Avian astroviruses are common, endemic viruses of chickens, turkeys, ducks, and geese. They mainly affect young birds, causing enteritis and growth retardation, although some can be vertically transmitted, causing hatchery disease (aka,, white chicks disease). Older birds are generally less susceptible to the viruses, and many strains are of low or no pathogenicity and so are frequently detected in healthy birds. Typically, astroviruses are specific to a host species for which they are often named (eg, chicken astrovirus), although crossover between host species is known to occur.
Astrovirus infections are typically harmful to very young birds, whereas older birds appear more resistant. They are associated with malabsorption diseases, such as malabsorption syndromeof chickens and poult enteritis complex in turkeys, and they often cause diarrhea, enteritis, and growth restriction in young birds. Although their main impact is economic due to production losses, they are also associated with fatal diseases, and they may involve organs outside of the gut (eg, nephritis with visceral gout in chicks and goslings and a fatal hepatitis of ducks). They have also been associated with hatchery disease of broiler chicks through vertical transmission, leading to embryo dwarfing, dead-in-shell embryos, and weak, pale, runted chicks.
Avian astroviruses are highly prevalent viruses, globally infecting diverse avian species, including many wild birds. Avian nephritis and chicken astrovirus infections are highly common in broilers, and broiler breeders often present as coinfections and with other enteric viruses such as fowl adenovirus and avian orthoreovirus. Avian astroviruses are contracted via the fecal-oral route and, in some cases, are known to pass vertically, potentially resulting in deformed embryos, but this is strain-dependent because pathogenicities vary widely and healthy chicks may hatch with high loads. Astroviruses continue to be shed in feces for several weeks after recovery from clinical disease and pass rapidly through flocks. Serologic surveys for turkey and chicken astroviruses have detected high levels of seroconversion in flocks affected with enteritis and growth but also lower levels in healthy flocks, demonstrating that subclinical infections are common.
Infections are rapid, and signs are variable, ranging from subclinical to mortality approaching 50% in young birds, depending on the age of the flock and pathogenicity of the strain. Signs can include:
Although mainly enteric infections, all astroviruses can be detected in other organs. The onset of duck hepatitis may be rapid, with death soon after signs first appear, including loose, whitish feces and severe opisthotonos. Immune suppression was reported in poults infected with turkey astrovirus serotype 2.
Chicks with malabsorption syndrome may have multiple, microscopic lesions in the intestinal wall filled with necrotic cells and mucin. Villous atrophy and altered crypt depth or mild crypt hyperplasia may be observed. This also occurs in infections by turkey astroviruses. especially in the jejunum. These infections can also result in mild to moderate atrophy of the bursa and thymus. In chick nephritis and gosling gout diseases, kidney lesions and widespread urate deposits on the viscera may be apparent. Ducklings may present with multiple hemorrhages of the liver, bile duct hyperplasia, and enlarged spleens.
Chicken astrovirus should be differentiated from avian nephritis virus and infectious bronchitis virus in cases of kidney disease. Molecular detection of astroviruses is most convenient by RT-PCR. Commercial conventional and quantitative RT-PCR assays are available for chicken astrovirus, avian nephritis virus, and turkey astrovirus serotypes 1 and 2. Duck astrovirus serotypes 1 and 2 should be differentiated from duck hepatitis virus 1, which is a picornavirus. Astroviruses can be difficult to grow in cell culture, so this is not generally a recommended diagnostic method. Some strains may be identified by electron microscopy from gut contents or propagated in embryonated eggs via yolk sac inoculation. A commercial ELISA is available that can screen sera for antibodies to group B chicken astrovirus. Serologic tests are not widely available and may be limited in range because of the high level of genetic diversity exhibited by astroviruses.
Typically, commercial vaccines do not exist for avian astroviruses, although autogenous vaccines have been reportedly used successfully. The unenveloped viruses are more resistant to disinfectant and can persist in poultry houses. Darkling beetles have tested strongly positive for chicken astrovirus, so good biosecurity, ample downtime between flocks, and fogging with formaldehyde are recommended. There are no treatments for avian astroviruses, so ideally breeders should have seroconverted against wild type infections before lay to prevent vertical transmission and provide maternally-derived antibodies to protect the offspring.
Astroviruses are highly common, evolve rapidly. and infect all types of poultry.
Astroviruses are associated with enteritis, diarrhea, growth retardation, and production diseases in young birds, including hatchery diseases.
No commercial vaccines or effective treatments for astroviruses exist, so biosecurity and good hygiene are important to control infections.