Molecular epidemiology and phylogenetic analysis of diverse bovine astroviruses associated with diarrhea in cattle and water buffalo calves in China

ALFRED, Niyokwishimira; LIU, Huan; LI, Mu Lan; HONG, Shao Feng; TANG, Hai Bo; WEI, Zu Zhang; CHEN, Ying; LI, Fa Kai; ZHONG, Yi Zhi; HUANG, Wei Jian

Astroviruses are small and non-enveloped with a single-stranded, positive-sense RNA genome of 6.4 to 7.3 kb in length consisting of three open reading frames (ORFs): ORF1a, which encodes the non-structural polyprotein 1a; ORF1b, which encodes the polyprotein 1ab, including the RNA-dependent RNA polymerase (RdRp) that is expressed by a ribosomal frameshift at the ORF1a/1b junction [1, 15] ; and ORF2, which encodes a viral capsid structural polyprotein. The current International Committee on Taxonomy of Viruses (ICTV) system divides the Astroviridae family into two genera, Mamastrovirus and Avastrovirus, which infect mammals and birds, respectively [5, 21] . Astrovirus was first identified in 1975 as the causative agent of infantile diarrhea in a maternity ward in England [1] . In 2011, astrovirus infection has expanded to 22 animal species or families, including various terrestrial domestic animals, wild animals and aquatic animals, (turkey, chicken, pig, cattle, dogs, cats, deer, ducks, mink, rabbit, sea lion, bats, etc.) [3, 5, 18, 21, 24, 28] . Bovine astrovirus (BAstV) was first described in England in 1978 from acute enteritis of calves [24] and initially considered to be avirulent, as experimental infection of two gnotobiotic calves with astrovirus-like virus did not cause diarrhea [24] . However, in 1984, two bovine astrovirus isolates from the U.S.A., US1 and US2, which were found to be antigenically related to an isolate in England (U.K.), were shown to cause infection and cytopathology of M cells of the dome epithelium covering the Payer's patches of the calf ileum [25] . In addition, astroviruses in co-infection with other enteric pathogens, such as bovine rotavirus (BRV) and bovine torovirus (BToV; Breda virus), were shown to increase the severity of astrovirus infection [26] . In 1985, a serologically based investigation classified bovine astroviruses into two serotypes: BoAstV-1 and BoAstV-2. Meanwhile, a study on the earliest three isolates of bovine astroviruses (UK, US1 and US2) implied that these isolates were different serotypes, suggesting that multiple serotypes of bovine astroviruses may exist in nature [25] . Astroviruses, as well as noroviruses, hepatitis A and E, and rotaviruses, are commonly reported as major causes of foodborne illnesses [2] . Astroviruses are commonly detected alongside other enteric viruses, especially noroviruses in alimentary specimens. A previous study suggested that BAstV may be excreted by up to 60-100% of calves on farms [4] , but a later study showed that only 5 (2.4%) of 209 rectal swabs collected from asymptomatic adult cattle tested positive for BAstV [21] . Unlike other enteric viruses, bovine astrovirus is characterized by asymptomatic viral shedding in stool samples without heavy diarrheal disease [4, 24] .