Date of Award
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Animal Nutrition and Food Science
First Advisor
John W. Barlow
Abstract
While all dairy producers rely on best management practices to support cow health, mastitis control is of the utmost importance in the prevention of intramammary infections (IMI) on organic farms. Understanding the epidemiology of mastitis pathogens leads to more effective measures which prevent or limit transmission of IMI. This work sought to better understand the epidemiology of the most relevant pathogens causing IMI in organic dairy cows in Vermont.
An observational study of organic dairy herds was conducted to identify differences in udder health outcomes between herds using different housing systems. For most of the udder health metrics and the two udder hygiene measures studied, herds using a bedded pack system either performed slightly better or were equivalent in comparison to the most commonly used facility types in Vermont. The diversity of bacterial species most frequently causing IMI on 10 organic farms in Vermont were then identified. Species from the genus Staphylococcus were responsible for the majority of mastitis in these herds. S. chromogenes was the most frequently found, followed by S. aureus. The diversity of staphylococcal species observed and the species-level effect on inflammatory reaction (as measured by somatic cell count, or SCC) was similar to conventional herds. Compared to no growth quarters, SCC was higher in quarters infected with nine of 10 staphylococcal species identified. Although increase in SCC was modest for most staphylococcal species, their widespread nature can still result in sizeable increases in bulk tank SCC. A large amount of variability was observed in SCC associated with S. chromogenes IMI, with some causing an elevation comparable to that of major mastitis pathogens and others similar to no growth quarters. The majority of S. chromogenes IMI were persistent. A study to better understand the epidemiology of this subgroup was completed, with two categories selected from persistent S. chromogenes IMI: 15 associated with high SCC and 15 with low SCC. Representative bacterial isolates from all 30 IMI were submitted for whole genome sequencing and strain-typed. Particular strain types of S. chromogenes were not found to be associated with either SCC category. Ten different strain types were identified, four of which were newly-described. The only antimicrobial resistance gene identified was blaZ, encoding for resistance to penicillin (10 isolates). Neither overall number of virulence factors nor blaZ carriage was found to be a significant predictor of SCC category. blaZ carriage, number and type of virulence factor appeared to be a function of strain type.
This work generated foundational knowledge about staphylococci causing mastitis on organic dairy farms in Vermont. Mitigating the effect of mastitis caused by staphylococci through targeted prevention and control measures helps dairy producers achieve quality price premiums and results in a higher-quality product for consumers.
Language
en
Number of Pages
305 p.
Recommended Citation
Jeffrey, Caitlin Elizabeth, "Udder Health On Organic Dairy Farms In Vermont: A Focus On The Epidemiology Of Staphylococci Causing Intramammary Infections In Dairy Cattle" (2024). Graduate College Dissertations and Theses. 1975.
https://scholarworks.uvm.edu/graddis/1975