Astrovirus MLB2 (AstV‑MLB2) is a genotype of human astrovirus belonging to the MLB clade of the family Astroviridae. It is a non‑enveloped, positive‑sense single‑stranded RNA virus with a star‑like icosahedral capsid, distinct from the classic serotypes that commonly cause infant diarrhoea.
Explanation
Astroviruses are 28‑30 nm virions with a positive‑sense RNA genome of about 6.4–7.7 kb divided into three open reading frames encoding non‑structural and structural proteins. MLB2 is closely related to MLB1 but forms a separate lineage within the mamastrovirus clade. It was identified through metagenomic analysis of fecal specimens from children with diarrhoea and has since been found in stool, plasma and cerebrospinal fluid. In a screening study of 943 fecal samples and 424 cerebrospinal fluid specimens, MLB2 sequences were detected in approximately 3 % of fecal samples and in one cerebrospinal fluid specimen. Some patients with MLB2 had high viral loads in blood and central nervous system tissue, demonstrating the capacity of the virus to disseminate beyond the gut. The capsid of MLB2 displays the characteristic star‑like morphology under electron microscopy. Its genome organization resembles that of other astroviruses, with ORF1a encoding a protease, ORF1b encoding the RNA‑dependent RNA polymerase and ORF2 coding for the capsid precursor. MLB2 infections appear to be transmitted via the fecal–oral route and have been reported mainly in children, including those who are immunocompromised. Although diarrhoea is the most frequent symptom, detection in cerebrospinal fluid suggests potential for neurotropic infection. Diagnostic assays require sequence‑based approaches because of its genetic divergence from classic astroviruses.
Examples and observations
MLB2 was first characterized in a child from Africa with acute diarrhoea. Subsequent investigations found the virus in multiple stool samples from the United States and other countries, often in co‑infections with rotaviruses or adenoviruses. In a notable case, MLB2 RNA was detected at high levels in blood and cerebrospinal fluid of a child with meningitis and in another patient with disseminated infection. The virus has also been found in immunocompromised adults. However, many infections seem to be asymptomatic or cause only mild gastrointestinal illness. Astrovirus MLB2 is a recently recognized human astrovirus that circulates worldwide and can infect both the gut and, rarely, extraintestinal tissues. Continued surveillance and sequencing will help clarify its epidemiology and clinical importance. Related Terms: Astrovirus MLB1, Astrovirus MLB3, Astrovirus VA1, Astrovirus VA2, Human Astrovirus 2