Transmission of relaxin and estrogens to suckling canine pups via milk and possible association with hip joint laxity

Objective - To determine whether abnormal laxity of hip joints of canine pups with genetic predisposition to hip dysplasia (HD+) is related to ingestion of milk-borne hormones. Animals - 7 female Labrador Retrievers with HD+ and 8 with low predisposition to hip dysplasia (HD-) and their offspring. Procedures - Immunoactive relaxin, estrogen, and estrogen precursor concentrations in milk of HD+ lactating bitches and in serum of their pups were compared with those of HD- bitches and pups. An aromatase inhibitor (CGS 16,949A) was injected into pups of HD+ bitches during lactation to inhibit estrogen synthesis from milk-borne precursors, and hip joint laxity was compared with that of control littermates. Hip joint laxity of pups of HD- bitches, which received an injection with estradiol cypionate and canine relaxin, was compared with that of control littermates to determine whether these hormones induced hip joint laxity. Results - High concentrations of estrogens and relaxin were found in milk of HD+ and HD- bitches throughout lactation. Serum concentrations of milk-derived relaxin and total estrogens were similar in all pups, but estradiol-17 beta was detected only in pups of HD+ bitches. Hip joint laxity was reduced in pups that received CGS 16,949A. Hip joint laxity was increased in pups of HD- bitches that received estradiol cypionate and relaxin. Conclusions and Clinical Relevance - Milk-borne maternal hormones and precursors were absorbed into the circulation of canine neonates and may play a role in hip joint laxity in HD+ pups. Phenotypic expression of hip dysplasia may therefore be preventable by antihormone treatment.