Astrovirus MLB1 (AstV‑MLB1) is a genotype of human astrovirus belonging to the species Mamastrovirus 8 in the family Astroviridae. It is a non‑enveloped, positive‑sense single‑stranded RNA virus that forms icosahedral capsids with a characteristic star‑like surface. The virus differs genetically from the eight classic human astroviruses and has been identified in stool specimens and occasionally in systemic samples from individuals with gastroenteritis.
Explanation
Astroviruses are small (28–30 nm) non‑enveloped viruses with a positive‑sense RNA genome of about 6.4–7.7 kilobases divided into three open reading frames: ORF1a encodes a serine protease, ORF1b encodes the RNA‑dependent RNA polymerase via a ribosomal frameshift, and ORF2 encodes the structural capsid protein. MLB1 belongs to the MLB clade that is phylogenetically distinct from the classic human serotypes and shares distant similarity with animal astroviruses. It was first detected in the stool of a three‑year‑old child in Australia and whole‑genome sequencing confirmed the typical astrovirus organization. Screening of stool samples from children in the United States showed that about 1.6 % of specimens contained MLB1, suggesting that the virus circulates globally. Under electron microscopy, the capsid of MLB1 exhibits a characteristic star‑like surface, a hallmark of the family. MLB1 infects the gastrointestinal tract and is transmitted through fecal contamination. Most infections cause mild diarrhoea or are asymptomatic, but it can occasionally be found in immunocompromised individuals. The virus is resilient to heat and acidic environments and lacks an envelope, so it is thought to be released by cell lysis. Its divergence from classic human astroviruses means that conventional diagnostic assays may miss it, requiring molecular sequencing for identification.
Examples and observations
The original case that brought MLB1 to attention involved a young child in Melbourne who presented with unexplained diarrhoea; the virus was identified during metagenomic analysis of the stool. Subsequent studies detected MLB1 sporadically in pediatric stool samples in North America and Asia, and in one study it accounted for about 1.6 % of diarrhoeal specimens. Infections are often co‑detected with other enteric viruses such as rotavirus or norovirus, making it difficult to assign causality. To date, no large outbreaks or severe systemic disease have been definitively attributed to MLB1. The current understanding of Astrovirus MLB1 is that it is a divergent human astrovirus genotype that circulates at low frequency and may contribute to mild gastroenteritis. Further epidemiological and virological studies are needed to clarify its prevalence, transmission patterns and potential impact on human health. Related Terms: Astrovirus MLB2, Astrovirus MLB3, Astrovirus VA1, Astrovirus VA2, Human Astrovirus 1