Afraid of African swine fever, how to prevent it?
The Information Office of the Ministry of Agriculture and Rural Affairs announced on August 3 that an African swine fever epidemic occurred in Shenbei New District, Shenyang City, Liaoning Province, which was the first African swine fever epidemic (ASFV) in China. After the outbreak, the Ministry of Agriculture and Rural Affairs started the Level II emergency response according to the Emergency Plan for African Classical Swine Fever Epidemic. According to the requirements, the local emergency response mechanism was launched, and measures such as blockade, culling, harmless treatment and disinfection were taken to prohibit all pigs and susceptible animals and products from being transported into or out of the blockade area. Shenyang has suspended the transportation of pigs throughout the city. At present, the epidemic has been effectively controlled.
Lanzhou Veterinary Research Institute of China Academy of Agricultural Sciences established a good cooperative relationship with OIE African swine fever reference laboratory as early as ten years ago, and established a laboratory with Polish National Veterinary Research Institute under the witness of Minister Han Changfu of the Ministry of Agriculture and Rural Affairs, introduced relevant diagnostic techniques and materials, and applied for and obtained the permission of Veterinary Bureau of the Ministry of Agriculture on 201/to carry out the research on African swine fever diagnostic techniques and the development of diagnostic kits.
At present, the research, development and storage of complete sets of technologies and kits for etiological and serological diagnosis of African swine fever have been completed, including conventional PCR and real-time fluorescence quantitative PCR kits, and indirect ELISA antibody detection kits, which have laid a solid foundation for the prevention and control of African swine fever ~ established a good cooperative relationship with OIE African swine fever reference laboratory, and established a laboratory with Polish National Veterinary Research Institute under the witness of Minister Han Changfu of the Ministry of Agriculture and Rural Affairs. Relevant diagnostic techniques and materials were introduced, and the application was made on 20 1 1 and the permission was obtained from the Veterinary Bureau of the Ministry of Agriculture to carry out the research on African swine fever diagnostic techniques and the development of diagnostic kits (agricultural medical memorandum [201] No.596). At present, the research and development of complete sets of technologies and kits for etiological and serological diagnosis of African swine fever have been completed, including conventional PCR and real-time fluorescence quantitative PCR kits, indirect ELISA antibody detection kits, etc., which laid a solid foundation for the prevention and control of African swine fever.
African Swine Fever (ASF) is an acute, hot and highly contagious infectious disease of pigs, also known as African Swine Fever or warthog disease. The clinical manifestations are high fever, loss of appetite, high mortality, cyanosis of skin, massive bleeding of lymph nodes and internal organs, abortion and edema of pregnant sows. The mortality rate can be as high as 100%. The disease belongs to the animal epidemic reported by OIE, and is listed as a kind of animal epidemic by the Ministry of Agriculture of China. African swine fever is one of the most terrible epidemics in pigs, mainly due to its high infectivity, diverse modes of transmission, high morbidity and mortality, and lack of specific vaccines. African swine fever has seriously affected the global pork production and trade. From 20 14 to 20 17, 800,000 pigs were culled by African classical swine fever in Eastern Europe and Russia alone. From 20 14 to 20 15, the pork exports of Poland, Latvia, Lithuania and Estonia decreased by 9. 5% is due to African swine fever.
First of all, pathogens
African swine fever is caused by DNA virus infection, which was once classified as a member of iridoviridae, but its DNA structure and replication mode are very different from iridovirus, similar to poxvirus. 1995, International Committee for Virus Classification (ICTV) listed it as a new virus family-African swine fever virus family. In July, 2005, the eighth report of the latest virus classification published by ICTV made it clear that African classical swine fever virus belongs to DNA virus directory (dsDNA), African classical swine fever virus family and African classical swine fever virus genus (ASFIVIRUS), and ASFV is also the only recognized member of African classical swine fever virus family. Unlike other DNA viruses, ASFV is a real arbovirus, which can reproduce in vertebrates and spineless propellers. In pigs, African swine fever virus can replicate in several types of cytoplasm, especially reticuloendothelial cells, monocytes and macrophages. The virus can reproduce in ticks, making it the main vector. After African swine fever virus infects animals, it does not produce neutralizing antibodies, so there is only one serotype. The latest research shows that there are 23 genotypes of African swine fever. Among them, the genotype 1 Yu 192 1 was first diagnosed in Kenya, mainly prevalent in eastern and central Africa, and then invaded Europe from 1957 to 1960, and became popular in Spain, Portugal and other countries. Genotype II invaded Georgia in 2007 and was subsequently introduced to Russia, Ukraine, Poland, Latvia, Lithuania, Estonia, Moldova, Czech Republic and Romania. The disease was first introduced to Cuba on 197 1; 1978 invaded Brazil and an epidemic occurred in American countries.
Second, epidemiology.
ASFV only infects pigs and wild boars. Sick pigs and infected pigs (recovered pigs and latent infected pigs) are the main sources of infection, and wild boar and ticks of Ixodes are the storage hosts of the virus. Sick pigs can detoxify before they have a fever; Some pigs with chronic infection can be detoxified for life; The virus can persist in the recovered pigs for one year and can be detoxified for more than six months. The disease is mainly transmitted through digestive tract and respiratory tract, and the main modes of transmission are contact transmission, food transmission and soft tick bite. There are other routes of transmission, and studies have shown that ASFV can spread through the air at close range (within 2 meters). There are other possible potential transmission routes, such as transmission by birds and rodents. There is no difference in the susceptibility of pigs of different breeds and ages. The incubation period after direct contact with infected pigs is 5- 19d, and the incubation period after biting by infected ticks is less than 5d, and symptoms will appear in 5-7d.
In Africa, ASFV can infect warthogs, jungle wild boar and wild boar. Many wild boars are easy to be infected with ASFV, but usually do not show obvious clinical symptoms. Wild boars in Europe and North America are susceptible to ASFV, and their clinical symptoms and mortality are similar to those of domestic pigs. However, the American spotted boar is an exception, and it is not sensitive to ASFV. Different strains have different pathogenicity. ASFV virulent strain can cause 100% pig death within 4 ~14d. The virus content in the blood of sick pigs is greater than 108 virus particles /ml, which mainly attacks lymphocytes, leading to lymphocyte apoptosis, vascular endothelial cell damage and bleeding. The mortality rate caused by moderate and strong strains is generally 30% ~ 50%, and the infection of weak strains only causes a small number of pigs to die.
African swine fever is not a disease of humans and animals. At present, there is no evidence that ASFV can be transmitted to people. Although African swine fever does not infect people, China is a big breeding country, and the number of pigs in the world accounts for more than half. After its invasion of China, it could not be effectively controlled in time. Once it spreads in a large area, the consequences will be unimaginable, seriously affecting economic and social stability.
Third, the pathogenesis
ASFV can enter pigs through oral cavity and upper respiratory system, and be infected in nasopharynx or tonsils. The virus quickly spread to mandibular lymph nodes and spread to the whole body through lymph and blood. After that, the virus can be detected in almost all tissues. The tissues with the highest virus content include mononuclear phagocyte system, reticular endothelial cells such as spleen and lymph nodes. Hemorrhagic lesions caused by ASFV are due to the release of cytokines by infected monocytes/macrophages, rather than directly leading to the loss of endothelial cells. ASFV infection is accompanied by thrombocytopenia, which may be caused by the production of damaged platelets or the consumption of platelets in peripheral blood vessels. In virulent infection, cells change rapidly, and all cells die before obvious stimulation reaction appears. When infected with attenuated virus, the stimulation reaction is easy to observe, the nucleus becomes larger and mitosis generally occurs.
Fourth, clinical symptoms
Pigs of different breeds and ages may appear. Most acute pigs died suddenly without obvious clinical symptoms, and some sick pigs had the same clinical symptoms as acute pigs. The mortality rate is as high as 100%. The incubation period of ASFV after acute infection is 5- 1.5 days d, and the first symptom is elevated body temperature (4 1-42℃). It is characterized by depression, anorexia, clustering, extensive bleeding and bleeding spots in the skin and mucosa of ears, nose, limbs and abdomen, cyanosis in the surrounding skin, and even hemorrhagic masses; The surface of feces is covered with blood and mucus; * * * Dysfunction or stiff gait, dyspnea, sick pigs usually live for 7 days after fever, and die 1-2 days after symptoms appear. The pregnant sow miscarried. The mortality rate is as high as 100%. The symptoms of subacute type are the same as those of acute type, but the symptoms are mild, the mortality rate is low, the duration is long, and the mortality rate of piglets is relatively high, with a course of 5-30 days. The mortality rate of chronic type is low, showing respiratory tract and pneumonia symptoms. Most pigs can recover after infection, but they are infected for life. Sick pigs usually survive for several months, but due to low immunity, they are prone to secondary bacterial infections and it is difficult to recover. The course of disease was 2- 15 months.
Verb (abbreviation of verb) diagnosis
According to the clinical symptoms and pathological changes, a preliminary judgment can be made. Because the clinical symptoms of African classical swine fever are very similar to those of classical swine fever, if there is no typical symptom-specific bleeding, it is almost indistinguishable only from the naked eye. Laboratory examination is necessary for differential diagnosis. In addition, it is also differentiated from highly pathogenic blue ear disease, porcine dermatitis nephritis syndrome (PDNS), bacterial septicemia, mycotoxin poisoning and coumarin poisoning. There are several methods for laboratory diagnosis: because the disease is mostly the most acute or acute type, it often dies before the appearance of specific antibodies, so it is very important to detect the etiology of African swine fever in the diagnosis. Virus isolation is the most commonly used etiological diagnosis technique. However, the diagnosis can only be carried out under the condition of high safety in P3 laboratory. In addition, serological tests such as erythrocyte adsorption and erythrocyte adsorption inhibition test, immunofluorescence, enzyme-linked immunosorbent assay and agar diffusion can also be used for the diagnosis of the disease. At present, Lanzhou Veterinary Research Institute of China Academy of Agricultural Sciences has completed the research, development and storage of complete sets of technologies and kits for etiological and serological diagnosis of African swine fever, including conventional PCR, real-time fluorescence quantitative PCR kits and indirect ELISA antibody detection kits, which are being mass-produced. Different kits are very convenient to use, and have the advantages of high accuracy and short time, which can be widely used in laboratory diagnosis. Diagnostic kits for African swine fever etiology (PCR, real-time fluorescence quantitative PCR) and serology (indirect ELISA for antibody detection) provide strong technical support for the prevention and control of African swine fever.
Differential diagnosis: ASFV disease is very similar to classical swine fever. The most acute type is usually characterized by severe bleeding of internal organs. The disease usually does not show clinical symptoms at the initial stage of body temperature increase; The most obvious lesion of acute type is splenomegaly, which can reach several times of the original. Spleen is big and black, extremely soft and fragile. Abdominal lymph node bleeding is serious, similar to hematoma, and edema appears in many places such as colon mucosa and mesentery. There are no vaccines and drugs for emergency vaccination and treatment.
Classical swine fever has obvious clinical symptoms when its body temperature rises. Lymph nodes are marble-shaped; Edema lesion is not obvious; When classical swine fever occurs in a large area, emergency vaccination against classical swine fever can alleviate the epidemic situation. It can be seen that African swine fever usually appears first, and then other symptoms, while African swine fever appears at the same time with the increase of body temperature, which can be used as a differential diagnosis between the two.
Six, prevention and control methods
Strengthen the quarantine of imported breeding pigs and pig products, and resolutely do a good job in the destruction and disposal of waste and garbage at international airports and ports. Once suspected African swine fever occurs, culling measures should be taken decisively, strictly blocked and thoroughly disinfected. The virus has strong resistance to the environment, and it still survives at 23℃ 120 days. In tropical areas, the virus in pigsty can survive for at least 2 weeks. There are fewer kinds of effective disinfectants. The most effective disinfectant at present is 10% phenol. Other disinfectants recommended by OIE include 0.8% sodium hydroxide, 0.3% formalin and 2.3% hypochlorite. Pig farmers should strictly prohibit the transportation of pigs from epidemic areas, pay attention to daily disinfection, try to keep pigs sealed, take isolation and protection measures, try to avoid contact with wild boar and ticks, and strictly prohibit the use of swill or kitchen waste from restaurants and canteens that have not been treated at high temperature to feed pigs. In case of unexplained death and abnormal increase of symptoms similar to swine fever, it should be reported to the local veterinary department in time.