【 Common types and comprehensive prevention and control of swine fever 】 Epidemic situation of African swine fever in Sichuan

1 Classification Classical swine fever is an acute, hot and highly contagious infectious disease caused by classical swine fever virus. It is the number one enemy of the pig industry. Since China successfully manufactured the classical swine fever vaccine many years ago, the disease has been effectively controlled. However, in recent years, with the wide application of vaccines and drugs and the emergence of virus strains with different virulence, the disease has undergone new changes, that is, there are three types of classical swine fever: acute, chronic and delayed.

1. 1 Acute classical swine fever mainly occurs in pigs that have not been immunized with classical swine fever vaccine. These pigs have no resistance to classical swine fever virus, and once infected, they will get sick quickly, and most of them die of acute attacks. These pigs showed typical symptoms of acute febrile hemorrhagic septicemia, which showed that the body temperature of pigs increased to 40.50 ~ 42℃, and the diagnosis was missed. Red spots or erythema of different sizes appear behind the ear, abdomen and inside the limbs, and the finger pressure does not subside. Feces are as hard as balls, urine is yellow, sick pigs lose appetite, mental disorders are serious and depressed, and some pigs have neurological symptoms such as convulsions. Most sick pigs died within 3 days, and there were changes such as diarrhea, thick and smelly urine, walking weakness and emaciation during a long course. Only these pigs died more than 65,438+0 weeks, and a few lucky survivors became stiff pigs, which were detoxified for a long time. At present, the immunity of classical swine fever is relatively common, and this type is relatively rare.

1.2 Chronic classical swine fever, also called atypical classical swine fever or mild classical swine fever, is the most common type of classical swine fever at present. It mainly occurs in pigs immunized with classical swine fever. These pigs should be resistant to classical swine fever virus, but due to unreasonable immunity of classical swine fever or some reasons, their resistance to immunosuppression decreased and they could not resist the invasion of classical swine fever virus. Because pigs still have some resistance, their symptoms are atypical, recessive, slow and mild. Symptoms such as mild fever, anemia, emaciation, poor appetite, constipation and diarrhea alternate. The incubation period from infection to onset of chronic swine fever can be as long as 1 1 week, and its course of disease is also long, accompanied by respiratory and nervous system symptoms. Dead pigs (mainly small and medium-sized pigs, especially suckling piglets) are constantly appearing in the pig herd. Sick pigs such as nameless high fever, dry ears, dry tail and rotten throat often appear in pigs, and some sick pigs have skin spots and punctate bleeding. Some have mild blood stasis under the abdomen or cyanosis of the lower limbs (purple spotted hoof, spotted disease), pigskin pigs. Compared with acute swine fever, the incidence and mortality of this disease are relatively low, mainly because of the high mortality rate of piglets. Many adult pigs often tolerate it, but they have become stiff pigs and have been detoxified for a long time. The appearance symptoms of pigs with chronic swine fever are extremely atypical, which is difficult to judge clinically. The author once encountered a chronic case of classical swine fever, which only had reduced food intake and no other symptoms. The drug was ineffective and died around 1 week. The classic course of chronic swine fever is depression, fever and anorexia, and then the clinical symptoms improve, as if they recovered and then suddenly deteriorated and died quickly, lasting 1 ~ 2 months.

1.3 delayed classical swine fever is a swine fever disease that sows are infected with low pathogenic classical swine fever in the third trimester of pregnancy and then transmitted to piglets. This kind of piglet looks like a normal pig from birth to weaning. After weaning, it gradually begins to get sick and usually dies within 6 months after birth. No pigs look healthy before the onset of the disease, but they constantly excrete viruses and infect other pigs. This kind of pig does not produce immunity after immunization with classical swine fever vaccine, and may get sick at any time. Once the disease breaks out, the same group of pigs may be destroyed, emergency immunization will not work, and economic losses will be great. This is a new type of classical swine fever which is the most harmful to the development of pig industry in recent years. At present, many delayed classical swine fever have been found in China's pig herds, all of which have been eliminated, which deserves the high vigilance of the majority of pig farmers.

The symptoms of delayed swine fever are similar to those of chronic swine fever, such as gradual depression, anorexia, normal or high body temperature, conjunctival inflammation, eyelid swelling in some pigs (easily misdiagnosed as edema), skin redness, local dyskinesia and unstable walking. Delayed classical swine fever rarely occurs alone, but usually occurs in combination with other diseases, such as eperythrozoonosis, blue ear disease, streptococcosis, infectious pleuropneumonia and Haemophilus parasuis. When drugs are used to treat secondary infectious diseases, it will stimulate the already weak pigs to speed up their attacks, and it seems that the more they are cured, the faster they will die. At the same time, the use of antipyretic drugs can sometimes relieve the superficial symptoms, but it has no effect on the course of disease, and there will be a special phenomenon that the symptoms are relieved but suddenly die. Pigs with mixed infection usually have purple skin or obvious bleeding spots after death, and typical pathological changes often appear in anatomy.

2 Pathogen of classical swine fever

The above is the basic situation of three types of classical swine fever. Although the types are different, they are all caused by the same strain of classical swine fever virus, but their pathogenicity and pathogenicity are different.

Acute swine fever is caused by no immunity to swine fever; There are many reasons for chronic swine fever, which can be divided into two aspects: unreasonable immunity and immunosuppression.

2. 1 The reasons for unreasonable immunity are as follows

2. 1. 1 Incorrect immunization program is the result of empirical estimation, and the situation of different pigs is different, so the reliability of blind copying is not high. In particular, some programs are immune to classical swine fever too frequently or mixed with multiple vaccines, which are easy to interfere with each other.

2. The main problems of1.2 vaccine are that the low temperature is not well controlled in the process of purchase, transportation, storage and use, or the titer decreases after repeated freezing; The vaccine loses its vacuum after dilution or is stored for too long and fails; Reasons such as expired vaccine or unqualified vaccine.

2. 1.3 false immunity. There are many reasons for this situation: flying injection during immunization, leaking pigs or insufficient injection depth; Sterilize the injection needle with tincture of iodine (using disinfectant can denature the vaccine protein and lose its immune activity); The dose of vaccine injection is not accurate. Generally, 2 doses are used in advance, 4 doses are usually used, and 6 doses are used by large breeders. Blindly increasing or decreasing the dose will have adverse consequences; Mixing needles during injection will not affect immunity, but it will spread diseases more harmfully.

2.2 The main reasons for immunosuppression are

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2.2.2 Drugs interfere with some commonly used drugs, such as dexamethasone, sulfonamides and antiviral drugs, which can cause immunosuppression or destroy the immune effect. However, the use of these drugs before and after epidemic prevention or the general use of these drugs at ordinary times will adversely affect the immunity of classical swine fever.

2.2.3 Immunosuppressive epidemic diseases, such as blue ear disease, circovirus disease and pseudorabies, which are emerging or prevalent in recent years, can inhibit the immune system from functioning, and when these diseases exist, the immunity will be ineffective. In addition, the widespread existence of eperythrozoonosis in pigs will cause the body's resistance to decline, leading to a decline in the antibody level of classical swine fever, and pigs with low antibody levels will also develop classical swine fever.

Compared with the first two, the main reason for delaying classical swine fever is that the antibody level of sows is not high enough and they are infected by classical swine fever virus during pregnancy.

3 Prevention and treatment of swine fever

For a long time in the future, classical swine fever will be the first serious disease that harms the pig industry. As there is no effective therapeutic drug at present, the disease can only be prevented and controlled, and strict comprehensive preventive measures should be taken for various pathogenic reasons, especially for large-scale pig farms, and special attention should be paid to the following points at ordinary times:

3. 1 It is an effective way to prevent the introduction of epidemic diseases from foreign pig farms. As long as the swine fever resistance of sows in this farm is measured regularly, the swine fever vaccine is injected in time, the anti-infection ability of sows is improved and the immunity of piglets is strengthened, the harm of swine fever can be guaranteed to be eliminated.

3.2 As long as the purchased pigs are carefully purchased, it is only the main way for the introduction of classical swine fever. If pigs must be purchased in disease-free areas, it is best to carry out laboratory tests if possible to prevent the introduction of sick pigs.

3.3 Vaccination against classical swine fever is the most effective prevention and control method at present. Because there are many factors that affect the epidemic prevention effect, we should pay attention to all links in the specific operation process. First of all, scientific immunization procedures should be formulated, and large-scale pig farms should be immunized according to antibody monitoring results. After the first immunization, it needs to be immunized again about 1 month to build a strong resistance. In order to prevent delayed classical swine fever, sows should be immunized during the empty pregnancy period, and antibodies should be determined to check the immune effect. Remember, there is no immutable epidemic prevention program in the world, and it must be applied flexibly according to different pig conditions. Secondly, we should do a good job in the procurement, transportation and preservation of vaccines. Classical swine fever vaccine is very cheap, so there are almost no fakes, but many vaccines fail because of improper preservation, such as prolonged high temperature, repeated thawing or loss of vacuum. Therefore, it is necessary to carry out vaccine quality inspection when purchasing vaccines, and at the same time adopt cold chain management such as low-temperature transportation and low-temperature preservation. Third, correct immunization operations should be carried out. In addition to preventing the above incorrect immunization operations, we should also pay attention to the fact that diluted vaccines must be used up within 2 hours in winter and within half an hour in summer, and the remaining vaccines should be resolutely discarded. Fourthly, eliminate immunosuppressive factors and advocate advanced immunization of classical swine fever to prevent the influence of maternal antibodies, otherwise the first immunization should be carried out at the age of 20 days. It is best to measure antibodies before immunizing purchased piglets to prevent interference and inhibition of existing antibodies. Do not use drugs that can cause immunosuppression before and after epidemic prevention, or use less drugs in peacetime to strengthen the prevention and control of immunosuppressive diseases. As long as the above matters are strictly implemented, classical swine fever can be effectively prevented, otherwise, vaccinated pigs will still have classical swine fever. Fifth, scientific immunization monitoring is the key to ensure the success of epidemic prevention. Conditional pig farms should be immunized with classical swine fever 7 ~ 7 ~15 days after immunization, and the reasons for abnormal supplementary antibody levels should be analyzed in time. It should be remembered that it is wrong to rely solely on immunity. Immunization can only improve the ability of animals to resist infectious factors of epidemic diseases, and no immunization can achieve the preventive effect of 100%, so comprehensive prevention and control must be relied on.

3.4 Disinfection is an effective means to kill classical swine fever virus. Many farmers also pay attention to disinfection, but they have not achieved the desired results. The key is that the selected disinfectant cannot and cannot be disinfected correctly. At present, most disinfectants on the market are quaternary ammonium salt disinfectants, which have little irritation, but the disinfection effect is not ideal. It is best to use efficient disinfectants such as sodium hypochlorite, chlorine dioxide, glutaraldehyde and potassium persulfate for disinfection. During disinfection operation, organic matter should be removed first, and then sprayed. The concentration of disinfectant should not be increased blindly, but it should be sprayed carefully, leaving no dead ends. At the same time, spray enough water for at least 20 minutes.

3.5 Handling sick pigs. After discovering suspected swine fever, we should quickly diagnose and promptly and decisively deal with sick pigs. Otherwise, blind treatment will only increase the spread loss of the epidemic. It may be effective to isolate the sick pigs immediately after the onset of the disease, and to vaccinate the same group of pigs (6-8 pigs) after the diagnosis, and to vaccinate the suspected sick pigs with high dose of classical swine fever vaccine (10-20 pigs or more). Emergency disinfection of piggery, environment and utensils, once a day 1 time, for more than 1 week, until the epidemic ends.