The occurrence of five major Newcastle disease virus genotypes (II, IV, V, VI and VIIb) in Bulgaria between 1959 and 1996.

Partial sequence and restriction enzyme cleavage site analyses of the fusion protein gene were used to genotype 47 Newcastle disease virus strains isolated between 1959 and 1996 in Bulgaria. Viruses belonged to five major genotypes that appeared to be associated with epizootics characterized by temporal and/or geographical restrictions. Genotype IV viruses (responsible for the European branch of the first panzootic) dominated the scene up to the early 1980s, interspersed with sporadic outbreaks caused by genotype II (US strains causing pneumoencephalitis) viruses. Genotype V viruses (transmitted by psittacines from South America) were first shown in 1973 and persisted until the late 1980s. Genotype VI (earliest members from the Middle-East 1968/70 outbreaks) was represented by scattered isolations between 1974 and 1996. A genotype VIIb (recent Middle East epizootic) virus was isolated as early as in 1984. Newcastle disease epizootics in Bulgaria were highlighted by multiple infection with more than one genotype at any one time.