Hepadnaviruses are partially double-stranded DNA viruses that infect a variety of species. The prototypical virus in this family is the human hepatitis B virus, which chronically infects approximately 400 million people worldwide and is a risk factor for progressive liver disease and liver cancer. The first hepadnavirus isolated from carnivores was a domestic cat hepadnavirus (DCH), initially identified in Australia and subsequently detected in cats in Europe and Asia. As with all characterized hepadnaviruses so far, DCH infection has been associated with hepatic disease in its host. Prevalence of this infection in the United States has not been explored broadly. Thus, we utilized conventional and quantitative PCR to screen several populations of domestic cats to estimate DCH prevalence in the United States. We detected DCH DNA in 1 out of 496 animals (0.2%) in the U.S. cohort. In contrast, we detected circulating DCH DNA in 7 positive animals from a cohort of 67 domestic cats from Australia (10.4%), consistent with previous studies. The complete consensus genome of the U.S. DCH isolate was sequenced by Sanger sequencing with overlapping PCR products. An in-frame deletion of 157 bp was identified in the N-terminus of the core open reading frame. The deletion begins at the direct repeat 1 sequence (i.e., the 5′ end of the expected double-stranded linear DNA form), consistent with covalently closed circular DNA resultant from illegitimate recombination described in other hepadnaviruses. Comparative genome sequence analysis indicated that the closest described relatives of the U.S. DCH isolate are those previously isolated in Italy. Motif analysis supports DCH using NTCP as an entry receptor, similar to human HBV. Our work indicates that chronic DCH prevalence in the U.S. is likely low compared to other countries.

Feline leukemia virus (FeLV) is a retrovirus that primarily affects domestic cats. Close interactions with domestic cats, including predation, can lead to the interspecific transmission of the virus to pumas, bobcats, or other feline species.

Background: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a recently emerged tickborne virus in east Asia with over 18,000 confirmed cases. With a high case fatality ratio, SFTSV has been designated a high priority pathogen by the WHO and the NIAID. Despite this, there are currently no approved therapies or vaccines to treat or prevent SFTS. Vesicular stomatitis virus (VSV) represents an FDA-approved vaccine platform that has been considered for numerous viruses due to its low sero-prevalence in humans, ease in genetic manipulation, and promiscuity in incorporating foreign glycoproteins into its virions. Methods: In this study, we developed a recombinant VSV (rVSV) expressing the SFTSV glycoproteins Gn/Gc (rVSV-SFTSV) and assessed its safety, immunogenicity, and efficacy in C57BL/6, Ifnar^−/−, and AG129 mice. Results: We demonstrate that rVSV-SFTSV is safe when given to immunocompromised animals and is not neuropathogenic when injected intracranially into young immunocompetent mice. Immunization of wild type (C57BL/6) and Ifnar^−/− mice with rVSV-SFTSV resulted in high levels of neutralizing antibodies and protection in a lethal SFTSV challenge model. Additionally, passive transfer of sera from immunized Ifnar^−/− mice into naïve animals was protective when given pre- or post-exposure. Finally, we demonstrate that immunization with rVSV-SFTSV cross protects AG129 mice against challenge with the closely related Heartland bandavirus despite negligible neutralizing titers to the virus. Conclusions: Taken together, these data suggest that rVSV-SFTSV is a promising vaccine candidate for SFTSV and Heartland bandavirus with a favorable safety profile.

Background: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a highly pathogenic bunyavirus with a high case-fatality ratio for which there is no approved vaccine. Studies have assessed different vaccine technologies. However, few studies have yet assessed the immunogenicity of heterologous prime-boost regimens. Methods: Here, we compare a lipid nanoparticle (LNP)-encapsulated nucleoside-modified mRNA-based vaccine encoding the SFTSV glycoproteins, Gn and Gc, to our recently described recombinant VSV SFTSV (rVSV-SFTSV) vaccine in single dose, homologous, and heterologous prime-boost regimens in mice. Results: We show that all regimens protect from pathogenic SFTSV challenge and elicit strong long-lasting antibody responses. Furthermore, strong cellular immunity is elicited by mRNA-LNP immunizations and by heterologous immunization with an rVSV-SFTSV prime and mRNA-LNP boost. Cellular responses robustly polarized towards a type 1 response, characterized by high levels of IFNγ, TNFα, and IL-2. Immunization with mRNA led to a mixed type 1/type 2 immune response, as determined by antibody isotypes IgG1 and IgG2c. We found that homologous immunization leads to stronger antibody responses while heterologous immunization drives a slightly stronger cellular response. Conclusions: Taken together, the vaccine platforms described here represent strong vaccine candidates for further development.

Severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV) are emerging tick-borne bandaviruses. They have high case fatality rates (10%), and no FDA-approved vaccines exist for disease prevention. SFTSV and HRTV are therefore identified as priority pathogens. A recombinant vesicular stomatitis virus (rVSV) vaccine, which replaces the original VSV glycoprotein with the SFTSV glycoproteins, shows early promise for SFTSV as it induces strong immune responses that are protective against lethal challenge. However, rVSV-SFTSV is highly attenuated in cell culture, which may be due to incompatibility between the assembly sites of SFTSV (the Golgi and ERGIC) and that of VSV (the plasma membrane). In this study, we identify a noncanonical COPI binding motif found in the cytoplasmic tail of SFTSV glycoproteins and demonstrate that an amino acid substitution in this motif (K1071A) inhibits binding to COPI. This mutation results in increased surface expression of SFTSV glycoproteins, improved incorporation onto VSV virions, and enhanced replication of rVSV-SFSTV in vitro. A mutation in a homologous site (K1074A) of HRTV has similar results, and rVSV-HRTV K1074A exhibits increased replication in vitro and in vivo . We show that vaccination with rVSV-HRTV K1074A results in improved induction of neutralizing antibody responses in immunocompetent C57BL/6 mice, and neutralizing antibodies elicited by vaccination are protective when administered to severely immunocompromised mice via passive transfer. Overall, our study identifies a mutation that improves the efficacy of the rVSV-SFTSV vaccine candidate and introduces the first vaccine candidate directly addressing HRTV infections. Importance: Severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland bandavirus (HRTV) are emerging tick-borne viruses with high fatality rates. FDA-approved vaccines and antiviral drugs are unavailable but critically needed. We identify an important mutation in the SFTSV glycoprotein that disrupts a previously unreported COPI binding site. The mutation improves the efficacy of the previously described recombinant vesicular stomatitis virus vaccine candidate for SFTSV (rVSV-SFTSV). We also develop an rVSV-HRTV vaccine and show potent induction of neutralizing antibodies and protection from lethal challenge. This is the first study directly addressing the lack of vaccines specifically targeting HRTV.

This retrospective study provides an analysis of the prevalence and detectability of canine distemper virus (CDV), feline leukemia virus (FeLV), and feline immunodeficiency virus (FIV) in ocelots (Leopardus pardalis) sheltered in a wild animal recovery center in Guayaquil, Ecuador. Blood samples of 19 rescued ocelots from 2019–20 were analyzed using FeLV p27 antigen enzyme-linked immunosorbent assays (ELISA) and commercial insulated isothermal reverse transcriptase PCR (iiRT-PCR) kits. Using this PCR we detected positive results for CDV (4/ 17; 23.5%) and FeLV (14/16; 87.5%), but not for FIV (0/8). Three previously positive cases of CDV and two of FeLV showed negative results on retesting 6 mo later. Moreover, a third analysis was conducted and was negative for CDV. Our results suggest that ocelots can recover from the local CDV and FeLV strains. An ELISA for the FeLV p27 antigen showed no capability to detect FeLV in ocelots that were confirmed positive by iiRT-PCR. Regional lineages, viral virulence, and host immune response capabilities should be addressed in further research to inform management and decision making for wildlife conservation.