The frequency of endemic fetal loss in dairy cattle: a review.

Estimates of the frequency of endemic fetal loss range from 0.4 to 10.6%, a 26-fold difference, in 26 studies of dairy herds over 5 decades. Sources of this difference include breed, geographic, study population, case definition and procedural differences. The definition of fetal loss was inconsistent among the studies and was often not clearly stated. Twelve of the studies cited only visually observed losses, while the remainder reported losses of all known pregnancies but using different gestational at-risk periods. Definitions of the resulting frequency measure and methods of computation varied widely among the studies. Fetal loss frequencies have been reported as ratios, proportions, incidence rates and cumulative incidence rates. Denominators used in frequency calculations have included the number of pregnant cows, number of cows in the breeding herd, number of cows that calved before or after the risk period and number of calves born. These procedural differences make comparisons among studies and an overall estimate of an expected frequency of endemic fetal loss in dairy cattle difficult at best. Procedural and definition differences aside, the median frequency of fetal loss from studies enumerating only observed abortions was 1.95% and from the studies that included both observed and unobserved abortions 6.5%. Based on this difference, approximately only 30% of endemic fetal losses are observed visually.