Hemorrhagic enteritis

Often the flock seems normal until birds in good body condition die suddenly. Depression and bloody droppings are seen in some outbreaks, although haemorrhagic faeces are not always present. At necropsy the intestines may be distended and filled with unclotted blood. The spleen varies: sometimes pale and small, often enlarged, congested and purplish with greyish necrotic foci. Microscopy shows Cowdry type B intranuclear inclusions in macrophages and lymphocytes. Secondary bacterial infections may follow in survivors.

Haemorrhagic enteritis of turkeys is caused by turkey haemorrhagic enteritis virus, a Siadenovirus; a closely related virus causes marble spleen disease in pheasants. The virus enters by the oral-faecal route and replicates mainly in the spleen, targeting macrophages and B lymphocytes, in which basophilic intranuclear inclusions form. Bleeding into the gut is not attributed to direct viral destruction of the intestinal lining but to an immune-mediated process in the lamina propria, leading to degeneration and sloughing. Infection is immunosuppressive, so even avirulent strains predispose birds to secondary bacterial disease, notably Escherichia coli infection and necrotic enteritis caused by Clostridium perfringens.

Turkeys are the host of concern. Disease affects birds aged 4 to 11 weeks, most often 7 to 9 weeks; maternal antibodies protect poults for their first 2-3 weeks. Spread is oral-faecal, and the virus is often brought onto a farm on contaminated clothing or equipment. It remains viable for up to 7 weeks in contaminated carcasses or faeces. The virus has been reported in European countries including Italy and Hungary and on turkey farms in Texas and Virginia in the United States. Avirulent strains can cause subclinical, immunosuppressive infection.

Necrotic enteritis caused by Clostridium perfringens, and bacteraemia due to Escherichia coli, Salmonella species or Pasteurella multocida. Intranuclear inclusions in splenic mononuclear cells and detection of the virus point to haemorrhagic enteritis; PCR is more sensitive than agar gel immunodiffusion for confirming it.

Spleen, small intestine and lamina propria, macrophages and B lymphocytes, immune system

Live vaccines made from avirulent virus strains, best given around 28 days of age, though live vaccines can be immunosuppressive; effective sanitation and biosecurity, including clean clothing and equipment.

There is no treatment. Mortality is usually 5-15% but can reach 60% with virulent strains; survivors may stay immunosuppressed and prone to secondary bacterial infection.