Effect of litter size and birth weight on naturally occurring myopia in the Labrador retriever

Abstract

Purpose To evaluate early environmental influences (e.g. litter size, birth weight, birth season) on adult refractive error in dogs. A previous familial aggregation analysis has shown that the distribution of refractive error in a large family of pedigree Labrador Retrievers has a significant genetic component, but that litter size and other residual/environmental factors also have significant effects. Methods Refractive error was measured by cycloplegic retinoscopy in both eyes of 166 dogs, 1-8 years of age and free of ocular pathology, from a large family of pedigree Labrador Retrievers. All dogs originated from the same breeding colony. The early records of these dogs included information on birth weight, maternal litter cohort, litter size and neonatal weight gain, measured daily for the first 6 weeks. These factors were analyzed to investigate their effect on adult refractive error. Results The average adult spherical equivalent refraction (SER) was -0.44D (-5.38D to +1.65D, n = 166): 35% were myopic (SER ≤ -0.50D), 58% emmetropic (SER = -0.49 to +0.99) and 7% hyperopic (SER ≥ +1.00D). Mean birth weight was 421±57g. Higher birth weight was weakly (R=0.3) correlated with more hyperopic adult refractions. Relative to large litters (≥ 7), dogs from small litters (< 7) gained more weight within the first 6 weeks of life and were on average 0.43D more myopic. Conclusion The dog provides a unique model for studying a wide range of environmental influences on the development of naturally occurring, high prevalence, low degree myopia.