Divergent astrovirus associated with neurologic disease in cattle

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Using viral metagenomics of brain tissue from a young adult crossbreed steer with acute onset of neurologic disease, we sequenced the complete genome of a novel astrovirus (BoAstV-NeuroS1) that was phylogenetically related to an ovine astrovirus. In a retrospective analysis of 32 cases of bovine encephalitides of unknown etiology, 3 other infected animals were detected by using PCR and in situ hybridization for viral RNA. Viral RNA was restricted to the nervous system and detected in the cytoplasm of affected neurons within the spinal cord, brainstem, and cerebellum. Microscopically, the lesions were of widespread neuronal necrosis, microgliosis, and perivascular cuffing preferentially distributed in gray matter and most severe in the cerebellum and brainstem, with increasing intensity caudally down the spinal cord. These results suggest that infection with BoAstV-NeuroS1 is a potential cause of neurologic disease in cattle.

Astroviruses are small, nonenveloped, positive single-stranded RNA viruses with a genome of 6.4–7.3 kb. The family Astroviridae comprises 2 genera, Mamastrovirus and Avastrovirus, known to infect mammals and birds, respectively. Since the first description of human astrovirus (HAstV) in children with diarrhea in 1975, a wide variety of astroviruses have been reported in multiple animals including humans, cattle, pigs, sheep, minks, dogs, cats, mice, sea lions, whales, chickens, and turkeys. Enteric astroviruses are transmitted through the fecal–oral route, and regardless of species, most infections are asymptomatic. In humans, the prevalence of exposure is very high, and astrovirus infection is a major cause of acute enteritis in infants. Clinical disease also can affect elderly and immunocompromised persons. In these persons, the clinical course of infection is acute, with 2–4 days of watery diarrhea and, less commonly, vomiting, headache, fever, abdominal pains, and anorexia. Astroviruses have been implicated twice in central nervous system (CNS) disease. One study demonstrated an HAstV-PS, which is distinct from the original HAstV and closely related to astrovirus HMO-C (AstV-HMO-C) and HAstV-VA1 in the brain tissue of a 15-year-old boy with X-linked agammaglobulinemia who had encephalitis. HAstV-PS was the only virus detected, and astrocyte infection was confirmed by anticapsid antibody staining. Serologic evidence of exposure to the closely related AstV-HMO-C was found in 36% of 5–10-year-old children in the United States, which reflects a common childhood infection and indicates that the encephalitis in this child was a likely consequence of his immunodeficiency. In an outbreak of so-called “neurological shaking disease” in mink, an astrovirus (Mink AstV-SMS) was detected in the brain tissues of multiple naturally and experimentally infected animals showing neurologic signs, including shaking and ataxia.

Cattle with neurologic signs are vigilantly screened to keep the food chain free of zoonotic pathogens, such as rabies virus, Salmonella spp., Listeria monocytogenes, Chlamydia spp., and the prion agent of bovine spongiform encephalopathy (BSE). In particular, BSE has become a major public health concern after recognition of the association between BSE and prion-associated disease in humans. Therefore, early and rapid recognition of the cause of neurologic disease is vital to the safety of the food chain. Etiologic diagnosis of CNS disease in cattle requires substantial effort; is costly; and usually presents a challenge because of the large number of pathogens or problems that can cause neurologic disease, including viruses, bacteria, parasites, prions, toxins, and metabolic disorders. Pathogens known to cause CNS disease in cattle include bovine herpesvirus 1 and 5 (BoHV-1 and BoHV-5), lyssavirus (rabies), ovine herpesvirus 2, L. monocytogenes, Histophilus somni, Escherichia coli, Salmonella spp., Chlamydia spp., Neospora caninum, amoebas, and prions. Brain tissue from a yearling steer with an encephalomyelitis and ganglioneuritis of unknown origin was analyzed by using viral metagenomics, which showed a divergent astrovirus distantly related to an ovine astrovirus. By retrospective analysis, this bovine astrovirus associated with neurologic symptoms (BoAstV-NeuroS1) was detected in the brains of 3 of 32 other cattle with encephalitides of undetermined etiology. Virus was detected by RNA by in situ hybridization within neurons in the brainstem, cerebellum, and/or spinal cord in all PCR positive samples from the 4 animals in this study.

Emerging Infectious Diseases
September 17, 2013

Original web page at Emerging Infectious Diseases

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