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A Clinical Trial Evaluating Prophylactic and Therapeutic Antibiotic Use on Health and Performance of Preweaned Calves

Veterinary Medicine Teaching and Research Center, University of California–Davis, Tulare 93274

Corresponding author: Anna Catharina Björnsdotter Berge; e-mail: caberge{at}ucdavis.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The objective of this clinical trial was to evaluate the influence of prophylactic and therapeutic antibiotics on health and performance in preweaned dairy calves on a calf ranch. One hundred twenty 1-d-old calves were enrolled into 3 management systems for antibiotic use and raised until 4 wk of age. Sixty calves were not eligible to receive prophylactic or therapeutic antibiotics. Thirty calves were eligible to receive individual antibiotic treatment for disease, but no prophylactic antibiotics in milk replacer. The remaining 30 calves received milk replacer medicated with neomycin and tetracycline HCl, and could be treated with antibiotics. Health status and treatments were monitored and recorded daily. The primary study outcomes were weight gain, morbidity, and mortality. The most important factor associated with morbidity and mortality was passive immune transfer through colostrum. In-feed antibiotics delayed onset of morbidity, decreased overall morbidity, and increased weight gain. Nonantibiotic therapies for clinical disease were associated with increased mortality and morbidity compared with antibiotic treatments. The study has shown that minimizing or eliminating the use of antibiotics in the feed requires measures to ensure adequate passive transfer of immunity, but that in the face of inadequate passive transfer of immunity, animal welfare may be endangered by replacing medicated milk replacer with nonmedicated milk replacer, and therapeutic antibiotics with nonantibiotic alternatives.

Key Words: antimicrobial • food animal production • herd health and medicine • dairy calf

Abbreviation key: APT = adequate passive transfer of immunity, FPT = failure of passive transfer of immunity, HR = hazard ratio, PH = Cox proportional hazard model.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Producers involved with rearing dairy-source bull or heifer calves are faced with numerous challenges, the most significant being protecting calf health in the pre-weaning period. Calf health is adversely affected by weather, nutrition, exposure to infectious agents and, most significantly, by the calf’s passive immune status.

The importance of adequate passive transfer (APT) for minimizing morbidity and mortality has been demonstrated in several studies (Quigley et al., 1997; Donovan et al., 1998a; Weaver et al., 2000). Although the health benefits of APT are unequivocal, the reality of calf rearing is that a high proportion of calves on calf ranches and dairies are colostrum-deprived (Gay et al., 1983; Fallon et al., 1987; Wilson et al., 2000). Calf ranches receive 1-d-old calves with a high proportion of them having failure of passive transfer (FPT) from multiple sources (Moore et al., 2002; Berge et al., 2003). One management strategy for successfully rearing these calves is to add antibiotics, such as neomycin and tetracycline, to the calf milk replacer. Although there are many commercial milk replacers supplemented with antibiotics available to small and large dairy farms, some large calf ranches choose to custom mix milk replacer and supplement it with antibiotics in quantities that exceed those commonly used as feed additives.

All uses of antibiotics in animal agriculture are under increased scrutiny due to concerns that such uses promote the development of bacterial antimicrobial resistance. The European Union has banned the use of several antibiotics for growth promotion; eventually all antibiotics for growth promotion will be banned in the European Union (European Commission, 1998; Commission Press Room, 2003). The World Health Organization recently recommended the cessation of use for all growth-promoting antibiotics (Ferber, 2003). Moreover, there is growing consumer interest in organic products, which provides an additional impetus to raise calves without antibiotics.

There are few studies evaluating health and performance of calves raised without in-feed or therapeutic antibiotics under field conditions. One study evaluated the clinical efficacy of tetracycline in milk replacer during an outbreak of enteric and respiratory disease. The calves receiving antibiotics in the milk replacer experienced lower morbidity compared with those not receiving any antibiotics in the milk replacer (Braidwood and Henry, 1990). In a study comparing a probiotic to tetracycline and neomycin supplementation of milk replacer, no differences in weight gain, feed efficiency, or fecal consistency in 45 calves were detected. The investigators concluded that it might be possible to replace antibiotics with nonantibiotic alternatives such as fructo-oligosaccharides, allicin, and probiotic bacteria (Donovan et al., 2002). Although published literature on efficacy is lacking, nonantibiotic treatments such as bismuth salts, kaolin-pectin, probiotic bacteria, enzymes, and charcoal are often used to treat calf diarrhea. These few studies have not sufficiently addressed the effect of rearing calves without prophylactic and therapeutic antibiotic use on calf health and productivity.

The objective of this study was to compare calf morbidity, mortality, and weight gain in preweaned calves reared with and without antibiotics for therapy and prophylaxis. The study hypothesis was that calf weight gain, morbidity, and mortality are not affected by antibiotics in the milk replacer or given as individual therapy.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Study Site
The clinical trial was conducted on a commercial calf ranch in the southern San Joaquin Valley in California. The ranch raised calves as heifer replacements, veal, and dairy beef, maintaining a daily inventory of approximately 2000 calves on milk. One-day-old calves from local dairies arrived daily and were placed in wooden housing units, each of which held 3 calves in individual hutches. Each unit had a plywood roof that could be folded down to provide protection from sun and rain, and slatted floors. The hutch units were placed on cement blocks that raised them approximately 30 cm off the ground. The 3 calves in each unit had the opportunity for nose-to-nose contact with their immediate neighbor. The calves destined for beef and heifer replacement were bottle-fed twice daily with 1.86 L of milk replacer composed of 23% protein and 18% crude fat (proprietary blend). Vitamins and minerals were added at each milk feeding according to a farm protocol. Starter grain in buckets was introduced to calves on d 3 and provided fresh daily. Fresh water was continually available in buckets. On the day of arrival, all calves received 1.86 L of plasma-derived colostrum supplement (Lifeline, APC, Ames, IA). Calves were vaccinated with a Salmonella-mutant core antigen bacterin (Endovac-bovi, Immvac, Columbia, MO). On d 14 of the trial, calves received a vitamin E and selenium injection (MuSe injection, 1 mL s.c., Schering-Plough, Kenilworth, NJ), and on d 18 they were vaccinated with modified live infectious bovine rhinotracheitis, bovine viral diarrhea, bovine respiratory syncytial virus, and parainfluenza 3 vaccine (Pyramid MLV 4 Vaccine, Fort Dodge, Overland Park, KS).

Enrollment and Treatment Allocation
One hundred twenty, random-source, 1-d-old calves from multiple dairies were purchased from a commercial calf supplier, and represented the typical dairy bull calf destined for beef. All calves were enrolled over a 2-d period, followed through 28 d of age, and then sold. No information on dairy source was provided with the calves, although no more than 3 calves could have come from a single farm. The calves from multiple dairies were commingled on the trailer and were randomly offloaded from the trailer by the calf dealer. No further randomization was attempted and calves were assigned to treatment groups and housing in the order they were removed from the calf trailer. Calves were housed in blocks according to study group allocation. This was done to ensure compliance with the feeding protocols. They were ear-tagged thereafter, weighed, had their navels dipped with 7% iodine solution, and received a veterinary visual inspection.

Calves in the trial were placed in new wooden housing units that had not previously housed calves. The veterinarian in charge of the trial and 2 technicians were responsible for all aspects of calf and trial management, including preparation of feed, feeding, and treatments. All equipment for feeding and treating study calves was separate from that used for calves belonging to the ranch. Calves were fed colostrum supplement and vaccinated according to farm protocol. Calves were bottle-fed 1.86 L of milk replacer at 0600 and 1700 h. Before the study, samples of water used to mix the milk replacer were collected and analyzed using standard microbiological techniques for the presence of Salmonella and Escherichia coli, with negative results (American Public Health Association, 1995; Farmer, 1999).

Health and Performance Monitoring
Calves were monitored for feed intake at all feedings, and received a recorded visual health appraisal twice daily by a veterinarian (blinded to treatment allocation) and by the manager of the calf facility who was responsible for treatments on the ranch. Calf health was assessed visually using objective criteria of appetite, fecal consistency, hydration status, respiratory effort, and attitude (Table 1). Subsequent to these evaluations, calves were designated as "normal" or "to be treated". Neither appraiser was involved with feeding or treatment administration. Treatments were administered by the veterinarian in charge of the study. Calves received treatment based on the health category strategy as described in Table 1, and according to their assigned experimental group. Calves were weighed on the first and last day of the trial using a scale accurate to the nearest 0.5 kg.

Experimental Approach
The assumptions for determining sample size were: Type I error = 0.10, Type II error = 0.10, and a mean number of 5 morbid days for calves not exposed to antibiotics compared with 3.5 morbid days for calves exposed to antibiotics in milk and treated with antibiotics, with a sample standard deviation of 2 d. Allowing for 15% loss to follow up, we enrolled 120 calves allocated to 4 study groups (30 calves/treatment group).

Study groups.
Sixty calves received no antibiotics in the milk replacer, and clinical disease was treated using only nonantibiotic alternatives. Bismuth subsalicylate (Biocode, Vedco, St. Joseph, MO), electrolytes (Biolyte, Pharmacia Animal Health, Kalamazoo, MI), kaolin-pectin (Kaolin-Pectin Plus, Agripharm, Grapevine, TX), activated attapulgite, and activated charcoal (UAA gel, Vedco) were used according to label dose to treat diarrhea; and flunixin meglumine (Banamine, 50 mg/mL, 2.2 mg/kg, Schering-Plough) was used to treat calves with a depressed attitude (Table 1). These 60 calves were split into 2 housing groups of 30 calves. One group was housed with other study calves along with commercial calves. The other group was housed away from the study calves and separated from the commercial calves. Because there were no health differences between these groups, these 2 study groups were treated as one group for all statistical analyses.

Thirty calves received no antibiotics in the milk replacer, but antibiotics were used to treat clinical disease. Treatment protocols followed standard protocols of the farm (Table 1). The primary antibiotic treatment was ceftiofur hydrochloride (2.2 mg/kg per d, 3 to 5 d; Excenel RTU, Pfizer Inc., NY). Penicillin G procaine (300,000 U/mL, 2.2 mL/100 kg per d, 1 to 3 d) and tilmicosin (10 mg/kg per d, 1 to 3 d; Micotil 300, Elanco Animal Health, Greenfield, IN) were available but used sparingly. In addition, nonantibiotic alternatives (described above) could be used concurrently with antibiotic treatments.

Thirty calves received medicated milk replacer containing tetracycline hydrochloride (22 mg/kg per d, TET-324, Agripharm) and neomycin sulfate (22 mg/kg per d, Neomix AG 325, Pfizer Inc.) according to protocols used on the ranch. The antibiotics were diluted in 20 mL of purified water and added directly to each bottle at the milk-mixing stage before addition of the milk replacer. These calves were treated with antibiotics and nonantibiotic alternatives as described (Table 1).

Fecal and Postmortem Samples
Using 2 sterile cotton-tipped swabs, fecal samples were taken per rectum on d 1, 14, and 28 for isolation of Salmonella. Samples were placed in 9 mL of tetrathionate pre-enrichment broth and incubated overnight at 37°C. The broth was plated on xylose-lysine desoxycholate agar and brilliant green agar and incubated at 37°C for 18 to 24 h. Plates were evaluated for suspect colonies of Salmonella. Two suspect colonies per plate was picked and transferred to triple sugar iron, lysine iron, and urea agar slants, incubated at 37°C for 24 h. Salmonella-positive colonies were serogrouped using the slide agglutination technique, and Salmonella enterica isolates were thereafter sent to the California Animal Health and Food Safety laboratory for serotyping. Antibiotic susceptibility to 12 antibiotics was tested using the disk diffusion method according to the National Committee of Clinical Laboratory Standards using E. coli ATCC 25922 and Salmonella Typhimurium (VMTRC bovine clinical isolate) as quality control standards (Bauer et al., 1966; NCCLS, 1999). The antibiotic discs included in the panel were amikacin 30 µg, amoxicillin/clavulanic acid 20/10 µg, ampicillin 10 µg, cephalotin 30 µg, ceftiofur 30 µg, chloramphenicol 30 µg, gentamicin 10 µg, nalidixic acid 30 µg, streptomycin 10 µg, sulfisoxazole 250 µg, tetracycline 30 µg, and sulfa-methoxazole/trimethoprim 23.75/1.25 µg.

Field necropsies were performed on calves more than 6 d old. Gross pathology was noted and samples from lung, liver, spleen, and intestines were taken for isolation of Salmonella. Results from the Salmonella fecal cultures from the 0-, 2-, and 4-wk-old calves, and the isolates from lung, liver, spleen, and intestines obtained from postmortem samples were assessed to evaluate risk factors for morbidity and mortality.

Passive Transfer of Immunity
On the day of arrival, a blood sample was collected from each study animal to evaluate passive transfer of immunity by measuring serum IgG using a single radial immunodiffusion assay (IgG Single Radial Immunodiffusion kit, VMRD Inc., Pullman, WA). The IgG values in mg/dL were calculated by comparing sample values to the standard curve obtained using a set of 4 standards (400, 800, 1600, 3200 mg/dL) applied to each plate. Passive immune transfer status was classified as IgG status 0 for IgG = 86 mg/dL (this corresponded to the y-intercept of the standard curve, and represented no evidence of IgG), IgG status 1 for values 87 IgG 999 mg/dL, and IgG status 2 for IgG 1000 mg/dL. Immunoglobulin G status 0 or 1 indicated FPT and IgG status 2 indicated APT (Tyler et al., 1996).

Measuring Serum Bacterial Inhibition Levels
Serum from calves at enrollment was tested for the presence of antimicrobial inhibitory substances using a serum inhibition bioassay (Bennett et al., 1966). The assay was a modified agar-well diffusion system containing Bacillus subtilis spores. The agar was poured over a 20 x 20 cm plate with 64 wells. A standard concentration of penicillin in 8 serial dilutions was added to the wells to determine a standard curve. One hundred microliters of serum was added in duplicate to the wells. The plates were incubated for 18 h at 37°C. The diameters of the inhibition zones surrounding the wells were measured and compared with the standard curve to estimate equivalent units of penicillin per milliliter. The assay is a general screen for serum bacterial inhibition and detects the presence of ß-lactams, tetracycline, and aminoglycosides among other inhibitory substances.

Data Analysis
All health and treatment data were recorded and entered into a spreadsheet program (Excel, Microsoft Corp., Redmond, WA) and analyzed using a statistical software package (SAS version 8.2, Proc LIFETEST, PHREG, and GLM; SAS Institute Inc., Cary, NC). For analytical purposes, calves removed from the trial for animal welfare reasons were treated as dead. Our primary outcome measures were mortality, morbidity, and survival weight-gain. For all models, predictive variables, and interaction effects with a P value < 0.10 were retained in the final models. The predictive variables included in the models were antibiotics in the milk-replacer; the calf individual treatment categorized as antibiotic, nonantibiotic or no treatment; passive immune transfer status categorized as IgG status 0, 1, or 2; and arrival weight.

Mortality.
Mortality was defined either as a calf dying or a calf removed from the trial because of severe disease. Mortality rates were calculated using Kaplan-Meier survival plots (days-to-death) and stratified by presence or absence of antibiotics in the milk replacer, individual calf treatments, and IgG status. Log-rank and Wilcoxon statistics were used to test equality over strata. Proportionality of the survival distributions was assessed by plotting ln(–ln(survival)) against the ln(calf-days). Multivariate assessment of survival time was developed using a Cox proportional hazard (PH) model. Hazard rate ratios and 90% confidence intervals for each covariate were determined (Hosmer and Lemeshow, 1999).

Weight gain.
A GLM procedure calculating least square means was used to compare differences in weight gain between the calves that survived to the end of the study in the different treatment categories. For all independent variables, least square mean estimates and 90% confidence intervals were calculated.

Morbidity.
A calf was considered diseased when it received a treatment. Treatments were administered when a calf was observed with a diarrhea score 2, respiratory disease score 2, and/or attitude score 1 (Table 1). Calf health was evaluated using 2 outcome measures. The first outcome evaluated was the number of days to first treatment using a PH model (Hosmer and Lemeshow, 1999). The second outcome measured the number of days a calf was untreated, standardized for the number of days the calf remained in the trial. This was calculated as the difference between the number of days the calf remained in the trial and the number of days the calf remained untreated. Four calf health categories corresponding to 7-d intervals were created from the difference calculation. Calves that died early or were treated for prolonged periods of time would thereby fall into categories 1 or 2, and calves that remained in the trial until d 28 with fewer than 6 treatment days would be in category 4. These categories standardized the morbidity data so that a calf leaving the trial early but untreated was not evaluated as "healthier" than a calf that survived until the end of the trial but received a treatment. Multivariate relationships between calf health categories and the predictive variables were assessed using a cumulative logistic regression model (Kleinbaum et al., 1988; Agresti, 2002). The model predicted the odds of the calf being in a lower health category (having fewer standardized untreated days) compared with all the higher health categories.

	RESULTS

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One hundred twenty calves were enrolled in the study over a 2-d period in February 2002. During the trial, the weather was dry except for 2 d of heavy rain. Nighttime temperatures ranged between 2 and 7°C, and daytime temperatures were between 18 and 27°C. Based on the results from the serum bacterial inhibition assays, no calves had detectable levels of antibiotic, suggesting that they had not received any antibiotic treatment before arrival on the farm.

Of the 120 calves enrolled, only 41% had APT (Table 2). Twenty-one calves died during the study, 6 within the first 5 d of the trial. Ten calves, not eligible for antibiotic treatments, were removed from the study for animal welfare reasons because their clinical status was deemed critical (Table 3). These calves were considered dead for subsequent analyses.

Salmonella was prevalent in the study calves throughout the follow-up period. On screening, 3 calves were positive for Salmonella on the day of arrival, 17 calves on d 14, and 3 calves on d 28. The Salmonella isolated included Salmonella Dublin (8), Give (8), Montevideo (5), Newport (1), and Reading (1). Salmonella was isolated from calves in all study groups, and there was no apparent pattern to the serotype distribution between groups.

Mortality
The Kaplan-Meier curves depicting mortality rate conditional on 1) receiving antibiotics in the milk replacer, 2) receiving antibiotic therapy, and 3) by passive transfer status are shown in Figures 1 to 3, respectively. As univariate analyses, mortality was highest in calves not receiving antibiotics in the milk replacer (Wilcoxon rank test; P < 0.02), receiving nonantibiotic therapy (P < 0.01), and with FPT (P < 0.01).

View larger version (11K): [in this window] [in a new window]   Figure 1. Kaplan-Meier plot showing survival proportion of 120 calves stratified on receiving antibiotics in the milk replacer (30 calves) or no in-feed antibiotics (90 calves) in a study of prophylactic and therapeutic use of antibiotics on a commercial calf ranch. Wilcoxon test of equality of strata, 2 = 5.04, P value < 0.02.

View larger version (12K): [in this window] [in a new window]   Figure 2. Kaplan-Meier plot showing survival of 120 calves stratified by treatments received (no treatments, antibiotic treatments, or treatments with nonantibiotic alternatives) in a study of prophylactic and therapeutic use of antibiotics on a commercial calf ranch. Wilcoxon test of equality of strata, 2 = 9.56, P value < 0.00.

View larger version (11K): [in this window] [in a new window]   Figure 3. Kaplan-Meier plot showing survival of 120 calves stratified on passive transfer of immunity status (IgG status 0 = 86 mg of IgG/dL of serum, IgG status 1 = 87 to 999 mg of IgG/dL of serum, IgG status 2 = 1000 mg of IgG/dL of serum) in a clinical trial evaluating effect of prophylactic and therapeutic antibiotic use on calf health. Wilcoxon test of equality of strata, 2 = 14.63, P value < 0.00.

The PH model evaluating mortality indicated that for the first 6 d of the trial, calves with IgG status 0 were more likely to die than calves with APT [hazard ratio (HR) = 17.80, 90% CI = 2.84 to 111.67; Table 4]. No mortality HR could be calculated for calves receiving antibiotics in the milk because none of these calves died during the first 6 d. Individual antibiotic treatments did not significantly alter the risk of early mortality. Between 7 and 28 d of age, antibiotics in the milk replacer had no significant influence on mortality. Treatments with nonantibiotic alternatives increased the hazard of dying (HR = 10.07, 90% CI = 1.81 to 56.06) compared with nontreated calves, whereas treatments with antibiotics did not change the hazard of death compared with a nontreated calf. Calves with IgG status 0 or 1 were more likely to die than calves with IgG status 2 (Table 4).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

Overall morbidity and mortality was high in this trial. Weather conditions may have been stressful on calves because of large night and day temperature fluctuations (Martin et al., 1975). The study ranch, as well as several other calf ranches in the area, experienced unusually high morbidity and mortality during this period. Several study calves had bloody diarrhea, possibly due to Salmonella infections. The S. Newport recovered from several necropsied calves was multiple-antibiotic resistant and has been associated with clinical disease in dairy cattle (Berge et al., 2004). This serovar was resistant to the third-generation cephalosporin ceftiofur, which was used as the primary antibiotic for calves with diarrhea in this study.

Passive Transfer of Immunity
Adequate passive transfer of immunity was the most important predictor of calf health. Several other studies have found APT associated with decreased morbidity and increased weight gain, but in these studies, APT had no specific influence on number of diarrhea days (Quigley et al., 1995; Rea et al., 1996; Weaver et al., 2000). Although the influence of APT on the development of diarrhea is not clear, the importance of APT for minimizing morbidity and mortality has been demonstrated in several studies (Quigley et al., 1997; Donovan et al., 1998a; Weaver et al., 2000). Failure of passive transfer of immunity has also been correlated with bacteremia in calves (Fecteau et al., 1997).

Our study confirmed published findings in that mortality and morbidity in calves was higher in calves that had FPT compared with calves with APT. We did not detect any difference in weight gain attributed to the IgG status of the calves. This may be explained in part by the loss of calves with FPT due to mortality and censoring. Donovan et al. (1998b) also showed that after adjustment for disease status, passive immune status had little influence on weight gain.

Antibiotics in the Feed
Our study indicated that morbidity was lower for calves receiving in-feed antibiotics than for calves not receiving medicated milk replacer. This suggests that antibiotic supplementation of the milk protected calves from disease events during the first month of life. Because no calves died in the antibiotic-supplemented group, we were unable to demonstrate statistically that in-feed antibiotics decreased mortality rate. As shown in other studies, antibiotics in the milk replacer were associated with increased weight gain (Morrill et al., 1977; Donovan et al., 2002). It should be noted that the dosage of neomycin and tetracycline added to the milk replacer in this study, although typical for many large calf ranches, exceeded that found in Type B and C medicated feeds (FDA, 2005). The antibiotic doses used in this trial were metaphylactic and required extended withdrawal times. It is not known whether lower doses would result in the same outcomes as we observed. Further studies to evaluate prophylactic antibiotic concentrations according to label use should be performed.

The beneficial effects of probiotics to calves have not been consistently demonstrated. Indeed, there are studies indicating that low levels of antibiotics in the feed increase feed consumption and weight gains compared with calves receiving probiotics (Morrill et al., 1977). Further research is needed to develop and evaluate nonantibiotic alternatives for improving calf health to minimize the use of prophylactic or metaphylactic antibiotics in the feed.

Therapeutic Treatments for Clinical Disease
Calves treated with nonantibiotic alternatives had increased the risk for morbidity and mortality, whereas calves treated with antibiotics had the same mortality and morbidity experience as calves that had no evidence of disease needing therapy during the trial. Calves receiving only nonantibiotic therapy were more likely to be in a lower health category compared with antibiotic-treated calves. Nonantibiotic therapy resulted in prolonged duration of treatment, possibly due to poor clinical response. When calves that were not eligible for any antibiotic treatments became critically ill, they were placed in separate hutches outside the study groups and then treated with antibiotics. Nine of 10 calves recovered after having received antibiotic treatments. This further supports the importance of therapeutic antibiotics for calf health and welfare. Our study indicated that ceftiofur hydrochloride was more effective than nonantibiotic alternatives in reducing morbidity and mortality. Given the scarcity of data in the peer-reviewed literature supporting the use of antimicrobials with labels for treating calf diarrhea, our data and others indicate that the extra-label use of antimicrobials (in this case, ceftiofur) may be justified in treating calves with diarrhea (Fecteau et al., 2003; Constable, 2004).

Calves that were treated for disease with antibiotics gained less weight, whereas calves treated with nonantibiotic alternatives gained similar weight to those calves not treated during the trial. These results are biased by the fact that the calves receiving nonantibiotic alternatives were more likely to die (as shown by our analysis of mortality) and their weights were not included in the analysis.

Flunixin meglumine in conjunction with antibiotic treatment has been shown to decrease the severity and total number of treatments in calves with bloody diarrhea and may be a valuable nonantibiotic treatment option (Barnett et al., 2003). We found the use of flunixin meglumine valuable in improving the well being of the calves, but the study design did not allow for further evaluation of this treatment.

Study Design
This trial was designed as a parallel-group clinical trial with 2 main treatment effects: exposure to antibiotics in the feed and exposure to antibiotics for clinical therapy. The hallmarks of a clinical trial are randomization of subjects to the treatment groups and objective assessment of outcomes using blinding (Institute of Medicine, 2001). The intent of randomization is to eliminate selection bias and to minimize the possibility that unmeasured covariates do not bias or confound the study. For logistical reasons, randomization for the trial was achieved by receiving calves from a single supplier who purchased calves from multiple sources and delivered them as a group. Selection bias was not an issue as study group enrollment was systematic and determined by the order of calves being removed from the trailer by the calf hauler (i.e., not influenced by us). Confounding bias was controlled in 2 ways, first through the randomization, and second through the analyses which accounted for the principle confounders of APT and arrival weight.

Management of Calves
Heifer replacement calves raised on dairies in California are not usually administered antibiotics in the milk (A. C. B. Berge, unpublished data, 2004). These heifers are not subjected to the same stressors, such as transportation and commingling, as calves on calf ranches. They are often fed fresh or hospital milk (milk withheld from human consumption because it is abnormal or contains antibiotics following treatment) that may be supplemented with milk replacer. In general, there may be more emphasis on providing heifers with colostrum as soon as possible after birth to ensure APT.

Applying the dairy model for calves raised on dedicated calf ranches would include efforts to minimize environmental stress (e.g., by not transporting neonatal calves), and improving passive transfer of immunity. These management changes could optimize calf health and may lead to a decrease in the use of prophylactic and therapeutic antibiotics. In the absence of these changes, antibiotics will continue to be an important tool to ensure the health and well-being of these calves.

	CONCLUSIONS

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Although ensuring that calves have APT continues to be the most significant management factor to ensure the health of neonatal calves, in-feed and therapeutic antibiotics played an important role in controlling morbidity and mortality in our study. Without the use of antibiotics for therapy or prophylaxis, it is difficult and labor-intensive to raise preweaned calves on a calf ranch without APT. If immunocompromised neonatal calves are under environmental stress and high infection pressure, such as exposure to calf-pathogenic Salmonella, antibiotic therapy appears necessary. Calf ranch systems need to address problems of colostrum-deprived calves, pathogen load, and environmental stress to successfully raise calves and minimize prophylactic and therapeutic antibiotic use.

	ACKNOWLEDGEMENTS

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The authors would like to thank the calf ranch for lending the use of their facilities and Tina Lindeque, Katrin Newman, Mary Gross, Chengling Xiao, Staci Barnett, Dulce Maria, and Sonya Vasquez for technical assistance. This project was supported by the California Dairy Research Foundation (CDRF-Project no. 0151W-01-DH), Center for Food Animal Health-SVM University of California, and NIFSI-CSREES Project no. 00-51110-9721.

Received for publication July 30, 2004. Accepted for publication February 9, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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