Infection following challenge of the lactating and dry udder of dairy cows with Actinomyces pyogenes and Peptostreptococcus indolicus

Challenge of 12 mammary glands of cows in mid-lactation with 10(7) colony forming units (cfu) of Peptostreptococcus indolicus on two occasions led to clinical mastitis in only four quarters. The bacteria were rarely recovered and disappeared from the secretion within 14 days. In challenges 7 days prior to drying off eight of 12 quarters became infected and at drying off all quarters challenged became infected. The infections established at drying off persisted well into the dry period. P. indolicus infection was also established in all of 12 dry glands challenged, but usually eliminated at calving or early in the next lactation. Isolation of P. indolicus was accompanied in about one-third of cases by changes in the appearance of the secretion. Intramammary challenge with Actinomyces (formally Corynebacterium) pyogenes led to clinical and subclinical infections in nine of 12 lactating glands and in all of six dry glands. Dry period infections with A. pyogenes were more severe and rarely eliminated even by antibiotic therapy. Infections during lactation were often eliminated either naturally or by antibiotic therapy. Intermittent recovery of A. pyogenes from the lactating mammary gland, without clinical signs of infection, was possible for up to 90 days after challenge. Combined infections with A. pyogenes and P. indolicus were clinically more severe with a higher frequency of systemic involvement. It was shown that in the non-lactating gland an acute mastitis, similar to 'summer mastitis' could be established either by simultaneous inoculation with A. pyogenes and P. indolicus or by subsequent inoculation of quarters excreting P. indolicus with A. pyogenes.     

Inflammatory reactions in the teat and udder of the dry cow

The inflammatory reactions in the teat and udder of the dry cow were studied by total and differential somatic cell counts (SCC) and by measuring bovine serum albumin, N-acetyl-beta-D-glucosaminidase (NAGase), plasminogen and plasmin. The teat and udder cisterns were surgically separated from each other in two udder quarters of each cow. Salmonella endotoxin was infused in one teat cistern and one udder quarter, and saline was infused in one teat cistern and one udder quarter. The inflammatory response was followed by several samplings post infusion. The reactions in the dry udder quarters were mainly in line with the results of similar studies in lactating glands. The differential SCC and the NAGase results were, however, somewhat different. The teats were capable of a strong inflammatory response. Reactions different from those seen in the glands were observed with regard to permeability changes and NAGase. The experimental model used showed promising results and is suitable for further studies of the inflammatory process.     

Effects of dry cow intramammary therapy on quarter infections in the dry period

Quarter milk samples were taken from 150 cows from three dairy farms in south-east Queensland at drying off, two, four and six weeks after drying off, at calving, and one, two and three weeks after calving. In each of the herds, the cows were randomly allocated to three groups of approximately equal size. One group had all the quarters of all the cows treated at drying off with a dry cow antibiotic infusion containing cloxacillin; the second group was given no treatment, and the third group had selected quarters treated on the basis of their high activity of N-acetyl-beta-D-glucosaminidase at drying off. Dry cow treatment resulted in a marked reduction in the number of infected quarters at two and four weeks after drying off, so that the comprehensively treated group had significantly less infected quarters at these times (P<0.02). Twelve dinical cases of mastitis were detected two weeks after drying off in the untreated groups, 10 in the untreated quarters of the selectively treated groups, and no cases in the comprehensively treated groups. These cases were due mainly to Streptococcus uberis and Streptococcus dysgalactiae. The number of infected untreated quarters increased markedly between drying off and two weeks later, but in all three groups there was a marked decrease in the number of infected quarters between six weeks after drying off and calving, suggesting that the mammary glands were more able to overcome infections at this time.     

Pathogenesis of Staphylococcus aureus mastitis: bacteriologic, histologic, and ultrastructural pathologic findings

Three lactating cows were experimentally infected with Staphylococcus aureus ATCC 29740 (Newbould 305 strain). Cows were euthanatized 2 to 216 hours after inoculation. Bacteriologic and microscopic examinations showed that S aureus attached to epithelial cells of the mammary gland in vivo. The histopathologic changes observed were progressive swelling, vacuolar degeneration of epithelial layers, and multiple foci of epithelial erosions and ulcers throughout the ductal system. The cellular response of the infected glands was demonstrated by a rapid increase in the number of somatic cells in the secretion and by accumulation of neutrophils below, within, and on the epithelium of the teat and lactiferous sinuses. The inflammatory response did not prevent infection nor subsequent pathologic changes in the inoculated glands.     

Role of eicosanoids, histamine, and serotonin in the pathogenesis of Klebsiella pneumoniae-induced bovine mastitis

By inoculating Klebsiella pneumoniae into the teat canals of mammary glands, coliform mastitis was induced experimentally in 6 lactating cows. Release of eicosanoids, histamine, and serotonin in plasma and milk was studied in response to 2 doses of K pneumoniae. A low dose (mean, 5,000 organisms/ml) was inoculated into cows 1 through 4, and a high dose (mean, 200,000 organisms/ml) was inoculated into cows 5 and 6. Milk and blood samples were collected before inoculation (0 hours), and hourly, from 3 to 24 hours after inoculation. Concentrations of prostaglandin F2 alpha (PGF2 alpha), prostaglandin E (PGE), thromboxane B2 (TxB2), histamine, and serotonin were measured in plasma and milk obtained from control (NaCl solution-inoculated) and infected quarters. Fluorometric analysis of milk from infected quarters revealed significantly increased histamine and serotonin concentrations regardless of the dose of K pneumoniae. The mean (+/- SEM) peak concentrations of histamine were significantly (P less than 0.01) increased from the preinoculation value of 44 (+/- 12) ng/ml to 312 (+/- 104) ng/ml in milk from infected quarters and 72 (+/- 24) ng/ml in milk from control quarters. The mean peak concentration of serotonin increased significantly from the preinoculation concentration of 436 (+/- 37) ng/ml to 1,754 (+/- 662) ng/ml and 4,867 (+/- 1,248) ng/ml in milk from control (P less than 0.02) and infected (P less than 0.001) quarters, respectively. However, serotonin concentration in milk from infected quarters was approximately 2.8 times greater than that in milk from control quarters. Concentrations of PGF2 alpha, PGE, and TxB2 in milk and plasma were evaluated by radioimmunoassay.(ABSTRACT TRUNCATED AT 250 WORDS)     

Persistence of Mycoplasma bovis infection in the mammary glands of lactating cows inoculated experimentally

To study the course of clinical mycoplasma mastitis and investigate its potential for persistence, 10(8) colony-forming units (cfu) of an Irish isolate of Mycoplasma bovis was inoculated aseptically into the right fore teat canal of three lactating cows. M bovis rapidly colonised the infected quarters and grew exponentially to more than 10(10) cfu/ml within the first three days, and spread to other quarters of each of the three cows within five to 10 days. After periods of between 24 and 72 hours the infected quarters became distended and sensitive to touch, and their secretions changed from containing visible particles, to a seropurulent exudate, to an aqueous suspension of fine particles which formed a sediment after a sample was collected. M bovis-specific antibody levels increased to varying degrees in all three cows. Subsequently, the concentrations of mycoplasma decreased to less than 10(7) cfu/ml in two of the cows, but remained at more than 10(8) cfu/ml to the end of the lactation of the other cow. Apparently normal milk was secreted by one of the cows within a month of the challenge, and by the other two cows at the start of their next lactation. However, in two of the cows subclinical M bovis infection persisted through the dry periods and into their next lactations.     

Changes in immunoglobulin levels in whey during experimental Streptococcus agalactiae mastitis

Total protein and immunoglobulin levels in the wheys of eight first lactation heifers, four vaccinated and four unvaccinated, were measured during three consecutive experimental intramammary infections with Streptococcus agalactiae. There were no significant differences between infections 1, 2 and 3 in the protein or immunoglobulin content of the uninfected quarters. Peak whey total protein of the infected quarters came earlier with each infection, until by the third they were seen after eight hours. During this acute phase a reversal of the normal milk IgG1/IgG2 ratio in all infected quarters was measured. Increases in whey IgA and IgM in the infected quarters of the vaccinates were also noted. A similar response only occurred following the third infection of the unvaccinated animals. All whey immunoglobulin levels returned to normal by 48 hours after infection, after which only IgG1 levels increased in infected quarters.     

Interleukin-1β infusion in bovine mammary glands prior to challenge with <Emphasis Type="Italic">Streptococcus uberis</Emphasis> reduces bacterial growth but causes sterile mastitis

Dairy cows are especially vulnerable to intramammary infection by the bacterial pathogen Streptococcus uberis in the dry period. Use of immunotherapeutic agents at drying off could increase cellular defences in the gland and prevent establishment of new S. uberis infections. This study investigated the potential of infusing recombinant bovine interleukin-1 beta (rbIL-1beta) in the mammary glands as a prophylactic agent against subsequent intramammary challenge with S. uberis in the early dry period. Immediately after the last milking at commencement of the dry period, one cow from each of 10 monozygous twinsets was infused with 10 microg of rbIL-1beta in two quarters and the other twin was infused with the carrier agent, sterile phosphate buffered saline. Twenty-four hours later, the quarters were infused with 10(3) colony-forming units (CFU) of S. uberis. Bacteriology, somatic cell count (SCC), concentrations of specific cytokines and antibody responses were monitored in mammary gland secretions and sera for the next 21 days. Infusion of rbIL-1beta into mammary glands at commencement of the dry period was associated with less new S. uberis intramammary infections, as determined by the number of quarters with bacterial growth. However, high SCC in quarters following infusion of rbIL-1beta masked the full beneficial effect of this procedure.     

Lymphocytes from one side of the bovine mammary gland migrate to the contra lateral gland and lymph node tissue

The four quarters of bovine mammary glands are completely separated and two quarters on each side (right or left) are connected to ipsi lateral supra mammary lymph nodes. It is not clear whether cells infused into the cistern of the mammary gland are capable of migrating to lymph nodes or the general circulation. To examine cell migration, a prescapular lymph node was removed from each of two lactating and three non-lactating dairy cows, and isolated lymphocytes were stained with Hoechst 33342. Autologous stained cells were infused into the mammary gland and then activated by intramammary infusion of zymosan-stimulated serum (source of C5a). After 17 h, Escherichia coli J5 bacterin was infused into the contra lateral mammary gland to mimic infection. After 43 h cows were euthanized and tissue samples (mammary quarters, right and left supra mammary, mesenteric, ileocecal and prescapular lymph nodes, liver and spleen) were collected for microscopic examination as well as flow cytometric analysis. Hoechst stained cells were detected not only in infused quarters, but also in contra lateral quarters as well as in both supra mammary lymph nodes. This indicates that cells infused into the mammary gland migrate to contra lateral tissues and supra mammary lymph nodes.     

Elimination kinetics of ceftiofur hydrochloride after intramammary administration in lactating dairy cows

OBJECTIVE: To determine the elimination kinetics of ceftiofur hydrochloride in milk after intramammary administration in lactating dairy cows. DESIGN: Prospective study. ANIMALS: 5 lactating dairy cows. PROCEDURE: After collection of baseline milk samples, 300 mg (6 mL) of ceftiofur was infused into the left front and right rear mammary gland quarters of each cow. Approximately 12 hours later, an additional 300 mg of ceftiofur was administered into the same mammary gland quarters after milking. Milk samples were collected from each mammary gland quarter every 12 hours for 10 days. Concentrations of ceftiofur and its metabolites in each milk sample were determined to assess the rate of ceftiofur elimination. RESULTS: Although there were considerable variations among mammary gland quarters and individual cows, ceftiofur concentrations in milk from all treated mammary gland quarters were less than the tolerance (0.1 microg/mL) set by the FDA by 168 hours (7 days) after the last intramammary administration of ceftiofur. No drug concentrations were detected in milk samples beyond this period. Ceftiofur was not detected in any milk samples from nontreated mammary gland quarters throughout the study. CONCLUSIONS AND CLINICAL RELEVANCE: Ceftiofur administered by the intramammary route as an extra-label treatment for mastitis in dairy cows reaches concentrations in milk greater than the tolerance set by the FDA. Results indicated that milk from treated mammary gland quarters should be discarded for a minimum of 7 days after intramammary administration of ceftiofur. Elimination of ceftiofur may be correlated with milk production, and cows producing smaller volumes of milk may have prolonged withdrawal times.     

Incidence of mastitis in beef cows after intramuscular administration of oxytetracycline

There is limited information on the value of antibiotic therapy for mastitis in beef cows. Effects of antibiotic treatment at weaning and the subsequent calving on calf weaning weight, milk somatic cell counts, milk components, and intramammary infection were studied in beef cows. Additionally, effects of number of infected mammary quarters, number of dry mammary quarters, type of intramammary pathogen, and parity on response variables were determined. Cows (n = 192) were randomly assigned to treatments in a 2 x 2 factorial arrangement; factors were time of treatment (weaning and after calving) and treatment (vehicle and vehicle plus antibiotic). Oxytetracycline (LA-200) or vehicle was administered intramuscularly following collection of quarter milk samples at weaning and calving. Percentage of infected cows and quarters averaged 43.4 and 16.4%, respectively, at calving and increased (P < 0.05) to 53.7 and 29.7% at weaning. Calves from cows with one or two dry quarters weighed 12.7 kg less (P < 0.05) at 90 d after calving and 18.7% less (P < 0.05) at 212 d after calving than calves from cows with no dry quarters. Calves from cows with three or four infected quarters weighed 17.5 kg less (P < 0.05) at 90 d and 25.5 kg less (P < 0.05) at weaning than calves from cows with two or fewer infected quarters. Infections by Staphylococcus aureus and coagulase-negative staphylococci were the most common and accounted for 67 and 78% of the infections. Percentages of infected cows and quarters, infections caused by S. aureus, and dry quarters increased (P < 0.05) with parity. No differences were found among antibiotic treatments for any of the response variables studied. Intramuscular oxytetracycline was not effective in the control of mastitis in beef cows under the conditions of the study.     

Tuberculin elicited cellular immune response in the lactating bovine mammary gland vaccinated intramammarily with Mycobacterium bovis

The development of a local antigen-specific sensitivity was monitored histologically and in secretions of the bovine mammary gland. Three cows in mid-lactation were immunized by injecting 50 microliter of a killed Mycobacterium bovis vaccine into the dorsal secretory tissue of the rear mammary glands; two cows served as unvaccinated controls. Ten weeks after vaccination, all cows were challenged by intramammary infusion of 1.0 microgram tuberculin in 5 ml phosphate-buffered saline (PBS). Three quarters of each cow received tuberculin at 72, 48, and 24 hours before slaughter; a control quarter received PBS at 72 hours. Vaccinated cows exhibited an intense, local cellular reaction to tuberculin in teat-end tissues at all times post infusion; PBS-infused glands were normal. A moderate leukocyte response in parenchymal tissues adjacent to the gland cistern of tuberculin-infused quarters was observed, but deep parenchymal tissues were normal; no effect on milk yield was found. Tuberculin-infused quarters exhibited histological responses in teat cisternal tissues similar to those in delayed-type hypersensitivity reactions. Leukocytic accumulation was primarily macrophages and lymphocytes with few neutrophils. Erythema was observed in the distal half of the cistern, and fibrin deposits were found in subepithelial connective tissues. The epithelium, although distended with leukocytes, was intact and numerous mitotic figures were present. Unvaccinated cows showed no response to challenge. Results demonstrated a marked and specific cellular response in sensitized cows to challenge with tuberculin.     

Effects of Staphylococcus aureus mastitis on bovine mammary gland plasma cell populations and immunoglobin concentrations in milk

Bovine mammary tissue and milk samples were examined to determine effects of chronic Staphylococcus aureus mastitis on the humoral immune response. Parenchymal and teat end tissues from lactating bovine mammary glands were stained immunohistochemically to determine distribution of immunoglobulin (Ig) G1-, IgG2-, IgA-, and IgM-producing plasma cells. Numbers of all Ig-producing plasma cells tended to be higher in tissues from S. aureus infected quarters compared with controls, but most differences were not statistically different. Numbers of IgG1-producing plasma cells at the Furstenberg's rosette area of infected quarters were significantly (P less than 0.05) higher than uninfected quarters. There were no significant differences in concentrations of Ig isotypes in milk from S. aureus infected and uninfected quarters. Data suggest that the antigenic effect of chronic S. aureus infection on the humoral immune response of the bovine mammary gland is minimal. Persistency of S. aureus infection may result, in part, from suboptimal stimulation or immunosuppression of the mammary immune system.     

Experimental infection of lactating bovine mammary glands with Streptococcus uberis in quarters colonized by Corynebacterium bovis

Twenty-seven quarters of 18 lactating dairy cows were inoculated intramammarily with 3.6 X 10(4) colony-forming units (CFU) of a strain of Streptococcus uberis isolated from a cow with clinical mastitis. Before quarters were inoculated, 22 were considered as naturally colonized with Corynebacterium bovis, and 5 were considered bacteriologically negative. Streptococcus uberis was isolated from all quarters within 2 days after inoculation, and all quarters developed clinical mastitis by 3 days after inoculation. Mastitis was acute, and most cows had increased rectal temperatures. The number of somatic cells increased significantly (P less than 0.05), and milk production decreased significantly. In many cows, rectal temperatures remained increased, and Str uberis was isolated from infected glands after intramammary and systemic antimicrobial treatments were given. A decreased number (110 CFU) of the same strain of Str uberis caused equally severe mastitis in 3 quarters colonized with C bovis and in 1 bacteriologically negative quarter in 2 cows. Streptococcus uberis was isolated from all inoculated quarters, and all quarters developed clinical mastitis by 2 days after inoculation. Two quarters colonized with C bovis and 2 bacteriologically negative quarters were inoculated once with 25 CFU and once with 240 CFU of a different strain of Str uberis (ATCC 27958). Streptococcus uberis was never isolated from inoculated quarters, and changes in milk yield or number of somatic cells were not observed.     

Pharmacological aspects of chloramphenicol administration by the intramammary route to lactating dairy cows

Concentrations of chloramphenicol (C M) were determined, by microbiological assay, in the milk and blood serum of 17 culled dairy cows after intramammary infusion of an approved parenteral CM product (Gloveticol) and in the milk of 16 lactating cows after treatment with two approved CM products for intramammary infusion, at dosages ranging from 1 to 30 g/cow. C M was quickly absorbed from the udder into the blood circulation; the doses of 12.5 and 25 g/cow were almost completely absorbed within 20 hours. Absorption half-life (t1/2ab) from fully functioning quarters was 57+/-18 minutes, and the t1/2ab from partially functioning quarters was 125+/-37 minutes. Mean peak serum C M concentrations were 6.1, 16.2, and 37.4 microg/ml after the cows had been infused with 5, 12.5, and 25 g, respectively. These values were considerably higher than the corresponding peak serum C M concentrations reported following intramuscular injection of equivalent doses of the drug. C M residues were not detectible microbiologically in milk from treated quarters 20 hours after treatment with 5 g or 6.25 g, and 36 hours after treatment with 15 g. Drug concentrations in the milk from the non-treated quarters were approximately 70 per cent of the corresponding serum drug levels. Serum CM concentrations of potential therapeutic value in the treatment of gram-negative bacterial infections, i.e. > 5 microg/ml, were maintained for 8 hours after cows had been infused with 12.5 g, and for 12 hours after infusion with 25 g. The implications of the improved systemic availability of C M infused by the intramammary route over the intramuscular route are discussed in terms of potential therapeutic efficacy, local irritation, and duration of drug residues.     

The survival of serum resistant Escherichia coli in the bovine mammary gland following experimental infection.

Serum resistant strains of Escherichia coli were injected into one or two quarters of the udders of eight healthy dairy cows. Animals receiving infection into two quarters showed variation in their ability to eliminate the bacteria. This variation extended from elimination from both glands to complete failure to remove the organisms from either gland. In most cases, the organisms were removed from one gland before the clinical signs of infection were observed, but persisted in the other gland for three to four days. Following a single infection most animals eliminated the organisms before the appearance of clinical signs, but one retained the bacteria for four days. The retention of bacteria within the gland for a period longer than the initial inflammatory response resulted in their survival within neutrophils; and in some of these glands spasmodic clinical signs of mastitis reappeared for up to at least 40 days after the initial infection. These signs were associated with the reappearance of the same serological strain of E coli. Bovine serum albumin levels in the milk were found not to constitute the most effective marker of the serum components involved in the bactericidal activity.     

Simultaneous intramammary and intranasal inoculation of lactating cows with bovine herpesvirus 4 induce subclinical mastitis

In this study, we examined whether an experimental bovine herpesvirus 4 (BHV4) infection can induce bovine mastitis, or can enhance bovine mastitis induced by Streptococcus uberis (S. uberis). Four lactating cows were inoculated intramammarily and intranasally with BHV4, and four lactating control cows were mock-inoculated. After 14 days, two of four cows from each group were inoculated intramammarily with S. uberis. No clinical signs were recorded in cows inoculated only with BHV4, and their milk samples showed no abnormal morphology, despite the fact that BHV4 replicated in inoculated quarters. Somatic cell count increased significantly in milk from three of six BHV4-inoculated quarters, compared to the non-inoculated quarters of the same cows (within-cow) and the quarters of mock-inoculated cows (control group) on days 8, 9 and 11 post-inoculation (pi). BHV4 was isolated from nasal swabs between days 2 and 9 pi. Clinical mastitis was observed in all four cows intramammarily inoculated with S. uberis. A preceding BHV4 infection did not exacerbate the clinical mastitis induced by S. uberis. S. uberis infections appeared to trigger BHV4 replication. From one quarter of each of two cows inoculated with BHV4 and S. uberis, BHV4 was isolated, and not from quarters inoculated with BHV4 only. In conclusion, BHV4 did not induce bovine clinical mastitis after simultaneous intranasal and intramammary inoculation. However, the BHV4 infection did induce subclinical mastitis in 50% of the cows and the quarters.     

Infection of the Bovine Udder with Bovine Herpesvirus

Infectious bovine rhinotracheitis — infectious pustular vulvovaginitis (bovine herpesvirus) grown in tissue culture was used as inoculum in trials to infect the lactating bovine udder. Six experiments were undertaken in which one or more quarters were infused with 1 ml. of tissue culture fluids containing 10⁶ to 10⁷ tissue culture infectious doses (TCID) of virus. In four of the experiments the inoculated quarters showed marked evidence of infection in the form of acute inflammation, swelling, reduced milk secretion and profound changes in the physical appearance of the milk. In each case virus was recovered in high titres in the milk from about the second until the tenth to fifteenth days following exposure. Uninfected quarters remained normal in appearance and virus could not be recovered from the milk.

In three of the experiments it was shown that serum and milk antibodies appeared shortly after the disappearance of virus from the milk. One experiment involving two animals showed that about 1000 TCID of virus were required to produce infection. In one experiment a cow having a pre-inoculation serum titre for bovine herpesvirus proved resistant to infection.

The experiments indicate that the bovine udder is readily susceptible to bovine herpesvirus in non-immune animals, and that the virus produces an acute, limited infection leading to a temporary disfunction of the gland. It appears that natural invasion of the udder through the teat canal is not readily accomplished by the virus.

     

Flunixin meglumine and flurbiprofen in cows with experimental Escherichia coli mastitis

The effect of flunixin meglumine and flurbiprofen on the course of experimental Escherichia coli mastitis was examined. Nine cows (within one month post partum) were inoculated intramammarily with 20 x 10(5) viable E coli in both rear quarters. Three cows remained untreated (controls); three cows received three injections of flunixin meglumine and three cows received flurbiprofen as two intravenous infusions. Flunixin meglumine and flurbiprofen were initially given before clinical signs were observed. Treatment was repeated if the cows' temperature increased by more than 1 degree C. In the untreated cows, rectal temperature and heart rate increased from three hours after infection, and rumen motility (both frequency and amplitude) decreased from four hours after infection. Treatment with flunixin meglumine or flurbiprofen almost completely abolished the febrile response during the first nine hours after infection, and the decrease in rumen motility was less pronounced in the treated animals. These results suggest that the decrease in rumen motility during E coli mastitis is at least partly due to a mechanism involving prostaglandin.     

Prevalence and associations between bacterial isolates from dry mammary glands of dairy cows

To assess the prevalence and patterns of bacterial isolates, cultures were made from the dry mammary glands of dairy cows in six commercial dairy herds in the UK. Milk samples were taken from all four quarters of 480 cows at drying off and at weekly intervals from 14 days before to seven days after calving. A major mastitis pathogen was isolated from at least one quarter of 220 (45.8 per cent) of the cows and from more than one quarter of 90 (18.8 per cent) of them. During the late dry to calving period, of the 957 quarters with three culture results, a major mastitis pathogen was cultured from 236 (24.7 per cent) quarters of 186 (38.8 [corrected] per cent) cows. The most commonly isolated major pathogen was Escherichia coli, followed by Streptococcus uberis and coagulase-positive staphylococci. There were significant differences between the patterns of isolates from different farms and in different calving months, suggesting that the rate of infection was partially dependent on external conditions. The isolation of E. coli, S. uberis or coagulase-positive staphylococci from a cow during the late dry/periparturient period was associated with an increased risk of that cow being culled in the next lactation. Bayesian general linear mixed models were used to assess the associations between the different bacterial species. The probability of isolating either E. coli or S. uberis was significantly greater when the other organism was cultured in a milk sample; this was also true of coagulase-positive staphylococci and S. uberis. When Corynebacterium species were isolated from a milk sample, the probability of isolating coagulase-positive staphylococci or S. uberis decreased significantly, and when coagulase-negative staphylococci were isolated the probability of isolating coagulase-positive staphylococci was reduced.