Turkey viral hepatitis (TVH) is a highly infectious disease affecting young turkey poults.
Etiology of Turkey Viral Hepatitis
Turkey viral hepatitis is caused by a picornavirus, designated as melegrivirus A (turkey hepatitis virus) in the genus Megrivirus. It has a positive-sense, single-strand RNA genome.
The virus is nonenveloped. It is thermostable; resistant to ether, phenol, and creolin, but not formalin; and susceptible to high, but not low, pH.
Epidemiology of Turkey Viral Hepatitis
Turkey hepatitis virus is shed in feces and transmitted by both direct and indirect contact. The virus is mainly transmitted by the fecal-oral route. It has been suggested that the virus can be vertically transmitted like other picornaviruses; however, there is no experimental evidence for this.
Morbidity and mortality rates vary according to the severity of stress. In poults < 6 weeks old, morbidity rates may reach 100% and mortality rates up to 25%. However, in most instances daily mortality rate is below 0.1%.
Clinical Findings of Turkey Viral Hepatitis
Turkey viral hepatitis is usually a subclinical disease that becomes apparent as a result of stresses due to environmental conditions or concurrent infection. Clinical signs in TVH-affected birds are not well defined.
Variable levels of listlessness may be observed in affected flocks; however, more commonly disease is characterized by sudden deaths of apparently healthy birds.
Breeder flocks may have decreased egg production, fertility, and hatchability; however, this has not been conclusively determined.
Gross lesions are confined to the liver and pancreas. Livers generally are enlarged. Lesions consist of focal, gray, sometimes depressed areas up to several millimeters in diameter.
Lesion distribution is variable; birds that die usually exhibit extensive lesions that often coalesce and may be masked by vascular congestion and hemorrhage.
Gross lesions in the pancreas are less frequently observed than hepatic lesions. Lesions in the pancreas generally are roughly circular, gray to pink, and may extend across a lobe. Gross lesions are present in only ~50% of all cases.
Histopathologic findings in the liver include coagulative necrosis of hepatocytes and inflammation characterized by an influx of mostly lymphocytes and macrophages. Lesions in the pancreas include focal necrosis of acinar cells and inflammation.
Diagnosis of Turkey Viral Hepatitis
A presumptive diagnosis of turkey viral hepatitis may be obtained by histologic examination; the presence of lesions in both the liver and pancreas is highly suggestive of this disease. However, similar lesions in the liver are produced by other pathogens, including Salmonella spp, Pasteurella multocida, avian adenoviruses, reovirus, and Histomonas meleagridis.
Virus isolation is accomplished by inoculation of 5- to 7-day-old embryonated chicken eggs by the yolk sac route. Infected chicken embryos have a high mortality, and dead embryos will be stunted with hemorrhages and liver lesions. Preferred clinical samples include liver, pancreas, spleen, kidney, intestinal contents, and feces.
Reverse transcription PCR assay can detect TVH viral RNA in tissues, intestinal contents, or feces.
Convalescent sera from naturally infected turkeys have been used for immunohistochemistry; however, no serologic tests are currently available.
Treatment and Control of Turkey Viral Hepatitis
There is no known treatment for turkey viral hepatitis.
Management procedures that decrease stress and other infections help prevent normally subclinical disease from developing into TVH.
Turkey viral hepatitis is a picornavirus disease of young turkeys characterized by the presence of hepatitis with or without pancreatitis. The disease also has been linked to decreased egg production, fertility, and hatchability in breeder hens.
Diagnosis of TVH is based on histologic evaluation, virus isolation, or detection of viral RNA by RT-PCR assay. Preferred clinical samples include liver, pancreas, spleen, kidney, intestinal contents, and feces.
No specific treatment or preventive measures are available. Prevention of stress and other infections helps prevent normally subclinical disease from developing into TVH.