A series of 33 human-avian and human-mammalian influenza virus reassortant clones possessing either HA or both HA and NA genes of the avian or mammalian virus was obtained by crosses of A/USSR/90/77 (H1N1) human virus with 5 avian and 1 mammalian influenza virus strains. All of the reassortants possessing NA genes of the H1N1 human parent virus and HA gene of an avian or mammalian parent virus had high values of infectivity/HA activity ratio. Since this feature could result from a limited virion aggregation, several reassortants were analyzed by velocity sucrose gradient centrifugation. In all cases tested, the reassortants of H3N1, H4N1, H10N1 and H13N1 composition were shown to be aggregated, whereas the preparations of the parent H1N1 virus and the reassortants possessing both HA and NA genes from the avian parents were represented mostly by single virions. The aggregates were formed at 4 degrees C and dissociated at 37 degrees C. The dissociation was blocked by an inhibitor of neuraminidase activity (2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid). The dissociation was reversible since the virions reaggregated at 4 degrees C; however, treatment with bacterial neuraminidase led to an irreversible dissociation of the aggregates. The tendency of the reassortants to aggregate correlates with an increased infectivity/HA ratio. No regular decrease in the neuraminidase activity in the virions of reassortants as compared to the parent H1N1 virus was revealed. The most likely explanation of the observed phenomenon seems to be an inefficient removal of sialic acid residues from the avian virus hemagglutinin by the human virus N1 neuraminidase.

Human-avian and human-mammalian influenza A virus reassortant clones with the neuraminidase (NA) gene of the A/USSR/90/77 (H1N1) strain and hemagglutinin (HA) genes of H3, H4 and H13 subtypes had been shown in an earlier publication to produce low HA yields in the embryonated chicken eggs. The low HA titers had been shown to be due, at least in part, to the formation of virion clusters at 4 degrees C; the clustering was removed by the treatment with bacterial neuraminidase [Rudneva et al., Arch. Virol (1993) 133: 437-450]. By serial passages of the reassortants in chick embryos non-aggregating variants were selected: the variants produced HA titers of the same order as A/USSR/90/77 parent virus. The assessment of the virus yields by the analysis of the partially purified virus preparations from fixed volumes of the allantoic fluid revealed that actual virion yields of the initial reassortants were lower than the yields of their passaged variants or of the parent viruses. The passaged variant of a reassortant possessing the HA gene of A/Duck/Ukraine/1/63 (H3N2) virus differed from the original (non-passaged) reassortant and from the parent A/Duck/Ukraine/1/63 virus in the reaction with a panel of monoclonal antibodies against H3 hemagglutinin. The data suggest that some HA-NA combinations may lead to an incomplete functional match between HA and NA and to the formation of low-yield reassortants, thus representing a possible limiting factor in the emergence of new HA-NA combinations in natural conditions.

We report twenty autochthonous sporadic cases of acute infection with HEV. The zoonotic etiology of the virus as well as the foodborne transmission of the infection is discussed. We found that aging and pre-existing underlying diseases are risk factors for a severe course of the HEV infection.

The present study aimed to determine the role of human parvovirus В19 (B19V) as an aetiological agent in measles and rubella negative fever/rash patients from Bulgaria between 2004 and 2013. A total of 1,266 sera from all over the country were tested for B19V IgM antibodies and all positives were further investigated by polymerase chain reaction (PCR). Overall, 280 sera (22%) were B19V IgM positive and 227 of these (81%) were also PCR positive. The highest number of IgM positives was found among five to nine year-old children (27%). Eight infected women gave birth to healthy children; one fetus was aborted with hydrops fetalis. Of the 55 genetic sequences obtained, 54 belonged to genotype 1a and one grouped as a genotype 2 outlier. Phylogenetic analysis of all available genotype 2 sequences covering the 994 nucleotide non-structural protein 1(NS1)/capsid viral protein 1 (VP1) unique region junction, showed that only one other sequence grouped with the outlier strain, forming a clearly distinct and well-supported cluster of genotype 2 (between-group genetic distance: 3.32%). In accordance with B19V nomenclature, this cluster may represent a new subgenotype 2b. The study showed that B19V infections may be falsely identified as rubella or measles in ca 22% of cases, emphasising the need for laboratory confirmation.

Aim: The aim of this study was to describe the etiological distribution, clinical and laboratory characteristics of patients with Fever of unknown origin (FUO), admitted to one Bulgarian hospital. Material and Method: A retrospective study was done for a period of four years. The modified criteria of Petersdorf and Beeson were applied. Complete history, physical examination and basic laboratory investigations were done. According to the potentially diagnostic clues, specific tests, immunological and imaging methods were performed. The invasive procedures were the last step. Results: Fifty-four patients met the inclusion criteria. The estimated causes were: infection 59.3%, neoplasm 3.7%, non-infectious inflammatory disease 14.8%, miscellaneous 5.5% and undiagnosed cases 16.7%, respectively. The mean age was 44.3 years. The leading clinical features were fever, chills, sweats and fatigue. The association of clinical signs, physical exam and laboratory results were discussed. Discussion: The infectious diseases were the leading cause of FUO. The etiological distribution of causes was near to results reported from South-East Europe. Some geographic, climatic, zoonotic and social factors influenced the results.