An evaluation of intramammary infection status in dairy heifers
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Contrary to expectation, IMI are prevalent in prepartum heifers. Though awareness of the problem has increased, there are still many questions surrounding the etiology and epidemiology of heifer IMI. For example, there is a paucity of studies evaluating IMI status in heifers prior to late gestation. Therefore, it is not known when heifers become susceptible to IMI or when the greatest risk of IMI occurs during the heifer life cycle. Furthermore, persistence of IMI identified during early lactation has been recently evaluated; however, data on the persistence of species first isolated from prepartum IMI, into early lactation, is lacking. Additionally, it is difficult to compare results between studies since there is no gold standard sampling method in primigravid and nulligravid heifers, no universally accepted IMI definition in heifers, and the methods that have been utilized for bacterial speciation vary in their reliability. The first study of this thesis, Chapter 2, addresses the knowledge gaps surrounding IMI prevalence in primigravid and nulligravid dairy heifers through the evaluation of 4 sampling techniques, including a teat canal swab, mammary secretions collected through the teat end, mammary secretion samples collected by cisternal puncture, and fine needle aspiration of the mammary tissue. Our second study (Chapter 3) aims to access the occurrence of IMI within 8 d postpartum and its relationship to the prepartum prevalence found in the first study. Results of these studies suggest that heifers are potentially susceptible to IMI by 6mo of age and, therefore, investigations are warranted to determine the prevalence and incidence of IMI in primigravid and nulligravid heifers. Furthermore, the IMI occurrence in heifers remained relatively constant between midgestation and 8d post-calving, whereas the IMI occurrence increased between initial sampling and 8d postpartum for the animals initially sampled as mature nulligravid heifers (ages 11-16mo). Therefore, while the focus of many treatment and prevention investigations have been primarily focused on late gestation, earlier treatments may be more effective in reducing the IMI prevalence in early lactation. Overall, diagnostic methods utilizing mammary sampling techniques that bypass the teat canal, such as a cisternal puncture or FNA technique, for prepartum IMI diagnosis may aid in developing a more comprehensive analysis of the etiology and epidemiology of IMI in nulligravid and periparturient heifers.
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M.S.