Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

S. C. Nickerson


Prevalence of mastitis and efficacy of intramammary treatment were determined in unbred and primigravid heifers (n = 116) from 4 dairy herds. Of 97 heifers from which secretion samples were obtained, intramammary infections were detected in 96.9% of animals and 74.6% of quarters. Clinical symptoms were observed in 29% of heifers. Staphylococcus aureus infections were detected in 37.1% of animals. Teat canal colonizations were confirmed in 93.1% of heifers and 70.7% of quarters. The most common isolates from mammary glands were Staphylococcus chromogenes, Staphylococcus hyicus, and Staphylococcus aureus. Average somatic cell counts in secretion from infected and uninfected quarters were 13,574 $\times$ 10$\sp3$/ml and 5,707 $\times$ 10$\sp3$/ml, respectively. Differential cell counts demonstrated that macrophages were the most prevalent cell type in infected and uninfected quarters, followed by lymphocytes and polymorphonuclear leukocytes. A total of 311 isolates from teat canal keratin and mammary secretion samples was tested for susceptibility to 12 antibiotics. Overall, more than 92% of isolates were susceptible to all 12 antibiotics tested. Thirty-five heifers were injected intramammarily with a nonlactating cow antibiotic in all quarters and 38 served as untreated controls. Prevalence of mastitis in treated heifers was reduced from 97.1% at treatment time to 40% at parturition, whereas prevalence in control heifers decreased from 100% to 97.4%. Prevalence of Staphylococcus aureus mastitis in treated heifers was reduced from 17.1% to 2.9%, while in untreated heifers, prevalence decreased from 26.3% to 15.8%. Somatic cell counts at parturition were also lower in treated heifers. Mammary tissues from quarters of unbred heifers infected with Staphylococcus aureus exhibited less alveolar epithelium and lumen, and more interalveolar stroma compared with uninfected quarters and quarters infected with non-aureus staphylococci. Staphylococcus aureus-infected quarters also exhibited greatest leukocyte infiltration into parenchymal tissues and cisternal linings. Results of this study demonstrated a high prevalence of mastitis in heifers, and a high susceptibility of isolates to several antibiotics. In addition, intramammary treatment before parturition was highly efficacious in controlling mastitis and reducing somatic cell count in early lactation.