Evaluation of genetic, common-litter, and within-litter effects on preweaning mortality in a birth cohort of puppies.

OBJECTIVE: To determine relative impact of genetic, common-litter, and within-litter factors on puppy mortality. ANIMALS: 2,622 Boxer puppies of 413 litters born during a 14-month period. PROCEDURE: For each puppy, pedigree was determined, and litter in which it was born was registered. Overall mortality and mortality per specific cause of death were analyzed by use of a model that included an additive genetic effect, common-litter effect, within-litter effect, and regression of mortality on inbreeding coefficient. Relative importance of the effects was determined from estimates of the variance in mortality explained by each factor. RESULTS: 22% of the puppies died before reaching 7 weeks old. Stillbirth was the most frequent cause of death, followed by infection. Most observed differences were attributable to within-litter factors, which explained 67% of the variance in death attributable to infection and < or = 96% of the variance in death attributable to asphyxia. Common-litter factors were more important than additive genetic factors. Variance attributed to common-litter factors ranged from 2% for cheiloschisis, palatoschisis, or cheilopalatoschisis to 30% for death attributable to infection, and variance attributed to additive genetic factors ranged from 0% for asphyxia to 14% for euthanatized because of white color. Inbreeding coefficient had a significant effect on death attributable to infection, which increased 0.26% for each percentage increase of inbreeding. CONCLUSIONS AND CLINICAL RELEVANCE: Additive genetic factors have less impact on preweaning mortality than common-litter factors, which in turn have less impact than within-litter factors. Mortality attributable to infection increases significantly with increases in inbreeding.