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American Journal of Epidemiology Vol. 117, No. 5: 621-631
Copyright © 1983 by The Johns Hopkins University School of Hygiene and Public Health

research-article

A PROSPECTIVE INVESTIGATION OF BOVINE LEUKEMIA VIRUS INFECTION IN YOUNG DAIRY CATTLE, USING SURVIVAL METHODS

MARK C. THURMOND1, KENNETH M. PORTIER2, DANIEL M. PUHR1 and MICHAEL J. BURRIDGE1

1Department of Preventive Medicine, College of Veterinary Medicine, University of Florida Gainesville FL. (Reprint requests to Dr. Thurmond at current address: Veterinary Medicine Teaching and Research Center, University of California, School of Veterinary Medicine, 18830 Road 112, Tulare, CA 93274
2Department of Statistics, University of Florida Gainesville, FL

The authors studied bovine leukemia virus infection in cattle bom between July 1, 1979, and June 30, 1981, and followed up to 27 months in the University of Florida Dairy Research Unit herd, a 200-cow milking herd. Cattle were tested monthly for antibodies to the virus by agar-gel immunodiffusion with the glycoprotein-51 antigen. Of 473 live calves entering the study at birth, 54 became infected during the course of the study. Rates of detection of bovine leukemia virus infection were examined using survival methods. Plotted cumulative hazard rates revealed a nonlogarithmic; survivorship function and three age-specific detection phases. These phases were 0–6 months, 7–16 months, and 17–26 months of age with estimated incidence rates of 5.25, 2.14, and 14.37 new detections per 10, 000 cattle-days at risk, respectively. Significant differences were found between incidence rates of the first and second phase (p = 0.0168) and between those of the second and third phase (p < 0.0001); corresponding relative risks were 0.41 and 6.71, respectively. The reduced Incidence rate of the second phase coincided with movement of calves from a crowded calf barn to pastures. The increased incidence rate of the third phase coincided with mixing of bred heifers with bovine leukemia virus-infected dry cows. Increased incidence rates did not coincide with common-needle vaccination or artificial insemination. Evidence was not found for milk-borne Infection. Age-specific detection rates were not associated with dam parity (p = 0.93), dam age (p = 0.79), breed (p = 0.60), or consumption of colostrum from cows infected with the virus (p = 0.23). However, detection rates were lower in cattle bom to bovine leukemia virus-infected cows than In those born to noninfected cows (p = 0.03).

bovine leukemia virus; cattle; prospective studies


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