During 1994-1999, infectious agents associated with different disease conditions were investigated in three separate outbreaks of disease in southern Thailand. The first outbreak was in native chickens from Nakhon Si Thammarat province resulting in sudden death with liver and kidney congestions. The second was in 38-day-old broilers from Krabi province. The lame birds showed signs of depression and bilateral hock joints swelling. The last case was in 19-week-old laying chickens from Phang-nga province manifested by depression, paleness and greenish-diarrhea. The causative agents were isolated in embryonating chicken eggs and chick embryo liver (CELi) cells. A characteristic cytopathic effect (CPE) of multinucleated syncytial cells and progressive detachment of cells from the monolayer into culture fluid was apparent in the first passage in CELi cells within 24 hours postinoculation (PI). The isolates were adapted to replicate in Vero cells and the CPE characterized by focal areas of cell fusion occurred 48 hours PI. The indirect fluorescent antibody test demonstrated viral antigens characterized by granular fluorescent masses in the cytoplasm of large multinucleated syncytial cells in both cell types. Cross-virus neutralization test revealed an antigenic relationship between the three separate isolates and avian reovirus strain S1133. Transmission electron microscopic study of 3 agents showed the nonenveloped, icosahedral particles, 60-80 nm in diameter with a double-capsid shell and it formed crystalline arrays in the cytoplasm of infected Vero cells. The viruses designated NK 917/ 37, Kb 538/40 and Pn 1212/42 were classified in the family Reoviridae. Coagulase-positive Staphylococcus aureus were also recovered from the lame bird of the second outbreak and considered as a secondary invader. These findings confirmed a variety of clinical signs caused by avian reovirus infection in three species of chicken in southern Thailand.
