Virological and serological studies of an epidemic disease in Bulgaria, 1975, were carried out. Epidemiologically, clinically and pathomorphologically, the disease simulated almost all known forms of poliomyelitis, acute stem encephalitis, encephalomyocarditis and aseptic meningitis. The studies completely rules out the participation of polioviruses and provided comprehensive evidence for the etiological role of a peculiar enterovirus subsequently identified as enterovirus (EV) type 71 known in the literature since 1974. Altogether, in 1975 and 1976 from 65 cases of poliomyelitis-like disease (PLD) 92 strains of EV71 were isolated, including 37 strains from the brain and medulla, 1 from the cerebrospinal fluid, 10 from mesenterial lymph nodes and tonsils and 44 from faeces. In addition, in 282 convalescent cases of the disease, diagnostic seroconversion or high titers of antibody to this virus were demonstrated. The most successful virus isolation was achieved by inoculation of green monkey kidney cell cultures and newborn white mice. Bulgarian strains of enterovirus 71 regularly caused paralysis in monkeys and morphological poliomyelitis-like lesions in their CNS, and paralysis and myositis with Zenker necrosis in newborn white mice, cotton rats, Syrian hamsters, and 3-week-old cotton rats. The diseased rodents had much more virus in their mucles than in brains.

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A new genus, bunyavirus, is proposed. As it cannot be allocated to a currently existing family, the establishment of a new one, Bunyaviridae, is suggested.

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Similar physicochemical characteristics were found with strains of Crimean hemorrhagic fever (CHF) and Congo viruses. The particle size, sedimentation coefficient, buoyant density, weight of the particles as well as morphology and morphogenesis of these viruses were similar to those of other members of the Bunyaviridae family. Data are presented on reproduction of CHF virus in cell cultures and on the inner structure of its virion.

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In 1975 in Bulgaria a severe epidemic of central nervous system (CNS) disease occurred. Clinically, histopathologically, and epidemiologically the cases resembled poliomyelitis, aseptic meningitis, meningoencephalitis, and, in some cases, encephalomyocarditis. About 21% of the 700 reported cases developed paralysis, 44 with fatal outcome [ref. 1]. In 65 cases, 92 strains of enterovirus of the same serologic type were isolated: 38 strains from the CNS, 10 from mesenteric lymph nodes and tonsils, and 44 from feces [ref. 1 2]. A typical representative strain, No. 258, isolated from the spinal cord of a 3-month-old baby who died on the 5th day of disease with signs of focal polioencephalitis, was selected for intensive study. Cross-neutralization tests established the antigenic identity of the Bulgarian 258 strain (Stanchev) with American strains of enterovirus type 71 (BrCr and JaFr strains) and Swedish strains of the same type (Nos. 52343, 52500, and 6041). From its biological [ref. 1,2], physicochemical [ref. 3], and antigenic properties, the etiological agent of this large epidemic can be classified as a member of enterovirus type 71, one of the most highly pathogenic of the recently recognized entero-viruses.