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Factors Affecting Cure and Somatic Cell Count After Pirlimycin Treatment of Subclinical Mastitis in Lactating Cows

¹ Pfizer Animal Health, Research and Development, 2870 Puurs, Belgium
² Pfizer Animal Health, Research and Development, Kalamazoo, MI 49001

Corresponding author: H. A. Deluyker; e-mail: hubert.deluyker{at}efsa.eu.int.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
This study investigated the associations of both bacteriological cure and quarter somatic cell count (SCC) after intramammary antibiotic treatment with treatment duration, cow characteristics, and pretreatment bacteriology and SCC. For the purpose of this paper, data from 2 treatment groups in each of 2 multi-location studies were selected. These studies were conducted to evaluate the efficacy of daily intramammary infusions with 50 mg of pirlimycin hydrochloride for the treatment of subclinical mastitis. Data from study 1 allowed for comparison of a group of cows that received pirlimycin intramammarily for 2 d with a group that received no treatment, and study 2 provided data for comparison of pirlimycin for 2 d with pirlimycin for 8 d. Quarter milk samples from cows with a high monthly SCC were tested for bacteriology and SCC. If one or more quarters had both a positive bacteriology and an SCC 300,000 cells/mL, the cow was enrolled and randomly allocated to a treatment group. Enrolled cows were monitored for clinical mastitis and other disease for 4 wk after treatment initiation. At 3 and 4 wk after treatment initiation, milk samples were taken from each enrolled quarter to determine the SCC and conduct a bacteriological culture. Bacteriological culture results were interpreted such that quarters where the same bacterial species was cultured before treatment and found in at least 1 of the 2 posttreatment samples were considered a failure. The analysis of SCC used a mixed linear model (SAS proc mixed) and the analysis of bacteriological cure used a mixed logistic model (SAS glimmix macro). Bacteriological cure rate was significantly higher for lower parity, lower number of colonies in the pretreatment culture, longer treatment duration, and for streptococci compared with Staphylococcus aureus. However, treatment regimen affected bacteriological cure differently in major than in minor pathogens and there was a significant interaction of treatment regimen with stage of lactation. Posttreatment SCC was significantly higher with increasing parity, in rear quarters, and with shorter duration of treatment. In the group of second and third parity animals, post-treatment SCC was more reduced in front quarters than in rear quarters. Also, the difference in posttreatment SCC between younger and older cows increased with higher pretreatment SCC. In conclusion, when predicting bacteriological cure following treatment of subclinical mastitis during lactation both treatment regimen and other risk factors need to be considered. The other risk factors may vary with treatment regimen. Posttreatment SCC was associated with treatment regimen, other risk factors, and interactions among the other risk factors; but these other risk factors did not vary significantly with treatment regimen.

Key Words: subclinical mastitis • pirlimycin • lactating cow • somatic cell count

Abbreviation key: CURE = bacteriological cure, FAIL = no bacteriological cure, lnSCCpre = natural logarithm of pretreatment SCC, lnSCCpost = geometric mean of SCC at 21 and 28 d after treatment initiation, Pirli2 x = 2-d pirlimycin treatment, Pirli8 x = 8-d pirlimycin treatment

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Bacterial IMI frequently persist for an extended time while causing subclinical inflammation of the mammary gland. Subclinical mastitis causes decreased milk production and lower milk quality; and contagious bacteria may spread to other quarters. Nevertheless, as long as bacterial infections remain subclinical they often go untreated during the lactation. Low cure rate has been cited as a reason why mastitis in its subclinical appearance should not be treated during the lactation. Treatment is instead postponed until a clinical flare-up occurs or until dry-off, a time when antibiotics are often routinely used to treat all quarters in all cows. With increasing pressure to deliver milk with a low bulk milk SCC, it may not be viable, economically, to wait until dry-off before measures are taken to lower the prevalence of IMI, alongside measures to reduce the incidence of new IMI. This may explain the high culling rates of high SCC cows in herds with a bulk milk SCC close to the upper legal limit (Barkema et al., 1998). Treatment of subclinical mastitis during lactation could represent an alternative to culling. However, responsible use of antibiotics demands that its use be limited to cases in which cure rates can be expected to be sufficiently high. Thus, there is an interest in identifying effective treatment regimens and other factors that affect cure.

Risk factors for bacteriological cure of Staphylococcus aureus infections following antibiotic treatment were reported previously. They included age, number of infected quarters, quarter location, stage of lactation, and severity of the inflammation (Wilson et al., 1972; Poutrel, 1978; Ziv and Storper, 1985; Owens et al., 1988; Sol et al., 1997). More recently, Dingwell et al. (2003) reported that the intensity of bacterial shedding was negatively associated with Staph. aureus cure following antibiotic treatment at dry-off. However, Wilson et al. (1972) did not find these factors to be significant for streptococcal species identified at that time, except for the degree of inflammation. Hence, it is worthwhile to determine whether previously identified risk factors for Staph. aureus cure apply to other bacterial species that also often cause subclinical mastitis.

Extending the duration of treatment with ß-lactams resulted in significant increases in bacteriological cure rates of Staph. aureus (Wilson et al., 1972; Ziv and Storper, 1985). Similarly, higher cure rates for infections caused by Staph. aureus and streptococci were reported following treatment with a lincosaminide (Gillespie et al., 2002; Oliver et al., 2003). Whereas treatment regimens that, on average, result in higher cure rates are desirable, it is nevertheless worthwhile to verify whether there are subpopulations that do not respond well to such treatment and to identify risk factors associated with a lower response.

Finally, treatment success should lead to reduction in SCC. Whereas previous studies investigated risk factors for bacteriological cure, risk factors for posttreatment SCC have, to our knowledge, not been investigated thus far.

The objective of this study was to determine whether risk factors for bacteriological cure and posttreatment SCC varied with bacterial species and with treatment regimen efficacy.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
For the purpose of this paper, data from 2 multi-location studies were analyzed. These studies were conducted in Member States of the European Union to evaluate the efficacy of daily intramammary infusions with 50 mg of pirlimycin hydrochloride for the treatment of subclinical mastitis. They complied with the guideline on Good Clinical Practice for the Conduct of Clinical Trials for Veterinary Medicinal products of the European Commission’s Committee for Veterinary Medicinal Products and with animal welfare regulations applicable in the countries where the trials were conducted. Data from study 1 allowed for comparison of a group of cows that received pirlimycin for 2 d (Pirli2x) to a group that received no mastitis treatment (untreated), whereas study 2 provided data for comparison of Pirli2xwith pirlimycin for 8 d (Pirli8x).

Herds
There were 54 herds in study 1 and 57 herds in study 2. The herds were located in the Netherlands, Sweden, Denmark, Spain, Italy, France, Germany, and the United Kingdom. Only herds with a regular history of subclinical mastitis were included. Farms were required to have proper record-keeping capabilities and cow identification, and be free of officially notifiable diseases.

Enrollment
In selected herds, the principal investigator monitored cows for subclinical mastitis using routine cow SCC results. A cow was considered to have subclinical mastitis (and thus potentially qualify for enrollment) if the SCC was greater than 250,000 cells/mL twice in the past 2 mo or greater than 400,000 cells/mL once in the past month (Dohoo and Meek, 1982). Somatic cell count results from milk samples taken less than 72 h postpartum were not considered for this purpose (Barkema et al., 1999). Cows were excluded from the study if they had teat lesions, were systemically ill, had clinical mastitis (IDF, 1987), had received antibacterial or anti-inflammatory therapy in the previous 30 d, were of sixth or higher parity, had reached a low milk yield, or were to be dried off within the next 30 d. In study 2, quarters with palpable udder lesions were also excluded. Pretreatment aseptic milk samples were collected from each quarter of the remaining cows to determine SCC and the presence of IMI, and the number of colonies was enumerated. Milk samples were refrigerated or frozen and shipped to incountry diagnostic laboratories within 24 h. Laboratory procedures and interpretations for bacteriological culture of milk were consistent with National Mastitis Council Guidelines (1990). Briefly, from each sample, one inoculum (0.05 mL of milk) was plated on Columbia base agar containing 5% sheep blood. Plates were incubated at 37°C and examined for bacterial growth at 24 and 48 h. Before study initiation, uniformity among laboratories in the identification of the bacterial strains was demonstrated through a test conducted by the Quality Assurance Unit of the Veterinary Laboratories Agency (Loughborough, UK). For each bacterial species, the number of colonies on the plate was recorded as 1 of the following 3 categories: actual number (10 cfu), ++ (11 to 100 cfu), +++ (>100 cfu). Quarters were enrolled if the SCC was 300,000 cells/mL and a mastitis pathogen was identified. Final enrollment occurred within 8 d of milk sampling. Enrollment was limited to a maximum of 20 cows per treatment group at each location.

Blocking, Randomization, Masking, and Treatment
Cows in a herd were ranked first by increasing parity and second by decreasing duration of lactation within parity. Next, treatments were randomly allocated to cows (not quarters) within blocks, consisting of the closest cows that were available for enrollment. Because treatment intervals and durations were different among the treatment groups, treatment administrators were not blinded. However, laboratory personnel who determined SCC and bacteriologic status were blinded to treatment assignment. Cows were infused using appropriate technique in each of the quarters that met the inclusion criteria immediately after milking. Cows were not milked for at least 8 h after infusion.

Clinical Observations and Definition of Bacteriological Cure
Cows were monitored for clinical mastitis and other disease in all 4 quarters for 30 d after enrollment. The principal investigator conducted clinical examinations on all cows at 22 to 23 and 29 to 30 d after enrollment. At these visits, sterile milk samples were taken from each enrolled quarter to be cultured for determination of SCC. If cows required treatment of clinical mastitis or another disease, monitoring was discontinued.

Bacteriological cure (CURE) was defined as absence of the bacterial species present pretreatment in both of the posttreatment milk samples from that quarter. Presence of the pretreatment bacterial species in at least one posttreatment sample was considered a failure (FAIL).

Statistical Analysis
The analysis of SCC used a mixed linear model (SAS Version 8.2, Proc Mixed, SAS Institute, Cary, NC), and analysis of CURE used a mixed logistic model (SAS Version 8.2, Glimmix Macro). Study, herd, and cow were considered random effects.

Somatic cell count was measured on each quarter and so the model was based on the quarter within cow. Bacteriological cure was measured on observation within quarter (i.e., a quarter could have multiple bacterial species isolated) and so the model was based on the observation within quarter. For the SCC analysis, when multiple bacteria were identified then the following order of precedence was used: Staph. aureus, Streptococcus uberis, other streptococci, CNS, Corynebacterium bovis, and other. For example, if Staph. aureus and C. bovis were identified in the same quarter, it was reported as Staph. aureus in the SCC analysis. Similarly, the number of colonies used would follow the same order of precedence.

For SCC, the natural logarithm of the somatic cell count divided by 100 was the variable analyzed. An initial analysis was conducted to determine how SCC changed between time of treatment and 21 and 28 d after treatment initiation. This analysis included the effects of treatment and CURE for each quarter. Two contrasts were computed: the first tested whether the SCC geometric mean at d 21 and 28 (lnSCCpost) was different from the pretreatment SCC (lnSCCpre), whereas the second tested whether the change from pretreatment differed by treatment duration. This analysis was also conducted for CURE and FAIL quarters.

The second analysis was conducted to determine the significant covariates associated with SCC and CURE. A forward stepwise method was used to include/exclude covariates (Table 1). The random effects were included in all models. At the first step of the stepwise procedure all covariates were tested in the model one at a time, the one resulting in the smallest P value was included in the model. The next step was to retest all the covariates in the model that included the covariate from step one. This procedure was continued until no additional covariates were significant (= 0.05). Once the main effects model was determined, a similar process was used for 2-way interactions. Each of the possible 2-way interactions was included in the model one at a time, keeping the one with the smallest P value. This was repeated until none of the remaining interactions was significant. Because the sample size in at least one of the categories of 3-way or higher-order interactions were quite small, these effects were not tested. Finally, the interactions between treatment and any other risk factors not included in the model were tested for inclusion in the model.

	RESULTS

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Bacteriological Cure
Bacteriological cure results by treatment group are in Table 2. The final model of the stepwise analysis, its parameters, and its least square means are in Tables 3, 4, and 5. Treatment, bacterial species, and their interaction were significant (Table 3). As expected, CURE rate was lower for Staph. aureus than for the other bacterial species. When compared with Pirli2x, CURE rates were significantly higher following Pirli8xand lower in the untreated group, for Staph. aureus, Strep. uberis, and other streptococci (Table 5). Cure rates for Pirli2xand Pirli8xdid not differ significantly for CNS, whereas for C. bovis and other bacterial species, Pirli2xCURE did not differ significantly from the other 2 treatment groups. There was also a significant interaction of treatment regimen with lactation stage (P = 0.008, Table 3). In the untreated group, the CURE percentage decreased as lactation progressed. The trend was opposite for Pirli2xas the percentage CURE increased throughout the lactation, whereas for Pirli8x, CURE was lowest in the beginning of the lactation but did not increase beyond 100 d. Percentage CURE was also significantly lower with increased parity and higher number of colonies, but these effects were consistent across treatments (P = 0.532 and P = 0.411, respectively). In the final model, treatment regimen did not interact significantly with number of affected quarters (P = 0.093), quarter location (P = 0.543), and lnSCCpre (P = 0.248).

View larger version (20K): [in this window] [in a new window]   Figure 1. Lowest and highest percentage of Staphylococcus aureus bacteriological cure for each parity xtreatment group, as predicted by the logistic regression model.

Figure 2 shows for each treatment by parity group combination, the lowest and highest posttreatment SCC when considering all the possible combinations of significant risk factors of the final model (Table 8). The results show that under both these best and worst cases, Pirli8xtreatment was about 50% lower than under the respective circumstances in the untreated group.

View larger version (23K): [in this window] [in a new window]   Figure 2. Lowest and highest posttreatment quarter SCC for each parity xtreatment group, as predicted by the linear regression model.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

The significant association of number of colonies present pretreatment with CURE did not differ between treatment regimens (P = 0.411). It might be a result of detection level differences (Sears et al., 1990), whereby the likelihood of a positive culture result is higher with high levels of shedding, or it may be real, i.e., that high level of shedding reduces likelihood of CURE. Because in the present study, a quarter was considered a CURE if in both posttreatment samples the original bacterial species could not be isolated, it is unlikely that the false-negative rate was high (Sears et al., 1990). Following treatment of mastitis during lactation, Owens et al. (1999) reported a statistically significant reduction of the Staph. aureus concentration present in milk. This was accompanied by a transient decrease in SCC that had returned to near pretreatment levels by d 28 post-treatment. In contrast, in the present study there was no evidence of a posttreatment return to high pretreatment SCC levels for cured quarters. Dingwell et al. (2003) reported number of colonies to be a risk factor for CURE with dry cow therapy. The present results suggest this is also the case for lactation therapy of subclinical mastitis.

The progressive increase in CURE with progression of lactation previously reported for Staph. aureus was only present in this study for Pirli2x. In the untreated group, the percentage spontaneous CURE decreased as lactation progressed, whereas in the Pirli8xgroup, an effect of treatment by stage of lactation was only apparent for the first 100 DIM, the period of peak milk production, when compared with the remainder of the lactation. Wilson et al. (1972) hypothesized that the stage of lactation effect was due to a more rapid elimination of the antibiotic with high milk yield in the first part of the lactation. This effect may be less important with treatment of clinical mastitis, where milk production is generally more affected (Sol et al., 2000). The significant interaction between treatment and stage of lactation in the present study suggests that this phenomenon varies with treatment regimen.

Whereas treatment regimen was significant for both CURE and lnSCCpost, there was a significant interaction of treatment with bacterial species for CURE. However, the lack of a significant association of lnSCCpost with bacterial species cultured (P = 0.217) indicates that the effect of treatment regimen on inflammation reduction did not differ across bacterial classes. This is further illustrated by the finding that following Pirli8xthere was a significant SCC decrease in quarters that did not cure bacteriologically (Table 2). The low CNS bacteriological cure rate and its failure to increase with longer duration of treatment could be due to reinfections with CNS (Timms and Schultz, 1984). It may be worthwhile to identify the most common CNS at the individual species level, particularly as some species could cause more inflammation than others (Laevens et al., 1997). Similarly, the lack of a difference in CURE rate between treatment regimens for C. bovis is not surprising because no extra measures were taken to prevent new infections during the study.

The significant effects of parity, stage of lactation, and number of colonies present pretreatment on CURE did not differ (P 0.414) between bacterial species. These additional risk factors explain why for example, the Staph. aureus bacteriological cure rates varied substantially within each parity by treatment combination (Figure 1).

For lnSCCpost, parity and treatment duration were again significant factors. In addition, quarter location, lnSCCpre, and the interaction of parity with lnSCCpre and quarter location were significantly associated with lnSCCpost. Effects of parity, quarter location, and lnSCCpre have been attributed to chronicity of infection (Wilson et al., 1972; Sol et al., 1997). The fact that within parity group 2 to 3, lnSCCpost in front quarters decreased to the level of quarters in parity1 cows, whereas the lnSCCpost in rear quarters of those cows remained at a level similar to the higher values in quarters of parity4 to 5 cows, supports this hypothesis (Table 8, Figure 2).

It is noteworthy that the P value of statistical significance for the interaction between treatment regimen and several other risk factors (quarter location, lnSCCpre, stage of lactation, and parity) was in the range of 0.104 to 0.184 for both CURE and lnSCCpost, suggesting that other risk factors for CURE and lnSCCpost may vary with treatment regimen. This aspect probably merits further research.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Treatment regimen affected bacteriological cure differently in major than in minor pathogens. Regardless of treatment regimen and the species isolated pretreatment, bacteriological cure rate was higher in younger animals and when fewer colonies had been isolated pre-treatment. Stage of lactation also affected bacteriological cure, but its effect varied with the treatment regimen.

Posttreatment SCC was associated with treatment regimen and with other factors, i.e., pretreatment SCC, quarter location, and the interaction of parity with quarter location and pretreatment SCC. These other factors did not vary significantly with treatment regimen.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The authors gratefully acknowledge the dedication of the investigators and the trial monitors at each site in each of the countries.

	FOOTNOTES

 
^* Current address: European Food Safety Authority, Genevestraat 10, B-1140 Brussels, Belgium.

Received for publication April 27, 2004. Accepted for publication September 3, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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This Article Abstract Full Text (PDF) Interpretive Summary Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Deluyker, H. A. Articles by Boucher, J. F. Search for Related Content PubMed PubMed Citation Articles by Deluyker, H. A. Articles by Boucher, J. F.