Astroviruses

Description of Pathogen

Astroviruses are nonenveloped, positive-sense, single-stranded RNA viruses in the family Astroviridae. ^, By electron microscopic examination, astroviruses are 28–30 nm in diameter with a smooth edge, and they sometimes have a characteristic star-like appearance in the center (Greek, astron meaning “star”) ( Fig. 240.1 ). ^, At least 8 distinct serotypes (HAstV 1–8) of human astrovirus are recognized, defined both antigenically and by genetic sequence differences, with several novel species described since 2008. Serotype 1 viruses are detected most commonly, but more than one serotype may circulate in communities during each season. ^, Non–serotype 1 viruses can predominate in a season, and greater serotype diversity can be found in developing countries. ^,

Epidemiology

Like rotaviruses and caliciviruses, astroviruses have worldwide distribution and have been detected as causes of sporadic gastroenteritis and outbreaks. In both economically developed and developing countries, astroviruses have been detected in 2%–9% of children treated for acute, nonbacterial diarrhea with prevalence higher in outpatient compared to hospitalized children. The lower proportions reported from some studies (<1%–3%) may reflect insensitive detection methods rather than true prevalence. ^{, ,} The mean prevalence of astroviruses globally is 11%, with detection usually higher in developing countries and in rural areas (23%) compared with urban areas (7%). Co-infections with astrovirus and other enteric pathogens are common. Since the 1990s, a decreasing trend in astrovirus detection has been observed, but methodologic differences over time make such longitudinal comparisons difficult.

Outbreaks of astrovirus gastroenteritis have been reported in closed settings such as schools, childcare centers, hospitals, nursing homes, and households, and they can be associated with high attack rates. ^{, ,} Astroviruses have been reported to be responsible for 5%–16% of cases of nosocomial gastroenteritis in children’s hospitals. ^{, ,}

Although astroviruses have been detected in all age groups, most infections occur in children aged <2 years. ^{, , ,} Most adults have serum antibodies against astroviruses that are acquired early in life. Disease in adults is uncommon. In volunteer studies, most adults challenged with astrovirus did not become infected or develop diarrhea. However, illness among teachers during school outbreaks has been described, perhaps as a result of a large dose of virus in this type of setting or a different mechanism of spread. ^, Outbreaks among older adults may reflect waning immunity associated with increasing age. Astroviruses also have been associated with disease in immunocompromised adults. Astroviruses have been found in stool from asymptomatic people but may reflect prolonged shedding of the virus.

Transmission is by the fecal-oral route, generally through person-to-person contact, but transmission occasionally can occur by contaminated food, water, or fomites. ^{, , ,} The infectious dose has not been established. In temperate climates, astrovirus shows a seasonal distribution similar to that of rotavirus with a peak in winter; seasonality is less clear in tropical settings but tends to peak during the rainy season. ^{, , , , , ,}

Clinical Manifestations

The median incubation period for astrovirus infections is 4.5 days. Symptomatic illness occurs more commonly and with increased symptoms in infants and young children, although asymptomatic infections can occur at all ages. ^, Clinical symptoms generally are milder but similar to symptoms caused by rotavirus, with 2–5 days of watery diarrhea, often accompanied by vomiting and less often by high fever and abdominal pain. ^{, , ,} Illness generally is mild and self-limited, but malabsorption and lactose intolerance have been reported following infection. ^{, ,} In highly immunocompromised children, astroviruses can spread systemically and cause severe, disseminated, lethal infections. Stools do not contain blood or mucus. Children can shed the virus 1–2 days before illness and for a median of 5 days after the onset of symptoms. ^{, ,} Prolonged diarrhea has been reported among children with malnutrition and in immunocompromised patients. Asymptomatic viral excretion among healthy children has been reported for 3 weeks when more sensitive detection methods have been used. Persistent excretion occurs in immunocompromised patients. Because illness is largely confined to young children and older adults, infection probably confers protection that is relatively durable.

Recent reports have associated astrovirus with encephalitis and meningitis in immunocompromised patients. Astroviruses also have been identified as potential, but uncommon, risk factors for intussusception in young children in several studies.