Dairy calf management,morbidity and mortality in Ontario Holstein herds. III. Association of management with morbidity

Associations between heifer calf management and morbidity, particularly scours and pneumonia, were studied on 104 randomly selected Holstein dairy farms in southwestern Ontario between October 1980 and July 1983. At the farm level, data were stratified by season, with two six-month seasons (winter and summer) per year. The odds of farms with particular management strategies having above median morbidity were calculated. At the individual calf level, the odds of a calf being treated, controlling for farm of origin and month of birth, were calculated for different management practices.Farm size, and policies related to anti-scour vaccination, offering free-choice water and minerals to calves, methods of feeding, and the use of medicated feeds significantly altered the odds of a farm experiencing above-median pneumonia rates. Farm policies with regard to anti-scour vaccination, offering free-choice salt to calves, age at teat removal, type of calf housing, and use of preventive antimicrobials significantly altered the odds of a farm experiencing above-median scours rates.Scours and pneumonia were significantly associated with each other at both the farm and the calf level. No significant associations were found between individual calf management practices and the odds of being treated for scours. Sire used, method of first colostrum feeding, navel treatment, use of anti-scour vaccine in the dam, and the administration of preventive antimicrobials significantly altered the odds of a calf being treated for pneumonia.     

Antimicrobial susceptibility of Escherichia coli isolates from dairy farms using organic versus conventional production methods

OBJECTIVE: To compare antimicrobial susceptibility patterns of Escherichia coli isolates cultured from fecal samples from cows and calves on dairy farms that used organic (ie, no or severely limited antimicrobial use) versus conventional production methods. DESIGN: Cross-sectional study. SAMPLE POPULATION: Fecal samples from 10 cows and 10 calves on each of 30 organic dairy farms and 30 neighboring conventional dairy farms in Wisconsin. PROCEDURE: E. coli isolates obtained from the fecal samples were tested for susceptibility to 17 antimicrobials by means of a microbroth dilution test. Prevalence of antimicrobial resistance was compared between organic and conventional dairy farms. RESULTS: E. coli was isolated from 1,121 (94%) fecal samples. Farm type (organic vs conventional) and animal age (cow vs calf) were significantly associated with odds that E. coli isolates would be resistant to various antimicrobials. After controlling for age, logistic regression analyses indicated that isolates from conventional dairy farms had significantly higher rates of resistance to ampicillin, streptomycin, kanamycin, gentamicin, chloramphenicol, tetracycline, and sulfamethoxazole than did isolates from organic dairy farms. However, no significant differences were detected for the 10 other antimicrobials that were tested. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that compared with isolates from conventional dairy farms, E. coli isolates from organic dairy herds have significantly lower prevalences of resistance to 7 antimicrobials; however, prevalence of resistance was not significantly different for 10 other antimicrobials. Resistance was more common for isolates from calves than for isolates from adult dairy cows.     

Farming practices in Sweden related to feeding milk and colostrum from cows treated with antimicrobials to dairy calves

Background

Milk produced by cows in receipt of antimicrobial therapy may contain antimicrobial residues. Such antimicrobial-containing waste milk must be withdrawn from human consumption and is therefore sometimes used as calf feed. Unfortunately, this approach might promote selection of antimicrobial resistant bacteria in the calves’ intestinal microbiota. The objectives of this study were therefore to obtain an overview of waste milk feeding practices on Swedish dairy farms and to investigate if these practices were associated with certain farm characteristics. A representative group of 457 Swedish dairy farmers participated in a web-based survey with questions about the use of colostrum and milk from cows treated with antimicrobials at dry off or during lactation, respectively, as calf feed.

Results

Colostrum (milk from the first milking after calving) and transition milk (milk from the second milking to the fourth day after calving) from cows treated with antimicrobials at dry off was fed to calves on 89% and 85% of the farms in the study, respectively. When antimicrobial therapy was given to cows during lactation, 56% of the farms fed milk that was produced during the course of treatment to calves, whereas milk that was produced during the subsequent withdrawal period was fed to calves on 79% of the farms. Surveyed farmers were less prone to feed such milk if the antimicrobial therapy was due to mastitis than other infections. In Sweden, a majority of antimicrobial treatments during lactation are systemic administration of benzylpenicillin and thus, the bulk of waste milk in Sweden is likely to contain residues of this drug. Feeding waste milk to calves was more common on non-organic farms, and on farms located in Southern Sweden, and was less common on farms with cows housed in cold free stalls barns.

Conclusions

Waste milk that may contain antimicrobial residues is, at least occasionally, used as feed for calves on a majority of surveyed Swedish dairy farms. Future work should focus on the effect of waste milk feeding on the occurrence of antimicrobial resistant bacteria in the calves’ intestinal microbiota.     

Dairy calf management,morbidity and mortality in Ontario Holstein herds. I. The data

Heifer calf management practices and clinical outcomes were studied on 104 randomly selected Holstein dairy farms in southwestern Ontario between October 1980 and July 1983. Data were collected at both the farm level (all farms) and the individual calf level (1968 calves, 35 farms).Farm-level management data were collected by means of questionnaires and farm visits. Calf-level management data were recorded by farmers on forms provided. Treatments for disease and mortality data were recorded by the farmers on the calves up to the age of weaning, the age at which calves were no longer fed milk or milk substitutes on a regular basis.Farm size ranged from 23 to 154 calvings per year. Farm-level mortality rates per six-month season (winter/summer) were skewed, with a mean of about 6%, a median of zero, and a range of 67%. Morbidity rates were similarly skewed. Four percent of liveborn heifer calves died, 20% were treated for scours, and 15% were treated for pneumonia before the age of weaning.Stated farm policies on calf rearing were implemented to varying degrees on different farms, as estimated from individual calf data. This posed particular problems for interpreting farm-level data, since the danger of committing an ecologic fallacy was high. The data set described in this paper formed the basis for an observational study of the interlationships of heifer calf management and clinical outcomes.     

Survey of the knowledge, attitudes and practice of Italian beef and dairy cattle veterinarians concerning the use of antibiotics

Between June and September 2002 a telephone survey of Italian beef and dairy cattle veterinarians was made to obtain information about their use of antibiotics and their perception of the problem of antimicrobial resistance. A total of 106 veterinarians, selected at random from the membership lists of two professional societies, were interviewed by telephone, using a structured questionnaire concerning their background, training and continuing education activities, and current type of practice; their diagnostic, treatment, and prophylactic practices for mastitis, calf scours and respiratory disease; and their perception of the threat posed by antimicrobial resistance. The median age of the interviewees was 42.5 (range 28 to 75) years; 62 per cent treated only dairy cattle, 10 per cent treated only beef cattle and 28 per cent treated both. Laboratory support was requested 'frequently' or 'always' by 67 per cent of the veterinarians when treating mastitis, by 37 per cent when treating calf scours and by 17 per cent when treating respiratory diseases. Twenty per cent reported using prophylactic antibiotics 'often' or 'sometimes' for calf scours, 28 per cent for respiratory diseases, and 62 per cent reported their use 'always' or 'often' for mastitis. Fluoroquinolones, phenicols or third/fourth-generation cephalosporins were prescribed as first-choice drugs by 54 per cent for calf scours, by 12 per cent for bacterial respiratory diseases and by 6 per cent for mastitis. Therapeutic failure was reported 'often' (21 per cent) or 'sometimes' (64 per cent) and was the main predictor in a multivariate model of the use of newer antibiotics. The level of awareness of the problem of antibiotic resistance was high, although more than half of the interviewees were confident that new antimicrobial drugs were already available to replace those of lower effectiveness.     

Dairy calf management,morbidity and mortality in Ontario Holstein herds. II. Age and seasonal patterns

Heifer calf management practices and clinical outcomes were studied on 104 randomly selected Holstein dairy farms in southwestern Ontario between October 1980 and July 1983. Data were collected at both the farm level (all farms) and the individual calf level (1968 calves, 35 farms).Age at death, and at first treatment for disease, during the first 20 weeks of life were described by life table methods and depicted graphically. Variations over calendar time in morbidity and mortality were described graphically.The percentage of calves at risk first treated for scours peaked during the second week of life, at 8.1% per week, then declined sharply, approaching zero by about six weeks. Pneumonia treatment rates peaked at about the sixth week of life, at 2.3% per week, but were more persistent than the rates for scours. Calves were at greater risk of dying during the first week of life than at any time thereafter; however, calf mortality in this population never exceeded 4% of the total.Treatment rates for scours and pneumonia were generally lower in spring and summer than during the autumn and winter. The mortality rates remained at < 5% per month and appeared to fluctuate in a random fashion.     

A mail survey of the efficacy of prophylactic medication in feed and/or water of feedlot calves.

A mail survey of feedlot owners was conducted to evaluate the efficacy of prophylactic antimicrobials, given in the water, or in the ration at preventing illness and/or death. One hundred and twenty-seven farmers from southwestern Ontario collaborated in the study. The percentage of calves requiring individual antimicrobial treatment, for any reason within 28 days of arrival was 22.6% (median 17.8%) and 0.6% (median 0.2%) died in that period. The use of medicated starter rations was not associated with either treatment or mortality rates until the effects of a number of other variables were controlled, analytically. Thereafter, the use of medicated feed was associated with a decrease in mortality rate, but was unrelated to morbidity rate. Overall, the use of medicated water was not associated with treatment or mortality rates. The use of sulphonamides was associated with decreased morbidity, but increased mortality rates. After controlling, analytically using multiple regression, the effects of other variables, the use of medicated water was associated with a significant increase in mortality rates. The other major factors which influenced mortality rates were the number of calves per group, the number of subgroups of calves in each group and whether the group contained cattle from different sources; all were related to increased mortality rates. During a two year period, more feedlot owners appeared to be using medicated rations as opposed to medicated water, as a means of providing antimicrobials to their newly arrived calves.     

An evaluation of antimicrobial therapy for undifferentiated bovine respiratory disease

A field trial of antimicrobial therapy for cases of undifferentiated bovine respiratory disease (UBRD) in beef calves was conducted at four Ontario feedlots. The primary purpose of the trial was to evaluate the efficacy of three different antimicrobials (oxytetracycline, penicillin, and trimethoprim-sulfadoxine) in the treatment of UBRD occurring within the first 28 days postarrival.

The response, relapse, and case fatality rates overall were 85.7%, 14.8%, and 1.4%, respectively, and were not significantly different among the three antimicrobials evaluated. Weight gains of calves treated with the different drugs were not statistically different over the feeding period. Calves that suffered a relapse posttreatment were first treated significantly earlier (p<0.001) in the postarrival period than those that did not relapse. Considered together, treated calves gained significantly less (p<0.05) over the first 28 days and throughout the entire feeding period than controls that were never sick. Cases of UBRD that responded to therapy and did not relapse had rates of gain that were not significantly different from the controls.

     

The effect of early calfhood health status on survivorship and age at first calving.

Survival to maturity and age at first calving were studied in heifer calves from 34 randomly selected Holstein dairy farms in southwestern Ontario. Calves were divided into cohorts on the basis of treatment for pneumonia, scours, other diseases, or no treatments, during the first 90 days of life. An effect of pneumonia and scours together, over and above the effects of each disease alone, was assessed by means on an interaction term in the statistical analyses. Heifers which had been treated for pneumonia during the first three months of life were 2.5 times more likely to die after 90 days of age than heifers which had not been treated for pneumonia, after controlling for the farm effect. Heifers with a calfhood history of being treated for scours were 2.5 times more likely to be sold for dairy purposes than other calves. Heifers which had been treated for scours were 2.9 times more likely to calve after 900 days of age than other heifers, after controlling for the farm effect.     

Description and evaluation of the influence of veterinary presence on the use of antibiotics and sulfonamides in dairy herds.

A longitudinal study was conducted over 2 years to identify types of antibiotics and sulfonamides used in Michigan dairy herds for disease prevention and treatment, to determine patterns of use of antibiotics and sulfonamides by herd size and animal age group, and to determine the influence of veterinary presence during diagnosis on the types of antimicrobials used for disease treatments. In order of frequency, the most commonly used preventive antibiotic and sulfonamides were penicillins, tetracyclines, aminoglycosides, and cephalosporins, making up over 86% of all antimicrobials used for disease prevention. The most commonly used therapeutic antibiotics and sulfonamides were penicillins, tetracyclines, aminoglycosides, and sulfonamides, making up over 81% of all antimicrobial drugs used for disease treatment. Cows received the greatest number of drugs, followed by calves (cattle from birth to weaning). Young stock (cattle from weaning to first calving) received the lowest number of drugs. All herds had similar patterns of drug use for the 3 age groups, regardless of herd size. With the exception of polymyxin and chloramphenicol, producers used antibiotics on their own more than with a veterinarian present or on the advice of a veterinarian. Overall, veterinary presence was significantly associated with increased use of tetracyclines, sulfonamides, and nitrofurans, and decreased use of penicillins and aminoglycosides. Implications for drug residue prevention strategies are discussed, with emphasis on the role of the practicing veterinarian.     

Stakeholder position paper: dairy producer

Bacterial antimicrobial resistance is a problem common to both animal and public health. An important public policy issue is to develop and implement prudent use practices where antimicrobials are used. As policy develops, there are questions regarding the use of antimicrobials in animal agriculture and whether these uses constitute prudent use. A series of papers assessing the risk to the public health from agricultural use of antimicrobials have consistently concluded that risk estimation is hampered by the lack of data that describe the amount, types, and uses of antimicrobials in animal agriculture. The absence of information has spurred efforts to develop a framework to collect these data. However, the reasons and benefit of collecting these data should be carefully defined. The dairy industry, contrasted to other major animal commodities, is not focused on meat production but on milk production. Milk production is constrained by disease and antimicrobial treatment is a common management tool, but unlike many other animal agricultural systems where the value and safety of the product is measured in the future; the value of milk is zero when an antimicrobial is used in a lactating cow and milk must be discarded because of residues. While there are exceptions, e.g. non-lactating cow therapy, this difference results in antimicrobials being used sporadically and directed at therapy rather than prophylactic uses. In the dairy industry, antimicrobial use data and its consequences may exist in sufficient detail or could be estimated from existing datasets without the expense of additional surveys. Finally, the main food product milk is mainly pasteurized and all shipments of milk from the farm to the processing plant are tested for the presence of antimicrobials. This makes the likelihood of farm-origin antimicrobials or bacteria appearing in finished product very low. This suggests that the use and quantity of antimicrobials in the dairy system has little impact on public health. This does not imply that the dairy industry does not have a significant role in developing guidelines for appropriate and careful application of antimicrobials, but the effort and cost to collect additional data should be used to fund efforts that improve our diagnostic and managerial skills. These data would change the use of antimicrobials by decreasing the rates of disease and ultimately decreasing prophylactic, metaphylactic and therapeutic use of antimicrobial. These studies and outcome are as important to the dairy industry as to public health.     

Antimicrobial susceptibility of Salmonella isolates recovered from calves with diarrhoea in Australia

OBJECTIVE To identify the common serotypes and antimicrobial resistance patterns of Salmonella spp. associated with diarrhoea in Australian dairy calves under the age of 6 weeks. DESIGN Cross-sectional study. METHODS Faecal samples were collected from outbreaks of diarrhoea in dairy and dairy beef calves less than 6 weeks old. Samples were screened for Salmonella using standard enrichment culture techniques. The antimicrobial susceptibility to 12 commonly used veterinary and human antimicrobials was assessed using the Kirby-Bauer disk diffusion method and the susceptibility profiles of dairy and dairy beef properties were compared using Fisher's exact test. RESULTS Salmonella ser. Dublin, S. ser. Typhimurium and S. ser. Bovismorbificans were the three most common salmonella serotypes isolated. The majority of properties had one serotype. Most of the Salmonella isolates were not resistant to any of the antimicrobials tested. No resistance was seen to amikacin and nalidixic acid, and only one isolate was resistant to ceftiofur or amoxicillin-clavulanic acid. The most common antimicrobial resistance was to streptomycin, ampicillin or combination sulfonamides. Multi-drug resistance was detected in S. ser. Anatum, S. ser. Bovismorbificans, S. ser. Muenster, S. ser. Newport and S. ser. Typhimurium. Isolates from dairy beef properties were more likely to be resistant to ampicillin, kanamycin, neomycin, sulfamethoxazole/trimethoprim and tetracycline (P < 0.05) and were more likely to exhibit multi-drug resistance. CONCLUSION The prevalence of antimicrobial resistance in Salmonella isolates from dairy calves in Australia is low compared with that reported overseas. From a human health perspective, resistance to antimicrobials used in the treatment of human salmonellosis was infrequent.     

Preventive and therapeutic efficacy of halofuginone-lactate against Cryptosporidium parvum in spontaneously infected calves: a centralised, randomised, double-blind, placebo-controlled study

The preventive and therapeutic efficacy of halofuginone-lactate (HFL) against Cryptosporidium parvum was evaluated in a study conducted from November 2004 to March 2005 on a dairy farm in Central Bohemia, Czech Republic, using 260 spontaneously infected calves. HFL (0.1 mg/kg/day) was administered orally for 7 days to 1-day-old and 8-day-old calves, respectively. In both treated groups the drug significantly, and in almost the same manner, decreased the intensity of diarrhoea (P<0.001) and faecal oocyst count (P<0.001) when compared to corresponding placebo groups. The only difference between both treated groups was the time of onset of symptoms of the infection. Over time, the clinical pattern of cryptosporidiosis in the animals treated at 8-14 days of age was similar to that seen in the groups receiving the placebo. In contrast, infection in the preventively treated group peaked about 10 days later but with the same intensity. The results of this study confirm the anticryptosporidial activity of HFL in calves, but show that the outcome of infection following preventive treatment is comparable to that observed in calves treated after the onset of symptoms.     

Comparative study of chloramphenicol absorption in calves after oral, intra-ruminal and intra-abomasal administration

The mechanisms responsible for the rapid decline of chloramphenicol plasma levels after oral administration in young calves during their first weeks of life were investigated. Chloramphenicol was administered by stomach tube, to four 2 week old calves on three consecutive days at a dose of 50 mg/kg. The plasma levels increased daily to a peak value on the third day. The minimum therapeutic concentration of 5 μg/ml, however, was barely obtained. Simultaneous estimation of the ruminal contents showed a parallel increase in chloramphenicol concentration. Thus it can be assumed that there is an inefficient absorption of chloramphenicol from the forestomachs of young calves. Chloramphenicol was not metabolized by the ruminal contents until the calves were 9 weeks old. Finally chloramphenicol was administered to 7 week old calves by the intra-abomasal route, intraruminal route and by mouth. Only with the intra-abomasal method could a therapeutically effective level be reached. This indicates that the rapid decline of chloramphenicol plasma levels in calves during their first weeks of life could be attributed to the delayed reticulo-rumen emptying and to inefficient absorption from the forestomachs.     

A field trial to evaluate the efficacy of a combined rotavirus-coronavirus/Escherichia coli vaccine in dairy cattle.

A field trial was designed to determine the efficacy of a combination rotavirus-coronavirus/Escherichia coli vaccine on dairy farms in southwestern Ontario. In Part A of the trial, 321 cows on 15 farms were randomly assigned to either vaccination or placebo groups. On eight farms, 50% of the dams were vaccinated, while on the other seven farms, 80% of the dams were vaccinated. In Part B of the trial, 26 farms were randomly assigned to either a total vaccination program or to no vaccination program. Mortality, disease occurrence and weight gains were recorded on all calves for the first two weeks of life. In Part A, 23.5% of all calves were treated in the first two weeks of life, 20.9% were treated specifically for scours and 3.6% of live-born calves died. Enteropathogenic E. coli was identified on 13 of the 15 farms, rotavirus on 11 and coronavirus on ten. At least one of the three potential pathogens was found on every farm. There were no significant differences between calves from placebo-treated and vaccine-treated dams with regard to the proportion treated for all diseases, or for scours, or the proportion which died. Neither were there differences in days to first treatment for all diseases (seven days on average), days to first scour (6.7 days), duration of treatments (3.9 days for all diseases, 3.7 days for scours), or estimated weight gains (0.5 kg/day to 14 days). These results were not altered when the presence or absence of enteropathogenic E. coli, rotavirus or coronavirus on the premises was accounted for.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of antimicrobial drugs in Swedish dairy calves and replacement heifers

A total of 3081 heifer calves in 122 Swedish dairy herds were monitored from birth to first calving, and all their diseases and treatments were recorded. There were 1268 cases of infectious diseases, of which 335 (26.4 per cent) were treated with antimicrobial drugs. Just over 90 per cent of the treatments were given during the first 210 days of the calves' lives, but only 61.8 per cent of the total quantity of antimicrobial drugs was consumed during this period. The most common diseases from birth until 210 days were respiratory diseases and diarrhoea; treatments for mastitis and interdigital phlegmon (foul in the foot) were more common after 420 days of age. Slightly more than 60 per cent of the treatments were given by the farmers without prior consultation with a veterinary surgeon.     

Emerging antimicrobial resistance in commensal Escherichia coli with public health relevance

In 2009, 1462 Escherichia coli isolates were collected in a systematic resistance monitoring approach from primary production, slaughterhouses and at retail and evaluated on the basis of epidemiological cut-off values. Besides resistance to antimicrobial classes that have been extensively used for a long time (e.g. sulphonamides and tetracyclines), resistance to (fluoro)quinolones and third-generation cephalosporins was observed. While in the poultry production chain the majority (60%) of isolates from laying hens was susceptible to all antimicrobials tested, most isolates from broilers, chicken meat and turkey meat showed resistance to at least one (85-93%) but frequently even to several antimicrobial classes (73-84%). In the cattle and pig production chain, the share of isolates showing resistance to at least one antimicrobial was lowest (16%) in dairy cows, whereas resistance to at least one antimicrobial ranged between 43% and 73% in veal calves, veal and pork. Resistance rates to ciprofloxacin and nalidixic acid in isolates from broilers were 41.1% and 43.1%, respectively. Likewise, high resistance rates to (fluoro)quinolones were observed in isolates from chicken meat and turkey meat. In contrast, ciprofloxacin resistance was less frequent in E.?coli isolates from the cattle and pig production chain with highest rate in veal calves (13.3%). Highest resistance rates to cephalosporins were observed in broilers and chicken meat, with 5.9% and 6.2% of the isolates showing resistance. In dairy cattle and veal, no isolates with cephalosporin resistance were detected, whereas 3.3% of the isolates from veal calves showed resistance to ceftazidime. Resistance to (fluoro)quinolones and cephalosporins in E.?coli isolates is of special concern because they are critically important antimicrobials in human antimicrobial therapy. The emergence of this resistance warrants increased monitoring. Together with continuous monitoring of antimicrobial usage, management strategies should be regularly assessed and adapted.     

Assessment of Antimicrobial Usage and Residues in Commercial Chicken Eggs from Smallholder Poultry Keepers in Morogoro Municipality,Tanzania

Occurrence of antimicrobial residues in commercial chicken eggs was determined in Morogoro municipality between January and February 2007. Twenty smallholder farmers were interviewed on the types of antimicrobials, reasons of use and their awareness on antimicrobial withdrawal period. Seventy egg samples were collected for qualitative antimicrobial drug residues analysis by use of agar well diffusion and Delvotest SP^® assays. It was found that farmers use antimicrobial drugs as prophylaxis and treatment of common chicken diseases namely fowl typhoid (85%), infectious bursa disease (Gumboro) (65%) infectious coryza (65%), collibacilosis (55%), coccidiosis (54%), Newcastle disease (50%), helminthosis (20%) and fowl pox (15%). Antimicrobials accounted for 85% of the drugs commonly used. It was also found that 65% of the farmers treat their chicken themselves. The common drugs were oxytetracycline (75%), egg booster (50%), amprolium (35%), sulphamethoxypyridazine (35%), sulphanilamide (25%), chlortetracyclines (10%), chloramphenicol (10%), sulphadiazine–trimethoprim (20%), duoxycycline (20%), sulphadiazine (25%) and flumequine (10%). Eighty per cent of the farmers had knowledge on antimicrobial withdrawal period sold eggs before withdrawal period and almost 85% were unaware of possible effects of antimicrobial residues in humans. All 70 eggs were positive to antimicrobial residues by Delvotest kit, but 21.4% positive with agar well diffusion test. It was concluded that the presence of antimicrobial residues in table eggs could be of public health significance to the egg consumers in Morogoro municipality.     

Antimicrobial Use in Feedlot Calves: Its Association with Culture Rates and Antimicrobial Susceptibility

Data were collected on anti-microbial usage and health problems, in beef feedlot calves. Although the association between them was not significant, many feedlot owners did not use recommended dosages or duration of treatment and this may have led to a reduced recovery rate.

Injectable tetracyclines were the most frequent antimicrobial for primary treatment of sick calves; whereas chloramphenicol was selected most frequently when the primary treatment appeared to be ineffective. Treatment with anti-microbials reduced the likelihood of isolating both Pasteurella multocida and Haemophilus somnus. Chloramphenicol therapy reduced the likelihood of isolating Pasteurella haemolytica.

Therapy with a particular antimicrobial, in the week prior to death, increased the level of resistance in P. haemolytica to that antimicrobial. Treatment with other antimicrobials also increased the level of resistance to that antimicrobial; although to a lesser degree. The lowest levels of resistance were observed in Pasteurella isolated from nontreated cattle. After adjustment for antimicrobial exposure, resistance to penicillin, tetracyclines and chloramphenicol occurred together more frequently than expected by chance alone.