Use of antimicrobials for food animals in New Zealand: updated estimates to identify a baseline to measure targeted reductions.

AIM: To describe the use of antimicrobial drugs for food animals in New Zealand, based on sales data reported to government for 2005-2018, to provide a baseline to determine the success of measures to reduce antimicrobial use for food animals and to compare usage to selected European countries. METHODS: Data were sourced from official government and industry reports to update previous estimates of use (as amount sold) of antimicrobial products applied to animals in New Zealand. The data included antimicrobial sales and animal populations, weighted where appropriate by breed and age class. Antimicrobial use was estimated based on the amount of active ingredient sold, per kg of animal biomass standardised to the probable weight at time of treatment or lifetime average but not slaughter weight (population correction unit; PCU). New Zealand data for 2017 and 2018 were adjusted to account only for antimicrobials used for farm animals by discounting horticulture use, companion animal use and export. RESULTS: Between 2014 and 2018 the estimated usage of antimicrobials in animals flattened to a 5-year rolling average of 10.40 mg/PCU. New data on use for companion animals and on exports of zinc bacitracin and tylosin, included previously in the gross New Zealand values, allow a more refined estimate of use of antimicrobials in food animals of 10.21 mg/PCU in 2018, the third lowest rate of use for countries reporting by the same methodology. The intensive industries of pigs and poultry combined use more than 500 mg/PCU whereas the extensive red meat and dairy industries use an estimated 6.25 mg/PCU. New Zealand uses proportionally more cephalosporins and macrolides, two of the critically important antimicrobial groups, when compared with European countries, but less quinolones. The most obvious difference is the extensive use of zinc bacitracin and tylosin fed to pigs and poultry in New Zealand but not in the European Union. CONCLUSIONS: Use of antimicrobials in food animals has stabilised to a rate of approximately 10.2 mg/PCU. This baseline should be used by the animal health industry to measure future success in its efforts to reduce and make more refined use of antimicrobial drugs as New Zealand works to deliver the government's Antimicrobial Resistance Action Plan. High rates of use of zinc bacitracin and some critically important macrolides represent clear targets in attempts to reduce usage.

Evaluating a commercial PCR assay against bacterial culture for diagnosing Streptococcus uberis and Staphylococcus aureus throughout lactation.

The performance of a commercial, real-time PCR assay was compared with traditional bacterial culture for the identification of Streptococcus uberis and Staphylococcus aureus in bovine milk collected at different stages of lactation. Initial validation tests using fresh and frozen quarter milk samples identified factors that affected the success of the PCR. Therefore, the standard protocol was adjusted for samples collected at the first milking postpartum (colostrum) and from clinical mastitis cases. The adjustment involved PCR testing both undiluted and diluted (1 in 10 with sterile water) DNA extracts. The performance comparison between culture and the PCR assay used milk samples collected aseptically from individual quarters of mixed-age spring-calving dairy cows, during early, mid, and late lactation. Bacterial culture results were used to select a subset of samples for PCR testing (n = 315) that represented quarters with a current or prior Strep. uberis or Staph. aureus infection. Compared with culture, PCR had a sensitivity of 86.8% and specificity of 87.7% for detecting Strep. uberis (kappa = 0.74) and 96.4% and 99.7%, respectively, for detecting Staph. aureus (kappa = 0.96). The dilution of DNA extracts for colostrum and clinical samples increased the relative sensitivity from 79.2% to 86.8% for Strep. uberis detection and from 92.9% to 96.4% for Staph. aureus, presumably through diluting unidentified PCR inhibitors. The sensitivity for detecting Strep. uberis using PCR, relative to culture, was similar throughout lactation (85-89%), whereas relative specificity was lowest immediately postcalving (64%) but improved in mid and late lactation (98%). Specificity estimates for samples collected in early lactation can be optimized by reducing the cutoff cycle threshold (Ct) value from the recommended value of 37 to 34. Although using this value improved specificity (77%), it reduced test sensitivity (77%). The PCR assay lacked agreement with culture in early lactation, specifically for diagnosing Strep. uberis. Thus, PCR should not be used as the only tool for diagnosing mastitis in early lactation.

Use of antimicrobials for animals in New Zealand, and in comparison with other countries.

AIM: To describe the use of antimicrobial drugs for food animals in New Zealand, based on sales data reported to government, changes over time, and in comparison with other countries and human use. METHODS: Data were sourced from official government and industry reports covering 26 European countries, Australia, Canada, New Zealand and the United States of America in 2012, the last year data were available for all countries. The data included antimicrobial sales, and animal and human populations. Antimicrobial use was estimated based on the amount of active ingredient sold, per standardised biomass (population correction unit). RESULTS: The estimated usage of antimicrobials for food animals in New Zealand for 2012 was 9.4 mg active ingredient/kg biomass. Total sales of antimicrobials between 2005-14 increased on average by 2.5% or 1.5 tonnes per year. Over the same time total animal biomass decreased by an estimated 4.3%, with the main decrease being in sheep (25%) and beef cattle (17%), while dairy cattle increased (28%). In the countries examined, the estimated usage of antimicrobials in food producing animals in 2012 varied from 3.8 to 341 mg active ingredient/kg biomass, in Norway and Italy, respectively, with use in New Zealand being the third lowest. Usage of antimicrobials for human health in New Zealand in 2012 was estimated at 121 mg active ingredient/kg biomass, being ranked sixteenth of the countries compared. Use in humans was 12.9 times the use in animals. CONCLUSIONS: New Zealand was the third lowest user of antimicrobials in animal production and used much less than in human medicine. This is the first report of baseline data which may be used by the New Zealand animal health industry to develop, and measure success in, approaches to maximise the life of antimicrobials for animal health and welfare. CLINICAL RELEVANCE: New Zealand veterinarians will soon have to make changes to adopt the World Health Organisation's global action plan to manage antimicrobial resistance. Having a benchmark of current antimicrobial use will inform priorities and allow measurement of the impact of future programmes.

Concentrations of buparvaquone in milk and tissue of dairy cows.

AIM: To determine the concentration of the anti-theilerial drug buparvaquone in the milk and tissue of dairy cattle following treatment with two different formulations, and to assess the effect of clinical theileriosis on the concentration of buparvaquone in milk. METHODS: Healthy lactating dairy cows (n=25) were injected once (Day 0) I/M with 2.5 mg/kg of one of two formulations of buparvaquone (Butalex; n=12 or Bupaject; n=13). Milk samples were collected from all cows daily until Day 35. Five cows were slaughtered on each of Days 56, 119, 147, 203 and 328, and samples of liver, muscle and injection site tissue collected. Milk samples were also collected from cows (n=14) clinically affected with theileriosis for up to 21 days after treatment with buparvaquone. Milk and tissue samples were analysed by liquid chromatography-mass spectrometry; limits of detection (LOD) were 0.00018 mg/kg for muscle and 0.00023 mg/L for milk. Concentrations of buparvaquone in milk and tissues were log10-transformed for analysis using multivariate models. RESULTS: In healthy cows, concentrations of buparvaquone in milk declined with time post-treatment (p<0.001), but were above the LOD in 11 of 25 cows at Day 35. Concentration in milk was higher one day after treatment in cows treated with Butalex than in cows treated with Bupaject, but not different thereafter (p=0.007). Concentrations of buparvaquone in muscle were below the LOD for four of five animals at Day 119 and for all animals by Day 147, but were above the LOD at the injection site of one cow, and in the liver of three cows at Day 328. Tissue concentrations did not differ with formulation nor was there a formulation by time interaction (p>0.3). Concentrations of buparvaquone in the milk of clinically affected animals were not different from those of healthy animals at 1 and 21 days post-treatment (p=0.72). Between 21 and 25 days post-treatment concentrations were below the LOD in 9/14 milk samples from clinically affected cows. CONCLUSIONS: Detectable concentrations of buparvaquone were found in the milk of some cows for at least 35 days and in the liver and injection site of some cows until at least 328 days after injection. There were no biologically meaningful differences in milk or tissue concentrations between the formulations, or in the milk concentrations for cows that were clinically affected compared with those that were healthy at the time of treatment.

Peripartum infection with Streptococcus uberis but not coagulase-negative staphylococci reduced milk production in primiparous cows.

The effect of an intramammary infection (IMI) at calving on the milk yield of heifers during their first 200 d in milk (DIM) was estimated by comparing monozygotic twins, where one member had a naturally occurring IMI detected at the first milking after calving and the other twin did not. Data collected weekly over a full lactation for 29 twin pairs were used to estimate the effects of a peri-calving Streptococcus uberis IMI on milk yield and composition. Data for 19 twin pairs were used to estimate the effects of pericalving coagulase-negative staphylococci (CNS) IMI. A heifer with a Strep. uberis IMI produced 200 kg (7%) less milk during the first 200 d of lactation compared with her uninfected twin, with significant differences evident throughout the 200-d period. Similar milk losses were recorded for heifers that developed CM or remained subclinical. An elevated milk SCC for infected heifers was only apparent for the first month (d 2-30), although SCC tended to remain high during the second (d 31-60) and third (d 61-90) months. Milk protein concentrations were greater in the Strep. uberis-infected twin over the 200-d period, whereas fat and lactose concentrations showed little change. An IMI caused by Strep. uberis was associated with a lower milk yield, whereas an IMI by CNS was not, despite CNS-infected twins having a higher SCC than their uninfected twin for the first 30 d of lactation.

Vaccines against bovine mastitis in the New Zealand context: what is the best way forward?

Mastitis is an important animal health disease which constitutes a serious problem for the dairy industry in New Zealand. Mastitis reduces milk yield and quality, necessitates the use of antibiotic therapy, with associated risks of contaminating the raw milk supply, and imposes a serious economic burden, currently estimated at NZ$300 million per year. Mastitis is caused by a variety of infectious agents. In the New Zealand context, with cattle grazing on pasture, Streptococcus uberis is a major bacterial pathogen, responsible for a significant proportion of clinical cases, especially during early lactation and the dry period. Other pathogens of significance include Staphylococcus aureus, Streptococcus dysgalactiae and Escherichia coli, as well as so-called 'minor pathogens', namely coagulase-negative staphylococci(CNS). Current strategies aimed at reducing cases of mastitis include improved hygiene in the farm environment, particularly with regards to the health and cleanliness of teats. Once mastitis occurs, antibiotic therapy is a favoured option, and as a prophylactic tool, in the form of dry-cow therapy, has also shown value. Prevention of mastitis using immunological tools such as vaccines lags behind the major vaccine breakthroughs that have been achieved in preventing and/or reducing the severity of numerous infectious diseases in animals. In this review, the current state of research in the area of development of vaccines against mastitis is summarised, with particular emphasis on bacteria important to the dairy farming industry in New Zealand. Few, if any, effective vaccines have been designed to prevent or mitigate intramammary infections. It is argued that novel approaches must be considered to search for vaccine candidates, and vaccines need to be designed and constructed within the special framework of their uses, in the mammary gland which offers a unique immunological environment. In addition, effective vaccines against mastitis due to Strep. uberis may be more likely to emerge from strategies that target the cell-mediated arm of the immune response rather than strategies that target specific antibody responses.

Variation in rubber chemistry and dynamic mechanical properties of the milking liner barrel with age.

The milking liner is the interface between the milking machine and the cow. Liner properties important to milking performance were investigated for liners of different ages using discriminating tests rather than the normal, rubber-industry quality control-based tests. Large variations in the liner mechanical properties occurred depending on where the sample was taken; stiffness increased 4-fold 40 to 50 mm below the top of the liner. This was related to changes in the chemistry of the rubber created by absorption of milk-derived products (MDP) into the rubber and losses of formulation components, particularly 50% of the plasticizer and all of the antidegradent 40 to 50 mm below the top of the liner, with age and use. The presence of MDP leads to calcium and phosphate deposits on the inner surface of the liner barrel where the MDP was absorbed. The detailed liner properties can be used to explain the forces on the cow's teat and its reactions and effects on milk flow behavior, and to guide future liner development.

Preventing bovine mastitis by a postmilking teat disinfectant containing acidified sodium chlorite.

A split-herd study was performed to determine if an acidified, sodium chlorite teat disinfectant, UDDERgold Platinum Germicidal Barrier Teat Dip (UG Pt, Ecolab Inc., Redmond, WA), was effective in preventing new intramammary infections (IMI) in lactating dairy cows compared with a licensed, iodophor teat disinfectant (Iosan, Novartis Animal Health, Ltd., Whittlesford, UK), and to show that the test product was tolerated equally well by teat skin. The study lasted 114 d and covered all weather conditions. The teats of 176 cows were dipped after each milking in UG Pt and the teats of 172 cows were dipped in Iosan, the positive-control product. Routine milk samples were taken from each quarter of every cow every 4 wk. Additional samples were taken from newly calved cows joining the trial and from cows with clinical signs of mastitis. Milk samples were cultured for the presence of bacteria and the cause of clinical mastitis. Each quarter was eligible for only 1 infection during the trial. The number of clinical cases was identical in each group (n = 13) and the number of subclinical infections was slightly lower in the UG Pt group than in the Iosan group (n = 27 and 31, respectively). These rates of infection suggest that the products did not differ in their ability to prevent a new IMI. At least 203 cows were assessed for skin integrity before the start of the trial and every 28 d throughout. The UG Pt teat dip had no adverse effects on teat condition. The prevalence of hyperkeratosis did not change with time for both groups (0.90 +/- 1.08 and 0.95 +/- 1.06 at wk 0 vs. 0.65 +/- 0.87 and 0.49 +/- 0.74 at wk 16 for fore and hind teats, respectively, for UG Pt and 1.02 +/- 1.25 and 1.16 +/- 1.11 at wk 0 vs. 0.51 +/- 0.71 and 0.45 +/- 0.65 at wk 16, respectively, for Iosan); no redness of the skin was observed in either group. Application of recommended statistical methods to demonstrate noninferiority was problematic.

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.

Errors associated with milk cell counting.

The accuracy of somatic cell counts in milk samples was investigated in four studies. First, the counts recorded by one milk buyer in one supply over six months ranged from 105,000 to 401,000 cells/ml with no apparent changes in the volume of milk consigned or the level of mastitis in the herd that would explain this wide range. Secondly, the counts in daily samples from one bulk milk supply for 28 days ranged from 84,000 to 282,000 cells/ml, again with no apparent changes in the performance of the herd to explain the wide range. Thirdly, the replicated counts recorded for one sample by three separate laboratories agreed closely; however, when a sample with a high cell count was interspersed then two of the three laboratories reported high cell counts suggestive of 'carry-over' in excess of the 2 per cent 'allowable' Finally, cell count data from three separate laboratories on samples from 21 cows for 33 days revealed problems with the misidentification of samples on the farm in 1 per cent of the samples, and misidentification and mishandling of 1 to 2.6 per cent of the samples in the laboratories. All three laboratories differentiated samples from cows with subclinical and clinical mastitis, but the mean cell count of the uninfected cows varied between the laboratories with one of them recording statistically significantly higher counts over the period.

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.

The effect of an intramammary teat seal on new intramammary infections.

As concern over the possible overuse of antibacterials increases, attention has focused on reduction of antibiotic usage and on nonantibiotic alternatives. A nonantibiotic intramammary teat sealant, Teat Seal (Cross Vetpharm Group Ltd., Tallaght, Dublin, Ireland), has been available in Ireland, in combination with an intramammary tube of cloxacillin. Teat Seal has been reformulated for use in cows with low cell counts as an alternative to antibiotic dry cow therapy at the end of lactation. The product is now marketed as Orbeseal (Pfizer Animal Health). A comparison between this teat sealant and no treatment was made on new intramammary infections and clinical mastitis, on all cows within four herds, and on low cell count cows in three herds. No cases of clinical mastitis in the dry period were observed in cows treated with Teat Seal (n = 197), whereas a significant number (6 cows) were observed in the untreated cows (n = 204). In all herds, significantly more new infections at calving were found in the untreated group (62 cows in the untreated group compared with 21 cows in the Teat Seal group). In those quarters where infections were first detected at calving, the incidence of clinical mastitis was significantly greater in the untreated group. Quarters in both treatment groups that were infected at drying off with Corynebacterium spp. or coagulase-negative staphylococci were not protected against new infections and had an increased risk of new infection by Streptococcus uberis. The results will inform those restricting their use of antibiotic dry cow therapy in alternative management strategies and the additional risk of new intramammary infection.

Evaluation of measurements of the conductivity of quarter milk samples for the early diagnosis of mastitis.

Measurements of the conductivity of quarter milk samples were made in 31 cows in a 70-cow herd in southeast England, for a period of 15 weeks. Over this period, 42 per cent of cow-weeks and 20 per cent of quarter-weeks had an increase in quarter milk conductivity of 10 per cent of more compared with the mean conductivity of the previous 14 milkings. Fourteen per cent of quarter-weeks had an increase in conductivity of 15 per cent or more. The geometric mean somatic cell count (cell count) was higher in quarter-weeks with a 10 per cent or greater increase in conductivity than in quarter-weeks with a conductivity change of less than 10 per cent. At a conductivity threshold of 10 or 15 per cent and a cell count threshold of 200,000 or 400,000 cells/ml the specificity of this system was estimated to be 85 to 92 per cent, the sensitivity 40 to 54 per cent, the negative predictive value 87 to 93 per cent and the positive predictive value 33 to 55 per cent. The positive predictive value of the individual quarter milk conductivity was insufficiently accurate to be used as the sole criterion for the selection of quarters for early antibiotic treatment.

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.

Changes in cow teat tissue created by two types of milking cluster.

We have investigated the responses of cow teats to machine milking in a study of relatively newly installed commercial milking parlours fitted with one of two types of milking cluster. The first was a common type with a large claw volume (> 200 ml), 15-16 mm i.d. long milk tube, 10 mm short pulse tube, cluster weight < 3-2 kg and used alternate pulsation. The second was a more traditional type with a 150 ml claw bowl volume, 13.5 mm i.d. long milk tube, 8 mm short pulse tube, cluster weight approximately 3.5 kg and used simultaneous pulsation. We scored approximately 50 cows in each of 20 herds, all within 60 s of cluster removal, for changes from the premilking teat condition: teat colour (creation of reddening or blueness), firmness, thickening at the base of the teat associated with the position of the liner mouthpiece, and whether the teat duct orifice was open. There were statistically significant differences in the proportion of cows displaying these four alterations in teat condition between herds using the two types of cluster. The more common type of cluster was always associated with better teat condition. The cause and effect of poorer teat condition have not been fully established and are likely to be multifactorial. The principal risk factors may be cluster weight, overmilking, vacuum applied during the overmilking phase and the design of the liner mouthpiece.

Detection of antimicrobial substances in individual cow and quarter milk samples using Delvotest microbial inhibitor tests.

The use of antibiotic therapy to treat and prevent udder infections of cows during the dry period is a key component of mastitis control in many countries. At the same time, the general public is becoming increasingly aware of potential hazards from antibiotic residues in foods. Consequently, Delvotest Cow Test (Royal Gist-brocades NV, Delft, The Netherlands), an on-farm version of Delvotest P, a microbial inhibitor test for antimicrobials, is being increasingly used by farmers to assess that milk from individual cows is fit for consignment to the bulk tank. Occasional reports of unexplained positive test results have led to suggestions of possible false-positive reactions in milk from individual cows. To investigate the potential causes of such positive test results, three separate investigations were undertaken. In a field survey of unexplained positive reports from farmers, 14 milk samples from six farms that tested positive were all found to contain antibiotic residues. In more formal investigations of individual quarter milk samples from an experimental herd, none of 134 milk samples from midlactation cows yielded positive reactions; for cows that had just calved, 16 of 144 milk samples were positive, and, of those, 13 had somatic cell counts > 4,000,000/ml. Natural inhibitors were responsible for 1 positive reaction, 8 positive reactions were related to incomplete milking, and 7 samples contained beta-lactam antibiotics. Positive reactions caused by antibiotic persisted in individual quarter samples for up to 7 d postcalving compared with 4 d for milk samples from the whole udder. Delvotest was sensitive to cephalonium, the active ingredient of Cepravin Dry Cow (Mallinckrodt Veterinary Ltd., Uxbridge, United Kingdom), which is the market-leading product in the United Kingdom. Test results yielded a partial purple color reaction in the presence of 8 micrograms/kg of cephalonium and a completely purple reaction at 16 micrograms/kg. These results confirm the validity of Delvotest when used to examine composite milk samples from individual cows supplying the United Kingdom dairy industry and suggest that, with proper attention to milk withdrawal periods and complete milking, there is no obvious risk of antibiotic contamination of milk.

Comparison of treatment of mastitis by oxytocin or antibiotics following detection according to changes in milk electrical conductivity prior to visible signs.

Mastitis was induced in dairy cows by infusion of 500 cfu of Streptococcus uberis into the mammary gland. Most infections developed to clinical disease, and the majority were predicted by changes in the electrical conductivity of the foremilk. The benefits of clinical prognosis and bacteriological cure were determined for cases that were treated when predicted to develop into clinical mastitis and compared with cases that were allowed to develop until milk clotted or until pyrexia before intramammary antibiotic treatment was used. Treatment prior to clinical mastitis included use of intramammary antibiotic or intramuscular oxytocin to allow stripping of residual milk to remove bacteria. All infections in which treatment was delayed resulted in clinical mastitis that was cured clinically and bacteriologically by sustained treatment using a broad-spectrum intramammary antibiotic preparation once daily but requiring a mean treatment time of 10 d. It was possible to prevent clinical mastitis from developing and to eliminate all infections in cows that were treated early when the developing disease was predicted by changes in the electrical conductivity of quarter foremilk and was treated aggressively by administering an intramammary antibiotic at each milking for 3 d. Treatment of 20 IU of oxytocin at six successive milkings of cows that were predicted to develop disease eliminated 25% of the infections, but 75% of the cows developed clinical mastitis. Those cases were resolved by sustained daily treatment using the same intramammary antibiotic. Elimination (100% clinical and bacteriological cure) of all infections caused by Strep. uberis was possible with early and aggressive or sustained use of the intramammary antibiotic. The early intervention using an intramammary antibiotic, when infection was first indicated by changes in the electrical conductivity of milk, was the most efficient method to achieve cure and led to quicker recovery of milk quality to a saleable standard.