Chicken infectious anaemia vaccinal strain persists in the spleen and thymus of young chicks and induces thymic lymphoid cell disorders.

The chicken infectious anaemia virus (CIAV) infection may induce immunosuppression and persistent infection. The use of vaccination in young chicks is still controversial due to its low immune efficiency. In order to verify the viral persistency of a vaccinal strain of CIAV and its associated-lymphoid cell disorders, 54 1-day-old specific pathogen free chicks were vaccinated (CIAV-VAC(®); Intervet, Millsboro, Delaware, USA) and haematologic examination, expression of viral VP3 gene, humoral response and phenotyping of lymphoid cells were studied in lymphoid organs at various times post vaccination (p.v.). No clinical signs were observed but light heteropaenia was detected in CIAV-vaccinated chicks. The VP3 gene of CIAV was detected by polymerase chain reaction in the thymus and spleen from day 7 until 28 days p.v. Thymic larger CD4(+)CD8(+) cells increased only at 7 days p.v. while smaller CD4(+)CD8(+) cells decreased after 14 and 28 days in CIAV-vaccinated birds. The CD4 expression, in contrast to that seen for CD8, decreased in thymocytes from the CIAV-vaccinated group. In the spleen and bursa, the percentage of CD8(+) cells increased at 7 and 28 days p.v. only, while CD4(+) cells decreased simultaneously. The vaccinated chicks also exhibited a higher number of splenic CD3(-)CD8(+) cells (natural killer cells). The anti-CIAV antibody responses, however, remained low in most vaccinated chicks and did not persist up to 18 days p.v. These results suggest that the vaccinal virus strain is clinically attenuated but persists in the thymus and spleen in some birds, inducing a low humoral immune response and altering thymopoiesis.