminescence response of bovine neutrophils. However, neither living bacterial suspensions nor concentrated culture supernatant from 4 untypable P haemolytica strains or a P multocida strain caused an inhibition of the luminol-dependent chemiluminescence response. The production of neither hemolysin nor leukotoxin by P haemolytica seemed to be plasmid mediated. Leukotoxin production is apparently a stable phenotypic characteristic of pathogenic P haemolytica strains, and the gene(s) coding for this activity is probably located on the bacterial host chromosome. Antibiotic susceptibility profiles were determined for the different bacterial strains. Studies of ampicillin and penicillin resistance in 8 P haemolytica (biotype A, serotype 1) strains provided evidence that the plasmid, with size of approximately 5,200 base pairs, may code for their resistance to these compounds.

Bovine pneumonic pasteurellosis: evaluation of interactions between bovine pulmonary lavage cells and Pasteurella haemolytica (biotype A, serotype 1), using a luminol-dependent chemiluminescence assay.

In vitro interactions of bovine pulmonary lavage cells (PLC) and pathogenic isolates of Pasteurella haemolytica biotype A, serotype 1, were examined, using a luminol-dependent chemiluminescence (LDCL) assay. The PLC containing high concentrations of bovine alveolar macrophages were incubated with living and heat-killed P haemolytica at bacteria to PLC ratio of approximately 1:1. Kinetics of the mean LDCL response of bovine PLC to heat-killed P haemolytica cells were characterized by a gradual increase in the amount of light emitted over 150 minutes followed by a slight decrease at 180 minutes. In contrast, the LDCL responses of reaction mixtures containing living P haemolytica were characterized by the development of a maximal response at 60 minutes followed by a continued precipitous decrease in light emission to background values by 150 minutes. Differences were not noticed in the LDCL response of PLC suspensions from the same cow to 3 P haemolytica isolates. In each instance, reaction mixtures containing heat-killed bacteria had a similar LDCL profile that was characterized by continuous production of light over 180 minutes, whereas all reaction mixtures containing living bacteria underwent a precipitous decrease in light emission, which eventually resulted in a complete cessation of chemiluminescence. The PLC suspensions from different cattle did not respond to bacterial stimuli uniformly, with respect to the amplitude or detailed nature of the LDCL profile. The time that lapsed between the addition of living P haemolytica to PLC suspensions and the complete cessation of chemiluminescence varied for different cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Survival of Corynebacterium pseudotuberculosis in axenic purulent exudate on common barnyard fomites.

Several inanimate surfaces (eg, plastic, wood, and steel) and particulate fomites (eg, wood shavings, hay, straw, and feces), common to the environment of confined small ruminants, were inoculated with Corynebacterium pseudotuberculosis in axenic purulent exudate that had been surgically removed from a naturally occurring case of caprine caseous lymphadenitis. Each inoculated fomite was incubated at 37, 22, and 4 C, and the length of time that C pseudotuberculosis survived was determined by isolation of bacteria from the fomite. The organism remained viable longer when caseous lymphadenitis abscess contents were mixed with particulate fomites than when spread on surfaces. Incubation at lower temperatures generally extended the survival potential of C pseudotuberculosis. Depending on the particulate fomite and the incubation temperature, viable C pseudotuberculosis organisms were isolated for mean periods ranging from 7 to 55 days, whereas recovery of bacteria from surfaces varied from 1 to 8 days.

Pasteurella haemolytica leukotoxin: physicochemical characteristics and susceptibility of leukotoxin to enzymatic treatment.

Sterile, concentrated culture supernatant from Pasteurella haemolytica (biotype A, serotype 1) strain 630 was subjected to physical, chemical, and immunologic treatments to determine their influence on leukotoxin (cytotoxin) activity contained in the supernatant. Each treated sample contained approximately 8 chemiluminescence inhibitory units of leukotoxin. Treatment effects were evaluated for their ability to inactivate leukotoxin activity. Leukotoxin activity in treated samples was determined by inhibition of the luminol-dependent chemiluminescence response of bovine neutrophils. Optimal leukotoxin synthesis by P haemolytica occurred when the bacteria were at the logarithmic growth phase, whereas stationary phase cultures contained minimal amounts of leukotoxin activity in their culture supernatant. Leukotoxin activity was heat labile; activity was substantially decreased when concentrated culture supernatant samples containing leukotoxin activity were incubated at 37 C for several hours. When concentrated culture supernatant was incubated at progressively decreasing temperatures, there was a progressive increase in the length of time that the leukotoxin retained its biologic activity. Samples stored at -70 C retained activity for at least 2 months. Leukotoxin activity was nondialyzable and was able to withstand considerable extremes in hydrogen ion concentration. Leukotoxin activity could not be pelleted when subjected to forces of 100,000 X g for 1 hour. Chemical and enzymatic studies suggested that P haemolytica leukotoxin contained carbohydrate and protein moieties. Chemical treatment with 0.2% sodium lauryl sulfate, 0.5% sodium deoxycholate, 7.5 mM EDTA and 8M urea with 8 mM 2-mercaptoethanol and enzymatic treatment with lipase, ribonuclease, and deoxyribonuclease had no discernible effect on leukotoxin activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Pasteurella haemolytica leukotoxin: comparison of 51chromium-release, trypan blue dye exclusion, and luminol-dependent chemiluminescence-inhibition assays for sensitivity in detecting leukotoxin activity.

Dilutions of concentrated, dialyzed Pasteurella haemolytica culture supernatant were caused to react with bovine neutrophil (PMN) suspensions, and then the trypan blue dye exclusion (TBDE), 51chromium (51Cr)-release, and luminol-dependent chemiluminescence-inhibition (LDCLI) assays were done to compare their relative sensitivities in detecting biological activity of P haemolytica leukotoxin (cytotoxin). The culture supernatant was concentrated approximately 200:1, and when caused to react as an undiluted preparation with bovine PMN, it was cytotoxic for 38.6% and 80.4% of PMN as determined by TBDE and 51Cr-release assays, respectively. This undiluted leukotoxin preparation caused 100% inhibition of the luminol-dependent chemiluminescence responses of bovine PMN. The LDCLI assay was the most sensitive of the 3 in vitro assays for P haemolytica leukotoxin activity--being approximately 17 times and 2,480 times more sensitive than the 51Cr-release and TBDE assays, respectively. The relative advantages and disadvantages of the 3 assays as in vitro systems for detecting and titrating leukotoxin activity and investigating the role of leukotoxin in disease pathogenesis and immunity are discussed. Because of its sensitivity, specificity, economy, technical ease, and potential for adaptation to automation, the LDCLI assay would seem to be the in vitro assay of choice for quantitating P haemolytica leukotoxin activity. To aid standardization of studies of leukotoxin between different laboratories, it is suggested that P haemolytica leukotoxin be quantitated and expressed as chemiluminescence inhibitory units.(ABSTRACT TRUNCATED AT 250 WORDS)

Pasteurella haemolytica leukotoxin: chemiluminescent responses of peripheral blood leukocytes from several different mammalian species to leukotoxin- and opsonin-treated living and killed Pasteurella haemolytica and Staphylococcus aureus.

A luminol-dependent chemiluminescence (LDCL) assay was used to assess the response of polymorphonuclear leukocyte (PMN) preparations from 4 species of ruminants (ie, cattle, sheep, goats, and antelopes) and 6 species of nonruminants (ie, swine, dogs, cats, rabbits, horses, and persons) to both opsonized and nonopsonized preparations of living and heat-killed Pasteurella haemolytica and Staphylococcus aureus and to opsonized and nonopsonized heat-killed strains of each bacterium in the presence of sterile culture supernatant (leukotoxin) from P haemolytica. The LDCL responses of PMN preparations from each of the species studied were greater for living than for heat-killed S aureus. The most efficient LDCL emission was observed with reaction mixtures containing opsonized living S aureus. Regardless whether they contained killed or living bacteria, the opsonized S aureus preparations elicited LDCL emissions more efficiently than did the corresponding nonopsonized preparations. Living P haemolytica cells and their sterile culture supernatant inhibited the LDCL emissions of phagocytically stimulated PMN preparations from ruminants, but not those from nonruminants. The LDCL response of ruminant PMN to nonopsonized living P haemolytica was characterized by the development of a peak response at 10 minutes of incubation followed by a precipitous decrease and a subsequent complete cessation of chemiluminescence. The peak LDCL response was higher for opsonized living P haemolytica than for nonopsonized living bacteria, and the increased response lasted longer. However, opsonization of living P haemolytica with the serum samples tested only temporarily spared the ruminant PMN preparations from the detrimental effects of leukotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine pneumonic pasteurellosis: chemiluminescent response of bovine peripheral blood leukocytes to living and killed Pasteurella haemolytica, Pasteurella multocida, and Escherichia coli.

A luminol-dependent chemiluminescence (LDCL) assay was used to evaluate the response of bovine polymorphonuclear leukocytes; (neutrophils [PMN]) to living and heat-killed Escherichia coli, Pasteurella multocida (type A, serotype 3), and P haemolytica (biotype A, serotype 1), and to heat-killed P haemolytica and sterile culture supernatant from living P haemolytica. Control cultures containing PMN that had not been phagocytically stimulated with bacteria had a modest increase in LDCL during the initial 10 minutes of incubation, followed by a gradual decline throughout the 120-minute incubation period. Bovine PMN emitted LDCL more efficiently when the cells were exposed to living E coli or P multocida than when they were exposed to the same bacteria killed by heat. The mean LDCL values for reaction mixtures containing living E coli or P multocida peaked at 30 minutes of incubation and remained above values for mixtures containing the same heat-killed bacteria. Kinetics of the LDCL response of bovine PMN to heat-killed P haemolytica were similar (although reduced in amplitude) to that observed with killed E coli or P multocida. The LDCL response of bovine PMN to living P haemolytica was not like that for E coli or P multocida, and was characterized by the development of a peak response at 10 minutes followed by a precipitous decrease in responsiveness and a subsequent complete cessation of LDCL. Addition of sterile culture supernatant from living P haemolytica to test samples containing heat-killed P haemolytica induced a response similar to that obtained with the living microorganism.(ABSTRACT TRUNCATED AT 250 WORDS)

Corynebacterium pseudotuberculosis exotoxin: fatal hemolytic anemia induced in gnotobiotic neonatal small ruminants by parenteral administration of preparations containing exotoxin.

Inoculation of live Corynebacterium pseudotuberculosis, culture supernatant, ammonium sulfate-fractionated crude exotoxin, or chromatographically purified exotoxin preparations into gnotobiotic small ruminants (n = 13) caused death of the ruminants within 48 hours. Characteristic changes observed in animals living greater than or equal to 2 hours after inoculation included hemorrhage and edema at the site of injection, severe hemolytic anemia and hemoglobinuria, dark red fluid in body cavities, lung edema, and icterus. The crude exotoxin preparation caused a syndrome of acute shock in 2 lambs that died within 15 minutes after inoculation. Clinical and pathologic responses of animals inoculated with culture supernatant and purified toxin were similar. Histopathologic evidence indicated that the exotoxin caused necrotic changes in the proximal convoluted tubules of the kidneys. Inoculation with live organisms caused multiple foci of suppurative inflammation in skeletal muscle and adjacent adipose tissue, whereas such changes were not observed in animals administered exotoxin preparations. Although C pseudotuberculosis exotoxin induced a hemolytic anemia in the experimental animals, it did not lead to in vitro lysis of ovine, caprine, or bovine erythrocytes, unless they had been sensitized with Rhodococcus (Corynebacterium) equi filtrate. The toxic sphingomyelin-specific phospholipase D from C pseudotuberculosis had a molecular weight of 31,000 daltons and an isoelectric point of approximately 9.6. The elution profile of exotoxin on a carboxymethyl Sephadex column was studied and the majority of the enzymatic activity was eluted by a NaCl gradient (0.25M to 0.7M) with a maximum at 0.35M NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Pasteurella haemolytica bacteriophage: identification, partial characterization, and relationship of temperate bacteriophages from isolates of Pasteurella haemolytica (biotype A, serotype 1).

Pasteurella haemolytica (biotype A, serotype 1) isolates (n = 15) from the upper respiratory tract of clinically normal cattle, as well as from lung lesions from cases of fatal bovine pasteurellosis, were examined for the presence of bacteriophage after irradiation with UV light. Treatment of all P haemolytica isolates with UV irradiation resulted in lysis of bacteria due to the induction of vegetative development of bacteriophages. The extent of growth inhibition and bacterial lysis in irradiated cultures was UV dose-dependent. Bacterial cultures exposed to UV light for 20 s reached peak culture density between 60 and 70 minutes after irradiation; thereafter, culture density declined rapidly, so that by 120 minutes, it was approximately 60% of the original value. When examined ultrastructurally, lytic cultures from each isolate revealed bacteriophages with an overall length of approximately 200 nm and that appeared to have a head with icosahedral symmetry and a contractile tail. Cell-free filtrate from each noninduced bacterial isolate was inoculated onto the other bacterial isolates in a cross-culture sensitivity assay for the presence of phages lytic for the host bacterial isolates. Zones of lysis (plaques) did not develop when bacterial lawns grown from the different isolates were inoculated with filtrates from the heterologous isolates.

Ovine caseous lymphadenitis: disease prevalence, lesion distribution, and thoracic manifestations in a population of mature culled sheep from western United States.

The prevalence, distribution, and severity of caseous lymphadenitis (CLA) lesions in 4,089 mature culled sheep were determined from a sample obtained by random selection of animal lots from a total population of 37,383 animals presented at an abattoir. The animals originated from 5 geographic regions comprising 9 western states. The prevalence rate for all forms of CLA was 42.41%. The prevalence of CLA lesions was significantly different in animals originating from 2 regions than it was in animals from the other 3 regions. The results indicate that CLA is a disease affecting a considerable number of mature culled sheep in different regions of western United States. Lesion distribution was catalogued according to a 4-compartment system: thoracic lymph nodes, lung parenchyma, abdominal tissues, and skeletal tissues including peripheral lymphatic tissues. Prevalence rates were determined for lesions occurring in the thoracic viscera (24.97%), skeletal tissues (23.09%), and abdominal viscera (11.79%). The prevalences of thoracic CLA lesions were significantly different for animals originating from 2 regions than for animals originating from the 3 other regions. Severe or advanced lesions were observed in the thoracic (3.74%) and abdominal viscera (1.27%). Involvements of CLA in the thoracic cavity occurred as abscesses of lung parenchyma, thoracic lymph nodes, or both. The bronchial and mediastinal lymph nodes were the most frequently affected thoracic lymph nodes. The liver was the most frequently affected abdominal organ, with the kidney being the next most commonly affected. The gross and histopathologic characteristics of CLA abscesses and tissues adjacent to the lesions were described.

Characteristics of the phagocytically induced respiratory burst in leukocytes from young adult and aged beagle dogs.

Phagocytically stimulated canine leukocyte suspensions obtained from 12 young adult and 43 aged individuals were examined for several physiological manifestations of the phagocytically induced respiratory burst. There was considerable variation in levels of oxygen consumption, glucose oxidation via the hexose monophosphate shunt (HMPS), and chemiluminescence response by both resting and phagocytically stimulated leukocytes from different individual animals in each age-group. Leukocyte suspensions from specific individuals in each age-group exhibited a weakly responsive respiratory burst. Chronological age could not be used as a predictor of either the specific (oxygen consumption and HMPS activity) or nonspecific (chemiluminescence response) manifestations of the respiratory burst. The kinetics of the chemiluminescence response were similar for all age-groups. Collectively, the results suggest that there is not an age-related alteration in the phagocytically induced respiratory burst of canine neutrophils and that cells from young adult and aged dogs have a comparable capacity to generate levels of highly reactive antimicrobial oxidizing agents. The increased relative susceptibility of aging dogs to microbial agents is apparently not related to an absent, abbreviated, or reduced leukocyte respiratory burst.

Delayed-type skin hypersensitivity and in vitro lymphocyte immunostimulation responses of swine following inoculation with Mycobacterium avium cell walls and a mycobacterial immunopotentiating glycolipid.

Miniature swine (n = 5 per group) were inoculated intradermally with mineral oil-in-water emulsions containing either 150 micrograms of mycobacterial immunopotentiating glycolipid P3 (EP3), 150 micrograms of lyophilized Mycobacterium avium (serotype 8) cell walls (E-MaCW), or 150 micrograms P3 and 150 micrograms M. avium cell walls (EP3-MaCW). Swine vaccinated with E-MaCW and EP3-MaCW developed antigen-sensitive lymphocytes detectable with delayed-type hypersensitivity (DTH) skin tests and lymphocyte transformation assays. Swine injected with EP3 were not sensitized. In general EP3-MaCW evoked a more pronounced in vivo DTH tuberculin skin test and in vitro lymphocyte transformation responses than E-MaCW. Time-course studies indicated a more persistent response in swine injected with EP3-MaCW than in those given E-MaCW. Commercial type Yorkshire swine (n = 5) inoculated intradermally with EP3-MaCW developed cell-mediated immune (CMI) responses to avian tuberculin detectable in vivo with delayed-type skin hypersensitivity and in vitro with lymphocyte immunostimulation responses.

Infectious bovine keratoconjunctivitis: bacteriologic, immunologic, and clinical responses of cattle to experimental exposure with Moraxella bovis.

The bacteriologic, immunologic, and clinical responses of 3- to 4-month old Holstein-Friesian calves to experimental exposure with Moraxella bovis type 10900 has been investigated. After u.v. radiation and intraconjunctival exposure with 1.9 X 10(7) microorganisms, each eye of 16 calves exhibited signs of blepharospasm, photophobia, and increased lacrimation. Bacteria were recovered from exposed eyes for 2-7 consecutive weeks before maximal clinical response occurred. The severity of the cases varied from eyes that exhibited mild signs to severe clinical cases with profuse lacrimation, conjunctival swelling, corneal opacity, and ulceration. By 70 days after exposure, M. bovis could not be recovered from any conjunctival swabs, and clinical signs were not observed. Four non-exposed control animals did not develop clinical signs nor was M. bovis recovered from conjunctival swabs. Lacrimal secretions collected at the time of and 1 week after maximal clinical response had significantly elevated levels of total protein as compared to those collected 3, 2, and 1 week before, and 2 and 3 weeks after maximal clinical response. A passive hemagglutination test, using tanned formalized sheep erythrocytes sensitized with M. bovis sonicate antigen, detected antibody in lacrimal secretions from 22 of 32 eyes. The appearance of specific antibody in lacrimal secretions correlated with the amelioration of clinical signs and the decline in numbers of M. bovis microorganisms recovered from conjunctival swabs.

Bacterial antibiotic resistance: frequency of gentamicin-resistant strains of Escherichia coli in the fecal microflora of commercial turkeys.

The relationship of subtherapeutic feeding and parenteral injection of antibiotics to the presence of antibiotic resistant strains of Escherichia coli in the fecal microflora of commercial turkeys has been investigated. Cloacal swabs collected from 137 commercial turkeys were examined for E coli resistant to gentamicin. Gentamicin-resistant E coli organisms were isolated and tested for resistance to ampicillin, chloramphenicol, kanamycin, streptomycin, and tetracycline. Strains of E coli resistant to gentamicin were identified in 118 of 137 (86.1%) specimens evaluated. There were 5 different antibiotic resistance patterns exhibited by the gentamicin-resistant strains of E coli. All strains showed a common antibiotic resistance pattern of gentamicin, kanamycin, and streptomycin. The results of the antibiotic susceptibility tests were compared to the known history of antibiotic usage in each flock. There was no significant correlation between the use of subtherapeutic concentrations of antibiotics and the frequency of gentamicin resistant E coli. However, the frequency of gentamicin-resistant E coli was closely related to the age of the bird, with birds less than 12 weeks of age being most likely to harbor E coli resistant to gentamicin. This age-dependent frequency of gentamicin-resistant E coli was associated with the common practice of dipping eggs in gentamicin and injecting newly hatched poults with gentamicin, but not with the feeding of subtherapeutic concentrations of antibiotics.

Experimental subacute foal pneumonia induced by aerosol administration of Corynebacterium equi.

Subacute pyogranulomatous pneumonia was experimentally induced in 3 neonatal foals following multiple challenge with aerosols containing Corynebacterium equi. On each of 7 consecutive days the foals were exposed to approximately 3.5 X 10(7) viable C equi in droplets small enough to reach the terminal airways. Clinical, pathological and bacteriological features of the induced syndrome were indistinguishable from those exhibited by cases with spontaneous subacute C equi foal pneumonia. Radiographic evidence of advanced pulmonary damage preceded the appearance of clinical signs and ante mortem cultures were not consistent in determining the presence of C equi infection. As observed in spontaneous cases of C equi foal pneumonia, there was lymphocytic hyperplasia in the T-dependent paracortical areas of bronchial lymph nodes and spleen, and granulomatous pulmonary lesions. These histological changes suggested predominant stimulation of cell-mediated immune processes in C equi infected foals. Lesions were restricted to the lungs and pulmonary lymph nodes and C equi was recovered from each foal's lung tissue at necropsy; the organism was also cultured from the trachea, mediastinal lymph nodes, mesenteric lymph nodes and caecal contents of one foal and from the liver of another foal. Three control foals exposed to saline did not develop evidence of pneumonia.