The effect of selective dry cow treatment on new intramammary infections

Dry cow therapy, or antibiotic treatment at end of lactation, is used to eliminate intramammary infections and prevent new infections during the dry period. It is one part of a total management system recommended in controlling intramammary infections in the dairy cow. Public health concerns advise prudent use of antibiotics, as their use may promote bacterial antibiotic resistance and leave antibiotic residues in the food chain. The effects of dry cow treatment and no treatment were compared, on new intramammary infections and clinical mastitis within two low cell count herds and two herds undergoing conversion to organic farming. The results will inform those restricting their use of dry cow therapy on the additional risk of new intramammary infection and aid in development of alternative management strategies. No cases of clinical mastitis in the dry period were observed in treated cows, whereas in the untreated groups a significant number were observed. Significantly more new infections at calving were found in the untreated group in all herds. In those quarters where infections were first detected at calving, the incidence of clinical mastitis was significantly greater in the untreated group in all herds. Clinical mastitis detection was significantly lower in organic herds. Untreated quarters infected at drying with Corynebacterium spp. or coagulase-negative staphylococci were found to have an increased risk of new infection by Streptococcus uberis or coliform bacteria. It can be concluded that dry cow therapy continues to lower significantly the rate of new dry period intramammary infection in herds with elevated somatic cell counts and a high prevalence of infection.     

Effect of an intramammary teat seal and dry cow antibiotic in relation to dry period length on postpartum mastitis

Infusion of either a long-acting antibiotic preparation (cefalonium) or the same antibiotic preparation combined with an internal teat sealant (bismuth subnitrite) were compared for the effect on new intramammary infections at calving and clinical mastitis in the first 100 d of lactation, in relation to dry period length. For all cows, a significant reduction in the incidence of new infections in quarters at calving (3.7 vs. 7.3%) was found for the combination treatment group (150 cows) compared with the antibiotic-alone treatment (133 cows). With a dry period of 10 wk or longer, significantly fewer new quarter infections (3.8 vs. 11.4%) were found in those cows receiving the combination treatment compared with antibiotic treatment alone. When the dry period was less than 10 wk, the incidence of new infections in quarters treated with the combination treatment was lower than for the antibiotic treatment alone (3.7 vs. 6%) but this was not a statistically significant difference. Fewer infections caused by Streptococcus uberis and coagulase-negative staphylococci were found in cows receiving the combination treatment compared with the antibiotic treatment alone (not significant). Coliform isolates were less likely in cows receiving the combination treatment with the longer dry period but the numbers of new intramammary coliform infections were low for both dry period categories. Few infections were caused by coagulase-negative staphylococci. The incidence of clinical mastitis in the first 100 d of lactation in quarters infected at calving was significantly lower (4 vs. 15 cases) for the combination treatment than for the antibiotic treatment alone for both dry period lengths. The clinical incidence in quarters in which a pathogen was not detected in either of the samples taken after calving was comparable between groups. No significant difference was found in the total clinical incidence after calving for both groups irrespective of dry period length.     

Efficacy of a novel internal dry period teat sealant containing 0.5% chlorhexidine against experimental challenge with Streptococcus uberis in dairy cattle

The incidence of clinical mastitis and infection status at calving was assessed in quarters treated with 1 of 2 internal teat sealants at the time of dry off. Two contralateral quarters per cow (n = 63 cows) were treated with a sealant that contained 0.5% chlorhexidine; the other quarters were treated with a commercial teat sealant. Ten cows were untreated (controls). On d 2, 4, and 16 after dry off, cows were challenged with Streptococcus uberis S210 strain. Cows were examined daily for 34 d after drying off and cases of clinical mastitis were recorded. Milk samples were collected for culture from any quarters that developed clinical mastitis during the first 34 d after drying-off and from all quarters on d −5 and 0 relative to treatment and at the first and twentieth milking after calving. The incidence of clinical mastitis during the examination period was lower in treated quarters (n = 7/252; 1.5%; lower incidence for those treated with chlorhexidine-containing teat sealant n = 3/126; 1.2%) than in untreated quarters (n = 13/40; 26.8%). The protection against intramammary infection after calving, adjusted for the effect of cow, was higher in quarters treated with the novel teat sealant (89/105; 15.2%; 95% CI = 9.6-23.4) than in those treated with the commercial teat sealant (71/104; 31.7%; 95% CI = 23.5-41.3) and untreated controls (6/28; 78.6%; 95% CI = 59.8-90.0), respectively. Quarters treated with teat sealants were less likely to have an intramammary infection after calving and had a lower incidence of clinical mastitis during the early dry period than did untreated controls in this challenge study.     

The effect of internal teat sealant products (Teatseal and Orbeseal) on intramammary infection,clinical mastitis,and somatic cell counts in lactating dairy cows: A meta-analysis

The objective of this study was to assess the efficacy of internal teat sealant products containing bismuth subnitrate (Teatseal and Orbeseal; Pfizer Animal Health, West Ryde, Australia) when used alone, or in the presence of antibiotic dry cow therapy (ADCT), before or at drying off on the incidence of new intramammary infections (IMI), clinical mastitis, and milk somatic cell count (SCC) during lactation. The literature search identified 18 English-language publications on the use of Teatseal in dairy cattle. A total of 12 studies with 17 subtrials or comparisons including 13 positive control subtrials (internal teat sealant and ADCT vs. ADCT) and 4 negative control subtrials (internal teat sealant vs. untreated) examining IMI were included in the analysis. Internal teat sealants, alone or in the presence of ADCT, reduced the risk of acquiring new IMI after calving by 25% [risk ratio (RR) = 0.75; 95% confidence interval (CI): 0.67 to 0.83]. Internal teat sealants reduced the risk of IMI by 73% compared with untreated cows (RR = 0.27; 95% CI: 0.13 to 0.55). The results of both meta-analyses of IMI, with positive and negative controls, were heterogeneous [I² (a statistic that describes the proportion of total variation in study effect estimates that is due to heterogeneity) = 65.4 and 92.1%]. No farm or cow factors studied significantly contributed to the heterogeneity of the results. A total of 16 studies (21 subtrials), including 14 positive control subtrials and 7 negative control subtrials, examining clinical mastitis were included in the analysis. Internal teat sealants alone and in the presence of ADCT reduced the risk of clinical mastitis after calving in lactating cows by 29% (RR = 0.71; 95% CI: 0.62 to 0.82), and 48% (RR = 0.52; 95% CI: 0.37 to 0.75), respectively. The results of the meta-analysis on clinical mastitis with positive controls were homogeneous (I² = 33.6%), whereas the results of studies with negative controls were heterogeneous (I² = 60.4%). No farm or cow factors studied that had sufficient data to evaluate significantly contributed to the heterogeneity of the results. The estimated linear score (LS) of milk SCC after calving in published studies (n = 3) and for studies that provided raw data (n = 2), was significantly lower for cattle treated with internal teat sealants and ADCT in 3 studies than for cattle treated with internal teat sealants only. The estimated LS of pooled raw data of 3 studies from 32 herds showed that the LS of cows treated with internal teat sealant and ADCT was not significantly different than those treated with ADCT only. This study found that the application of internal teat sealants in the presence of ADCT or the use of internal teat sealants alone at dry off significantly reduced the incidence of IMI and clinical mastitis in lactating dairy cows compared with respective control groups. Further studies are needed to investigate the effect of internal teat sealants on postpartum milk SCC in lactating dairy cows.     

Effect of localised antibiotic infusions applied to the teat-canal and teat sinus at drying-off on mastitis in the dry-period and at calving

An experiment using three New Zealand herds and a total of 632 cows, examined the effect of localised prophylactic treatments with antibiotic at drying-off on the incidence of new intramammary infection during the dry period and at calving. Antibiotic was infused either into the teat canal (0.22 g of dry-cow formulation) or the teat sinus (3.1 g of lactating-cow formulation) of uninfected quarters to eliminate any bacteria present in these locations at the last milking of lactation. These treatments were compared with a negative control (nil treatment) and a positive antibiotic control (infusion of 3.6 g of dry-cow formulation). All antibiotic formulations used the same active ingredient, sodium cloxacillin. No significant reduction in new dry period clinical mastitis was observed for the two localised treatments whereas the positive control treatment achieved 100% reduction in new clinical mastitis compared with untreated control quarters. A 41% reduction (P < 0.05) in new Streptococcus uberis infections at calving was associated with the teat canal antibiotic treatment, compared with an 82% reduction (P < 0.001) for the positive antibiotic control. Both localised treatments showed a reduced incidence of new intramammary infection (P < 0.001) when pooled across periods and pathogens. Teats receiving either the teat canal antibiotic treatment or a full infusion of long acting dry-cow antibiotic had a lower incidence of open teat canals (P < 0.05) at 3 weeks after drying-off.     

The use of a cephalonium containing dry cow therapy and an internal teat sealant, both alone and in combination

The dry period is a critical time in the lactation cycle, being the optimum time to cure existing intramammary infection (IMI) as well as encompassing the periods of highest susceptibility to new infection. Currently, IMI in the dry period is controlled with antibiotic dry cow therapy. The aim of this randomized control trial was to investigate different dry cow therapy regimens by stratifying cows by likely infection status at drying off in herds with low somatic cell count (SCC; bulk milk SCC <250,000 cells/mL) in southwest England. All quarters in 890 cows were recruited. The recruited cows were categorized as either infected or uninfected on the basis of SCC and clinical mastitis history. Ipsilateral quarters within each cow were randomly allocated to receive 1 of 4 different treatment regimens according to their infection category. Quarters in high-SCC infected cows were allocated to receive antibiotic dry cow therapy either alone or in combination with an internal teat sealant; quarters in low-SCC uninfected cows were allocated to receive teat sealant either alone or in combination with antibiotic dry cow therapy. All quarters were sampled for bacteriology at drying off and again within 10 d post-calving. Quarters were subsequently monitored for clinical mastitis for the first 100 d of lactation. The mass of residual sealant was assessed immediately post-calving to allow assessment of the association of sealant retention with treatment efficacy. Models were constructed to assess the efficacy of the different regimens in preventing IMI. Apparent cure rates of existing IMI with major pathogens were consistently >90% in quarters receiving antibiotic. Combination treatment of high-SCC infected cows resulted in an increased likelihood of being pathogen free post-calving (odds ratio = 1.40; 95% credibility interval = 1.03-1.90). The benefits of combination treatment of low-SCC uninfected cows were less clear. With respect to clinical mastitis, combination treatment of high-SCC infected cows resulted in a decreased likelihood of developing clinical mastitis in the first 100 d of the subsequent lactation (odds ratio = 0.68; 95% credibility interval = 0.48-0.98). The retention of the internal sealant was adversely affected by its use in combination with antibiotic dry cow therapy.     

A comparison of broad-spectrum and narrow-spectrum dry cow therapy used alone and in combination with a teat sealant

The dry period is a critical time in the lactation cycle, offering the optimum time for cure of existing intramammary infection (IMI), while also encompassing the periods of highest susceptibility to new intramammary infection. Until recent years, intramammary infection in the dry period has been controlled with the use of antibiotic dry cow therapy. The aim of this study was to investigate 3 different dry cow therapy regimens, in low-somatic cell count (SCC; bulk milk SCC < 250,000 cells/mL) herds in southwest England. A total of 489 cows was recruited to the study and randomly allocated to receive either the broad-spectrum antibiotic cefquinome, a combination treatment comprising the narrow-spectrum antibiotic cloxacillin and an internal teat sealant, or the narrow-spectrum antibiotic cloxacillin alone. All quarters were sampled for bacteriology at drying off and again in the week immediately postcalving; 2 quarters were also sampled 2 wk before the estimated calving date to allow an assessment of infection dynamics during the dry period. Quarters were subsequently monitored for clinical mastitis for the first 100 d of lactation. Conventional multilevel (random effects) models were constructed to assess the efficacy of products in preventing IMI. Survival analysis was used to examine factors that influenced the risk of clinical mastitis using conventional Cox proportional hazards models. No differences were identified between the treatment groups in terms of cure of IMI caused by the major pathogens. Quarters in both the combination and cefquinome-treated groups were more likely to be free of a major pathogen or enterobacterial pathogen postcalving. With respect to clinical mastitis, the cefquinome-treated group was less likely to develop clinical mastitis than was the cloxacillin treated group.     

Evaluation of selective dry cow treatment following on-farm culture: Risk of postcalving intramammary infection and clinical mastitis in the subsequent lactation

The objective of the study was to evaluate the utility of a Petrifilm-based on-farm culture system when used to make selective antimicrobial treatment decisions on low somatic cell count cows (<200,000 cells/mL) at drying off. A total of 729 cows from 16 commercial dairy herds with a low bulk tank somatic cell count (<250,000 cells/mL) were randomly assigned to receive either blanket dry cow therapy (DCT) or Petrifilm-based selective DCT. Cows belonging to the blanket DCT group were infused with a commercial dry cow antimicrobial product and an internal teat sealant (ITS) at drying off. Using composite milk samples collected on the day before drying off, cows in the selective DCT group were treated at drying off based on the results obtained by the Petrifilm on-farm culture system with DCT + ITS (Petrifilm culture positive), or ITS alone (Petrifilm culture negative). Quarters of all cows were sampled for standard laboratory bacteriology on the day before drying off, at 3 to 4 d in milk (DIM), at 5 to 18 DIM, and from the first case of clinical mastitis occurring within 120 DIM. Multilevel logistic regression was used to assess the effect of study group (blanket or selective DCT) and resulting dry cow treatment (DCT + ITS, or ITS alone) on the risk of intramammary infection (IMI) at calving and the risk of a first case of clinical mastitis between calving and 120 DIM. According to univariable analysis, no difference was observed between study groups with respect to quarter-level cure risk and new IMI risk over the dry period. Likewise, the risk of IMI at calving and the risk of clinical mastitis in the first 120 DIM was not different between quarters belonging to cows in the blanket DCT group and quarters belonging to cows in the selective DCT group. The results of this study indicate that selective DCT based on results obtained by the Petrifilm on-farm culture system achieved the same level of success with respect to treatment and prevention of IMI over the dry period as blanket DCT and did not affect the risk of clinical mastitis in the first 120 d of the subsequent lactation.     

Effectiveness of an internal teat seal in the prevention of new intramammary infections during the dry and early-lactation periods in dairy cows when used with a dry cow intramammary antibiotic

The objectives of this study were to determine the effect of infusion with an internal teat seal at dry off, when used as an adjunct to long-acting antibiotic infusion at dry off, on the risk for acquiring a new intramammary infection (IMI) during the dry period, prevalence of IMI and linear score (LS) after calving, and risk for experiencing a clinical mastitis event between dry off and 60 DIM. A total of 437 cows from 2 dairy herds, with no clinical mastitis and 4 functional quarters, were enrolled at dry off. Prior to the final milking, all quarters were sampled for bacteriological culture and SCC analysis. After milking, all 4 quarters were infused with a commercially available long-acting dry cow antibiotic. Two contralateral quarters were then infused with an internal teat seal (Orbeseal, Pfizer Animal Health, New York). Following calving the teat seal was stripped out at first milking. Duplicate milk samples were collected between 1 to 3 DIM and again between 6 to 8 DIM for culture and SCC analysis. Quarters treated with Orbeseal had significantly lower prevalence of IMI at 1 to 3 DIM (tx = 22.8%, control = 29.1%), had significantly fewer quarters that acquired a new IMI between dry off and 1 to 3 DIM (tx = 20.2%, control = 25.4%), and had significantly fewer quarters affected by a clinical mastitis event between dry off and 60 DIM (tx = 5.9%, control = 8.0%). Multivariable analysis showed a significant effect of treatment, with treated quarters being 30% less likely to develop a new IMI between dry off and 1 to 3 DIM, 31% less likely to have an IMI present at 1 to 3 DIM, 33% less likely to experience a clinical mastitis event between dry off and 60 DIM, and having significantly lower linear score measures at 1 to 3 DIM and 6 to 8 DIM, compared with control quarters.     

Control of new intramammary infection at calving by prepartum teat dipping

Cows were subjected to teat dipping in a commercial iodophor germicide twice daily for at least 7 days immediately before calving to assess efficacy of the practice in reducing new intramammary infection at calving. In a cold season and in a warm season, groups of about 35 cows successively calving were treated and were compared with similarly sized and selected control groups. There were no advantages for teat dipped over control cow groups for incidence of new intramammary infections at calving, for new infections that persisted for longer than 14 days after calving, or for new infections that required antibiotic therapy. Incidence of new infection in warm weather (27.3% of quarters) was twice that in cool weather (13.6%). The most frequently isolated pathogens were coagulase-negative staphylococci and Streptococcus uberis. Staphylococcus aureus infections were most likely to persist into the new lactation (83%) than were infections by other pathogens (38 to 57%).     

Herd management factors that affect duration and variation of adherence of an external teat sealant

During 1998 and 1999, a purposive sample of Ontario dairy herds was enrolled in a study to examine management factors associated with adherence of an external teat sealant. A total of 74 herds were recruited that had previously complied with a provincial Sentinel Herd mastitis study. All herds were sent a management survey and a commercially available external teat sealant product. The sealant was applied to all cows in first lactation or greater that were scheduled to go dry. Adherence of the teat sealant was scored on a scale of 1 to 5 (1 = sealant completely removed) for the first 12 d following dry off. Complete data were analyzed from 806 cows in 48 herds (mean of 17 cows/herd). Mean duration of adherence of the teat sealant was 4 d (range 1 to 7 d). Of the management factors recorded, only changing the feed to reduce milk production prior to drying off was significantly associated with 0.78-d prolonged adherence. No factors related to dry cow housing, bedding material, or floor surface were associated with the duration of adherence. The variance around the duration of adherence was examined. The median variance of adherence was 3.0 d², which was equivalent to a standard deviation of 2.6 d. Herds in which dry cows were housed in tie-stall barns, and the herds that used a penicillin and novobiocin combination dry cow antibiotic had the greatest variation. We concluded that modifying the ration to reduce milk production enhanced adherence of an external teat sealant. Teat-end preparation prior to application of the teat sealant is an important factor to consider when choosing to adopt this dry period mastitis prevention strategy. This study demonstrates that beyond cow- and quarter-level factors, herd management factors can influence the duration and variation of sealant adherence experienced among different herds.     

The effect of intramammary antibiotic therapy at calving on udder health traits

The effect of intramammary antibiotic therapy at calving on mastitis infection prevalence, linear score milk somatic cell count, and milk NAGase activity, 30 d postpartum, and on milk production, 90 to 120 d postpartum, was tested. Cows (n = 175) were split into treatment and control groups at drying off. All cows received commercial dry cow therapy. At calving, treated cows received commercial lactating cow therapy in all quarters after the first two milkings; control cows were not treated. Composite milk samples were aseptically collected from all cows at drying off, calving, and 30 d postpartum. Udder health traits: linear score milk SCC, NAGase activity, and bacterial content in milk, were determined on all samples. The first three DHI milk weights were recorded for all cows. Treatment and control cows had similar prevalences of intramammary infections during the dry and 30-d postpartum periods. Least squares means of linear score milk SCC and NAGase activities were similar at drying off and calving. Cell count scores were similar between groups; NAGase activities were higher in control cows at 30 d postpartum. Control cows tended to produce more milk postpartum. Results demonstrated no advantage of intramammary therapy at calving in improving milk production or udder health.     

Invited review: Selective use of antimicrobials in dairy cattle at drying-off

Administering intramammary antimicrobials to all mammary quarters of dairy cows at drying-off [i.e., blanket dry cow therapy (BDCT)] has been a mainstay of mastitis prevention and control. However, as udder health has considerably improved over recent decades with reductions in intramammary infection prevalence at drying-off and the introduction of teat sealants, BDCT may no longer be necessary on all dairy farms, thereby supporting antimicrobial stewardship efforts. This narrative review summarizes available literature regarding current dry cow therapy practices and associated impacts of selective dry cow therapy (SDCT) on udder health, milk production, economics, antimicrobial use, and antimicrobial resistance. Various methods to identify infections at drying-off that could benefit from antimicrobial treatment are described for selecting cows or mammary quarters for treatment, including utilizing somatic cell count thresholds, pathogen identification, previous clinical mastitis history, or a combination of criteria. Selection methods may be enacted at the herd, cow, or quarter levels. Producers' and veterinarians' motivations for antimicrobial use are discussed. Based on review findings, SDCT can be adopted without negative consequences for udder health and milk production, and concurrent teat sealant use is recommended, especially in udder quarters receiving no intramammary antimicrobials. Furthermore, herd selection should be considered for SDCT implementation in addition to cow or quarter selection, as BDCT may still be temporarily necessary in some herds for optimal mastitis control. Costs and benefits of SDCT vary among herds, whereas impacts on antimicrobial resistance remain unclear. In summary, SDCT is a viable management option for maintaining udder health and milk production while improving antimicrobial stewardship in the dairy industry.     

Risk factors for isolation of Staphylococcus aureus or Streptococcus dysgalactiae from milk culture obtained approximately 6 days post calving

Milk culture results at approximately 6 d post calving were assessed in a 2-year retrospective single-cohort study in 178 Norwegian herds. A combined teat dipping and selective antibiotic therapy trial was performed in these herds where cows with composite milk somatic cell count (CMSCC) >100,000 cells/ml before drying-off (geometric mean of the last three CMSCC test-days) and isolation of Staphylococcus aureus or Streptococcus dysgalactiae were selected for either short-acting lactation antibiotic treatment or long-acting dry cow antibiotic treatment. Milk culture results at approximately 6 d post-calving were available from 437 treated cows and 3061 non-treated cows before drying-off and separate multivariable logistic regression models were ran for these two groups. Risk factors associated with isolation of Staph. aureus 6 d post calving for non-treated cows were CMSCC >400,000 cells/ml before drying-off v. <400,000 cells/ml (Odd ratio (OR) = 2.4) and clinical mastitis (CM) in the previous lactation v . non-treated (OR=1.5). Risk factors associated with Staph. aureus 6 d post calving for treated cows was a CMS > 200,000 cells/ml before drying-off v. <200,000 cells/ml (OR=2.3) and CM in the previous lactation verus non-treated (OR=1.7). For non-treated cows it was 1.7 times more likely to isolate Str. dysgalactiae 6 d post-calving if the CMSCC was > 50,000 cells/ml compared with <50,000 cells/ml. For treated cows it was 3.7-5.8-times more likely to isolate Str. dysgalactiae 6 d post calving if given short-acting lactation formula at quarter level compared with long-acting dry cow formula used at cow level. Regular use of iodine post-milking teat disinfection (PMTD) did not influence the isolation of Staph. aureus 6 d post calvin, but it was less likely to isolate Str. dysgalactiae 6 d post calving if iodine PMTD was used regularly rather than irregularly. The external teat sealant had no effect on either of the two bacteria. This study indicates that the CMSCC limit for sampling cows before drying-off can be reduced to 50,000 cells/ml in herds with a Str. dysgalactiae problem. Iodine PMTD should also be recommended in these herds. Cows with a CMSCC > 400,000 cells/ml prior to drying-off should receive long-acting dry cow formula irrespective of the milk result.     

Adherence and efficacy of an external teat sealant to prevent new intramammary infections in the dry period

The efficacy and adherence of an external teat sealant applied at drying off was evaluated in 2 studies between 1997 and 1999. At drying off, 2 quarters were randomized to receive intramammary dry-cow antibiotic therapy, and the remaining 2 quarters were treated with either a single or double application of external teat sealant. Approximately 3 d before calving, all teats that had been dipped at drying off were redipped in a single coating of teat sealant. Adherence of the teat sealant was scored for the first 2 wk after drying off, and physical traits of the teat skin and teat ends were recorded. Quarter milk samples were collected 1 wk before drying off, at drying off, 0 to 7 d, and 14 to 21 d postcalving. Somatic cell counts were determined from quarter samples taken at d 7 and 14 to 21 d after calving. Data were analyzed from 172 dry periods of 162 cows. The mean time of sealant adherence following drying off application was 3 ± 0.13 d. Double sealant application significantly increased the duration of adherence by 0.67 d. Teats that had teat sealant applied twice at drying off and that had up to 3 d of adherence had the lowest linear score (LS) at 14 to 21 d (1.89 ± 0.31) of all quarters. The LS of quarters that received antibiotic therapy only was 2.27 ± 0.19. The majority of intramammary infections identified at drying off were caused by the minor pathogens, coagulase-negative staphylococci and Corynebacterium bovis (51 and 23%, respectively). The results from this study indicate that duration of sealant adherence to the teat-end should be considered when evaluating the impact of teat sealant treatment at drying off on the level of infection after calving. Double sealant application, cooler seasons, and longer teat lengths were associated with a significant increase in the duration of sealant adherence to the teat-end.     

The use of national-level data to describe trends in intramammary antimicrobial usage on Irish dairy farms from 2003 to 2015

In this study, we used national-level data to describe trends in on-farm intramammary antimicrobial usage in Ireland from 2003 to 2015. We calculated actual sales of intramammary tubes and the quantity of active substance sold, by year, product type [lactation or dry cow therapy (DCT)], antimicrobial group, World Health Organization antimicrobial classification, and from 2009 to 2015, prescribing route. We also estimated on-farm usage of lactation and dry cow intramammary antimicrobials using defined daily dose (DDDvet) and defined course dose (DCDvet) calculations, and dry cow coverage. Sales of tubes of antimicrobial for DCT have increased, and the estimated national dry cow coverage in 2015 was 1,022 DCDvet per 1,000 cows per year. An increase has also occurred in sales of teat sealant (2015 sales: 66.7 tubes with teat sealant for every 100 tubes with antimicrobial for DCT). In contrast, the number of tubes of antimicrobial sold for lactation use has decreased to 1,398 DDDvet and 466 DCDvet per 1,000 animals per year. Sales in intramammary tubes with at least one critically important antimicrobial (CIA) have either risen since 2007 (DCT) or fallen (lactation therapy). Increases were observed in both the number of dry cow and lactation tubes containing CIA considered of highest priority for human health. Differences between prescribing routes with respect to CIA usage were observed. This study provides detailed insight into on-farm usage of intramammary antimicrobials in Ireland. It demonstrates positive national progress but also highlights areas for review. In particular, blanket dry cow treatment in Ireland should be reconsidered. It is not possible to investigate farm-level variation in antimicrobial usage from national sales data. In several countries, measurement and benchmarking have been critical to progress in reducing antimicrobial usage in farm animal production. Central collation of data on farm-level antimicrobial use is also needed in Ireland to allow objective measurement and benchmarking of on-farm usage. More generally, standardized indicators to quantify antimicrobial usage in farm animals are urgently needed to allow country-level comparisons.     

Prophylactic effects of two selective dry cow strategies accounting for interdependence of quarter

Infusion of a long-acting antibiotic preparation at drying off in dairy cows as a prophylactic therapy is usually recommended for all quarters where it is in use. Studying the effectiveness of such treatment using quarter as the unit of analysis assumes that each quarter within a cow has a risk of being infected independent of the other quarters of the cow. Failure to account for interdependence of quarters within a cow may lead to inaccurate variance estimates and errors in assessing treatment effects. Data from two trials assessing different dry-cow strategies were examined for interdependence of infection between quarters. Logistic regression with a variance inflation factor or a multilevel analysis was used to assess the effect of antibiotic and internal teat-sealant dry cow strategies. Parity and infection status at drying off were covariates in the analysis. Interdependence of the risk of quarter infections within control-group cows was demonstrated in both dry-cow antibiotic and teat-seal trials. However, cows that received either of these treatments did not demonstrate interdependence. Treated quarters in both trials were 3.0 times less likely to acquire a new infection at calving compared with the untreated controls. Quarters in cows of parity 3 or greater were also at an increased risk in the antibiotic treatment trial. In both trials, quarters with either Corynebacterium spp. or coagulase-negative staphylococci infections at drying off had an increased risk of a new intramammary infection at calving. This study has demonstrated the beneficial and comparable effects of antibiotic and teat seal dry cow strategies; both decreased the risk of intramammary infection at calving. The application of dry-cow strategies at the cow level and not the quarter level is also supported.     

Interdependence of udder quarters for new intramammary infection during the dry period in cows submitted to selective antibiotic therapy

The aim was to investigate the interdependence of udder quarters within cow towards the incidence of intramammary infections during the dry period in herds under selective dry-cow antibiotic therapy. A total of 368 cows among 28 herds were included in a survey. Quarter milk samples collected at the last milking before drying-off and on day 3 after calving were submitted to microbiological procedures. An expected distribution of cows according to their number of newly infected quarters was calculated based on a binomial probability distribution from the overall quarter incidence (or from the quarter incidence in each herd) and compared with the observed distribution. Incidence of newly infected quarters ranged from 0.0 to 39.3%, depending on the herd (median: 17.7%). Interdependence of quarters towards new infection during the dry period was observed whatever the pathogen type, for both treated and untreated cows. Calculation of an expected distribution of cows according to their number of newly infected quarters using the quarter incidence in each herd (instead of the overall quarter incidence) reduced the distance to the observed distribution, but interdependence was still observed. Our results support the application of selective antibiotic therapy at the cow level rather than at the quarter level.     

Evaluation of the efficacy of an internal teat sealer during the dry period

The efficacy of an internal dry period teat sealer containing bismuth subnitrate (Product A; Teatseal, Cross Vetpharm Group Ltd, Ireland) was compared with a long-acting antibiotic preparation containing cephalonium (Product B; Cepravin Dry Cow, Schering-Plough Ltd, UK), by assessing the number of new intramammary infections (IMI) acquired during the dry period and the number of cases of clinical mastitis during the first 100 d of lactation. Selection of study animals was based on historical data. No cases of clinical mastitis and all routine cow level somatic cell counts <200,000 cells/ml during the previous lactation were used to select cows likely to be uninfected with a major pathogen at drying off. Compared with the antibiotic tube, quarters that received the teat sealer acquired significantly fewer new IMI caused by Escherichia coli, all Enterobacteriaceae, and all major pathogens combined. There was no significant differences in the number of IMI caused by any other major pathogen. There was no significant difference in the severity or number of quarter or cow cases of clinical mastitis between product groups. Sixty quarters (3.2%) were infected with major pathogens at drying off, 27 (2.9%) in teat sealer and 33 (3.5%) in antibiotic tube cows. The dry period cure rate was not significantly different (63% product A, 70% product B). This is the first controlled study to demonstrate the efficacy of an internal bismuth teat sealer in protecting quarters against new dry period IMI caused by major mastitis pathogens, particularly environmental organisms, under UK field conditions.     

Effect of using internal teat sealant with or without antibiotic therapy at dry-off on subsequent somatic cell count and milk production

The objective of this study was to assess the effect of treating cows with teat sealant only compared with antibiotic plus teat sealant at drying off on weekly somatic cell count, potential intramammary infection, and milk production across the entire subsequent lactation. In 3 research herds in the south of Ireland, cows with SCC that did not exceed 200,000 cells/mL in the previous lactation (LowSCC) were randomly assigned to 1 of 2 treatments at drying off: internal teat sealant alone (ITS) or antibiotic plus teat sealant (AB+ITS). Cows with SCC that exceeded 200,000 cells/mL in the previous lactation were treated with AB+ITS and included in the analyses as a separate group (HighSCC). Weekly individual animal composite SCC records were available for 654 cow lactations and were transformed to somatic cell scores (SCS) for the purpose of analysis. Data were divided into 3 data sets to represent records obtained (1) up to 35 DIM, (2) up to 120 DIM, and (3) across the lactation. Foremilk secretions were taken from all quarters at drying off, at calving, 2 wk after calving, and in mid-lactation and were cultured to detect the presence of bacteria. The LowSCC cows treated with ITS alone had higher daily milk yield (0.67 kg/d) across lactation compared with LowSCC cows treated with AB+ITS. The LowSCC cows treated with ITS alone had higher SCS in early, up to mid, and across lactation compared with LowSCC cows treated with AB+ITS. We detected no difference in weekly SCS of LowSCC cows treated with ITS alone and SCS of HighSCC cows. The least squares means back-transformed SCC across lactation of the LowSCC cows treated with ITS alone, LowSCC cows treated with AB+ITS, and HighSCC cows were 41,523, 34,001, and 38,939 cells/mL respectively. The odds of LowSCC cows treated with ITS alone having bacteria present in their foremilk across lactation was 2.7 (95% confidence interval: 1.91 to 3.85) and 1.6 (1.22 to 2.03) times the odds of LowSCC cows treated with AB+ITS and of HighSCC cows treated with AB+ITS, respectively. In this study, Staphylococcus aureus was the most prevalent pathogen isolated from the population. Recategorizing the threshold for LowSCC cows as ≤150,000 cells/mL or ≤100,000 cells/mL in the previous lactation had no effect on the results. The results indicate that herds with good mastitis control programs may use ITS alone at dry-off in cows with SCC <200,000 cells/mL across lactation with only a small effect on herd SCC.