Co-infection of Chicken Infectious Anaemia Virus (CIAV) and Marek's Disease Virus (MDV) in SPF chicks reveals altered viral dynamics, severe immunopathology and compromised host immune responses.

Co-infections with immunosuppressive viruses remain a major constraint to poultry health, yet their interactive pathogenesis is poorly understood. This study delineates the virological and immunopathological interplay between Chicken Infectious Anaemia Virus (CIAV) and Marek's Disease Virus (MDV) in specific pathogen-free (SPF) chicks during early life. Seventy-two-day-old chicks were allocated into four groups: CIAV (day 1 infection), MDV (day 14 infection), CIAV + MDV (sequential co-infection), and uninfected control; and evaluated for survivability, viral load and gene expression, cytokine responses, histopathology of lymphoid organs and antibody response against CIAV on 7th, 14th, 28th and 42nd days post CIAV infection. Co-infection resulted in significantly reduced survivability (27.78%) compared to single infections (66.67% in CIAV and 88.89% in MDV) by the end of 6 weeks. CIAV replication was markedly enhanced in co-infected birds, whereas MDV load declined, indicating possible depletion of MDV target cells by CIAV-induced immunosuppression. Viral gene expression (VP3 and MEQ) corroborated tissue-specific viral kinetics. Histopathological analysis revealed profound lymphoid depletion and degenerative changes, most severe under co-infection. Immunologically, CIAV and co-infected groups exhibited significant downregulation of key cytokines (IL-1β, IFN-γ, IL-2, IL-4) and diminished humoral responses. In contrast, MDV infection alone induced significant upregulation of IFN-γ and IL-2 expression, which was markedly attenuated during co-infection with CIAV. Collectively, these findings demonstrate that CIAV-driven immune suppression reshapes MDV pathogenesis and host immune responses, leading to early chick mortality and immune dysfunction in surviving chicks. The study provides novel insights on the progression of concurrent viral infections in chicks and highlights the need to explore other concurrent viral infections likely to inflict in field conditions so as to design effective control strategies for poultry health.