Environmental chemicals in dogs: evidence review and new data linking dietary exposure and reproductive performance.

The domestic dog is proposed as a sentinel species for the adverse reproductive effects of environmental chemicals (ECs), sharing the human environment and some components of dietary exposure, whilst exhibiting similar reproductive changes as humans. Here we present a narrative review of published information incorporating new data that potentially link EC exposure to changes in reproductive function in the dog. We compare these findings with similar trends in humans and other species. Our conclusions include; (1) within a single UK assistance-dog breeding population, a decline in sperm motility in Golden Retrievers but not Labradors, and an increase in cryptorchidism in Labradors and crossbred dogs that persisted after accounting for genetic effects; (2) detection of di(2-ethylhexyl) phthalate (DEHP), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs) in commercial dry and wet dog food and identification of the same chemicals in dog testes, and of PCBs and PBDEs in the ejaculate; (3) new data on concentrations of PCB and PBDE congeners in bitch's milk, in which, as in human breast milk, PBDE 47 was the dominant PBDE congener; (4) a within-individual association between testicular EC concentrations and histopathological severity in dogs across different geographical locations; (5) no evidence of declining reproductive performance in female dogs over the same period, in contrast to the changes observed in males. These observations support further evaluation of the dog as a sentinel species for human exposure to environmental chemicals and identify possible future research opportunities.