Introduction

Caused by an avian retrovirus. This condition has until now been seen only in meat-type chickens, with considerable strain-to-strain variation. It has occured in Europe, North and South America. Morbidity is low, though there is high mortality of affected birds.

Transmission is by congenital infection from antibody-negative females, bleeding and vaccination needles; lateral transmission by faecal-oral route (this declines as the bird ages). The incubation period is 10-20 weeks. Congenitally infected birds tend to remain antibody negative, shed virus and develop tumours. Virus survival is poor but sufficient to allow cross contamination in hatcheries and on farm in rear.

Signs

• Depression.
• Emaciation.
• Loss of weight.
• Persistent low mortality.
• Enlargement of abdomen, liver.
• Many are asymptomatic.

Post-mortem lesions

• Liver enlargement, often with tumour foci.
• Splenomegaly and enlarged kidneys also occur.
• Most characteristically are chalky white tumours in the bone marrow, particularly of the sternum, ribs, sacral joint.
• Microscopic - tumours usually contain well-differentiated myelocytes. Two cell types may be found in the same tumour.

Diagnosis

History, age, lesions, histology, ultimately identification of virus by isolation and/or PCR. Differentiate from Marek's disease, Lymphoid Leukosis.

Treatment

None.

Prevention

Checking of antigen in the albumen is a basis for eradication - most but not all, birds with egg antigen will be antibody negative. 'Shedders' - 80% produce infected chicks, 'non-shedders' - only 3% produced infected chicks. PCR testing of embryonally infected chicks using DNA testing is uniformly positive for blood and faecal samples. There is also evidence that it is slightly more sensitive than conventional testing. Serology - an Elisa test is aviailable to identify antibody positive birds. Prevent/ reduce cross-infection in hatchery and on farm.

Critical hatchery practices:

• Separate infected and uninfected lines.
• Handle clean lines before infected lines, preferably on separate hatch days and in separate machines.
• Separation in vaccination.
• Minimise stress.

Farm practices:

• Brood and rear lines separately and maintain separate for as long as possible.
• Minimise group sizes.
• Delay live vaccine challenges.
• Avoid migration errors (birds unintentionally moving between pens).