Interaction between Marek's disease and chicken infectious anaemia viruses

The inoculation of three-day-old SPF chicks with a suspension of lymphocytes obtained from five-month-old layers showing "break of immunity" due to Marek's disease (AZ9/98 isolate) induced a syndrome characterized by stunted growth, anaemia, low haematocrit values at 14 and 21 days (< 20%), early mortality (also up to 100% in less than 20 days). The lesions were bone marrow anaemia, total thymus and marked bursa atrophy. Severe immunodepression with frequent bacterial infections was also evident. Contemporary presence of MDV and CIAV in AZ9/98 isolate was demonstrated by inoculation on tissue cultures of MDCC-MSB1 and CKC and, indirectly, by inoculation of chicks bearing CIAV maternal antibody. No infection with IBDV and Reovirus was demonstrated. In two experiments, a bivalent vaccine (HVT Fc126 + MDV CVI 988) against MD, administered to one-day-old chicks, prevented early mortality due to exposure to AZ9/98 isolate. In a first experiment the protection against MD was very high (90-95%) after exposure to AZ9/98 isolate 7 days, by injection or by contact. In a second experiment, the protection reached 70% with exposure at 6 days and 43% with exposure at 3 days to CIAV and at 6 days to the AZ9/98 isolate, by injection. While the unvaccinated birds showed generalized MD lesions, the vaccinated ones showed, mostly, only paralysis with nerve lesions. This research let us conclude that in AZ9/98 isolate: 1) MDV had a normal virulence (v), according to Witter's classification; 2) CIAV had a very high virulence, because of high mortality even when injected alone; 3) MD vaccination prevented not only MD, but also the pathogenic effect of CIAV at different degrees.