To determine pre and post calving major infectious microorganisms in dairy heifer mammary glands, a study was conducted for a period of 18 months in 6 large industrial dairy farms in Mashhad, north east of Iran. Composite sterile milk samples were taken on day 5±5 before and 10±5 after calving (168 and 157 samples, respectively) from 168 Holstein dairy heifers. Heifers had no apparent pre-parturient problems. Milk samples were refrigerated and sent to the food microbiology lab of the faculty of veterinary medicine for culture and antibiotic sensitivity test. At the time of sampling, CMT was done for all the quarters and positive samples were sent to the lab for SCC. If a heifer was showing signs of clinical mastitis before or during parturition, milk sample was taken and sent to the microbiology lab. Washing, drying, milking and teat deeping was practiced regularly in all the farms engaged in the research. Pre-calving milk samples had either thick, thin, or normal consistency. Normal samples had mostly milky-creamy appearance. Average milk pH of the 3 groups had no significant difference (p>0.05) but, pH variations in the samples from normal group were much less than the other 2 groups. Thick and thin samples had a pH of 6.74 and 7.31, respectively. Mean TBC of the thick and thin samples were 3.1 × 105 and 4.9 × 105, respectively. Mean TBC of the thin samples was different from the other two groups (p<0.05) but it was not different between normal and thick samples (p>0.05). Thirty five percent of the pre-calving and 47.11% of the post-calving milk samples contained mastitis bearing bacteria and the difference was significant (p<0.05). Post calving samples had lower but not significant SCC than the pre calving samples (p>0.05). Mean SCC of the samples infected with coagulase negative Staph. chromogens, Hyicus, Haemolyticus, Saprophyticus and Epidermidis, were lower than those infected with Staph. aureus. Mean SCC in E. coli containing samples collected after calving was lower than those samples containing Staph. aureus. Mean SCC in post calving samples from quarters infected with Staph. chromogens, hyicus and aureus were, 7.8 × 105, 8.5 × 104 and 9.2 × 106, respectively. In conclusion, a high proportion of the peri-parturient heifers had infected quarters. Coagulase negative Staph., Staph. aureus, Enterobacter sp. and E.coli were important microorganisms causing IMI in heifers.

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