Avian astroviruses (AstVs) have been detected in several species of poultry and typically cause diarrhea, enteritis, and growth retardation in young birds. In turkeys, AstVs are commonly associated with poult enteritis complex, whereas in chickens, AstVs are associated with nephritis, white chick syndrome, and runting and stunting syndrome. Diagnosis is generally made by RT-PCR assay. There are no treatments or commercial vaccines available, so prevention relies on good biosecurity, environmental mitigation, and good management practices.
Avian astroviruses are common endemic viruses of chickens, turkeys, ducks, and geese. They are transmitted by the fecal/oral route and primarily affect young birds, causing enteritis and growth retardation (see runting and stunting image). However, vertical transmission has been demonstrated for AstVs that cause white chick syndrome. White chick syndrome causes decreased hatchability of broiler breeder progeny and increased hatch of weak, white chicks.
Older birds are generally less susceptible to disease, and because many AstVs are of low or no pathogenicity, they are frequently detected in healthy birds. AstVs are typically specific to a host species for which they are often named (eg, chicken astrovirus), although crossover between host species is known to occur.
Chicken astroviruses (CAstVs), through vertical transmission, have also been associated with hatchery disease of broiler chicks manifesting as embryo dwarfing, dead-in-shell embryos, and weak, pale, runted chicks.
Courtesy of Dr. Guillermo Zavala and Dr. Holly Sellers.
Epidemiology and Transmission of Astrovirus Infections in Poultry
Young birds have an age-related susceptibility to astrovirus infection resulting in disease. AstVs have been isolated from and detected in both chickens and turkeys with enteric disease. Clinical signs in both species are very similar and consist of diarrhea and stunting.
Turkey astroviruses (TAstVs) are associated with several enteric disease syndromes, including poult enteritis mortality syndrome and poult enteritis complex. Clinical signs of TAstVs develop in turkeys between 1 and 3 weeks old and can last up to 2 weeks.
CAstVs are associated with runting and stunting syndrome (RSS) in broilers and have been detected during the first week of life. RSS is characterized by substantial growth depression and the hallmark histological lesion characterized by cyst formation in the intestinal crypts of Lieberkuhn. A novel CAstV, isolated in cell culture, has been identified as an etiological agent of RSS following experimental reproduction of the disease in susceptible broilers.
While both CAstVs and TAstVs are detected in birds with enteric diseases and are considered to play an important role in causing clinical signs of disease, they have also been detected at a lower rate in apparently healthy birds. With CAstV and TAstV infection, mortality rate is usually low; however, the morbidity rate can be moderately high, depending on the severity of disease. Immunosuppression has been described in poults infected with TAstV-2.
Clinical Findings of Astrovirus Infections in Poultry
Onset of clinical signs with astrovirus infection is rapid following infection, with CAstV RNA staining in the duodenum evident as early as 6 hours after infection in experimentally infected birds. Clinical signs vary and range from subclinical infection to death, with mortality rates approaching 50% in young birds, depending on the age of the flock at the time of infection and the pathogenicity of the virus. Clinical signs can include the following:
diarrhea
enteritis
wet litter
frothy cecal contents
thin-walled intestines
dehydration
abnormal feathering
litter-eating
stunting
body weight suppression
high feed conversion rates
Courtesy of Dr. Holly Sellers.
Courtesy of Dr. Holly Sellers.
Although AstV infections are mainly enteric, all astroviruses can also be detected in other organs.
Lesions
Cystic enteropathy is a hallmark lesion in clinical cases of RSS in broilers and is most prevalent in the duodenum during the first 1–5 days after hatching. Other microscopic lesions associated with AstV infection of chickens and turkeys include multiple dilated crypts in the intestinal mucosa filled with necrotic cells and mucin. Villous atrophy and altered crypt depth or mild crypt hyperplasia may also be observed.
Astrovirus infections may result in mild to moderate atrophy of the cloacal bursa and thymus. In cases of avian nephritis in baby chicks, kidney lesions and widespread urate deposits on the viscera may be apparent.
Courtesy of Dr. Holly Sellers.
Diagnosis of Astrovirus Infections in Poultry
RT-PCR assay
Virus isolation
ELISA and virus neutralization assay
Molecular detection of astroviruses is routinely performed by RT-PCR assay. Commercial conventional and quantitative RT-PCR assays exist for CAstV, avian nephritis virus, and TAstV serotypes 1 and 2, but none of the commercial kits are widely available in the US. In-house RT-PCR assays are offered by several avian diagnostic laboratories.
Due to the genetic variability of the astrovirus genome, the target of RT-PCR primers for diagnostic testing should be taken into consideration. AstVs can be isolated from avian epithelial cells; however, not all AstVs can be cultivated in cell culture. Isolation from and propagation in chicken or turkey embryos via yolk sac inoculation has been successful for both chicken and turkey AstVs.
Electron microscopy has been used to visualize AstVs in intestinal contents; however, the majority of astroviruses lack structural features used for definitive identification and therefore are usually identified simply as small round viruses. A commercial ELISA kit is available for detection of group B chicken astrovirus antibodies but is not licensed in all countries and may require a special permit to import.
Virus neutralization (VN) testing can detect antibodies against group-specific AstVs; however, this testing is not widely available in avian diagnostic laboratories. VN can detect astrovirus-specific antibodies if the laboratory has the specific astrovirus isolate required.
Prevention and Treatment of Astrovirus Infections in Poultry
AstV isolates in autogenous vaccines
Increased ventilation and fresh litter
Proper brooding temperatures
Good biosecurity
Commercial vaccines do not exist for avian astroviruses; however, anecdotal reports document successful use of autogenous vaccines containing chicken astroviruses. AstVs are resistant to inactivation by heat and many disinfectants, so prevention should focus on biosecurity and environmental management practices to mitigate disease.
AstV has been detected in darkling beetles, so pest management in poultry houses, good biosecurity, extended downtime between flocks, and thorough cleaning and disinfection with appropriate products are recommended.
Zoonotic Risk of Astrovirus Infections in Poultry
Zoonotic risk from avian astroviruses is extremely low, and the viruses are not considered harmful to humans. However, antibodies against TAstV serotype 2 have been detected in poultry workers, indicating that astroviruses have the potential to cross species barriers.
Key Points
Astroviruses are very common, evolve rapidly, and infect all types of poultry.
Astroviruses are associated with enteritis, diarrhea, growth retardation, and production diseases (including hatchery diseases) in young birds.
No commercial vaccines or effective treatments for astroviruses exist, so biosecurity and good hygiene are important to control infections.
For More Information
Cattoli G. Astrovirus infections. In: Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, associate eds. Diseases of Poultry. 14th ed. Wiley; 2020: 416-420.
Schultz-Cherry S, Vaillancourt JP. Astrovirus infections. In: Brugere-Picoux J, Vaillancourt JP, Shivaprasad HL, Venne D, Bouzouaia M, eds. Manual of Poultry Diseases. French Association for the Advancement of Science; 2015:200-203.
