1 etiology
Classical swine fever virus belongs to flaviviridae and pestivirus.
The virus has strong resistance to the external environment, and can survive not only in frozen conditions, but also in smoked meats. But it is not heat-resistant, and only some strains can tolerate the temperature of 56℃. PH & lt3.o or pH > 1 1. O can be inactivated; Sensitive to ether, chloroform and β -propenolide; It can be inactivated with 2% sodium hydroxide, 1% formalin, sodium carbonate (4% anhydrous or 10% crystalline sodium carbonate +0. 1% detergent), ionic and nonionic detergents, and phosphoric acid containing 1% iodophor.
2 epidemiology
The disease occurs in most Asian countries and regions, Central and South America, Africa and some European countries.
Under natural conditions, pigs and wild boars are the only hosts of classical swine fever virus. Other animals are resistant. Pigs are susceptible regardless of age, sex and breed. It can happen all year round. At first, one or several pigs became ill, and then it gradually increased, reaching the peak after 1 ~ 3 weeks, and the incidence rate was 80% ~ 100%. Treatment is ineffective and the mortality rate is extremely high. It is popular or local.
The virus mainly infects through the digestive tract and respiratory tract. It can also be transmitted vertically through conjunctiva, wound, spermatic cord infection and placenta. Infected by direct contact with secretions, excreta, semen and blood of infected animals; Or actively communicate through the transactions of farm visitors, veterinarians and pigs; Indirect transmission through contaminated pens, utensils, vehicles, clothes, equipment and blood collection needles; Feeding pigs with uncooked swill can also lead to transmission.
The sources of infection are sick pigs, pigs with virus after recovery and pigs with virus in incubation period. All tissues, blood, secretions and excretions of sick and dead pigs; Congenital infection of piglets, persistent toxemia, detoxification for several months; Porcine cell vaccine carrying virus and natural attenuated strain can be the source of infection.
3 Clinical symptoms
Pigs exposed to classical swine fever virus in fetus may be infected for life, and the incubation period is usually several months. The incubation period of piglets exposed to virus is 7 ~ 10 days, which is usually 5 ~ 10 days after infection, but chronic infection cases are usually infected after 3 months.
Acute type: high fever (4 1 ~ 42℃). Loss of appetite and occasional vomiting. Sleepy and crowded. Dyspnea, cough. Inflammation of conjunctiva, purulent discharge from both eyes. Extensive hyperemia and bleeding of skin and mucosa all over the body. Cyanosis of the skin, especially in the limbs (ears, tail, limbs and nose and mouth). First, short-term constipation, volleyball-like fecal mass with mucus (pus or pseudomembranous fragments); After diarrhea, gray-yellow thin feces are discharged. Most of them died within 5 ~ 15 days after infection, and the mortality rate of piglets could reach 100%.
Chronic type: the body temperature is high and low, showing a relaxed heat type. Constipation or diarrhea appear alternately, mainly diarrhea. Rash, scab, ear, tail and limb necrosis. The course of disease is long, lasting more than 1 month, and the mortality rate is low, but it is difficult to fully recover. Immortal pigs often become stiff pigs. It is more common in the middle and late epidemic period or in areas where classical swine fever often occurs.
Mild type: For many years, a so-called "nameless high fever" has been distributed in some areas, and it has been proved that it is mostly swine fever. Because of its long incubation period, mild and atypical symptoms, the mortality rate is generally less than 50%, and antibacterial drugs are ineffective, so it is called "mild" swine fever. The sick pigs showed transient fever (generally 40 ~ 465,438 0℃, and a few were above 465,438 0℃) with no obvious symptoms. Long-term infection of sows, low conception rate, abortion, stillbirth, mummified fetus or abnormal fetus; Piglets are born with congenital infection, die or become stiff pigs.
4 Pathological changes
Acute type: leukopenia and thrombocytopenia. Systemic bleeding and congestion, especially skin, lymph nodes, larynx, bladder, kidney and ileocecum (ear root, neck, lower chest and abdomen, inner limbs). Spleen is not swollen, and there is a hemorrhagic infarction with a slightly prominent dark purple edge on the surface, which is a characteristic lesion of classical swine fever, but it is generally uncommon, and only 50% ~ 70% of cases have infarction. It is common that lymph nodes are swollen and bleeding all over the body, bleeding around the cut surface is obvious, and mixed anemia changes, showing red and white marble shape, which is more common in submandibular, neck and abdominal lymph nodes.
Chronic type: mainly necrotizing enteritis, generally forming concentric button-shaped ulcer on ileocecum, cecum and colon mucosa, protruding from the mucosal surface, dark brown in color and sunken in the center. There is usually no bleeding or inflammatory lesions. Systemic lymphatic atrophy. Thymus atrophy and osteophyte are common in piglets.
5 diagnosis
5. 1 According to the typical clinical symptoms and pathological changes, a preliminary diagnosis can be made, and the final diagnosis needs further laboratory diagnosis.
5.2 Laboratory diagnosis
In international trade, the designated diagnostic methods are peroxidase neutralization test (NPLA), fluorescent antibody virus neutralization test (FAVN) and enzyme-linked immunosorbent assay, and there is no alternative diagnostic method.
Pathogen identification: After the organs and tissues of sick pigs are frozen, they are examined by direct immunofluorescence staining, or the virus is isolated by cell culture, and the virus is detected by immunofluorescence or peroxidase. The diagnosis was confirmed by monoclonal antibody.
Serological tests: peroxidase neutralization test, fluorescent antibody virus neutralization test and enzyme-linked immunosorbent assay.
Sample collection: To identify pathogens, tonsils (the most suitable samples), lymph nodes (pharynx and mesentery), spleen, kidney, distal ileum and blood (EDTA anticoagulation) of living sick animals should be collected. The above samples should be sent to the laboratory as soon as possible under cold storage conditions (but not frozen).
Serological detection should collect serum samples from suspected recovered animals, piglets born by sows and monitoring pigs.
5.3 Differential diagnosis
It should be differentiated from African swine fever (which is difficult to distinguish clinically and needs to be sampled for inspection), bovine viral diarrhea virus infection, salmonellosis, swine erysipelas, acute pasteurellosis, other viral encephalomyelitis, streptococcosis, leptospirosis, coumarin poisoning and other diseases.
6 prevention and control
There is no cure for this disease. Infected pigs must be culled and animal carcasses must be destroyed. In areas where classical swine fever is prevalent, the use of attenuated classical swine fever vaccine can effectively reduce economic losses, but it cannot effectively eliminate classical swine fever. Antibodies against classical swine fever can be used in areas where there is no classical swine fever or where a classical swine fever eradication plan is carried out.