The Extent of Placental Pathology Is Negatively Correlated to Birth Weight in Ewes Infected with the Wild-Type Strain of Chlamydia Abortus

The placenta is the organ that allows the exchange of oxygen and nutrients between maternal and foetal blood, supplying the requirements of the growing foetus. Consequently, any factor that alters placental integrity may affect foetal nutrition, viability and lamb birth weight. Reproductive diseases, such as ovine enzootic abortion (OEA), impact on foetal viability due to the reduction in the functional area for maternofoetal exchange. This study aimed to consider the impact of pathological features of OEA placental lesions on lamb birth weight and indirectly on foetal viability. To investigate the relationship between birth weight and various OEA-related parameters, data from 562 lambs/foetuses from animals experimentally challenged with Chlamydia abortus strain S26/3 and from uninfected animals were analysed. The parameters investigated included the number of foetuses/lambs delivered (single/multiple), foetus/lamb sex, length of gestation, the proportion of placentas affected by lesions (percentage of gross placental pathology), foetal viability (live/aborted) and the number of C. abortus organisms shed in post-parturition vaginal excretions. The results suggest that the length of gestation and the proportion of placentas affected by lesions are the main contributors to birth weight variability, whereas the other factors, including foetal viability (live or aborted outcomes), were found to be less relevant co-variables. The study determined the strongest positive and negative correlations between birth weight were with the length of gestation and the extent of placental pathology, respectively. These results may indicate that economic losses associated with OEA infections result not only from aborted foetuses but also from the surviving lambs that are born weaker and consequently are more susceptible to diseases.