Isolates of Marek's disease virus (MDV) from vaccinated flocks in Argentina were characterized as very virulent (vv) and very virulent plus (vv+) strains. Experimental infection with these viruses caused a high incidence of Marek's disease in both resistant N-2a line and susceptible P-2a line birds. Vaccine viruses from each of the three Marek's disease viral serotypes were evaluated alone and in various combinations for protection against challenge with a vvMDV called NULP-1. Vaccination of P-2a birds with HVT did not protect satisfactorily against any of the vv and vv+MDV strains isolated. However, CVI988/Rispens vaccine alone or combined with serotype 2 and/or serotype 3 vaccine strains enhanced protection significantly against NULP-1. Serotype 2 plus serotype 3 vaccines also provided significant protection when challenged with this strain. This is one the first reports of the occurrence of vvMDV and vv+MDV in Argentina and Latin America. It is also a preliminary evaluation of the synergistic protective effect of different vaccine viruses with local MDV strains. However, further studies are needed to evaluate the real role of these and other Marek's disease isolates in 'vaccination failures' and the influence of serotype and virus strain on synergism between Marek's disease vaccine viruses.

SUMMARYThree chicken infectious anaemia virus (CIAV) isolates were obtained from broiler flocks with anaemia and poor performance, and were designated AIP-1, 2 and 3. All isolates were resistant to treatment with chloroform and were able to pass through 50-nm pore-size filters. The CIAV isolates induced signs and lesions of chicken infectious anaemia (CIA): thymus atrophy, bone marrow aplasia, low haematocrit values, and body weight reduction when inoculated into 1-day-old susceptible chicks. Microscopic lesions were a reflection of macroscopic observations. CIAV-specific antigens have been demonstrated in tissues of experimentally-infected chicks using monoclonal antibodies specific for CIAV. The presence of antibodies against CIAV in breeders was determined by indirect immunofluorescence tests. Although the chicks derived from breeder flocks that possessed antibodies against CIAV, they were not refractory to infection. These findings, the characteristics of the virus and the virus-induced lesions, demonstrate that CIAV is present in Argentina.