Neurotropic astrovirus in cattle with nonsuppurative encephalitis in Europe

Abstract

Encephalitis is a frequently diagnosed condition in cattle with neurological diseases. Many affected animals present with a nonsuppurative inflammatory reaction pattern in the brain. While this pattern supports a viral etiology, the causative pathogen remains unknown in a large proportion of cases. Using viral metagenomics, we identified an astrovirus (bovine astrovirus [BoAstV]-CH13) in the brain of a cow with nonsuppurative encephalitis. Additionally, BoAstV RNA was detected with reverse transcription-PCR and in situ hybridization in about one fourth (5/22 animals) of cattle with nonsuppurative encephalitis of unknown etiology. Viral RNA was found primarily in neurons and at the site of pathology. These findings support the notion that BoAstV infection is a common cause of encephalitis in cattle. Phylogenetically, BoAstV-CH13 was closely related to rare astrovirus isolates from encephalitis cases in animals and a human patient. Future research needs to be directed toward the pathogenic mechanisms, epidemiology, and potential cross-species transmission of these neurotropic astroviruses.

FIG 1

Identification of the novel bovine astrovirus BoAstV-CH13 in the brain tissue of a cow with nonsuppurative encephalitis. (A) The viral genome is composed of 3 open reading frames (ORFs), short 5′ and 3′ untranslated regions (UTRs), and a poly(A) tail [-A(n)]. ORF1a and ORF1b encode nonstructural proteins, and ORF2 encodes structural proteins. The numbers of amino acids (aa) in each translation product are shown. Bars, target sequences for ISH (blue, probe A; black, probe B) and RT-PCR (red). (B) Assembly of NGS reads revealed a contig of 6,540 nt that included the entire coding region of the viral genome. The graph presents the average coverage of this contig for each nucleotide. Bars, fragments that were confirmed by Sanger sequencing. (C) Whole-genome sequences of BoAstV-CH13 (reference), ovine astrovirus (OvAstV), and astroviruses isolated from brain tissues of mink (MiAstV-SMS), a human patient (HuAstV-PS), and a steer in the United States (BoAstV-NeuroS1) were compared by sliding window pairwise identity plots. Scale, nucleotide positions within the astrovirus genome.

FIG 2

Histopathological lesions and BoAstV-CH13 RNA detection in the facial nucleus in the medulla oblongata of a cow with nonsuppurative encephalitis. (A) Affected animals exhibited gliosis and neuronal necrosis. (B) High magnification of the marked area in panel A, showing necrotic neurons. (C to E) In situ hybridization for BoAstV-CH13 with probe A (C), probe B (D), and an unrelated control probe (E). (F and G) Negative-control sections of a healthy cow, analyzed with probe A (F) and probe B (G). Dark-blue labeling, presence of viral RNA. Magnification, ×10 (A and C to G) or ×40 (B).

FIG 3

Maximum-likelihood tree constructed from aligned amino acid sequences of the full-length capsid protein, showing the phylogenetic relationships between BoAstV-CH13 and other representative astroviruses. GenBank accession numbers are given in parentheses. HuAstV, human astrovirus; FeAstV, feline astrovirus; CaAstV, canine astrovirus; BoAstV, bovine astrovirus; OvAstV, ovine astrovirus; PoAstV, porcine astrovirus; MiAstV, mink astrovirus; AvAstV, avian astrovrius (avian nephritis virus); DuAstV, duck astrovirus; TuAstV, turkey astrovirus; HMO-AstV, human-mink-ovine-like astrovirus. ◆, isolates from brains of neurologically diseased mammals.