Signs of infection with NDV vary greatly depending on factors such as the strain of virus and the age, species of the host and health .
Incubation period for the disease ranges from 2 to 15 days. The infected bird may exhibit several signs, including respiratory signs (gasping, coughing), nervous signs (depression, inappetence, drooping wings,muscular tremors, twisting of head and neck, circling, complete paralysis), greenish, watery diarrhea,swelling of the tissues around the eyes and neck, misshapen, rough- or thin-shelled eggs and reduced egg production which are important.
In acute cases, the death is very sudden, and, in the beginning of the outbreak, the remaining birds do not seem to be sick. In flocks with good immunity, however, the signs (respiratory and digestive) are mild and progressive, and are followed after seven days by nervous symptoms, especially twisted heads.