Among chicken diseases, the one people are most concerned about is probably avian influenza. It is very common on our farms, and we cannot avoid it. When discussing avian influenza, we must first understand that avian influenza in broilers is fundamentally different from avian influenza in layer chickens. To date, the most frequently occurring form in broilers is still mild avian influenza, namely H9.
Second, the avian influenza virus can remain latent—that is, what we have always told you about: the primary disease. Why do we say this? If you observe your flock carefully, when they are 10 days old, the signs are very clear. Changes in the eyes show increased tearing, narrowed eyelids, and redness at the back of the eyes. These are early signs of damage to the respiratory and circulatory systems. At this point, avian influenza has already begun to emerge from a subclinical infection. If this disease process occurs on a farm with good environmental management, it progresses slowly. If environmental management is inadequate, the disease can progress very quickly, with signs including nasal discharge, worsening coughing, and then various other chicken diseases following. The primary disease manifests as the latent presence of the avian influenza virus. H9 itself is not highly aggressive; its presence alone can readily trigger other diseases, known as secondary diseases, or act together with other pathogens to attack the chicken and cause severe damage to its organs. We usually see only the secondary diseases and overlook the primary disease, so prevention and treatment address the symptoms rather than the root cause, and the best clinical results are not achieved. In earlier cases of avian influenza, in addition to classic pancreatic necrosis, necrotic spots the size of pinpoints, bleeding at the edges of the pancreas, testicular necrosis, and severely swollen kidneys, autopsies also showed peritonitis and airsacculitis. The lungs could also develop a covering—known as “lung encapsulation”—that looked like jelly or thin plastic film and appeared glossy.
In later cases of avian influenza, the signs changed: lung encapsulation became less common, while unilateral or bilateral lung necrosis became more frequent, directly resulting in viral pneumonia and making treatment more difficult. In terms of the proportion of cases, subclinical infections remain the most common, followed by typical cases. Highly virulent cases are less common, but they do occur and cause significant losses for poultry farmers.
Regardless of the type of Newcastle disease, epidemiologically, the virus itself has not changed much. In other words, Newcastle disease can be prevented through effective vaccines, appropriate methods, thorough disinfection, and a good environment. From this perspective, most cases of Newcastle disease on farms are associated with improper management, inadequate vaccination, poor environmental conditions, and incomplete disinfection.