Swine fever (HC) is one of the major diseases that currently harm the development of China's swine industry. Although we carried out comprehensive prevention and control measures based on vaccination, we effectively controlled the occurrence and prevalence of swine fever. However, in recent years, the prevalence of swine fever has changed greatly, and atypical and mild swine fever has emerged. Causes considerable harm to the hog cholera virus infection (subclinical infection), placental infections, congenital tremors in born pigs, and reproductive disorders in pregnant sows. It has brought great difficulties to the prevention and control of swine fever and caused huge economic losses.
1 disease characteristics and clinical symptoms
1.1 The epidemic forms are mainly distributed, and sometimes local epidemics are occurring at a slow rate. The litter of litters is not a full litter, but only a few piglets.
1.2 All ages of pigs can be affected, but the incidence of piglets before and after weaning is more, especially 30-40 days old piglets are seriously ill.
1.3 The incidence rate is not high, the incubation period is prolonged, sometimes up to several weeks, immunized pigs have unexplained immune failures, and swine fever with untypical symptoms and pathological changes are not found.
1.4 The lack of typical clinical symptoms, rarely appear dense skin bleeding, purulent conjunctivitis, a high rate of missed heat, severe diarrhea and other symptoms. More performance of light heat or heat, mild hypotension in the abdomen or cyanosis in the lower extremities, after the recovery can occur dry ears, dry tail or even ear shell off, anorexia, mental depression, dry hard stools, hi sleep and so on.
1.5 The duration of the disease is prolonged, mostly in more than half a month, and some 2-3 months slowly self-healing, become dead pigs, the mortality rate is about 30%.
1.6 Sows with persistent swine fever virus (subclinical infection) can infect piglets through the placenta. Abortions, stillbirths, retained fetuses, fetal malformations, mummified fetuses, weakly litter or partially healthy piglets. When sows are used for breeding, they are not allowed to be used repeatedly.
1.7 Sustained (sub-clinical) sows born with sows showed congenital tremors, healthy piglets, and swine fever virus infection in the body, resulting in immune tolerance and low immune response to vaccinated swine fever vaccine. These pigs can survive for 2-11 months and most of them can live for more than 6 months. The diseased pigs have no obvious symptoms. Subsequent mild anorexia, depression, conjunctivitis, dermatitis, diarrhea, ataxia, posterior body paralysis, and death can be seen.
1.8 There are no typical renal, bladder hemorrhage and splenic hemorrhagic infarctions in necropsy of most diseased pigs. Lymph nodes may be edematous, or peripheral bleeding may be seen as point-like hemorrhage, and mesenteric lymph nodes are more obvious. Some only see the lightening of the kidney and a small amount of needle-point bleeding.
1.9 Characteristic lesions are tonsils, ileocecal valves, spleen, stomach, and gallbladder. Light tonsillar edema, more than 5 days of disease, the majority of necrosis, ulceration and suppuration, the slice did not see the structure of tonsillar, a large number of leukocyte infiltration and tissue necrosis. During the course of 3-5 days, there was ulceration at the ileocecal valve and a visible button-shaped ulcer with a long duration. Gallbladder enlargement, gallbladder mucosa with protruding tan or black stained necrosis, thick bile, coal tar-like. Gastric area with flaking or bleeding; some ulcers. Fetal pigs and piglets infected by the placenta showed lymphedema, thymus atrophy and other prominent lesions.
1.10 Pigs with persistent atypical swine fever and hog cholera virus infections are difficult to diagnose only with clinical symptoms, autopsy, etc. They must be confirmed by laboratory tests.
2 Reasons for the occurrence Due to differences in pig feeding and management status, immunization procedures, and immunity levels in different regions, the causes of the persistent infection of atypical swine fever virus and swine fever virus are very complicated. The main causes of the disease are the following.
2.1 The seedlings are not widely used and the immune density is not high. Over the years, due to the prevention and control of swine fever, comprehensive control measures based on vaccination have been implemented to better control the spread and prevalence of swine fever. However, in recent years, due to the imperfect organization of anti-epidemic organizations, outdated equipment, and tight funding, the inoculation density of swine fever vaccine is not high and the scope is not wide. Some pigs only missed vaccines, or some villages did not receive vaccines. The level of immunity is low.
2.2 The immunization procedure is irrational. Based on the prevalence of swine fever, the level of swine population and the law of maternal antibody growth, a scientific immunization program was developed. The first immunization is generally used at the age of 20 days, the second immunization is performed at 55-60 days, and the immunization procedure is no longer immunized within 1 year. However, in raising pigs, especially in rural areas, it is difficult to seriously implement the above immunization procedures. It is often the case that the timing of the first exemption and the second exemption is improper, or that two immunizations become only one injection, which affects the The effect of swine fever vaccine.
2.3 Insufficient dose of vaccine immunization. The dose of swine fever vaccine is closely related to the protection of the vaccine. When the dose of vaccine is low, piglets are challenged with virulent swine fever. The virulent virus can reproduce and become poisonous in pigs, presenting a subclinical infection when the immune dose is increased to 80. With -100 PD50 (about 320-400 RID), subclinical infections can be suppressed after challenge, and all resistant pigs are not toxic. In the production, atypical swine fever can still appear after injecting a dose of our country's immunity-free attenuated vaccine. A large dose of vaccine can be used to obtain good immunity. The European Pharmacopoeia stipulates that when immunized with a C strain vaccine, the intramuscular injection dose is 100 PD50 (400 RID). China's swine fever vaccine factory inspection to 50,000-fold dilution can cause rabbit body heat reaction is qualified, that is, containing 50,000 RID per milliliter of stock solution. The prescribed dose is 150 RID (37 PD50), which is much lower than the international standard. Therefore, the use of this dose in HC unstable areas can not cut off the vicious circle caused by HC subclinical infection. It is very necessary to increase the vaccine immune dose to control atypical swine fever.
2.4 Poor quality of the vaccine and improper vaccination operations. The freeze-dried vaccine for swine fever that is used in production is manufactured after passing the efficacy test. However, some factors such as high transport and storage temperatures, vaccine vacuum loss, vaccines exceeding the expiration date, dilutions that are not required, and vaccines that cannot be used up soon after dilution It often affects the quality of the vaccine. Experiments showed that in the summer morning, the fully-conformed vaccine was diluted, that is, there were 150 rabbit body response (RID) per head. In the evening test, there were only 15 rabbit body reactions per head, and the potency lost 90% in one day. . Therefore, if the diluted vaccine cannot be used up immediately, it must be stored in an ice bottle. In addition, when the vaccine is injected, the disinfection is not strict, the injection dose is inaccurate, and the site and method of inoculation are improper, which will reduce the immune effect of the vaccine.
2.5 Sows that did not eliminate breeding barriers in time. After gestation sows are infected with swine fever virus, they often do not show clinical symptoms, but they can infect the piglets through the placenta. These piglets become persistent infected pigs, can develop immune tolerance, and have low immune response to the swine fever vaccine. Immune failure often occurs, and this situation is of great importance in epidemiology. Frequent immunization failures, atypical swine fever, and reproductive problems on a farm have a great relationship with the continuous infection of swine fever virus by sows. In addition, some people regard the continuous infection of swine fever virus as a kind of infection that can not remove the virus after acute infection, and it is the main source of outbreak of “spontaneous swine fever†in pigs. Therefore, the timely elimination of sows that continue to infect sows and Syphilis syndromes is particularly important.
2.6 Atypical swine fever is thought to occur after swine has been infected with a swine fever variant strain. This mutant strain is a virulence variant strain that appears under immune pressure. The virulence of swine fever virus isolated from atypical swine fever-infected pigs is relatively weak. After returning to susceptible swine, three generations pass and the virulence can be strong. This shows that the popular atypical swine fever virus is only one of the virulence. Changes. Recently, some people have analyzed the genetic relationship of classical swine fever virus in China, which shows that the homology of the E2 whole-body nucleic acid sequence of the swine fever virus strain with the vaccine strain is 82.2%-84.3%, indicating that the swine fever virus is attenuated. There are differences in the level of protective antigens among epidemic strains. Whether this difference affects the immune protection of porcine attenuated attenuated vaccine still needs to be further confirmed.
3 Prevention and control In order to do a good job of prevention and control of atypical swine fever and swine fever, we should implement the principle of "prevention first", combine practical local conditions, formulate practical prevention measures, and control and extinguish this disease.
3.l Implement comprehensive control measures based on vaccination. Strengthen the feeding and management of pigs, establish a strict veterinary hygiene system, prevent the introduction of diseased pigs, frequently disinfect pig houses, utensils, vehicles, and clothes, and cut off the spread of diseases. Observe the dynamics of the herd and the health status of the pigs. Immune surveillance of swine fever should be carried out on conditional swine farms. The immune status of the swine fever should be monitored in a timely manner and the swine fever vaccine should be given in due course. Pigs should be diagnosed as soon as possible, and the affected pigs should be immediately culled and detoxified to eliminate the source of infection.
3.2 Develop scientific immunization procedures to increase the immune density. All localities should scientifically formulate swine fever immunization procedures based on local conditions. Particular attention should be paid to avoiding maternal antibody interference and to arrange the first free time reasonably. Generally, the following two immunization procedures can be used: first immunization at 120 days, second immunization at 55-60 days, and repeated vaccination during the year. The immunization at 20 o'clock was preceded by immunization with the swine fever vaccine immediately after birth. After 2 hours of injection, the newborn pigs were fed with colostrum and immunized once at about 60 days of age. It is best to develop an immunization program based on the results of the immune surveillance. In the implementation of the above-mentioned immunization programs, the majority of rural villages pay attention to “supplementing month and month†to avoid the occurrence of missed injections, so as to ensure the density of the seedlings that are more than 90%, so that the pigs have relatively normal antibody levels. In addition, the quality of the vaccine should be assured and the factors that affect the vaccine's potency during storage, transportation, and use should be avoided.
3.3 Increase the immunization dose. At present, in order to eliminate the subclinical infection of swine fever, Europe has adopted a method of increasing the dose of vaccine. After the dose of vaccine is increased, clinical and sub-clinical infections of swine fever can be well prevented. Especially in the infected areas where atypical swine fever and swine fever are persistently infected, the dose of vaccination should be increased on the premise of strictly implementing exemption procedures. Four doses of the vaccine can be administered at one time as required by the piglet freeze-dried vaccine manufacturing protocol.
3.4 Detect and eliminate subclinical infected pigs. It is critical to purify the pig farms for pig farms where swine fever occurs frequently and for large-scale pig farms. Some pig farms have changed different immunization procedures and still cannot control the occurrence of swine fever. Detection and elimination of sub-clinically infected pigs, especially infected sows, should be adopted. This quickly stops the prevalence of swine fever. In order to detect subclinically infected pigs, swine fever monoclonal antibody ELISA was used. In order to achieve the purpose of testing, the sow serum samples taken from the relevant quarantine departments may be sent for diagnosis, and piglets may also be diagnosed if piglets born to sows with reproductive difficulties have died. When confirming swine fever, the sow should be eliminated. In the case of immunization monitoring in pigs, if a sow that has indeed been immunized or repeatedly immunized and the antibody level is still low, should be culled.
3.5 In order to effectively control the reproductive disorder caused by classical swine fever virus, a novel swine inferno subunit inactivated vaccine containing E2 glycoprotein-labeled antigen before sow mating can not only provide good protection, but also It is helpful for the identification of serology between immunized pigs and naturally infected pigs and is currently applied in European and American countries.
3.6 Pay attention to the ecological environment and play an important role in the prevention and control of diseases such as swine fever. Large-scale pig farms have become the main force in China's pig-breeding industry, but many large-scale pig farms are surrounded by large numbers of rural livestock and poultry, and pig farms are also dealing with excrement, secretions, and sewage from pigs. Poor capacity has caused the ecological environment for live pigs to deteriorate and pollution has become more serious, which has caused the spread of pathogens. Therefore, the ecological environment plays an important role in the prevention and control of swine fever and other diseases, implementing "biosafety technologies" and implementing "biosecurity measures" to prevent the occurrence and prevalence of swine fever and other diseases under intensive feeding conditions. Significance.
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