The efficacy of 18 disinfectant teat dips was tested on teats artificially contaminated with a milk suspension of Staphylococcus aureus. A solution of Na hypochlorite with 40 g/l available chlorine was significantly more bactericidal than one containing 1 g/l available chlorine and than most other disinfectants tested. The method was not able to distinguish differences in efficacy between solutions containing 40 g/l and 10 g/l available chlorine nor between these and some of the iodophors containing 5 g/l available iodine. The addition of 190–416 g/l (15–33 % v/v) glycerol significantly reduced the bactericidal properties of 3 iodophors (5 g/l available iodine), but soluble lanolin at approximately 20 g/l did not appear to lower the efficiency of NaOCl (45 g/l available chlorine) or of an iodophor (5 g/l available iodine).

Diagnosis of microbial infections in the udders of cows in commercial dairy farms for large experiments cannot be without error. Limitations of sampling method and routine prevent collection of the necessary information for sure diagnosis. However, with an organized method of repeated bacteriological examinations using consistent and proven methods of aseptic sampling the errors were shown to be very low. A method based on bacteriological tests on aseptic milk samples was used in 32 herds (approximately 2000 cows) for a 3-year period. This is described and examined in terms of other criteria to validate its use in experimental work. With this method it was not difficult to differentiate between those quarters which regularly shed pathogens and those which did not. Other evidence indicated that it was reasonable to assume that this classification accurately distinguished between infected and uninfected quarters. The errors using this method were quite small: when measuring the state of infection of all quarters in the herds the errors did not exceed 1 %. Some small modifications to the method described are suggested to improve further its diagnostic accuracy.

Large differences in new infection rates occurred with half-udder milking machines in which one half of the cluster operated substantially free from vacuum fluctuations while the other half was subjected to various combinations of different types of fluctuation in vacuum. To increase the rate of new infection all teats were contaminated before and after each milking by immersion in a suspension of Streptococcus agalactiae and Str. dysgalactiae. A series of short-duration trials indicated that neither irregular nor cyclic fluctuations in vacuum acting alone were able to increase new infection rate. However, either the Nyhan and Cowhig irregular fluctuation, or a variant of it in combination with cyclic fluctuations in vacuum, was able to do so under the particular experimental conditions used.

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.

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.

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 sensitivities of isolates of bovine staphylococci, corynebacteria and streptococci to various antibiotics were determined. The growth of various strains of Staphylococcus aureus was inhibited by the following range of concentrations of antibiotic: cloxacillin, 0·07 to 0·6 μg/ml; penicillin G, 0·018 to > 250 μg/ml; streptomycin, 1·25 to > 250 µg/ml; novobiocin, 0·15 to 25 μg/ml; chlortetracycline, 0·6 to 10 µg/ml. The concentrations of cloxacillin required to inhibit growth were between 0·15 and 1·25 μg/ml for strains of Streptococcus agalactiae; 0·07 to 0·3 μg/ml for strains of Streptococcus dysgalactiae and 0·15 to 0·6 μg/ml for strains of Streptococcus uberis.

The corynebacteria were generally sensitive to the penicillins (ampicillin, penicillin G, phenethicillin and cloxacillin), but showed least sensitivity to cloxacillin which had a similar activity to that of chlortetracycline, neomycin and oleandomycin. Chloramphenicol and streptomycin were less active than the penicillins and novobiocin showed little activity against Corynebacterium ulcerans.

A clear relationship was confirmed between the sensitivity of staphylococci to cloxacillin and penicillin G. Some indication was found of an association between the sensitivity of staphylococci to cloxacillin and the success of cloxacillin intramammary therapy in the non-lactating udder. There was no evidence of a change in the sensitivity of Staph. aureus following exposure to cloxacillin in the non-lactating udder.

A series of experiments has been carried out in order to develop antibiotic preparations which, when infused into the udder at drying-off, eliminate established infection and prevent new infection from occurring in the dry period. It has been shown that the persistence of antibiotic in the dry udder is affected by the solubility of the antibiotic salt, the quantity of antibiotic infused and the base in which it is formulated. Whereas 0.2 g cloxacillin as the sodium salt infused in a 3% aluminium monostearate in mineral oil base after the last milking of lactation persisted for less than 1 week, 0.5 and 1.0 g cloxacillin in the form of the benzathine salt, in a similar base, usually persisted for 3 weeks.

Secretions removed from the udders of dry cows inhibited the growth of Bacillus subtilis and Sarcina lutea but not 2 strains of staphylococci.

Penicillin G at a dose level of 1 and 5 million units infused in a quick release base or an aqueous solution into quarters of cows after the last milking of the lactation persisted in infused quarters for about 4 days. From 1 h after infusion penicillin was detected in the venous blood and from 2 h after infusion in non-infused quarters.

The effect of bacterial infections, produced by infusions of Streptococcus dysgalactiae and of Staphylococcus pyogenes into one quarter of the udder, on milk yield and composition in heifers has been investigated. After a quarter became infected there was invariably a decrease in the yield of milk and in the concentrations of lactose and potassium, while there was an increase in those of sodium, chloride and non-casein proteins. These effects persisted into the 2nd lactation if the infection was not eliminated. If the infection was eliminated during the 1st lactation or during the dry period before 2nd calving there was a complete recovery in the composition of the milk in the 2nd lactation, but the recovery in milk yield was not complete.

Fifty-seven inoculations of a strain of Staphylococcus aureus were made into the outer 4 mm of the teat ducts of 8 cows. The inocula ranged from 10 to 600 colony-forming units (CFU) and were made with a stainless steel instrument designed to overcome the disadvantages of glass rods and cotton swabs hitherto used.

A single inoculation resulted in either no colonization of the duct, in temporary colonization for up to 7 days, or in colonization followed by intramammary infection.

The recovery of the organisms depended on the size of the inoculum. When 70–100 CFU were placed in the teat duct no organisms were recovered from 12 of 24 quarters after the 1st post-inoculation milking. When 500–600 CFU were used, organisms were recovered from all of 31 quarters for at least 3 milkings, and from 61% for 6 milkings or more, in spite of dipping the teats in a strong disinfectant twice daily.

Intramammary infection developed in 1 of 12 quarters (8%) inoculated in the teat duct with about 600 CFU when the animals were milked twice daily, and in 5 of 19 (23%) quarters if the 1st post-inoculation milking was omitted.

There was no evidence of sensitization resulting from previous infection.

Small numbers of bovine staphylococci, usually less than 30, were infused into the teat cistern of 17 cows secreting less than 100 000 leucocytes/ml milk. Infection was assumed to have occurred if the organisms multiplied sufficiently to be recovered in the foremilk and inflammation was regarded as demonstrated if there was an increase in leucocytes. If the staphylococci were recovered at the 1st post-infusion milking they were consistently recovered thereafter and inflammation always followed but was never evident at the 1st post-infusion milking. Inflammation was usually evident by the 3rd but in one instance was delayed to the 11th post-infusion milking. There was never inflammation in the absence of infection. When there was a high probability that the infusion contained one organism or more, infection occurred in 2 of 10 quarters infused with coagulase-negative staphylococci and 23 of 25 quarters infused with coagulase-positive staphylococci.

A significant positive relationship was found between the number of staphylococci found at the 1st post-infusion milking and the number of leucocytes at the 3rd postinfusion milking.

Small volumes of milk each containing an average of less than 10 colonyforming units of Staphylococcus aureus were injected into the teat cisterns of 39 quarters of 10 cows.

Typical staphylococci were recovered from only 14 quarters following overnight incubation of milk stripped manually from each quarter 10 min after injection. There was an inverse relationship between the total leucocyte count of the milk taken from the quarter before infusion and recovery of staphylococci.

From 4 of 8 quarters from which Staph. aureus could not be recovered by enrichment it was possible to recover the organism by centrifuging the milk and culturing the deposit of leucocytes and bacteria.

It is suggested that release of ingested staphylococci from defunct phagocytes may be important in initiation of infection, failure of antibiotic therapy and recurrence of infection.

When the teats of dry cows were dipped in a culture containing 3–25 × 106 mastitis organisms/ml only 25% of 71 quarters acquired an intramammary infection. Small numbers of organisms applied to teats caused negligible infection. The degree of contamination also determined the survival of the organisms on the exterior of teats, which was much greater at the teat apex than on the body of the teat.

There was no evidence that swabbing the teat apex increased the chance of intramammary infection. Washing the udder of infected cows with a disinfectant increased the chance of recovering the organisms from the teat skin.

A within-cow comparison was made of methods of control of udder infection in dry cows whose teats were deliberately contaminated with cultures of mastitis organisms. Very nearly complete protection against infection with staphylococci and streptococci was obtained by infusion of antibiotics through the teat canal. Disinfection alone, i.e. dipping the teats in 2% chlorhexidine, gave good protection against staphylococci but no protection against Streptococcus uberis.

Data available for fifty cows mainly infected with Staphylococcus pyogenes have been analysed to determine the effects of udder infections and mastitis on milk, fat and solids-not-fat yield and on fat and solids-not-fat percentages over full lactations.

It was found that lactation yields of milk, solids-not-fat and fat were depressed by 10, 11 and 12% respectively in the presence of infection. These depressions were significant (P < 0·05). The composition of the milk averaged over whole lactations was not significantly affected (P > 0·05), although the mean solids-not-fat percentage of the milk was significantly depressed by 0·196±0·092 during the period of infection.

It is suggested that the depressions in lactation yield and changes in milk composition would have been greater in the absence of antibiotic therapy for mastitis.

1. An experiment was carried out to study the effect on udder health and milk yield when half-udders of dairy cows were milked with either a moulded or an extruded teat-cup liner.

2. There was a higher incidence of mastitis in quarters that were milked with moulded than with extruded liners, but the incidence of new infection was similar for each group of quarters.

3. The types of liners used had no measurable effect on teat orifice erosion or leucocyte content of the milk, nor was the decline in milk yield with advance in lactation significantly different for the groups of quarters.

We wish to thank Dr A. T. R. Mattick and Mr A. S. Foot for advice and encouragement, and Miss H. R. Chapman for assistance in the cleaning of teat-cup liners. We are indebted to Miss M. S. Hawkins and Messrs T. M. Higgs and D. S. Simpkin for technical assistance.

A description has been given of a dairy herd, its breed composition, history and management over a period of 8 years. A close study of infection and mastitis was made in the herd, and the bacteriological methods used have been outlined. The data collected have been analysed and the methods of analysis have been described. There were 530 complete lactations for which full records were available and, because of the frequency of milk sampling and the care with which the bacteriological and other tests were made, these data are considered to be suitable for a precise study of the effect on the incidence of infection and mastitis of stage of lactation, season of the year, age and previous infections. These factors will be the subject of a later report.

1. An experimental study was made of the effect of drying-off by ‘stop’ or intermittent milking on the incidence of new infection in the early dry period of dairy cows. Ninety-one and ninety-seven dry periods were examined following the treatments, and the results were analysed on a cow and quarter basis.

2. The incidence of new infection was similar for groups of cows dried off by ‘stop’ or intermittent milking.

3. The quarters of cows that were not infected on drying-off showed a significantly higher incidence (P <0·05) of new infection following ‘stop’ than following intermittent milking.

4. Of the cows that were not infected at the time of drying off, those which had a previous history of infection showed a significantly higher (P <0·01) incidence of new dry-period infection than cows that had not been infected before.

5. There was a much higher incidence of new infection in cows that were already infected on drying-off compared with cows free from infection at that time.

6. About half of all the new dry-period infections persisted until the next lactation, and about one-third of these resulted in mastitis in the affected quarter.

7. The method of drying-off had no effect on infections that were already established in quarters going dry.

8. The types of pathogen causing infection were similar for both methods of drying-off.

9. The method of ending lactation did not affect milk yield in the lactation following the dry period studied.

1. The detailed udder health records of 530 lactations have been analysed on a monthly and lactation basis, so that separate studies of the first infection rate, re-infection rate, total new infection rate and total infection and mastitis incidence for both cows and quarters could be made. These data have been used to study the relationship of stage of lactation, age and season of the year with udder infection and mastitis.

2. With advancing lactation there was a decline in the rate of first infection, reinfection, total new infection and in mastitis in infected cows and quarters.

3. In early lactation there was a marked tendency for infected cows and quarters to develop clinical symptoms.

4. During the last 4 months of lactation there was no significant rise either in infection or in mastitis.

5. There was an increase in infection and mastitis with advancing age, but infected older cows were not more liable to develop clinical symptoms than younger cows.

6. There was an indication that the longer a quarter was free from infection the greater were its chances of remaining so.

7. No major seasonal trends were found in the incidence of udder infection and mastitis, although the incidence was slightly higher in the summer months.