Thus, in the member countries of the European Economic Community, reporting the disease is mandatory, and if it occurs, drastic measures—including culling, quarantine, and vaccination—must be taken to prevent its spread.

The causative agent of this disease is a paramyxovirus, which is an RNA virus.

The incubation period of the Newcastle disease can vary from two to eleven days, with an average of five or six days.

The incubation time and severity of the process decreases gradually with the age and degree of immunity, passive or active.

The symptoms that may occur can be summarized in four main groups:

—General symptoms:

Lethargy, drowsiness, indifference to external stimuli, bruised combs, a marked decrease in feed and water intake, and a decline in egg production.

—Digestive symptoms:

Within a few hours, a patient may experience loose stools that progress to profuse, grayish, or even reddish diarrhea.

—Respiratory symptoms:

Respiratory movements increase, breathing becomes labored, and the animal needs to open its mouth to take in more air. They exhibit wheezing, and as the disease progresses, nasal discharge appears.

—Nervous system symptoms:

These symptoms are very characteristic of the disease and, in some cases, are more prominent than those mentioned above. The bird exhibits chronic tremors and spasms, as well as paralysis of the legs and wings.

The most significant lesions are likely to occur in the digestive tract. Purple or dark red hemorrhages associated with necrosis are found in the intestinal wall, particularly in the posterior portions of the duodenum, jejunum, and ileum.

In the respiratory it appears, in some cases, a discharge catarrhal or serous in the upper parts, larynx, and trachea, and in the previous parts of the lung may be submitted forms neumónicas.

Since this is a viral disease, there is currently no effective treatment; the best course of action recommended when an outbreak occurs on a farm is to administer broad-spectrum antibiotics to minimize, as much as possible, the harmful effects that may be caused by opportunistic pathogens that complicate the primary problem.