Pigeon circovirus (PiCV) is a widespread circovirus of domestic pigeons and is frequently discussed in relation to young pigeon disease syndrome. However, PiCV detection alone does not establish disease causation, since viral DNA can be found in both clinically normal and diseased birds. This review reorganises current evidence around viral genomic diversity, transmission ecology, pathogenesis, immune-cell infection, host immune response, and diagnostic interpretation. Whole-genome studies show that PiCV populations are genetically diverse and frequently recombinant, especially in dense management systems where young pigeons from different sources are housed together. Although experimentally validated PiCV pathotypes are not yet established, genomic data provides a basis for testing whether particular lineages, recombinants, or capsid variants differ in tissue tropism, replication efficiency, immune recognition, or disease association. Pathological and immunophenotyping studies support a model in which PiCV has tropism for the bursa of Fabricius and other lymphoid tissues, is associated with lymphoid depletion, and preferentially affects B-cell-rich compartments. This can compromise humoral immune competence and may help explain the frequent occurrence of secondary or concurrent infections in clinically affected young pigeons. Diagnostic interpretation should therefore integrate viral load, tissue distribution, histological lesions, immune-cell findings, and clinical context rather than relying on PCR positivity alone. Future studies should link viral genomics with transcriptomics, immune phenotyping, spatial pathology, and longitudinal clinical outcomes.
Harvey M. Santos (Mon,) studied this question.