Question of the Day

Buy only chicks vaccinated against Marek disease. This chicken has ocular evidence of Marek disease. There is no treatment, but the problem is preventable when chicks are vaccinated at hatching or before 2 weeks of age.

Diffuse infiltration of mononuclear cells can turn the irises of affected chickens from a normal yellow color to a pale tan to gray color (a.k.a. “gray eye”). Cellular infiltration can also occur in many other tissues (skin, nerves, viscera), and there may be palpable tumors in the muscle and skin.  

Marek disease is common in commercial poultry. Dx usually based on enlarged nerves and lymphoid tumors in the viscera. Lymphoid leukosis is a key differential.  

Absence of bursal tumors helps distinguish Marek disease from leukosis. But if you find bursal tumors, Marek disease is still on the DDX list pending other tests, like immunochemistry. Chicks as young as 3 weeks can show Marek disease, but lymphoid leukosis is typically seen in chickens more than 14 weeks old.

Image courtesy of the USDA.