Ambili Ramachandran
Humans and Viruses 2004
Prof. Robert Siegel
Stanford University
Astroviridae
+

Stool sample from an individual with gastroenteritis.
Negative-stain Transmission Electron Microscopy. Bar = 50 nanometers.
F.P. Williams, U.S. EPA



INTRODUCTION
CLASSFICATION and TAXONOMY
TRANSMISSION and PATHOGENESIS
CLINICAL PROFILE
PREVENTION and MANAGEMENT
UPDATES for 2003-2004
REFERENCES and LINKS
PATHOGEN CARDS




INTRODUCTION
Welcome to the Astroviridae homepage! This webpage was produced for Humans and Viruses, a course taught by Dr. Robert Siegel in the Program in Human Biology at Stanford University. The purpose of this webpage is to expand on information collected by previous students and to serve as a reference for other aficionados of virology.

Past Astroviridae webpages
Astroviridae 1998 - Caitlin Pickart
Astroviridae 1999 -- Jessica Davy
Astroviridae 2000 -- Win Han

A full listing of student webpages for all 22 human virus families is available at the Stanford Virology website.
CLASSIFICATION and TAXONOMY

Family: Astroviridae
Genus: Mamastrovirus
Species: Human Astrovirus, sertoypes 1-8

Family Facts

+Positive sense single-stranded RNA (+ssRNA)
+Nonenveloped
+Icosahedral capsid
+Small size (28-30 nanometers in diameter)
+Round viruses with characteristic starlike surface appearance when viewed by electron microscopy, hence the name Astroviridae. Stars may be five- or six-pointed.
TRANSMISSION and PATHOGENESIS
Astrovirus is most frequently transmitted through a fecal-oral route. Contaminated food, water, or fomites have been suspected in several breakouts.
Adults usually do not develop gastroenteritis when deliberately given the virus in volunteer studies.
In outbreaks in military barracks and childcare settings, adults may have been exposed to high doses of virus and only then have fallen ill.
Astrovirus is believed to replicate in the intestinal tissue of the jejunum and ileum.
CLINICAL PROFILE

Clinical disease

Astrovirus infection causes gastroenteritis, which is characterized by the production of copious, watery diarrhea, followed for complaints such as nausea, vomiting, fever, malaise, anorexia, and abdominal pain for up to 4 days

Incubation period

Volunteer studies with healthy adults found that the average incubation period for astrovirus infection was 3 to 4 days.

Duration of illness

Diarrhea lasts for 2 to 3 days, and other complaints such as vomiting, fever, anorexia, and abdominal pain can last 4 days.

Outcome

Astrovirus infection is generally mild and self-limiting, rarely leading to severe dehydration, hospitalization, or death. Symptoms usually resolve on their own, although individuals may continue to shed virus in their feces for days afterward. Persistent gastroenteritis occasionally occurs.

Epidemiology

+A ubiquitous virus, astrovirus infections are found worldwide.
+Young children, especially in childcare settings, are most likely to develop clinical illness.
+Elderly patients in nursing facilities and military recruits have also experienced outbreaks of astroviral gastroenteritis. Astrovirus is an important cause of enteric disease in immunocompromised individuals, too.
+Similar to rotavirus, astrovirus infections tend to occur during winter months in temperate regions and in the rainy season in tropical regions.
+More refined detection techniques, such as reverse transcription-polymerase chain reaction (RT-PCR), have demonstrated that astrovirus infection is more common and important to viral gastroenteritis than previously shown.
PREVENTION and MANAGEMENT

Prevention

+There is no vaccine for astrovirus
+Personal hygiene and decontamination of outbreak settings are important to reducing transmission and infection
+Individuals can continue to shed astrovirus in their feces several days after illness resolves, and so should continue to take precautions after recovering from overt illness.

Management

+There is no anti-viral treatment for astrovirus infection, and astroviral disease rarely requires hospitalization
+However, young children who are at risk due to preexisting malnutrition or illness should be treated with oral rehydration therapy (ORT) to avoid severe consequences of gastroenteritis. ORT is also used for rotaviral infections and other diarrheal diseases, such as cholera.

UPDATES for 2003-2004

(1) Walter JE, Mitchell DK. "Astrovirus infection in children." Current Opinion in Infectious Diseases, 16:247-253 (2003)

Walter and Mitchell review astrovirus as an agent of enteric disease in children. They report on how techniques such as enzyme immunoassay (EIA) and reverse transcription-polymerase chain reaction (RT-PCR) have improved surveillance and diagnosis. Identification of viruses by electron microscopy (EM) has considerable limitations; for example, only 10% of astrovirus particles display the characteristic starlike morphology used to identify the virus in EMs. RT-PCR, which is even more sensitive than EIA, can detect astrovirus even at low doses. These techniques have extended epidemiological knowledge of related enteric viruses such as astroviruses, caliciviruses, and rotavirus, and have helped clarify the burden of disease attributable to each viral species. Astroviruses make up a higher percentage of diarrheal diseases than previously thought. EIA and RT-PCR are also useful for serotyping viral strains and measuring persistent viral shedding in recovering individuals.

(2) Caballero S, Guix S, Morsy El-Senousy W, Calicó I, Pintó RM, Bosch A. "Persistent gastroenteritis in children infected with astrovirus: association with serotype-3 strains." Journal of Medical Virology, 71:245-250 (2003)

Researchers in Barcelona, Spain, used competitive RT-PCR to quantify the presence of astrovirus in stool samples. Using this technique, the authors found that average viral titers for astrovirus serotype-3 exceeded titers for other serotypes. Serotype-3 astrovirus also appeared to cause a larger proportion of cases of persistent gastroenteritis. Higher viral titers of serotype-3 could be due to greater replication of virus and shortcomings of the immune system to stop halt viral infection sooner.

(3) Koci MD, Moser LA, Kelley LA, Larsen D, Brown CC, Schultz-Cherry S. "Astrovirus induces diarrhea in the absence of inflammation and cell death." Journal of Virology, 77(21):11798-11808 (2003)

Koci and colleagues investigated the effect of turkey astrovirus type-2 (TastV-2) on the histopathology of the intestine and thymus in young turkeys. Although all infected turkeys experienced severe diarrhea, examination of intestinal tissue revealed minor lesions with little evidence of widespread inflammation. In keeping with these findings, the authors determined that TastV-2 infection did not increase cell death. Tumor growth factor-beta (TGF-B), an immunosuppressive cytokine, had elevated activity in the serum of infected birds relative to controls and may be responsible for the absence of inflammation in the intestines. Koci et al hypothesize that TGF-B helps maintain the epithelial barrier of the intestines. It is still not know how TastV-s induces diarrhea in hosts, especially given the lack of evidence of inflammation or cellular damage.

(4) Koci MD, Kelley LA, Larsen D, Schultz-Cherry S. "Astrovirus-induced synthesis of nitric oxide contributes to virus control during infection." Journal of Virology, 78(3):1564-1574 (2004)

Koci et al studied turkey astrovirus type-2 (TastV-2) in young turkeys as a means of understanding mechanisms of astroviral pathogenesis and host immune response. The authors found that astrovirus replicated in the intestines, but that virus could be detected throughout the body. In terms of immunity, researchers did not find any significant differences in antibody production or T-cell counts between infected and control animals, suggesting that the adaptive immune response was not crucial to clearing TastV-2 infection. On the other hand, Koci and colleagues found that TAstV-2 stimulated an innate immune response, namely activation of macrophages to produce nitric oxide (NO), and that NO inhibits TAstV-2 replication.
Koci el al propose developing the turkey as a small animal model for astroviral infection, however, astroviruses are strikingly species-specific in their range and immune response, and the generalizability of results from turkeys to other animals, especially humans, is questionable. For example, in humans, both the humoral and cell-mediated arms of the adaptive immune system respond to astroviral infection. As the authors further suggest, the turkey model might be better suited to simulate pathogenesis and immune response in immunocompromised hosts.

(5) Méndez-Toss M, et al. "Prevalence and genetic diversity of human astroviruses in Mexican children with symptomatic and asymptomatic infections." Journal of Clinical Microbiology, 42(1):151-157 (2004)

Samples from five different locations in Mexico were collected from October 1994 to March 1995 and analyzed to determine the distribution patterns of astroviruses, rotaviruses, and adenoviruses. Stool samples were collected from children younger than five years of age with or without diarrhea. When samples were serotyped using EIA, investigators found all eight serotypes except serotype 5 among the five locations. There was no correlation, however, between the infecting serotype and symptomatic status. Serotypes 1 and 3 were most common among children with diarrhea, and serotype 3 was also the common among asymptomatic children. More than one serotype was frequently found in circulation in each location.

Refernces and Links

+Matsui SM, Greenberg HB. "Astroviruses." Chapter 28 in Fields Virology, 4th edition. Fields B, Knipe D, Howley P, eds. Lippincott-Raven: Philadelphia, 2000 (875-893)

+The Astrovirus Homepage of the Institute for Animal Health
Links to laboratories engaged in astrovirus research.

+ICTV - International Committee on the Taxonomy of Viruses
Useful for classification information of human and animal astroviruses.

+All the Virology on the WWW
Links related to astrovirus biology and disease (notice the link to Jessica Davy's Humans and Viruses webpage!)

PATHOGEN CARDS

Click here to see pathogen cards for Human Adenovirus 2, Human Herpesvirus 6a, and Human Papillomavirus 33.



This page last updated March 10, 2004. Comments.