Efficacy of a herd-specific vaccine against Staphylococcus aureus to prevent post-partum mastitis in dairy heifers

A placebo-controlled field study was performed to evaluate the effect of a herd-specific vaccine against Staphylococcus aureus on intramammary infection (IMI), somatic cell count (SCC) and clinical mastitis. Three hundred and twenty-one heifers were assigned to two groups. Heifers in the vaccination group (n = 164) were vaccinated twice, i.e. 5 and 2 weeks before their expected calving date. Animals of the control group (n = 157) received the same treatment with a placebo containing no bacterial antigen. Quarter milk samples were collected immediately after parturition prior to the first machine milking, 3-4 weeks after calving and before the onset of treatment in animals with signs of clinical mastitis during the first 3 months after calving. For comparison of SCC the data from the monthly milk test records were evaluated. The prevalence of S. aureus in quarter milk samples taken at calving and 3-4 weeks post-partum did not differ significantly between the vaccine and control group. Incidence of clinical mastitis during the first 3 months after calving and the prevalence of S. aureus in quarter milk samples taken before the onset of treatment did not differ significantly between the groups. The SCC was lower in vaccinated than in control heifers. However, the difference was only significant on the third milk test day. Regarding prevalence of IMI with S. aureus and incidence of clinical mastitis the use of a herd-specific vaccine against S. aureus did not prove to be efficient on this farm.     

Dynamics of Staphylococcus aureus infections during vaccination with an autogenous bacterin in dairy cattle

The effect of an autogenous vaccine against Staphylococcus aureus on S. aureus prevalence and mastitis, as well as on somatic cell count (SCC), was studied in a dairy herd with a high prevalence of S. aureus. The vaccination group (n = 35; 22 cows and 13 heifers) and the control group (n = 36; 23 cows and 13 heifers) received the vaccine or a placebo, respectively, according to the following protocol: all animals: basic immunization (twice, 3 weeks apart); cows: booster dose at the time of drying off, 5 and 2 weeks before calculated calving date; heifers: booster dose 2 and 5 weeks before calculated calving date. The vaccine or the placebo was administered subcutaneously in the area of the supramammary lymph nodes. Quarter milk samples were collected monthly and subjected to SCC and bacteriological evaluation. At this time, the animals were also checked for signs of clinical mastitis. Non-clinical S. aureus mastitis diagnoses were based on udder quarter SCC and a positive S. aureus culture. In order to compare the SCC in individual whole milk samples, records from the monthly milk quality testing were evaluated. Cow and udder quarter prevalence of S. aureus intramammary infections calculated for the experimental animals and quarters, respectively, did not differ between groups. However, during the lactation period following the boostcr dose, the prevalence of S. aureus increased in both groups (P < 0.05). The cumulative incidence of various mastitis diagnoses (clinical, subclinical, latent infection) due to S. aureus on an animal basis did not differ between groups. On an udder quarter basis, the cumulative incidence of subclinical mastitis was higher in vaccinated animals than in control animals (33.8 versus 26.0%; P < 0.05). This was mainly due to a higher cumulative incidence of subclinical mastitis in vaccinated than control heifers. The SCC in composite milk samples did not differ between groups, but increased as lactation progressed. The herd prevalence of S. aureus differed considerably throughout the study period, but declined consistently to below 10% at the end of the study period. Recent herd checks revealed a prevalence of S aureus infections of < 5%. It is concluded that the autogenous bacterin tested in this study did not have the desired effect on the prevalence of S. aureus infections and mastitis or SCC. The decline in S. aureus prevalence was very probably due to other factors than specific immunization against S. aureus.     

Vaccination against experimental staphylococcal mastitis in ewes

Experiments were carried out in ewes using a new vaccine developed for the prevention of mastitis caused by Staphylococcus aureus. The vaccine comprised three major components: (i) killed S aureus cells which had been cultured to induce synthesis of pseudocapsule; (ii) toxoided staphylococcal beta haemolysin and (iii) the adjuvant dextran sulphate. Ewes systemically vaccinated twice during pregnancy developed significantly elevated circulating levels of IgG1 and IgG2 anti-pseudocapsule antibody, as well as increased serum titres of anti-beta haemolysin. Five different strains of S aureus were used to challenge both vaccinated and control ewes by the intramammary route during the ensuing lactation. The incidence of acute gangrenous mastitis and nonacute, clinical mastitis was significantly lower in vaccinated than in control groups after challenge with each strain. Vaccinated ewes produced significantly more milk than control ewes after challenge with four of the five strains of S aureus.     

Vaccination against experimental staphylococcal mastitis in dairy heifers

Vaccination-challenge experiments were carried out with dairy heifers using new, killed cell-toxoid-adjuvant Staphylococcus aureus vaccines. The organisms in the vaccines were cultured under conditions which simulated in vivo growth and induced expression of a pseudocapsule. Dextran sulphate which promotes synthesis of IgG2 antibody was included in the vaccines as the primary adjuvant. Vaccinated heifers developed very high levels of both IgG1 and IgG2 anti-pseudocapsule antibody in serum, however, titres of neutralising antibody against toxoided haemolysins were generally low. Vaccinated and unvaccinated control heifers were challenged by intramammary infusion of three virulent strains of S aureus in four experiments. Vaccinated heifers were more resistant to clinical mastitis following challenge than were controls, and the vaccinates had significantly greater milk production than controls following challenge. The most promising vaccine had dextran sulphate combined with mineral oil as the adjuvant injected intramuscularly.     

Development of a Staphylococcus aureus vaccine against mastitis in dairy cows. I. Challenge trials

A vaccine composed of three field isolates of Staphylococcus aureus (S. aureus) derived from cases of mastitis in cows was developed. The vaccine was administered to nine uninfected cows while 10 other cows were used as controls. All cows were challenged with a highly virulent S. aureus strain administered into two quarters of each cow. Quarters were tested for clinical signs, secretion of S. aureus, and somatic cell count (SCC). No systemic effects were observed in any of the cows, vaccinated or control. Vaccinated cows had 70% protection from infection compared with fewer than 10% in the controls. Moreover, all quarters challenged in the vaccinated cows, regardless of whether they were successfully infected or not with S. aureus, exhibited very mild inflammatory reactions, identified by their low SCCs (<100,000).     

The effect of intramuscular and intramammary vaccination of cows on antibody levels and resistance to intramammary infection by Staphylococcus aureus.

Cows were vaccinated simultaneously by the intramuscular and intramammary route with formolised Staphylococcus aureus cells of strains BB, Mexi and 3528. Vaccination resulted in slight increases in serum agglutinin titres, but the levels of the agglutinins in milk and colostrum were not higher in vaccinated cows than in the unvaccinated controls. Vaccination did not result in higher levels of IgM, IgG1, IgG2 or IgA immunoglobulins in serum, colostrum or milk as compared with controls. Vaccinated cows, particularly those in which strain 3528 was used, showed some resistance to infection following challenge with low numbers of viable S aureus Mexi, but there was no resistance to infection when similar numbers of the virulent BB strain were used for challenge. It is concluded that vaccination is unlikely to prevent infection of the bovine udder by S aureus.     

Effect of prepartum intramammary treatment with pirlimycin hydrochloride on prevalence of early first-lactation mastitis in dairy heifers

OBJECTIVE: To determine whether prepartum intramammary treatment of dairy heifers with pirlimycin hydrochloride would reduce the prevalence of intramammary infection (IMI) and lower the somatic cell count (SCC) during early lactation or improve 305-day mature equivalent milk production. DESIGN: Prospective clinical trial. ANIMALS: 183 Holstein-Friesian heifers (663 quarters) from 2 dairy farms. PROCEDURE: Heifers were assigned to treatment and control groups. Treated heifers received a single 50-mg dose of pirlimycin in each mammary quarter approximately 10 to 14 days prior to parturition. Prepartum mammary gland secretions and postpartum milk samples were collected for bacterial culture. Postpartum milk samples were also collected for determination of SCC or California mastitis testing and were tested for pirlimycin residues. Mature equivalent 305-day milk production data were recorded. RESULTS: Treated heifers in herd A had a higher overall cure rate, higher cure rates for IMI caused by coagulase-negative staphylococci (CNS) and Staphylococcus aureus, lower SCC, and lower prevalence of chronic IMI, compared with control heifers. Treated heifers in herd B had a higher overall cure rate and cure rate for IMI caused by CNS, compared with control heifers, but postpartum California mastitis test scores and prevalence of chronic IMI did not differ between groups. Mature equivalent 305-day milk production did not differ between herds or treatment groups. No pirlimycin residues were detected in postpartum milk samples. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that prepartum treatment of dairy heifers with pirlimycin may reduce the prevalence of early lactation IMI, particularly IMI caused by CNS, without causing pirlimycin residues in milk.     

Control of heifer mastitis: antimicrobial treatment-an overview

Initial studies in Louisiana, USA to determine the prevalence of mastitis in breeding age dairy heifers demonstrated that intramammary infections (IMIs) were present in 97% of heifers and 75% of quarters. Most common isolates were Staphylococcus aureus, Staphylococcus hyicus, and Staphylococcus chromogenes; somatic cell counts (SCCs) ranged from 12.4 to 17.3 x 10(6)ml(-1). Histologic examination of Staph. aureus-infected quarters demonstrated significant reductions in alveolar epithelial and luminal areas, and increases in connective tissue and leukocytosis, illustrating limited secretory development and marked inflammation. A one-time infusion of various nonlactating cow antibiotic preparations into infected quarters during different stages of gestation but >45 days prepartum resulted in cure rates for Staph. aureus IMI of 67-100%. Mean SCC was 50% lower at calving for treated heifers, and milk yield over the first 2 months of lactation was 10% greater than that of untreated controls. Subsequent multiple herd studies, however, revealed that use of nonlactating cow therapy was beneficial only in herds exhibiting a high prevalence of heifer mastitis and not in low prevalence herds. Results of lactating cow antibiotic therapy infused 1-2 weeks prepartum demonstrated cure rates of 59-76% vs. 26-31.7% in untreated controls. In some studies, milk production during the first lactation in treated heifers was approximately 10% higher than untreated controls, and SCC were significantly lower; however, in other studies, prepartum treatment was successful in reducing prevalence of infection but had no effect on SCC or milk yield during the subsequent lactation. Thus, treatment of heifers is advantageous because the cure rate is much higher than during lactation, there is no milk loss, and risk of antibiotic residues minimal; however, successful therapy may not necessarily result in lowered SCC and increased milk production in all herds.     

Efficacy of intramammary treatment in unbred and primigravid dairy heifers

A total of 73 breeding-age and primigravid Jersey heifers in 4 herds was randomly allotted to treatment and control groups according to expected calving date. Thirty-five heifers were injected intramammarily with a nonlactating cow product containing penicillin/streptomycin. Thirty-eight heifers served as untreated controls. Of the 35 treated heifers, 34 (97.1%) were infected at time of treatment. In the untreated control group, all 38 heifers (100%) were infected at treatment time. At parturition, prevalence of intramammary infection in treated heifers decreased to 40%, whereas in the control group, prevalence remained about the same (97.4% of heifers). Prevalence of Staphylococcus aureus mastitis in treated heifers was reduced from 17.1% to 2.9% after treatment. In the control group, prevalence of S aureus mastitis decreased from 26.3% to 15.8%. Heifers treated during the second trimester of pregnancy had the greatest reduction in prevalence of mastitis and in somatic cell count at parturition, compared with controls. Findings indicated that intramammary treatment during pregnancy in primigravid heifers was effective in reducing prevalence of mastitis and somatic cell counts at parturition.     

A comparative randomized field trial on intramammary and intramuscular dry cow antibiotic treatment of subclinical Staphylococcus aureus mastitis in dairy cows

The efficacy of two dry cow treatment (DCT) regimens for subclinical Staphylococcus aureus mastitis was evaluated in naturally infected dairy cows. At dry-off, cows were assigned to two treatment groups by randomized blocks on the basis of parity and somatic cell count (SCC). Two antibiotic DCT regimens were used, namely: (1) a single intramammary infusion containing sodium nafcillin, procaine benzylpenicillin and dihydrostreptomycin; and (2) systemic cefquinome administered intramuscularly, twice at a 24-h interval. In the intramammary (IMM) treatment group, the S. aureus intramammary infection (IMI) rate was reduced from 40% (56/140 quarters) before dry-off to 20% (28/140) after calving. Seventy per cent (39/56) of the S. aureus-positive quarters were negative after calving, and 13% (11/84) of the negative quarters were positive after calving. In the systemic treatment group, the S. aureus IMI rate increased from 39% (29/74 quarters) before dry-off to 55% (41/74) after calving. Twenty-eight per cent (8/29) of the S. aureus-positive quarters were negative after calving and 45% (20/45) of the negative quarters were positive after calving. The odds ratio of an S. aureus-positive quarter being negative after calving in the IMM group relative to the systemic therapy group was 44.6 (95% confidence interval = 2.1-909.1, P < 0.01). Parity, quarter, milk SCC and N-acetyl-beta-D-glucosaminidase were tested in the model, and were found to have no significant effect on S. aureus cure rates or new IMI rates. The IMM treatment resulted in a higher cure rate compared with that observed in previous studies. The very low cure rate after systemic cefquinome treatment was comparable to the spontaneous cure rate observed in untreated controls in previous studies. The unfavourable results of the cefquinome systemic DCT might reflect inadequate pharmacokinetic properties of the drug regarding poor udder penetration in subclinical mastitis and short antimicrobial effect compared with the IMM treatment.     

Effect of intramammary injection of rboGM-CSF on milk levels of chemiluminescence activity, somatic cell count, and Staphylococcus aureus count in Holstein cows with S. aureus subclinical mastitis

The effect of intramammary injection of recombinant bovine granulocyte-macrophage colony-stimulating factor (rboGM-CSF, 400 microg/10 mL) on quarter milk levels of chemiluminescence (CL) activity, and somatic cell count (SCC) and shedding pattern of Staphylococcus aureus was investigated. Ten Holstein cows, naturally infected with S. aureus were used, with either early-stage or late-stage subclinical mastitis. Injection of rboGM-CSF caused a remarkable increase in milk CL activity with a peak at 6 h after the cytokine injection in the early- and late-stage groups. In the early-stage group, milk SCC stayed around preinjection level at 6 h, rose significantly on days 1 and 2, and was followed by a smooth and significant decline to an under preinjection level (below 200 000 cells/mL) on day 7 postinjection. Alternatively, in the late-stage group, milk SCC rose significantly at 6 h after the cytokine injection and maintained high levels thereafter. The milk S. aureus count decreased drastically by the cytokine injection in the early-stage group. The bacterial count was moderately decreased in the late-stage group, but increased back to preinoculation levels on day 7 after the cytokine injection. The results suggest that the rboGM-CSF has a potential as a therapeutic agent for S. aureus infection causing subclinical mastitis of dairy cows, if the cytokine is applied at the initial stage of infection.     

Vaccination of cows with rough Escherichia coli mutants fails to protect against experimental intramammary bacterial challenge

Vaccination of cows with rough Escherichia coli mutants fails to protect against experimental intramammary bacterial challenge. Vaccine A, a heat-killed Re mutant of strain K12, (UB 1574), was administered as a single parenteral and local dose to 5 cows with 3 control animals and Vaccine B, a heat-killed mutant of O111:B4 (J5) was administered as three parenteral doses into 5 cows with 5 control animals. Following intramammary challenge with a smooth wild-type strain (P4), an acute, severe clinical mastitis developed in all 14 quarters (9 vaccine A and 5 vaccine B) of the vaccinated animals which was indistinguishable from that in the 11 quarters of the control animals. Following vaccine B there was an elevation in serum IgG1 and IgG2 antibody to the common core antigen of endotoxin which, in contrast to the control animals, showed a further increase after intramammary infection.     

Effect of a trivalent vaccine against Staphylococcus aureus mastitis lymphocyte subpopulations, antibody production, and neutrophil phagocytosis

The effect of a novel bovine mastitis trivalent vaccine, containing Staphylococcus aureus capsular polysaccharide type 5 (T5), 8 (T8), and 336 (T336), on lymphocyte subpopulations, antibody production, and neutrophil phagocytosis was evaluated. Twenty pregnant heifers were immunized with either the trivalent alone, trivalent emulsified in Freund's incomplete adjuvant (FICA), trivalent in aluminum hydroxide, or adjuvant only (FICA). Immunization was done 30 d before the expected calving date followed by 2 boosts in a 2-week interval. Compared to FICA, serum antigen-specific immunoglobulin (Ig)G1 and IgG2 were significantly increased in all the vaccinated groups before parturition and sustained until 3 wk postpartum. In comparison with the trivalent alone, formulation with either adjuvant enhanced production of IgG2, but not IgG1. Immune sera, which contained the highest amount of antibodies, slightly increased neutrophil phagocytosis to the 3 serotypes of killed S. aureus, but most of the differences were not significant due to large variation between the cows. The percentage of CD4+ lymphocyte was significantly higher in vaccinated groups than that of FICA 4 wk after the primary immunization. In comparison with FICA, cows inoculated with trivalent vaccine and adjuvants had an increased percentage of CD8+ lymphocytes at 2 time points, 2 wk before and after calving. Our results indicated that the whole cell trivalent vaccine, with or without adjuvants, is able to elicit antibody responses specific to the 3 capsular polysaccharide antigens. The increase of T8-specific IgG2 was more noticeable when the vaccine was emulsified with adjuvants.     

Staphylococcus aureus vaccine against mastitis in dairy cows,composition and evaluation of its immunogenicity in a mouse model

Bovine mastitis caused by Staphylococcus aureus (S. aureus) is a most important infection disease that affects both the quality and the quantity of milk production. Antibiotic therapies formulated for intramammary use are generally unsuccessful in eliminating existing S. aureus infections. Vaccination is a logical approach to the control of S. aureus udder infections. However, to date commercially available S. aureus vaccine have shown limited efficacy under field conditions, mainly due to the paucity of information regarding relevant antigens which will induce a broad spectrum immunization. In the present paper the attempt to develop a new vaccine designated MASTIVAC I is described. MASTIVAC I is composed of three strains of S. aureus namely: VLVL8407; ZO3984 and BS449 which were isolated from clinical and sub-clinical cases of bovine mastitis. A mouse model was used to evaluate the S. aureus specific antibody production and protection of mice against virulent S. aureus strains. The results obtained showed that this vaccine exhibits a broad spectrum of antigenic and immunogenic properties that protects mice from homologues and hetrologous S. aureus challenge.     

金黄色葡萄球菌性奶牛乳房炎多价疫苗的研制和免疫效果评价

为研制用于预防金黄色葡萄球菌性奶牛乳房炎的免疫疫苗,使用4株血清型为外膜多糖336型(336PS)、荚膜多糖5型(CP5)和荚膜多糖8型(CP8)强毒力金黄色葡萄球菌试制出金黄色葡萄球菌多价疫苗,并通过抗体水平监测、攻毒保护试验以及两个牧场的田间试验对免疫效果进行了评价。抗体水平监测显示,首免后30 d即可产生较高的抗体水平,二次免疫后有效抗体水平可持续到产后210 d;人工攻毒保护试验显示,疫苗对金黄色葡萄球菌引起的临床型乳房炎的保护率为89%;田间试验结果表明,该疫苗对奶牛临床型乳房炎的保护率在75.2%～83.3%之间,对隐性乳房炎的保护率在49.9%～52.2%之间,免疫组日产奶量较对照组平均提高1.26 kg以上。上述结果表明,所试制的疫苗可以显著降低泌乳牛金黄色葡萄球菌引起的临床型和隐性乳房炎发病率并提高产奶量。     

Effect of intramammary injection of RbIL-8 on milk levels of somatic cell count, chemiluminescence activity and shedding patterns of total bacteria and S. aureus in Holstein cows with naturally infected-subclinical mastitis

Summary The effect of intramammary injection of recombinant bovine interleukin-8 (rbIL-8, 1 mg/10 ml of saline) on quarter milk levels of somatic cell count (SCC), chemiluminescence (CL) activity and counts of total bacteria and Staphylococcus aureus (S. aureus) was investigated, using 10 Holstein cows with an early stage or a late stage of subclinical mastitis naturally infected with S. aureus. In the late-stage group, milk SCC and CL activity had significant rises with maximum levels at 6 h, following maintained high levels thereafter post-cytokine injection. The counts in milk total bacteria and S. aureus were insignificantly decreased, being increased back on day 7 post-cytokine injection. Thus, the cytokine was inefficient for the late-stage subclinical mastitis. However, in the early-stage group milk SCC and CL activity declined to under pre-injection levels on day 7 after marked and significant rises at 6 h and day 1 post-cytokine injection. The milk total bacterial count decreased significantly on days 0.25 and 2. Furthermore, the milk S. aureus count was decreased significantly on days 1, 2, 3 and 7 by the cytokine injection. These results suggest that the rbIL-8 has a potential as a therapeutic agent of the subclinical mastitis of dairy cows, if the cytokine is applied at an initial stage of infection.     

Impact of intramammary infections in dairy heifers on future udder health, milk production, and culling

Dairy heifers represent the future of a dairy herd, and are expected to freshen with a healthy and well-developed udder, capable of producing an optimal amount of high quality milk. A high proportion of heifers have infected mammary quarters at calving, with coagulase-negative staphylococci (CNS) being the most common cause. Staphylococcus aureus and environmental pathogens are also found. The aim of this paper is to summarize how intramammary infections during (late) gestation and early lactation impair the development of the mammary gland and negatively affect future udder health and milk production. Heifers calving with either subclinical or clinical mastitis are also at a higher risk to be culled in first lactation. The magnitude of the effect is most likely related to the virulence of the causative pathogen, the persistence of the infection when milk production has started, and the time of onset of infection. Histological changes in udder tissue from quarters infected with S. aureus are more pronounced than those in udder tissue from CNS-infected quarters. The longer the infections exist and the longer they persist into lactation, the larger the impact on heifers' future udder health and milk production will be. In general, CNS infections are cleared early in lactation and some studies show that CNS do not have a large impact on future milk production and udder health. Future research should elucidate to what extent pathogen-specific as well as host-related factors affect the persistence of IMI in early lactating heifers.     

奶牛乳腺炎金黄色葡萄球菌疫苗研究进展

奶牛乳腺炎是由多种因素引起的严重影响奶牛业发展的重要疾病，至今尚未有彻底解决的办法。金黄色葡萄球菌是引起奶牛乳腺炎的重要病原菌，从安全、经济，高效角度出发，探索防治乳腺炎新方法，是今后奶牛乳腺炎防治研究的新趋势。同时，金黄色葡萄球菌减毒苗在动物模型上很大程度上防御了同源及异源菌株的攻击。文章从目前研究现状和最新进展趋势等方面对奶牛乳腺炎金黄色葡萄球菌疫苗的研究进行综述。     

Development of a Staphylococcus aureus vaccine against mastitis in dairy cows. II. Field trial

A recently described new Staphylococcus aureus vaccine "MASTIVAC I" (Patent no. PTC/IL98/00627) against S. aureus udder infection elicited protection against experimentally induced infection in cows. In the present paper we describe a large-scale vaccination field trial. A total of 452 Israeli Holstein heifers were included in the study over two consecutive years. Approximately half of the heifers (228) were vaccinated while the others (224) served as a control group. Antibody response was detected in all vaccinated animals 4-5 weeks post-primary immunization and it was sustained throughout the experimental period (300-330 days). S. aureus infection could be detected in only 3 out of 228 animals (1.3%) in the vaccinated group and in 6 out of 224 (2.7%) in the control group. These numbers were too low to be statistically evaluated. However, when somatic cell counts (SCC) and milk yields were considered, a significant difference was found between the two groups, namely, the vaccinated cows in first and second lactation had 42 and 54%, respectively, lower SCCs and milk yields 0.5 kg per day higher than the non-vaccinated control cows. These results suggest that the new vaccine elicits a non-specific health improvement of the udder in addition to specific protection against S. aureus.     

Periparturient use of parenteral micronised procaine penicillin to reduce the risk of clinical mastitis in heifers after calving

Mastitis in primiparous heifers immediately postpartum can be both a significant welfare concern and a heavy economic loss. Interventions successfully researched include intramammary therapies. This study considered the clinical and practical effects of a parenteral approach to controlling clinical mastitis in heifers immediately postpartum. The objective of this field trial was to determine whether preventative treatment of heifers with a single parenteral treatment of 15 million iu micronised procaine penicillin within 12h after calving would reduce the incidence of clinical mastitis in early lactation as detected by farmers. All heifers (n=609) calving on three commercial dairy farms in New Zealand during the spring of 2006 were randomly allocated to either treatment or no treatment. Treatment (15 million iu micronised procaine penicillin) was given at the first milking following calving. All clinical mastitis was recorded. Treatment reduced the odds of having clinical mastitis within the first 7 days in milk by over half (Mantel-Haenszel adjusted OR=0.456; p=0.044); and reduced the odds of having mastitis within the first 100 days in milk by just under half (Mantel-Haenszel adjusted OR=0.518; p=0.027). Treatment had a significant effect on increasing the median days to clinical mastitis (p=0.019; beta=1.961, LCI 1.117, UCI 3.445). Preventative treatment of heifers immediately following calving with 15 million iu micronised procaine penicillin parenterally could be of benefit as part of a control programme aimed at reducing the incidence of clinical mastitis in heifers in their first lactation.