Relationship between placental characteristics and puppies' weight in toy and small sized dog breeds

Introduction: Puppy mortality is reported to be a significant problem in the dog and can be above 20%, when no supervision at delivery, additional nursing or hand rearing are performed (1). Low birth weight identifies puppies at higher risk and might be caused by placental factors, such as the extension of the exchange area and the degree of vascularization (2). The relation between the newborn puppy and the features of its placenta has been poorly investigated in the dog. Methods: Twenty bitches, 9 toy- sized (i.e. < 5Kg) and 11 small- sized (i.e. 5 to 10 Kg), were included in this study. During natural whelpings or c- sections, puppies were identified and their order of birth, sex and weight were recorded. Puppies weights were registered at birth (D0) and daily until Day 6 (D6). Placentas were weighed after trimming of extraplacental membranes and umbilical cord; a single picture was taken and assessed using the NIS- Elements Br Microscope Imaging Software. The Total Placental Area (TPA), the Transfer Zone Area (TZA) and the Marginal Hematoma Area (MEA) were calculated and their surface expressed in mm². Immunohistochemistry using monoclonal mouse anti CD31 antibody (Cat. n° sc-101861, Santa Cruz Biotechnology; TX, USA) was used to identify fetal and maternal vessels in the placental labyrinth zone. A vascularization index (VI) was determined for each placenta and the Total Vascular Area (TVA) was estimated. Puppies birth weight values were classified into quartiles, separately from small and toy breeds. The first quartile (Q1) represents the lowest 25% of registered values. Results: Puppy's birth weight was positively correlated with placental weight (p < 0.001, r = 0.689). A positive correlation was found between puppy's birth weight and TPA (p < 0.001, r = 0.786), TZA (p < 0.001, r = 0.772), and TVA (p < 0.001, r = 0.482). Furthermore, a positive correlation was found between placental weight and TPA (p < 0.001, r = 0.661), TZA (p < 0.001, r = 0.583), and TVA (p < 0.001, r = 0.333). In small-sized breeds, the placentas of Q1 puppies had a lower weight and a smaller TZA and TVA compared to not- Q1 ones (p < 0.05). In toy breeds no differences in placenta characteristics of Q1 and not- Q1 puppies were observed. The VI was higher in toy-sized compared to small-sized bitches placentas (p < 0.01). No effect of breed size, parity, litter size, or sex of the puppy was observed on early growth rate. As in humans (3), placental weight and the extension of the transfer zone correlates closely with placental total vascular area which can be considered a significant determinant of puppy's birthweight in normal pregnancies. In toy- sized breeds, the greater capillary density observed might support the development of fetuses proportionally bigger compared to small-breed ones. Reference values for placental weight, total placental area, transfer zone area, total vascular area and index of vascularization were described in toy and small-sized dog breeds