Methods of reducing the incidence of udder infection in dry cows

A retrospective on claims regarding clinical mastitis in the subsequent lactation, after use of an internal teat sealant in the dry period

Infusion of an internal teat sealant into the mammary gland of the dairy cow at drying off has been claimed to reduce the incidence of clinical mastitis over many months in the subsequent lactation, despite the absence of any ingredient of the sealant remaining for that long. However, these claims have been poorly substantiated, often by lack of identification on when the infection occurred in the period from calving to disease, if the infection was present at post calving sampling and if the pathogen causing the disease was that causing an earlier infection. Moreover, no hypothesis on how any effect on clinical mastitis might occur has been advanced in any of the publications claiming the effect. That the effect might occur is only reported in a minority of publications, and the possibility that this is relatively specific to Gram-negative pathogens is reviewed.

Cow-level risk factors for clinical mastitis in the dry period in cows treated with an internal teat sealant alone at the end of lactation

AIMS

To assess potential cow-level risk factors for clinical mastitis during the dry period in cows infused with internal teat sealant (ITS) alone at drying off, and associations with elevated somatic cell counts (SCC) at the first herd test, or clinical mastitis in the subsequent lactation.

METHODS

Ten herds reporting an incidence of clinical mastitis during the dry period of >3% amongst cows infused with ITS alone at the end of the 2018/2019 lactation were enrolled. Cow data collected included age, breed, milk yield and SCC at the last herd test before drying off, drying off date, clinical mastitis records during the dry period and in the subsequent lactation, and SCC at the first herd test following calving.

RESULTS

Overall, 2,401 cows were infused with ITS alone and 196/2,401 (8.2%) cows were diagnosed with clinical mastitis during the dry period. In the final multivariable model for risk of clinical mastitis there was an interaction between age and milk yield at the last herd test (p < 0.001), with cows aged >4 years and producing ≥10 L/cow/day having a higher risk of clinical mastitis than younger cows or those producing <10 L/cow/day. The risk of clinical mastitis was higher for cows dried off in the last week of lactation than for cows dried off >1 week before the end of lactation for the herd (p = 0.033). The percentage of cows with elevated SCC (>150,000 cells/mL) at the first herd test in the following lactation was higher amongst those diagnosed with clinical mastitis over the dry period compared with those that were not (p = 0.048). The hazard of clinical mastitis in the first 60 days after calving was also higher in cows diagnosed with clinical mastitis in the dry period than those that were not (p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE

The risk of clinical mastitis in the dry period following infusion of ITS alone at the end of lactation was associated with cow age, milk yield before drying off and timing of drying off. Cows diagnosed with clinical mastitis over the dry period had a higher risk of clinical mastitis and of elevated SCC in the subsequent lactation. This was an observational study so no causal inferences can be made, however herdowners and veterinarians should follow current recommendations regarding management of milk yield before drying off, and selection of cows for treatment with ITS alone.

Milk Microbiota: What Are We Exactly Talking About?

The development of powerful sequencing techniques has allowed, albeit with some biases, the identification and inventory of complex microbial communities that inhabit different body sites or body fluids, some of which were previously considered sterile. Notably, milk is now considered to host a complex microbial community with great diversity. Milk microbiota is now well documented in various hosts. Based on the growing literature on this microbial community, we address here the question of what milk microbiota is. We summarize and compare the microbial composition of milk in humans and in ruminants and address the existence of a putative core milk microbiota. We discuss the factors that contribute to shape the milk microbiota or affect its composition, including host and environmental factors as well as methodological factors, such as the sampling and sequencing techniques, which likely introduce distortion in milk microbiota analysis. The roles that milk microbiota are likely to play in the mother and offspring physiology and health are presented together with recent data on the hypothesis of an enteromammary pathway. At last, this fascinating field raises a series of questions, which are listed and commented here and which open new research avenues.

Retrospective analysis of Austrian health recording data of antibiotic or nonantibiotic dry-off treatment on milk yield, somatic cell count, and frequency of mastitis in subsequent lactation

Typically, dairy cows are dried off at the end of lactation. During the dry period, intramammary infections may be cured or persist, and new infections may occur. Traditionally, antibiotics (AB) have been used at dry-off. However, blanket antibiotic dry-off treatment may no longer be justifiable and, recently, selective use of antibiotics at dry off has been proposed and different decision criteria suggested. The objective of the study was to evaluate cows receiving antibiotic treatment at dry off (AB group) compared with cows dried off without antibiotics (non-AB group) using a large data set. Primary outcome parameters were milk yield, somatic cell count (SCC), and frequency of mastitis in the subsequent lactation. Additionally, we aimed to calculate cut-off values to determine, at the cow level, whether antibiotic dry-off treatment is indicated. A data set of 88,534 lactations was used; 27,723 cows were dried off using antibiotics (AB group; 31.3%) and 60,811cows were dried off without antibiotics (non-AB group; 68.7%). Milk yield in previous and subsequent lactations was higher in the AB group. Cows in the AB group produced, on average, 91 kg more milk in the subsequent lactation. The average SCC during the final 90 d of the previous lactation and at dry-off did not differ between the 2 groups. The probability of drying off using antibiotics increased almost linearly with higher milk yield and with higher SCC. The use of antibiotics resulted in an average decrease in SCC of 1,500 cells/mL in the subsequent lactation. The frequency of clinical mastitis during the previous lactation was higher in the AB group than in the non-AB group. Independently of the dry-off treatment, this difference remained in the subsequent lactation within 90 d in milk. The use of antibiotics at dry-off had no significant effect on the frequency of mastitis within 90 d in milk of the subsequent lactation. The study indicates that Austrian farmers and veterinarians are applying selective dry-cow treatment using milk yield and, to a lesser extent, SCC and mastitis frequency, for decision-making. However, even though a large data set was used, cut-off values with sufficient diagnostic value for selective dry-cow treatment could not be identified. It is likely that cut-off values must be identified at the herd level or in combination with additional parameters (e.g., results of bacterial culture).

The changing face of mastitis control

Over the past 125 years, mastitis has been one of the most studied conditions of dairy cows. This is confirmed by a search of peer-reviewed literature covering this period, which revealed more than 12,000 papers. Yet mastitis is still one of the most relevant and problematic diseases to treat and control in practice, which is testament to the complexity and mutability of the condition. Although progress has been made, recent research suggests that substantial areas require better understanding. Here, Martin Green and Andrew Bradley reflect on the control of mastitis over past decades and look forward to an interesting future.

Population medicine and control of epidemics

Population medicine is an important component of veterinary care in livestock (farm animals) and companion animals (pets). This chapter covers some of the chemotherapeutic approaches undertaken at population level to control infectious diseases in domestic animals. Optimisation of health, productivity and welfare in livestock commonly entails implementation of whole-herd or whole-flock strategies to effectively counter the negative impact of infectious diseases. Gastro-intestinal and liver parasites of grazing cattle and sheep are endemic in most parts of the world and can result in significant production losses. Strategically timed anthelmintic treatments are instituted with the double objective of reducing worm burdens in infected animals and ensuring reduction of pasture contamination with infective larvae. Mastitis is another major endemic problem, particularly in cattle, which causes significant economic losses to dairy farmers globally. As a painful inflammatory condition of the cow's udder, clinical mastitis also raises animal welfare concerns. Prevention of clinical mastitis requires rigorous post-milking hygiene, identification and culling of chronically infected cows, attention to the cow's environment and therapeutic management of udder health during the dry period. A third condition that can cause high levels of morbidity and mortality is bacterial respiratory disease. Pneumonia in young livestock is often exacerbated by stressful transportation and co-mingling of animals from different herds. The welfare consequences and production losses can be significant. Antimicrobial treatment of pneumonic animals and, when appropriate, of in-contact animals living in the same air-space is an integral part of whole-herd respiratory disease management. The role of the veterinary profession is to also ensure that principles of population medicine are understood and adhered to by pet owners. The increase in pet ownership and the importance of the human-animal bond in modern developed societies give rise to zoonotic risks, which require vigilance and intervention. Regular internal parasite control in dogs and cats, particularly in endemic areas, contributes to animal welfare and minimises public health hazards.

Rate of infection of milked and unmilked udders

The teats of milked and unmilked quarters of 6 cows were deliberately contaminated twice daily by immersion in a suspension of streptococci and staphylococci. Only the unmilked quarters became infected, 7 with Staphylococcus aureus and 3 with Streptococcus uberis. Neither previous infection with the same strain or a different strain of pathogen nor existing infection with Corynebacterium bovis appeared to influence the results.

The effect of intramammary infection during the dry period on the milk production of the affected quarter at the start of the succeeding lactation

Intramammary infection present in quarters of dairy cows, either throughout the dry period or originating in the dry period, reduces the milk yield after calving. Relative to the milk yield of equivalent non-infected quarters of the same udder the reduction was found in this work to be about 35%. The relative milk yield of quarters found infected in late lactation was depressed by 48% but if the infection was eliminated during the dry period, the depression in milk yield was only 11% after parturition.

Prevalence of subclinical mastitis and associated risk factors in smallholder dairy cows in Tanzania

A cross-sectional study was carried out on 200 randomly selected farms in each of the Iringa and Tanga regions of Tanzania to estimate the prevalence and risk factors for subclinical mastitis in dairy cows kept by smallholders. Subclinical mastitis was assessed using the California mastitis test (cmt), and by the bacteriological culture of 1500 milk samples collected from 434 clinically normal cows. The percentages of the cows (and quarters) with subclinical mastitis were 75.9 per cent (46.2 per cent) when assessed by the cmt and 43.8 per cent (24.3 per cent) when assessed by culture. Factors significantly associated with an increased risk of a cmt-positive quarter were Boran breed (odds radio [or]=3.51), a brought-in cow (rather than homebred) (or=2.39), peak milk yield, and age. The stripping method of hand milking was associated with a significantly lower prevalence of cmt-positive quarters (or=0.51). The cmt-positive cows were more likely to be culture positive (or=4.51), as were brought-in (or=2.10) and older cows.

The importance of the nonlactating period in the epidemiology of intramammary infection and strategies for prevention

The dry period is a high risk period for the acquisition of new environmental IMIs, with over 60% of new IMIs occurring at this time. It also offers an ideal opportunity to improve udder health, through the judicious use of antibiotics to treat high SCC cows. The prophylactic use of antibiotics in food producing animals is increasingly being called into question; DCT is the primary example of such antibiotic use in the dairy cow. A holistic approach to management of the dry cow is a vital part of mastitis control, and should encompass environmental and nutritional management as well as the blanket use of therapeutic and prophylactic medicinal products. In high SCC cows, it is important to select cows appropriate for treatment (as opposed to culling), and there is a good clinical justification for the use of antibiotics to treat existing IMIs. In low SCC cows, teat sealants provide a viable alternative to antibiotic DCT, and careful consideration should be given to their use.

Dry cow therapy with a non-antibiotic intramammary teat seal - a review

: Dry cow antibiotic therapy is used to eliminate existing intramammary infections and to prevent new infections in the dry period. It is implemented as part of a total management system known as the 'Five-Point Plan' for mastitis control. Recent public concerns over the widespread prophylactic use of antibiotics, coupled with an increasing interest in organic farming, have lead to a re-evaluation of the treatment of cows at drying-off. As a result, attention has focussed on the use of novel alternatives to antibiotic therapy at the end of lactation. One such therapy involves the application of a non-antibiotic bismuth-based intramammary teat seal designed for use in cows with low cell counts at the end of lactation. Like the keratin plug that forms naturally in teats of cows that have been dried-off, teat seal forms a physical barrier to invading pathogens. To date, a number of independent studies have shown that teat seal is as effective as traditional dry cow antibiotic products in preventing the occurrence of new infection during the dry period in cows with somatic cell counts of ≤200,000 cells ml-1 at drying-off. This paper reviews the efficacy of teat seal in preventing dry period mastitis in both conventional and organic dairying systems.

Management factors associated with the incidence of clinical mastitis over the non-lactation period and bulk tank somatic cell count during the subsequent lactation

AIM

To evaluate associations between management decisions related to the control of mastitis, including the infusion of antibiotics at the end of lactation (dry-cow therapy; DCT), on the incidence of clinical mastitis over the non-lactating period and the bulk tank somatic cell count (BTSCC) in the subsequent lactation.

METHODS

Dairy herd owners (n=158) provided information via a retrospective survey about (a) the proportion of their herds treated with DCT; (b) DCT management, including: number of occasions on which cows were dried off; manipulation of feed and water intake around drying off; infusion technique (partial vs full depth insertion of cannula); and hygiene before and after DCT infusion; (c) occurrence of mastitis and frequency of occurrence following drying off and in the subsequent lactation; (d) number of cows culled for mastitis-related conditions; (e) reasons for culling; (f) incidence of clinical mastitis; and (g) stock purchase policy with regard to mastitis. The BTSCC for each vat of milk supplied for the 1999/2000 and 2000/2001 seasons, and records of antibiotic purchases were collated for each herd. The probability that >2% of cows within a herd were diagnosed with clinical mastitis over the dry period was initially examined using univariate analysis (i.e. chi2 or logistic regression) and associated factors (p<0.2) were offered to a reverse stepwise logistic regression model. Factors hypothesised as being associated with the average lactation log10 BTSCC for the 2000/2001 season were initially examined using univariate analysis (i.e. ANOVA or linear regression analysis) and associated factors (p<0.2) were then tested using a forward manual model-building approach.

RESULTS

Increasing the percentage of the herd treated with DCT at the end of lactation was associated with reduced probability that >2% of a herd would be diagnosed with clinical mastitis over the non-lactating period and with a lower BTSCC in the subsequent lactation (p<0.01). A lower BTSCC was associated with small herds (<150 cows; p<0.05), not reducing feed intake around drying off (p<0.05), checking for clinical mastitis over the dry period in the milking parlour rather than at pasture (p<0.05), partial insertion of the DCT cannula (p<0.01), and use of 'change in udder shape' during lactation as a diagnostic criterion for mastitis (p<0.05). The incidence of clinical mastitis over the dry period was positively associated with reduced feeding around drying off (p=0.05) and the estimated volume of milk being produced at the time of drying off (p=0.014).

CONCLUSIONS

Use of dry cow therapy was associated with fewer cases of clinical mastitis over the non-lactating period and reduced BTSCC over the subsequent lactation. Reduced BTSCC was also associated with smaller herds, use of partial (compared with full depth) insertion of the DCT cannula, not reducing feed intake at the time of drying off, checking for clinical mastitis over the dry (non-lactation) period in the milking parlour, and use of udder shape for diagnosis during lactation. Control of clinical mastitis and BTSCC involves a range of management practices that need to be used in conjunction with DCT.

KEYWORDS

Dairy cows, mastitis, dry-cow therapy, somatic cell count, management practices.

Bovine mastitis: an evolving disease

Mastitis remains a major challenge to the worldwide dairy industry despite the widespread implementation of mastitis control strategies. The last forty years have seen a dramatic decrease in clinical mastitis incidence but this has been accompanied by a change in the relative and absolute importance of different pathogens. Escherichia coli and Streptococcus uberis are now the two most common causes of bovine mastitis and are an increasing problem in low somatic cell count herds. This paper reviews the changes in incidence and pattern of mastitis in the UK over the last four decades and discusses some of the possible explanations for these changes. It focuses in particular on apparent changes in the behaviour of E. coli and its ability to cause persistent intramammary infection; which may be as a result of bacterial adaptation or the unmasking of previously unrecognized patterns of pathogenesis. The prospects for novel approaches to mastitis control are discussed, as are the current and future challenges facing the industry.

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.

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

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

Comparison of tilmicosin and cephapirin as therapeutics for Staphylococcus aureus mastitis at dry-off

Forty-four cows (26 Jerseys and 18 Holsteins) that had at least 1 mammary quarter that was naturally (n = 12) or experimentally (n = 84) infected with Staphylococcus aureus were allotted to three treatment groups of approximately equal number at the end of lactation. Cows were dried off by abrupt cessation of milking, and dry cow therapy was administered as an intramammary infusion of cephapirin benzathine at 10 ml per quarter, an intramammary infusion of tilmicosin (solution containing 300 mg/ml) at 5 ml per quarter, or a subcutaneous injection of tilmicosin at 5 mg/kg of body weight on the day of drying off and another injection 4 d later. Mammary secretions were monitored during the dry period and postpartum for antimicrobial residues, intramammary infection (IMI) status, and somatic cell counts. Results demonstrated the following percentage cures for IMI caused by Staph. aureus at 28 d postcalving based on individual mammary quarters: cephapirin benzathine, 78.1%; tilmicosin infused, 74.2%; and tilmicosin injected, 9.1%. During the first 4 wk after drying off, the mean concentration of tilmicosin in mammary secretions from cows infused with the antibiotic remained approximately 10-fold higher than that in secretions from cows injected with the antibiotic (3.43 vs. 0.32 ppm), and, by the time of calving, concentrations for cows treated with both methods were below the dilution limit of the assay (< 0.1 ppm). Results demonstrated that intramammary infusion of tilmicosin was equally as effective as cephapirin benzathine in curing IMI caused by Staph. aureus at drying off; however, the subcutaneous injection of tilmicosin at the dose used was not effective as a dry cow therapeutic against Staph. aureus.

The prophylactic effect of a teat sealer on bovine mastitis during the dry period and the following lactation

AIM

To determine the prophylactic efficacy of a teat sealer, administered at drying off, in reducing new intramammary infections in the dry period and the following lactation.

METHODS

A total of 528 cows with late lactation somatic cell counts <2 00,000 cells/ml was identified in three commercial herds. Of these, bacteriological examination showed 482 cows were uninfected in all four quarters and 46 were infected in only one quarter. At drying off, uninfected quarters were randomly allocated to the following treatments: no infusion (negative controls), infusion with a bismuth subnitrate based teat sealer, infusion with teat sealer + antibiotic, or infusion with a cephalonium-based dry cow antibiotic (positive control). New infections were identified during the dry period by periodic udder palpations and at calving by bacteriological culture.

RESULTS

All three infused treatments reduced the incidence of new intramammary infections due to Streptococcus uberis, both during the dry period and at calving, by about 90% (p <0.01). The majority of the infections were due to Streptococcus uberis. For all treatments, a 50% lower incidence of clinical mastitis over the first 5 months of the ensuing lactation was reported by farmers. X-ray imaging of 19 teats showed that the teat sealer material was retained, at least in part, in the lower teat sinus over about 100 days of the dry period.

CONCLUSIONS

Closure of the teat canal from day one of the dry period as achieved by the teat sealer was as effective in reducing new dry period infections as the infusion of a long-acting dry cow antibiotic formulation. The lower incidence of new infections in the ensuing lactation among the infused quarters implies that fewer subclinical infections persisted from the dry period. Use of teat sealers at drying off appears to offer the same prophylactic efficacy as the dry cow antibiotic approach.

The prophylactic effect of a dry-cow antibiotic againstStreptococcus uberis

The prophylactic use of a dry-cow antibiotic for reducing the incidence of mastitis due to Streptococcus uberis was studied in four seasonally calving dairy herds involving 378 cows. The treatment was a long-acting dry-cow antibiotic preparation administered immediately after the last milking of lactation. New intramammary infections were identified by comparing the bacteriological status of quarters at drying off with that after calving, or through manual udder palpation during the dry period. The administration of dry-cow antibiotic to uninfected quarters at drying off reduced the overall incidence of new infections with Streptococcus uberis from 12.3% for untreated quarters to 1.2% of quarters (p<0.01). The reduction was significant (p<0.01) for both dry-period and post-calving infections. The susceptibility of uninfected quarters to new infection by Streptococcus uberis appeared to be unrelated to the infection status of a cow at drying off. Clinical infections during the dry period were most prevalent (97%) in quarters identified as having open teat canals. Fewer open teat canals (p<0.05) were observed among antibiotic treated quarters over the first 4 weeks of the dry period. Treated quarters had a lower (p<0.05) incidence of new clinical infection during the ensuing lactation and lower somatic cell counts. This did not affect production levels of milk, milk fat or protein. The results clearly indicated a prophylactic benefit for the dry cow antibiotic treatment against new Streptococcus uberis infections during the dry period.

Efficacy of intramuscular oxytetracycline as a dry cow treatment for Staphylococcus aureus mastitis

To determine the efficacy of intramuscular oxytetracycline as a supplemental dry cow treatment for Staphylococcus aureus mastitis, 37 Holstein cows were randomly assigned to two treatment groups: intracisternal infusion with a commercial preparation of cephapirin benzathine at drying off (20 cows) and infusion with cephapirin benzathine at drying off and intramuscular oxytetracycline at 11 mg/kg once daily on d 7, 8, 9, and 10 after drying off (17 cows). Milk samples collected 7, 14, 30, and 60 d after calving were plated for bacterial isolation within 24 h after collection and after 24 to 72 h of storage at -20 degrees C. Quarters were defined as infected if S. aureus was isolated from the fresh and frozen cultures from any one sample collected before drying off. An infected quarter was defined as cured if S. aureus was not isolated from the fresh or frozen culture from milk samples obtained following calving. The rate of cure by 30 d after calving for systemic oxytetracycline (in combination with cephapirin treatment) was 29.4% for infected quarters and 29.4% for infected cows, compared with 27.5 and 25.0%, respectively, for the cephapirin treatment only. Results including the culture at 60 d after calving were 21.2 and 22.5%, respectively, for combination therapy and cephapirin therapy only. Systemic oxytetracycline, in combination with intramammary dry cow treatment, did not improve the rate of cure for S. aureus mastitis.

Antibiotic resistance patterns and pigment production of streptococci of serological group B isolated from bovines and humans

All 127 streptococci of serological group B isolated from bovines and humans were resistant to bacitracin (0.04 U) and gentamicin and susceptible to penicillin. Of the bovine isolates examined 10%, 8%, 22%, 2% and 30% were resistant to bacitracin (10 U), cefoxitin, clindamycin, erythromycin and tetracycline respectively. Of the human isolates 52%, 21%, 38%, 3% and 65% were resistant to bacitracin (10 U), cefoxitin, clindamycin, erythromycin and tetracycline respectively. Most of the bovine strains were susceptible, most of the human strains were resistant to bacitracin and tetracycline. A relation of tetracycline resistance to group B streptococcal microcapsule was proposed. Pigmentation studies revealed generally a weak pigmentation of bovine and a strong pigmentation of human group B streptococci. Antibiotic resistance patterns together with the ability to form a yellow to orange-red pigment might be used for characterization of individual cultures of group B streptococci.

Udder health in the periparturient period

The periparturient period is associated with rapid differentiation of secretory parenchyma, intense mammary growth, copious synthesis and secretion, and marked accumulation of colostrum and milk. Udder health during this time is an important factor associated with the production of maximum quantities of high quality milk. Intramammary infections that occur during the dry period can adversely affect udder health, resulting in decreased milk production, altered milk composition, and impaired mammary function. Bovine mammary glands are markedly susceptible to new infections during the periparturient period, especially prior to parturition. Many infections that occur at this time are associated with clinical mastitis during early lactation. Methods of controlling mastitis in the dry period have focused primarily on the use of antibiotics. However, antibiotic therapy at drying off does not appear to prevent new infections in the periparturient period. This is most likely due to lack of persistence of antibiotics. Furthermore, antibiotics used currently are less effective against environmental pathogens, in particular coliform bacteria, which can cause a high proportion of intramammary infections during the periparturient period. Methods of controlling bovine mastitis during the periparturient period is an important area that requires additional research. Procedures need to be developed that are effective against a variety of bacteria, including environmental mastitis pathogens, if additional control is to be achieved.

Teat skin normal flora and colonization with mastitis pathogen inhibitors

Isolates of bacteria from normal teats were used to attempt colonization of teats of dry cows or neonatal calves. Isolates for inoculation were chosen on the basis of ability to inhibit mastitis pathogens in vitro, with the ultimate goal of in vivo inhibition of mastitis pathogens at the teat surface. Three bacterial normal flora isolates (Corynebacterium xerosis, Bacillus sp. and Aerococcus viridans) persisted less than 10 days on the teats of dry cows. The fourth isolate, Staphylococcus hominis 1, was studied in greatest detail because studies characterizing the normal teat flora showed staphylococci to be the predominant flora. The S. hominis 1 isolated used for inoculation was an inhibitor of Gram-positive mastitis pathogens. It was a biotype not found on these teats prior to inoculation, thus facilitating identification of the inoculated isolate on sequential sampling. Colonization of newborn calves, before other bacterial flora became established, resulted in recovery of inoculated S. hominis 1 for an average of 51 days or longer. On dry cow teats it was detected for up to 28 days. On several occasions the inoculated S. hominis 1 was found in pure culture. Since many new infections occur during the dry period, the colonization of dry cow teats with S. hominis 1 organisms inhibitory for Gram-positive pathogens should be tested as an adjunct to other methods of mastitis prevention.

Management of dry cows to reduce mastitis

The objective of mastitis control during the dry period is to have as few infected quarters as possible at calving. This depends on enhancing elimination of infections present at drying off and on reducing the new infection rate during the dry period; prevention of new infection appears to offer greater long-term benefits. Incidence of new infection is high in the dry period with periods of high susceptibility shortly after dry off and again prepartum. Pathogens of both contagious and environmental origin cause new dry period infections; however, exposure to contagious pathogens probably decreases with cessation of regular milking, whereas exposure to environmental pathogens continues throughout the dry period. Varying susceptibility over the dry period may be affected by bacterial loads on the teat skin, characteristics of the teat canal, and internal protective mechanisms. At present, antibiotic therapy at the end of lactation is the most effective means of eliminating existing infections and preventing new infections. Although there are reasons to prefer selective therapy, present evidence favors a recommendation for treatment of all cows at the time of drying off. A shortcoming of present therapy regimens for the dry period is that they provide little or no protective effect against new infection prepartum. Other mastitis control methods and management practices have not been shown conclusively to reduce new dry period infections. However, it appears that reduction of exposure to environmental pathogens during dry period should be recommended. More effective means to reduce new infections in the prepartum period are needed.

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%).

Effects of antibiotic treatment of nonlactating dairy cows on antibiotic resistance patterns of bovine mastitis pathogens

Antibiotic resistance patterns of the major groups of bovine mastitis pathogens (Streptococcus agalactiae, other streptococci, Staphylococcus aureus, and Staphylococcus epidermidis) were examined by determining the minimum inhibitory concentration (MIC) of 13 different antibiotics against bacterial isolates from dairy cattle. The bacterial strains were obtained from milk samples from each cow in 21 New York state dairy herd surveys. In 12 herd surveys (high antibiotic-use group), all 365 cows received antibiotic infusions into the udder at the cessation of each lactation cycle. The 324 animals in the other nine herd surveys (low antibiotic-use group) did not routinely receive antibiotics during the nonlactation period. The MICs from the two groups were compared by calculating for each bacterial group the average MIC, the antibiotic concentration necessary to inhibit 90% of the isolates, and the antibiotic concentration necessary to inhibit 50% of the isolates. Increased resistance to all 13 antibiotics was observed with Streptococcus agalactiae isolates from the high antibiotic use herds. However, there was relatively little difference between the two groups in the resistance patterns of the other bacterial species examined. The most important finding of the study was the identification of a multiple beta-lactam resistance phenotype in Streptococcus agalactiae.

Mastitis--progress on control

Mastitis has been investigated for over 100 yr, but progress in control has been slow. The reasons for this are complex. Mastitis is caused by several types of infection, each with a different etiology. The possible methods for control are eradication, immunization, therapy, breeding resistant cows, or by improved management factors, but in practice success has been achieved only with the latter. The first practical control was based on specific hygiene methods coupled with the improved use of antibiotic therapy. Later this was augmented by better designs of milking equipment and improved housing. By the control method it is possible to eradicate or reduce to few cows infections caused by staphylococci, Streptococcus agalactiae, and Streptococcus dysgalactiae, but control of Streptococcus uberis is much less effective and coliform mastitis is unaffected. Research indicates that these varying degrees of success are due to basic differences in the various types of infection. There is poor control with infection caused by pathogens that emanate from sources other than the mammary gland and multiply mainly in bedding material (environmental pathogens). Further progress probably depends on the degree of exposure of udders, particularly to these pathogens.

Susceptibility of the bovine udder to bacterial infection in the dry period

The teats of 18 cows were externally exposed to infection with mastitis organisms by dipping them daily for 1 or 2 weeks in a mixed suspension of Streptococcus dysgalactiae and Str. uberis at the start, the middle or the end of the dry period. The teat sinuses of quarters which remained uninfected after external exposure at the beginning or the middle of the dry period were then infused with the same strains of streptococci. Of the 9 quarters infected following external exposure 8 occurred in animals exposed at the start and one in the middle of the dry period. Thirty-six new infections occurred after infusion of bacteria into the teat sinuses of 38 quarters. In a further trial with 10 cows, Staphylococcus aureus and Str. zooepidemicus were inoculated into the distal 3 mm of the streak canals of 5 cows immediately after drying-off and into those of 5 cows which had been dry for 28 weeks. Animals were slaughtered 48 h later and infection determined by teat puncture. Five infections occurred in cows which were at the start of the dry period and only one in the cows dry for 28 weeks. It is suggested that these differences in the rates of new infection between the early dry period and a very extended dry period are due to differences in the ease with which bacteria can penetrate the teat canal. At the later stage bacterial growth through the teat canal appeared to be inhibited.

Control of mastitis by hygiene and therapy

Hygiene and therapy are two important components of a program to control mastitis. Conscientious application of these practices significantly reduces intramammary infection, especially when they are applied in concert with superior management. The two components operate independently, and response is maximum when both are applied. Hygiene acts by reducing the frequency of infection. The primary effect of therapy is to increase the rate of eliminating established infections, although dry cow treatment also provides prophylactic benefits. The most effective hygiene and therapy practices are dipping of teats after milking and treating each quarter at the end of lactation.

The susceptibility of milked and unmilked udder quarters to intra-mammary infection

The teats of quarters which were milked, and those in which milking had ceased, of 16 cows were exposed to bacteria twice daily for 2 weeks by dipping them in a suspension of streptococci and staphylococci.

Of 32 quarters in which milking had ceased 22 became infected in this period. Eighteen infections were with Streptococcus dysgalactiae, 2 with Streptococcus uberis, 1 with Staphylococcus aureus and 2 with organisms not used in the culture. Only 3 of 32 milked quarters became infected in the experimental period: 2 infections were with Str. aureus and 1 with an organism not used in the culture. Animals differed in their susceptibility to infection.

Quarters which had been unmilked for 2 weeks were as susceptible to new infectionas those in which milking had just ceased.

In this experiment, factors such as intramammary pressure, leakage of milk, concentration of bacterial inhibitors, composition of secretion and cell counts of the secretion did not appear to influence the rate of infection. Yield at drying-off, milking rates and teat patency were not related to the differences in susceptibility between animals. It is suggested that the main reason for the lower new infection rate in the milked quarters is that pathogens entering the streak canal or teat sinus are usually flushed out with the secretion during milking and do not cause infection.