Viruses of the genus Orthohepadnavirus, family Hepadnaviridae, which are known to infect primates, bats, and rodents, are familiar to almost everyone, given the major public health concerns associated with the human Hepatitis B virus (HBV), a member of this family and genus. People chronically infected with HBV have an increased risk of other hepatic diseases such as cirrhosis of the liver and hepatocellular carcinoma. A vaccine is available for hepatitis B in people, but it is still uncertain how this virus causes tumors.
The new feline hepadnavirus, similar to HBV, described in this report was first identified in a 7 year-old male neutered domestic shorthair cat who belonged to one of the authors of this paper. The cat became sick with clinical signs of vomiting and weight loss. A large mid-abdominal mass was found on physical examination, and subsequently the cat was euthanized and necropsied. Necropsy and histopathology results demonstrated that the cat had a multicentric, large cell, high grade, B-cell lymphoma. He was also found to be feline immunodeficiency virus (FIV) positive based on serology and polymerase chain reaction (PCR) of tumor DNA.
Lymphoma tissue from this cat was then used in a virus discovery project, and a novel hepadnavirus was isolated from the lymphoma sample. The complete 3187 base pair circular genome of this virus was then assembled; the virus has been named domestic cat hepadnavirus. This new hepadnavirus shares 73-94% amino acid identity with the core protein, surface protein, and polymerase of other known hepadnaviruses. However, it is not closely related genetically or phylogenetically to other members of the genus Orthohepadnavirusand is therefore considered to be a new and separate species within this genus.
Hepadnaviruses have a tropism for the liver, but immunosuppressed individuals are likely to be viremic as well. The new feline hepadnavirus was also identified on PCR of frozen whole blood from the cat. The authors also collected whole blood from 60 more FIV-positive cats and found the feline hepadnavirus in 10% (n=6) of these animals as well as in the blood of 2/63 (3.2%) FIV-negative cats. In humans, HBV antigenemia exhibits a similar pattern: 7-10% of HIV-positive people sampled in Europe and the USA have been found to have persistent HBV antigenemia, while only 1% of the general human population in these same geographic areas is HBV antigenemic.
Not only is this hepadnavirus the first of its kind to be discovered in cats, it is the first such virus to be identified in any carnivorous animal, and also the first found in a companion animal. Hepatobiliary diseases as well as lymphomas of various organ systems are relatively common in domestic cats; what role, if any, might this newly discovered virus have in the etiology of some of these diseases? The study of domestic cat hepadnavirus, its natural history, epidemiology, association with comorbidities such as retroviral infections, lymphoma and other tumors, and hepatopathies in cats, has great potential as a fruitful area for future feline health research. [PJS]