Viruses in unexplained encephalitis cases in American black bears (Ursus americanus)

Abstract

Viral infections were investigated in American black bears (Ursus americanus) from Nevada and northern California with and without idiopathic encephalitis. Metagenomics analyses of tissue pools revealed novel viruses in the genera Circoviridae, Parvoviridae, Anelloviridae, Polyomaviridae, and Papillomaviridae. The circovirus and parvovirus were of particular interest due to their potential importance as pathogens. We characterized the genomes of these viruses and subsequently screened bears by PCR to determine their prevalence. The circovirus (Ursus americanus circovirus, UaCV) was detected at a high prevalence (10/16, 67%), and the chaphamaparvovirus (Ursus americanus parvovirus, UaPV) was found in a single bear. We showed that UaCV is present in liver, spleen/lymph node, and brain tissue of selected cases by in situ hybridization (ISH) and PCR. Infections were detected in cases of idiopathic encephalitis and in cases without inflammatory brain lesions. Infection status was not clearly correlated with disease, and the significance of these infections remains unclear. Given the known pathogenicity of a closely related mammalian circovirus, and the complex manifestations of circovirus-associated diseases, we suggest that UaCV warrants further study as a possible cause or contributor to disease in American black bears.

Fig 1

A. The genome schematic of UaPV. Clear boxes represent untranslated regions, while the dashed lines represent splicing. The arrows represent ORFs. B. Maximum likelihood tree of NS1 aa chaphamaparvovirus sequences. Bar, 0.2 amino acid substitutions per site. Bootstrap values below 60 were removed.

Fig 2

A. Genome map of Ursus americanus circovirus (UaCV) including Rep (blue) and Capsid (purple) genes. B. Maximum likelihood trees of the Rep (left) and Capsid (right) aa circovirus sequences. Bar, 0.2 (Rep) and 0.5 (Capsid) amino acid substitutions per site. Bootstrap values below 60 were removed.

Fig 3. ISH detection of UaCV nucleic acid in tissue sections.

Case 1. UaCV probes (A, D) and negative control (DapB) probes (B, E); hematoxylin counterstain. A. In sections of liver, UaCV ISH signal is detected in scattered polygonal cells (presumed Kupffer cells or endothelial cells) in hepatic sinusoids. Inset: higher magnification of a cell exhibiting positive probe hybridization. B. No signal is detected with negative control (DapB) probes. C. Immunohistochemistry for PCV2 in the liver of a pig demonstrates a similar pattern of immunoreactivity. (UC Davis SVM archives. IHC performed at California Animal Health and Food Safety Laboratory, Davis CA.) D. Sections of brain exhibit punctate UaCV ISH signal in and around small-caliber blood vessels. E. No signal is detected with negative control probes. Bars = 25μm.