Once the disease was introduced into susceptible pigs, only a few pigs suddenly became ill and died in the early stage, and then the number of pigs increased, which quickly spread to the whole group within 1 ~ 3 days, most of them were acute cases, and finally subacute and a few chronic cases appeared, with extremely high mortality.

The incubation period of this disease is related to virus virulence and pig resistance, which is usually 5 ~ 6 days, the shortest is 2 days, and some are as long as 21 days. Generally, it can be divided into three types according to the duration of the disease: the most acute, acute and chronic.

(1) The most acute type is rare, with sudden onset in pigs, mainly characterized by high fever, loss of appetite, mucosal congestion, weakness and death soon.

(2) Acute type is the most common, and the temperature of sick pigs rises to 4.5 ~ 42℃. Sick pigs are listless, with rough fur, chills and prone to lying down, slow and weak walking, loss of appetite, increased drinking water, occasional vomiting, initial constipation, diarrhea, foul smell and loose stools, red spots or erythema of different sizes at the back of the ears, abdomen, inner limbs and other parts of sick pigs, inflammation of the male foreskin, and neurological symptoms in pigs.

(3) Chronic type is mostly transformed from acute pigs. Loss of appetite, constipation and diarrhea alternate. The sick pig was emaciated, listless, weak in the hindquarters, hobbled, and finally died of exhaustion.

In addition, mild classical swine fever has occurred in recent years. Mild classical swine fever is caused by a weak strain of classical swine fever virus, which is characterized by long incubation period and course, low fever, respiratory and neurological symptoms, low morbidity and mortality, and adult pigs can recover, while the mortality rate of piglets is high.