astrovirus infections

International Common Names

• English: bovine astrovirus infection; caprine astrovirus infection; diarrhea due to astrovirus infection; duck virus hepatitis type II; enteritis in calves; neonatal diarrhoea; ovine astrovirus infection; porcine acute gastroenteritis; porcine astrovirus infection; viral gastroenteritis

Astroviruses were first discovered in the diarrhoeic faeces of children in 1975, but have since been isolated from many mammals, including cattle, swine, sheep, deer, cats, dogs and mice, as well as in poultry such as turkeys and ducks, and in captive ostriches; the viruses are associated with gastroenteritis in man, cattle, swine, sheep, deer, cats, dogs, mice, and turkeys. The most important clinical implication is as a causal agent of hepatitis in ducks, which causes high mortality; to date, this disease occurs commonly in the UK. Each host species has its own astrovirus, which differ from each other antigenically. Cross-infection and cross-protection do not normally occur. Different serotypes occur within affected host species. The economic importance of the viruses (except in ducks) is difficult to assess because the virus is frequently isolated in conjunction with other pathogens, and frequently without clinical symptoms. Furthermore, in most experimental infections of animals, disease is usually mild or inapparent.

Multiple haemorrhages in the livers of ducks can be observed. The kidneys are swollen, and the spleen is enlarged (Gough and Stuart, 1993). Microscopically, hepatocytes show necrotic cytoplasm. Other organs are normal (Gough and Stuart, 1993).

After experimental infection of turkeys with turkey astrovirus, caeca were distended with fluid and gas, and there was gaseous fluid in the intestinal tract, which also shows loss of muscular tone (Thouvenelle et al., 1995a; McNulty and McFerran, 1996).

Bovine astroviruses are not pathogenic (Bridger, 1990); astroviruses have been isolated from calves with diarrhoea, but after experimental infection with bovine astrovirus alone no diarrhoea develops, which suggests little or no pathogenicity of astroviruses in cattle (Kurtz and Lee, 1987). Only after concomitant experimental infection of gnotobiotic calves with other viruses such as rotavirus or Breda virus, did scour occur. In contrast, ovine, turkey, and feline astroviruses do produce diarrhoea during experimental infections (Bridger, 1990). Experimental infection of cats causes mild diarrhoea with fever (Kurtz and Lee, 1987; Bridger, 1990; Geyer et al., 1994).

In sheep, the incubation time after experimental infection with ovine astrovirus was about 4 days (Snodgrass et al., 1979), after which yellowish diarrhoea occurred that lasted a few days. Outbreaks of diarrhoea in lambs caused by astroviruses have been recorded to occur naturally (Snodgrass and Gray, 1977). In pigs, deer, dogs, cats and mice, astroviruses have been found to be associated with diarrhoea, but frequently other viruses were also isolated that could equally contribute to the observed clinical symptoms, such as coronaviruses, caliciviruses or rotaviruses. In young turkeys and ostriches, astroviruses have been found to be associated with scours, but likewise other viruses were isolated from the same animals, including adenoviruses, enteroviruses, or group D rotavirus in turkeys (Kurtz and Lee, 1987; Saif et al., 1989, 1990; Bridger, 1990; Reynolds, 1992; McNulty, 1993; Shivaprasad et al., 1994; McNulty and McFerran, 1996). Experimental inoculation of 1-day-old turkey poults with turkey astrovirus produces diarrhoea, reduced weight gain, and reduced absorption of d-xylose (Bridger, 1990; Thouvenelle et al., 1995b).

Astroviruses cause hepatitis type II and are important pathogens in ducks (Kurtz and Lee, 1987; Gough and Stuart, 1993). Disease caused in ducks is similar to hepatitis type I, caused by an enterovirus (Gough and Stuart, 1993). Ducks die within a few hours after appearance of illness, frequently preceded by convulsions and opisthotonus (Gough and Stuart, 1993).

In human astrovirus infections, fever, malaise, and vomiting occurs, followed by diarrhoea, after an incubation period of about 3-4 days (Kurtz and Lee, 1987). The episode of diarrhoea normally lasts 2-3 days and sometimes 14 days, but may last much longer in immunocompromised persons (Kurtz and Lee, 1987).

Eleven of 22 cattle herds examined in the UK had antibody to bovine astrovirus and 30% of cattle tested in the USA were positive (Bridger, 1990). In Japan, 50 of 128 randomly sampled adult pig sera reacted positively to a swine astrovirus-specific ELISA, but prevalence between herds varied between zero and 83% (Shimizu et al., 1990).

Astroviruses have been found in 29% of healthy turkey poults, and in 78% of diseased turkey poults in the USA in one study (McNulty, 1993). In another study in the USA, 86% of 43 turkeys aged 6-35 days were astrovirus-positive in flocks with diarrhoea, and in 41% of 22 healthy flocks (Bridger, 1990).

Astroviruses are mainly transmitted horizontally by the faecal-oral route (Kurtz and Lee, 1987). There is no evidence for vertical transmission (Kurtz and Lee, 1987; McNulty, 1993).

Astroviruses are species-specific. Experimental infections of ducks with astroviruses from chickens or turkeys did not cause mortality, wheras subsequent infection of the same animals with duck astrovirus caused hepatitis (Gough and Stuart, 1993). Generally, the prevalence of astroviruses seems to increase with age in most species, but clinical complications seem most severe in young animals after primary infections, alone or in conjunction with other viral enteral pathogens. Infected ducks are refractory to re-infection (McNulty and McFerran, 1996).

In a sample of healthy dogs, 2% were found to harbour astroviruses in their faeces (Bridger, 1990).

Generally, astrovirus infections cause mild or inapparent disease, or gastro-intestinal disorders. The ability of astroviruses to cause disease appears to differ between isolates from different species. As other pathogens are isolated from cases of gastro-enteritis in cattle, the economic impact of astroviruses is difficult to assess.

Astrovirus infection can cause decreased growth (stunting) and diarrhoea due to malabsorption in hatchling turkey poults, with transient maldigestion (Reynolds, 1992; Thouvenelle et al., 1993). Duck astrovirus infections may lead to 50% mortality in ducklings of 1-2 weeks, and up to 25% mortality in ducklings of 3-6 weeks of age (Jordan et al., 1986; McNulty and McFerran, 1996).

Astroviruses are species-specific, and animal astroviruses pose no risk for humans; humans are only susceptible to human astroviruses.

There are no chemotherapeutics or other measures reported to be efficacious for control of astrovirus infections in poultry (Reynolds, 1991). Antiviral drugs are not available, although experimentally, certain plant extracts have been shown to demonstrate in vitro antiviral activity against astroviruses (Kudi and Myint, 1999).