Structural variation of La Crosse virions under different chemical and physical conditions.

La Crosse (LAC) virions exposed to different pHs (7.3, 6.2, and 5.4) and temperatures (37 degrees, 20 degrees, and 4 degrees) were preserved in thin layers of vitreous ice and observed by electron cryomicroscopy. Our results indicate that, at lower pH, virus particles interact with each other to form aggregates. In some cases, particles could be interpreted to have fused together. At neutral pH and higher temperatures morphological changes consistent with deformation in some particles were observed. We suggest that low pH conditions are sufficient for membrane fusion events to occur with LAC virions.

[The circulation of viruses of the California encephalitis and Bunyamwera groups (Bunyaviridae, Bunyavirus) on the northeastern Russian plain]. / Tsirkuliatsiia virusov kompleksov Kaliforniiskogo éntsefalita i Bun'iamvera (Bunyaviridae, Bunyavirus) na severo-vostoke Russkoi ravniny.

In the Komi ASSR, 102.2 thousand mosquitoes, 207 small mammals, 1487 blood serum specimens from people, 793 cow blood sera, 140 blood serum specimens from reindeer were collected in June-August, 1988. Twenty seven virus strains isolated from mosquitoes were classified into Bunyaviridae family according to the data of electron microscopic studies. Identification of the isolates by CFT and IFA showed 18 of the strains to belong to California encephalitis complex and 9 to Bunyamwera complex. Serological studies by neutralization test demonstrates a high frequency of contact of the human population and domestic animals with viruses of the California complex practically in the entire study area: an average of 45% among human subjects, 48% among cattle and 33% among reindeer. Such high values of the immune portions indicate the activity of the discovered natural foci of this complex. The results for Batai virus from the Bunyamwera complex do not indicate its high activity: approximately 2% of positive findings in human subjects and about 4% in the cattle. The results permit a prognosis of California encephalitis and Bunyamwera complex viruses spread in the northern part of Western Siberia.

Electron microscopy of vitrified-hydrated La Crosse virus.

La Crosse (LAC) virions were cryopreserved by rapid freezing in a thin layer of vitreous ice. The vitrified-hydrated LAC virions were subsequently imaged at -170 degrees C in a transmission electron microscope equipped with a low-temperature specimen holder. This cryoelectron microscopic technique eliminates the artifacts frequently associated with negative staining. Images of vitrified-hydrated LAC virions clearly revealed surface spikes as well as bilayer structure. Size measurements of the vitrified-hydrated LAC virions showed heterogeneity, with diameters ranging from 75 to 115 nm. Regardless of the particle size, the spike was about 10 nm long, and the bilayer was about 4 nm thick. The spikes are interpreted to be one or both of the glycoproteins, and the bilayer is interpreted to be the membrane envelope of the virus. In contrast to the pleomorphic appearance of the negatively stained LAC virions, the vitrified-hydrated LAC virions showed uniform spherical shapes regardless of their sizes.

Experimental model of transovarial transmission of Tahyna virus in Aedes aegypti mosquitoes.

The progeny of 31 viruliferous Aedes aegypti females infected with Tahyna virus by sucking on viraemic newborn mice was investigated for virus presence. Out of 1587 individuals of the F1 generation, 16 suspensions representing the progeny of 7 females were positive in 146 trials. Individuals of the F1 generation failed to transfer the virus by sucking. Electron microscopy revealed the presence of Tahyna virus particles in the cytoplasm of maturing oocytes inevitably confirming the transovarial transfer of the virus by germinal cells of Aedes aegypti mosquitoes.

Segmented genome and nucleocapsid of La Crosse virus.

La Crosse (LAC) virions purified by velocity and equilibrium gradient centrifugation contained three single-stranded RNA species. The three segments had sedimentation coefficients of 31S, 25S, and 12S by sodium dodecyl sulfate-sucrose gradient centrifugation. By comparison with other viral and cellular RNA species, the LAC viral RNAs had molecular weights of 2.9 x 10(6), 1.8 x 10(6), and 0.4 x 10(6). Phenol-sodium dodecyl sulfate-extracted LAC virion RNA was not infectious for BHK-21 cell cultures under conditions in which Sindbis viral RNA was infectious. Treatment of LAC virus with the nonionic detergent Triton X-100 and salt released three nucleocapsid structures, each containing one species of virion RNA. The nucleocapsids had sedimenation coefficients of 115S, 90S, and 65S. Negative-contrast electron microscopy of the nucleocapsids indicated that they were convoluted, supercoiled, and apparently circular. They had a mean diameter of 10 to 12 nm and modal lengths of 200, 510, and 700 nm (some were even longer). By chemical and enzymatic analysis of purified viral RNA, one type of 5' nucleotide (pppAp) present in the proportion of one per RNA segment was identified. After periodate oxidation, each virion RNA species was labeled by reduction with [3H]sodium borohydride. Taken together, these results suggest that although the nucleocapsids appear as closed loops, the viral RNA has free 5' and 3' ends and is, therefore, not circular.

Structural proteins of La Crosse virus.

Preparations of La Crosse virus, a member of the California encephalitis group of bunyaviruses, were found to possess three major virion proteins. Two of the proteins were glycosylated (G1 and G2) and were located on the surface of the virus particles. These two glycoproteins were present in equimolar amounts and possessed apparent molecular weights of 120 X 10(3) and 34 X 10(3). Virion nucleocapsids, isolated by a nonionic detergent and salt treatment, contained another major protein, N (molecular weight = 23 X 10(3)). A large, but minor, protein species L (molecular weight = 180 X 10(3)) was also found in virus preparations. The approximate number of protein molecules per virion has been determined. Electron microscopy of purified La Crosse virus indicated that the virus particle (mean diameter, 91 nm) is enveloped and possesses irregular surface projections (length, 10 nm).