Hepatitis B virus (HBV) is a human pathogen of global concern, while a high diversity of viruses related to HBV have been discovered in other animals during the last decade. Recently, the novel mammalian hepadnavirus, tentatively named domestic cat hepadnavirus (DCH), was detected in an immunocompromised cat. Herein, a collection of 209 cat sera and 15 hepato-diseased cats were screened for DCH using PCR, resulting in 12.4% and 20% positivity in the tested sera and necropsied cats, respectively. Among the DCH-positive sera, a significantly high level of co-detection with retroviral infection was found, with the highest proportion being co-detection with feline immunodeficiency virus (FIV). Full-length genome characterization of DCH revealed the genetic diversity between the nine Thai DCH sequences obtained, and that they phylogenetically formed three distinct monophyletic clades. A putative DCH recombinant strain was found, suggesting a possible role of recombination in DCH evolution. Additionally, quantitative PCR was used to determine the viral copy number in various organs of the DCH-moribund cats, while the pathological findings were compared to the viral localization in hepatocytes, adjacent to areas of hepatic fibrosis, by immunohistochemical (IHC) and western blot analysis. In addition to the liver, positive-DCH immunoreactivity was found in various other organs, including kidneys, lung, heart, intestine, brain, and lymph nodes, providing evidence of systemic infection. Ultrastructure of infected cells revealed electron-dense particles in the nucleus and cytoplasm of hepatocytes, bronchial epithelial cells, and fibroblasts. We propose the intracellular development mechanism of this virus. Although the definitive roles of pathogenicity of DCH remains undetermined, a contributory role of the virus associated with systemic diseases is possible.

Canine distemper virus (CDV), a pathogen causing fatal disease in a wide range of carnivores, can be classified into several geographically-related lineages. It is unclear how genetic recombination contributed to the evolution and emergence of the novel CDV strains and the evolutions of these strains are not fully yet investigated. In this study, the complete genome sequences of eight CDV viruses, isolated from domestic dogs in Thailand, were investigated. Interestingly, most of the identified CDV strains (CDV1-3, -5, -8 TH/2014) clustered as a novel Asia-4 lineage, while the CDV4, -6, -7 TH/2014 belonged to the Asia-1 lineage. Recombination analysis revealed that the CDV4 TH/2014 is a putative recombinant virus from the Asia-1 and America-2 parent viruses. In contrast, no recombination events were detected in the Asia-4 lineage, indicating that it is a distinctive lineage. Evolutionary analysis suggested that the CDV Asia-4 lineage had emerged since 1924 and shared common ancestor with the America-2 lineage. Pressure analysis revealed that CDV nucleotides were under negative selection pressure for its rapid adaptation. These findings demonstrate the evolution of CDV Asia-4 lineage and identified the Asia-1 recombination event. The information regarding genetic diversity of CDVs is essential for further CDV’s research and monitoring.

Tilapia lake virus (TiLV) is a notable contagious agent that causes massive economic losses in the tilapia industry globally. Evaluations of the histological changes associated with TiLV infection are not only crucial for diagnosis, but also to gain an understanding of the disease. We therefore synthesized a rabbit polyclonal immunoglobulin G antibody against TiLV and developed an immunohistochemical (IHC) procedure to detect TiLV localization in the tissues of infected fish for comparison with in situ hybridization (ISH) testing. A total of four different sample cohorts derived from TiLV-infected fish was used to validate the IHC procedure. The TiLV IHC application was successfully developed and facilitated nuclear and cytoplasmic immunolabelling in the intestines, gills, brain, liver, pancreas, spleen, and kidneys that corresponded with the ISH results. Apart from the ISH results, TiLV-IHC signals were clearly evident in the endothelial cells of various organs, the circulating leukocytes in the blood vessels, and the areas of tissue inflammation. Among the tested sample cohorts, the intestines, gills, and brain had IHC-positive signals, highlighting the possibility of these organs as common TiLV targets. Immunological staining pattern and distribution corresponded with the TiLV viral load but not the inoculation route. The TiLV IHC was also capable of detecting TiLV infection in the experimentally challenged ornamental cichlids, Mozambique tilapia, giant gourami, and naturally infected tilapia, indicating the dynamic range of IHC for TiLV detection. Overall, our study delivers the first IHC platform to detect TiLV infection and provides novel evidence of cellular tropism during TiLV infection. Our findings also reveal the TiLV distribution pattern of infected fish and propose the endotheliotropism and lymphotropism of this virus, which requires further elaboration. Importantly, this new IHC procedure could be applied to study the pathogenesis and interaction of TiLV in future research.

Tilapia tilapinevirus, or tilapia lake virus (TiLV), is a highly contagious virus found in tilapia and its hybrid species that has been reported worldwide, including in Asia, the Americas, and Africa. In this study, we experimentally challenged Mozambique tilapia (Oreochromis mossambicus) with a virulent TiLV strain, VETKU-TV01, at both low (1 × 10³ TCID₅₀/mL) and high (1 × 10⁵ TCID₅₀/mL) concentration. After the challenge, the Mozambique tilapia showed pale skin with some hemorrhage and erosion, lethargy, abdominal swelling, congestion around the eye, and exophthalmos; there was a cumulative mortality rate at 48.89% and 77.78% in the groups that received the low and high concentration, respectively. Quantitative PCR and in situ hybridization confirmed the presence of TiLV in the internal organs of moribund fish. Notably, severe histopathological changes, including glycogen depletion, syncytial hepatic cells containing multiple nuclei and intracytoplasmic inclusion bodies, and infiltration of melanomacrophage into the spleen, were frequently found in the Mozambique tilapia challenged with high TiLV concentration. Comparatively, the infectivity and pathology of the TiLV infection in Mozambique tilapia and red hybrid tilapia (Oreochromis spp.) were found to be similar. Our results confirmed the susceptibility of Mozambique tilapia, which has recently been determined to be a vulnerable species, to TiLV infection, expanding knowledge that the virus can cause disease in this fish species.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been the cause of human pandemic infection since late 2019. SARS-CoV-2 infection in animals has also been reported both naturally and experimentally, rendering awareness about a potential source of infection for one health concern.