Field Evidence for Vertical Transmission of Avian Metapneumovirus Subtype B in Naturally Infected Broiler Breeder Chickens in Egypt

Avian metapneumovirus (aMPV) represents a serious respiratory pathogen of poultry and is associated with considerable economic losses in breeder flocks worldwide. Although horizontal transmission is well established, the contribution of vertical transmission remains poorly understood, especially under field conditions in chickens. In this study, we aimed to assess whether aMPV could be transmitted vertically in unvaccinated broiler breeder flocks that tested positive by PCR in Egypt. Therefore, 10 broiler breeder flocks (≥30 weeks) from seven Egyptian governorates were screened for aMPV subtypes A and B. From each flock, tracheal swabs were collected from breeder hens, along with 20 fertile eggs and 20 newly hatched chicks. All samples, including tracheal swabs, chicken tissues (trachea, lungs, reproductive tract, and spleen), eggshells, internal egg contents, and embryonic tissues were analyzed for aMPV RNA using subtype-specific RT-qPCR. All breeder flocks tested positive for aMPV subtype B, but not subtype A. No aMPV RNA was found in eggshells, internal egg contents, embryonic tissues, or tissues from newly hatched chicks. In conclusion, Despite PCR detection of aMPV in breeder hens, the absence of viral RNA in eggs and their progeny provides field evidence that vertical transmission of subtype B is unlikely to play a significant role in virus spread in commercial broiler breeder flocks. These results support horizontal transmission as the primary route of aMPV spread and highlight the need for further longitudinal and genomic studies to better elucidate aMPV transmission dynamics.