Astrovirus MLB3 (AstV‑MLB3) is a genotype of human astrovirus in the MLB clade of the family Astroviridae. Like other astroviruses it is a non‑enveloped, positive‑sense single‑stranded RNA virus with a distinctive star‑like capsid. It is phylogenetically distinct from both the classic human serotypes and the related MLB1 and MLB2 genotypes.
Explanation
Astroviruses are small, 28‑30 nm particles that consist of a single‐stranded positive‑sense RNA genome of roughly 6.4–7.7 kilobases comprising three open reading frames encoding a serine protease (ORF1a), an RNA‑dependent RNA polymerase (ORF1b) and the capsid protein (ORF2). MLB3 is part of the MLB lineage, which diverges markedly from the eight classic human serotypes. It was detected during molecular screening of stool specimens from children with and without diarrhoea. In a survey of 949 stool samples from children in Kenya and The Gambia, astroviruses were found in 9.9 % of samples, and MLB3 represented the most frequent novel type with a prevalence of about 2.6 %. Interestingly, MLB3 was more commonly detected in asymptomatic control subjects than in children with diarrhoea, suggesting that infection may often be subclinical. The genome organization and predicted proteins of MLB3 are similar to other astroviruses, and the capsid exhibits the characteristic star‑like appearance in electron micrographs. Transmission is presumed to occur via the fecal–oral route. Because MLB3 is genetically distinct from common astroviruses, detection requires PCR or sequencing using pan‑astrovirus primers. The clinical significance of MLB3 is still uncertain, but current data indicate that it may not be an important cause of gastroenteritis.
Observational notes
MLB3 was identified in stool samples collected from African and Asian pediatric populations and has since been reported in surveillance studies elsewhere. The virus is often present alongside other enteric pathogens and has not been linked to severe disease. Its higher prevalence among healthy control children compared with those with diarrhoea in one study suggests either that MLB3 causes very mild symptoms or that it may be part of the gut virome without obvious pathogenic effects. Astrovirus MLB3 appears to be a widespread but largely asymptomatic human astrovirus genotype. Ongoing surveillance and functional studies will be important to determine whether it has any pathogenic potential or interacts with other enteric infections. Related Terms: Astrovirus MLB1, Astrovirus MLB2, Astrovirus VA1, Astrovirus VA3, Human Astrovirus 3