| The purpose of the studies included in this dissertation were: to evaluate the relationship between shedding of etiologic agents and clinical signs of disease, to investigate the effect of passive immune status on the expression of clinical disease, and to examine the effects of environmental and managerial determinants on neonatal dairy calf mortality rates. The studies were done using 452 calves from 48 dairy herds which represented a stratified random sample of Ohio dairy producers.;Significant findings included identification of Salmonella in 10 calves from 7 herds. Serotypes identified include: S. enteritidis, S. typhimurium, S. mbandaka, S. montevideo, S. dublin, and S. agona.;Another significant finding was identification of the following environmental and managerial determinants of neonatal mortality: herd size, days on a nipple feeder, navel disinfection, type of housing, and whether or not each calf observed with diarrhea was treated with antibiotics. These factors explained approximately 39% of the variation in mortality between herds.;Studies examining the relationship between shedding of etiologic agents and disease showed that many calves shed coronavirus, and cryptosporidia without clinical signs of disease. Diarrheal prevalence was highest in the second week of life, however, with the exception of cryptosporidia in the second week of life, more calves with normal feces shed coronavirus and cryptosporidia than did calves with diarrhea. No statistically significant relationships were found between shedding of coronavirus, cryptosporidia, or rotavirus, either singly or in combination, and diarrhea or increased rectal temperature. Coronavirus and cryptosporidia shedding varied by herd size but with no clear pattern. No association between neonatal calf mortality rates and prevalence of rotavirus, coronavirus, and cryptosporidia shedding could be demonstrated in herds with at least 10 calves tested.;Forty-four percent of calves tested had serum IgG levels less than 5 mg/ml. Calves with low levels of IgG were more likely to exhibit diarrhea, regardless of whether they were shedding rotavirus, coronavirus, cryptosporidia, or none of the agents. Calves with low levels of IgG were no more likely to shed rotavirus, coronavirus, or cryptosporidia, than calves with serum IgG levels higher than 5 mg/ml. |
