CIAV is one of the most important ubiquitous immunosuppressive pathogen in chicken, which leads to great economic loss in poultry industry worldwide. Here we experimentally investigated the pathogenesis of two live-attenuated CIAV vaccines. One-hundred-twenty-five, 1-day-old specific-pathogen-free (SPF) chicks were allocated into five-groups (25-chicks-each). Group-1 was orally vaccinated with attenuated CIAV vaccine-strain in drinking water. Group-2 was kept as a contact non-vaccinated-group to group-1. Group-3 was intramuscularly vaccinated with attenuated CIAV vaccine-strain. Group-4 was kept as a contact-non-vaccinated-group to group-3, and group-5 served as a control group. Three-chickens from each group were sacrificed at 3rd, 7th, 15th, 22nd, 29th, 36th and 41st days-post-vaccination (dpv). Tissue specimens from bone-marrow, thymus, spleen, cecal-tonsils, bursa-of-Fabricius and liver were collected for histopathology, lesion-scoring and immunohistochemistry fluorescence detection of CIAV-antigen. Liver-specimens were also collected at 7th, 15th, 21st dpv for real-time-PCR (qPCR), to quantitate the CAV genomic-DNA. Mortalities occurred in both vaccinated groups and their contact groups. Group-4 had the highest no. of mortalities. Frothy diarrhea, ulcer, S/C-hemorrhages, cannibalism and signs of mild depression were the most developed clinical-signs in group1, 2, 3 and 4. Pale bone-marrow and atrophy of thymus were the most characteristic post-mortem changes in group 1, 2, 3 and 4. Microscopically, changes characteristic for CIAV were observed, and the most affected organs were bone-marrow, thymus, liver and spleen; respectively. The demonstrated histopathological lesions started as early as the 3rd dpv in almost all organ of concern in all 4-groups. Immunofluorescence labelling of CIAV-antigen demonstrates virus in all examined organs in both vaccinated and contact groups, with the bone-marrow as the strongest positive. CIAV-genome detected in livers of vaccinated and contact groups at concerned days. Indeed, live attenuated CIAV-vaccines (either by oral or parenteral route) are pathogenic to 1-day-old chicks and they could spread horizontally and transmitted to contact-chicks, inducing mortalities and immunosuppression.