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

Here, we detected a high prevalence of bovine astrovirus by RT-PCR from rectal swabs collected from young calves of cattle and water buffalo with diarrhea. However, more studies are needed to determine whether the persistent diarrhea observed in these calves was mainly associated with the high prevalence of astroviruses, as previous studies reported that bovine astroviruses were not directly associated with severe diarrhea in calves under natural conditions [4, 24] . However, other reports [14, 26] indicated that bovine astrovirus may evolve to severe diarrhea in co-infections with other gastrointestinal viruses, as in the case of BAstV co-infection with BRV or BToV (Breda virus) [26] . Nonetheless, the epidemiology of bovine astrovirus remains unclear, especially considering the limited number of studies of cattle and water buffalo. While excretion of BAstV may occur in up to 60-100% of calves on farms [4] , only 5 (2.4%) of 209 rectal swabs collected from asymptomatic adult cattle were positive for BAstV [21] . In the present study, sampling at different time points demonstrated a high prevalence of up to 56.52% among calves with diarrhea. Moreover, in our complementary study, astrovirus was detected alongside other gastrointestinal viruses, including BEV, BCoV, BRV and BVDV, in 87.5% of cases. Surprisingly, 12.5% of these cases were positive for bovine astrovirus, but yet negative for other tested gastrointestinal viruses. In contrast, titers of BRV and BToV, which have been previously reported as principal gastrointestinal co-infecting viruses with bovine astrovirus, were minimal or undetectable in this study. From these results, it is clear that astrovirus may be directly associated with diarrhea or possibly linked to other factors, such as poor hygienic conditions or associated with other non-viral pathogens, such as bacteria or parasites, which are known causative agents of diarrhea in calves. Similar results were reported in Korea, where co-infection of bovine astrovirus with gastrointestinal viruses, other than BRV and BToV, was associated with clinical symptoms of diarrhea in 20-and 14-day-old calves [14] .