The lack of a mouse model has hampered an understanding of the pathogenesis and immunity of Marburg hemorrhagic fever (MHF), the disease caused by marburgvirus (MARV), and has created a bottleneck in the development of antiviral therapeutics. Primary isolates of the filoviruses, i.e., ebolavirus (EBOV) and MARV, are not lethal to immunocompetent adult mice. Previously, pathological, virologic, and immunologic evaluation of a mouse-adapted EBOV, developed by sequential passages in suckling mice, identified many similarities between this model and EBOV infections in nonhuman primates. We recently demonstrated that serially passaging virus recovered from the liver homogenates of MARV-infected immunodeficient (SCID) mice was highly successful in reducing the time to death in these mice from 50 to 70 days to 7 to 10 days after challenge with the isolate MARV-Ci67, -Musoke, or -Ravn. In this study, we extended our findings to show that further sequential passages of MARV-Ravn in immunocompetent mice caused the MARV to kill BALB/c mice. Serial sampling studies to characterize the pathology of mouse-adapted MARV-Ravn revealed that this model is similar to the guinea pig and nonhuman primate MHF models. Infection of BALB/c mice with mouse-adapted MARV-Ravn caused uncontrolled viremia and high viral titers in the liver, spleen, lymph node, and other organs; profound lymphopenia; destruction of lymphocytes within the spleen and lymph nodes; and marked liver damage and thrombocytopenia. Sequencing the mouse-adapted MARV-Ravn strain revealed differences in 16 predicted amino acids from the progenitor virus, although the exact changes required for adaptation are unclear at this time. This mouse-adapted MARV strain can now be used to develop and evaluate novel vaccines and therapeutics and may also help to provide a better understanding of the virulence factors associated with MARV.

A thorough characterization of the genetic diversity of viruses present in vector and vertebrate host populations is essential for the early detection of and response to emerging pathogenic viruses, yet genetic characterization of many important viral groups remains incomplete. The Simbu serogroup of the genus Orthobunyavirus, family Bunyaviridae, is an example. The Simbu serogroup currently consists of a highly diverse group of related arboviruses that infect both humans and economically important livestock species. Here, we report complete genome sequences for 11 viruses within this group, with a focus on the large and poorly characterized Manzanilla and Oropouche species complexes. Phylogenetic and pairwise divergence analyses indicated the presence of high levels of genetic diversity within these two species complexes, on a par with that seen among the five other species complexes in the Simbu serogroup. Based on previously reported divergence thresholds between species, the data suggested that these two complexes should actually be divided into at least five species. Together these five species formed a distinct phylogenetic clade apart from the rest of the Simbu serogroup. Pairwise sequence divergences among viruses of this clade and viruses in other Simbu serogroup species complexes were similar to levels of divergence among the other orthobunyavirus serogroups. The genetic data also suggested relatively high levels of natural reassortment, with three potential reassortment events present, including two well-supported events involving viruses known to infect humans.

(16,48) and are considered the model of choice for vaccine and therapeutic safety and efficacy studies. However, for studies that require preliminary analyses based on large numbers of samples, the number of primates required is often ethically problematic and prohibitively expensive, especially for the number of animals required to analyze a batched throughput of therapeutic candidates. In addition, mechanistic pathogenesis studies are not feasible due to the lack of genetically modified or transgenic monkeys. Therefore, comprehensive studies were undertaken to define the genomics, pathogenesis, and virulence of the MARV and RAVV mouse models that can be useful to address the above-mentioned gaps.Human or nonhuman primate marburgvirus variants (wild type [wt]) are not lethal to adult immunocompetent mice but can be selected by the in vivo passage of the viruses into naïve mice (46, 47). The identification of lethal mouse (lm) marburgvirus variants was elusive until the development of a novel approach that incorporated an initial propagation of the marburgvirus in scid BALB/c mice (here referred to as scid mice) prior to the infection and passage of the viruses in immunocompetent BALB/c mice.Filoviruses are considered "emerging or reemerging pathogens" (reviewed in reference 24) or the etiological agents of MHF as a result of a change or evolution in a previously identified organism or as a known MHF pathogen whose incidence rapidly increases in a given geographic area. The description of key consensus genomic changes within the lm marburgvirus variant quasispecies identified during the selection process of lm marburgvirus variants provides the basis to describe the potential ability of marburgvirus to naturally select variants that will propagate in new hosts for emergence or reemergence. Genomic analyses of lm marburgvirus variants are consistent with previous reports of mechanisms for other lethal host RNA viral variants by in vivo selection that augment pathogenesis or virulence either in new hosts or in tissue tropism(s) within a susceptible host (reviewed in reference 35). These mechanisms include unidirectionally biased hypermutagenesis of pyrimidines as a result of Ade-to-Ino editing in multiple genes, a distinct order of mutational occurrence, mutations in multiple genes, extensive hypermutation in untranslated regions (UTRs), and switching to host codon usage. Each of these mechanisms will be described separately; however,

The International Committee on Taxonomy of Viruses (ICTV) organizes the classification of viruses into taxa, but is not responsible for the nomenclature for taxa members. International experts groups, such as the ICTV Study Groups, recommend the classification and naming of viruses and their strains, variants, and isolates. The ICTV Filoviridae Study Group has recently introduced an updated classification and nomenclature for filoviruses. Subsequently, and together with numerous other filovirus experts, a consistent nomenclature for their natural genetic variants and isolates was developed that aims at simplifying the retrieval of sequence data from electronic databases. This is a first important step toward a viral genome annotation standard as sought by the US National Center for Biotechnology Information (NCBI). Here, this work is extended to include filoviruses obtained in the laboratory by artificial selection through passage in laboratory hosts. The previously developed template for natural filovirus genetic variant naming ( ///-) is retained, but it is proposed to adapt the type of information added to each field for laboratory animal-adapted variants. For instance, the full-length designation of an Ebola virus Mayinga variant adapted at the State Research Center for Virology and Biotechnology “Vector” to cause disease in guinea pigs after seven passages would be akin to “Ebola virus VECTOR/C.porcellus-lab/COD/1976/Mayinga-GPA-P7”. As was proposed for the names of natural filovirus variants, we suggest using the full-length designation in databases, as well as in the method section of publications. Shortened designations (such as “EBOV VECTOR/C.por/COD/76/May-GPA-P7”) and abbreviations (such as “EBOV/May-GPA-P7”) could be used in the remainder of the text depending on how critical it is to convey information contained in the full-length name. “EBOV” would suffice if only one EBOV strain/variant/isolate is addressed.

The complete genome sequences of 2 closely related plaque-derived variants of Marburg virus (MARV) species Lake Victoria marburgvirus, strain Musoke, indicate only a few regions of the RNA genome as underlying the differences between the 2 viruses. One variant is >90% lethal for guinea pigs and the other much less virulent, when guinea pigs are challenged with 1000 pfu of virus. Only 4 mutations that result in amino acid changes were identified, 1 in viral matrix protein VP40 and 3 in L, the RNA-dependent RNA polymerase. In addition, 6 differences were identified in noncoding regions of transcribed mRNA, and 1 silent codon change was identified in the L gene. Interestingly, the amino acid mutation identified in VP40 occurs in a nonconserved loop structure between 2 domains that are homologues only among MARV species. The L gene mutations were equally intriguing, clustering near a highly conserved motif in viral RNA-dependent RNA polymerases.