P u r i f i c a t i o n a n d C o n t r o l o f C l a s s i c a l S w i n e F e v e r i n L a r g e - S c a l e P i g F a r m s
P u r i f i c a t i o n nClassical swine fever (CSF) is caused by infection with the classical swine fever virus, which belongs to the genus Pestivirus in the family Flaviviridae. It is currently one of the main infectious diseases in large-scale pig farms in China, and is also an important infectious disease requiring priority prevention and control. According to the Catalogue of Classification of Animal Pathogenic Microorganisms issued by Order No. 53 of the Ministry of Agriculture of the People's Republic of China in May 2005, classical swine fever virus is classified as a Category II animal pathogenic microorganism and is a highly pathogenic pathogen. It is worth noting that although large-scale outbreaks of typical acute classical swine fever in large-scale pig farms are no longer common, the occurrence of classical swine fever, especially atypical classical swine fever, is still widespread, and due to deficiencies in diagnostic techniques, classical swine fever outbreaks are often overlooked and misjudged. This article will introduce the necessity of classical swine fever purification, the favorable conditions, and purification strategies in the work practice of large-scale pig farms.1 Purification of Classical Swine Fever in Large-Scale Pig Farms1.1 Pig Farms Three large-scale breeding pig farms were located in northern China (Hebei A), central China (Henan B), and southern China (Jiangxi C), respectively, with good geographical conditions and isolation measures, standard feeding management, and strict disinfection measures. Each had more than 600 foundation sows and practiced self-breeding and self-rearing.1.2 Operation Methods Tonsil biopsy sampling was used to collect tonsil samples from breeding pigs. The entire production line was sampled within 1-2 d, and samples were sent to the laboratory at low temperature within 24 h for testing. Touch smears were prepared in the laboratory, and classical swine fever fluorescent antibody (purchased from the China Institute of Veterinary Drug Control) staining was used to detect classical swine fever virus antigen. Testing was completed within 1-2 d, and all positive pigs were culled at one time within 1-2 d after the test report was delivered. Then the entire breeding herd was universally immunized once with classical swine fever attenuated tissue vaccine (rabbit spleen and lymph node vaccine, 2 doses per head). Thereafter, every batch of retained replacement pigs underwent tonsil biopsy testing, and positive animals were not allowed to be used for breeding.1.3 Preliminary Purification Results The results of classical swine fever purification and testing in the 3 pig farms are shown in Table 1. In Hebei A pig farm, testing half a year after purification showed that the positive rate decreased by 70.09%. Statistical analysis (χ2 test, SPSS software) showed that the classical swine fever virus carriage rate decreased significantly (P<0.01). At the same time, through purification, the production indicators of the pig farm improved significantly: the sow farrowing rate increased by 0.8%, the live litter rate increased by 4%, the average number of live piglets per litter increased by 1.3, and the overall mortality rate decreased by 13.6%. In Henan B pig farm, two tests conducted 5 months apart showed that the positive rate of classical swine fever virus in breeding pigs decreased by 83.30% (P<0.01). After culling following the first test, the average number of healthy piglets per litter increased from 9.28 to 9.56, and the survival rate of growing pigs increased from 81.4% to 83.9%. In Jiangxi C large-scale pig farm, the author first immunized the entire breeding herd once with classical swine fever attenuated tissue vaccine (rabbit spleen and lymph node vaccine, 2 doses per head), and then tested and culled pigs with reproductive disorders. A second test of the whole herd showed that the positive rate decreased by 90.20% (P<0.01), and positive virus-carrying breeding pigs were likewise culled. While carrying out classical swine fever purification, the pig herd was also purified of pseudorabies based on gE-ELISA monitoring results, and targeted prevention and control measures were taken against porcine reproductive and respiratory syndrome (PRRS). In a very short time, the production performance of the breeding herd was greatly improved: the average number of live piglets per litter increased from 5.2 to 9.2, and the proportion of stillborn, deformed, and weak piglets decreased from 49.50% to 10.44%, greatly improving the production efficiency of the pig farm. The above purification practices in 3 large-scale pig farms show that adopting a "testing and culling" strategy in large-scale pig farms is a powerful measure for controlling classical swine fever. 2 Necessity of Classical Swine Fever Purification 2.1 Classical Swine Fever Outbreaks Are Still Widespread in China Due to the widespread application of the lapinized attenuated classical swine fever vaccine, large-scale outbreaks of classical swine fever have been effectively controlled, but at the current stage classical swine fever outbreaks are still widespread. Liu Youchang et al. used the classical swine fever virus detection ELISA kit produced by IDEXX of the United States to test more than 600 blood samples from breeding boars and sows submitted from 10 pig farms in Hebei, Tianjin, Beijing, Liaoning, Inner Mongolia, and other places from January to August 2005 for classical swine fever virus, and found that virus-carrying breeding pigs existed in all 10 pig farms. Excluding suspicious pigs, the positive rate in each pig farm was 3%-70%, with many at 10%-20%, averaging 20.5% [1]. Researcher Qiu Huishen of the China Institute of Veterinary Drug Control and others in recent years tested breeding pigs from 29 large and medium-sized pig farms in 13 provinces and municipalities in China. The results of tonsil classical swine fever fluorescent antibody staining showed that classical swine fever virus carriage in breeding pigs existed in all tested pig farms. The national average positive rate of classical swine fever antigen in breeding pigs was 11.12%, with the highest reaching 30.7% and the lowest 4.4% [2]. Wei Xiuyu et al. used an enzyme-labeled method in 2004 to detect classical swine fever antigen in submitted samples from 234 pig farms, and 64 were positive, with a positive rate of 27.4%; meanwhile, indirect hemagglutination was used to test 4,735 serum samples, and the qualified rate was only 69.1% [3]. In 2005, the author investigated the virus carriage status of breeding pigs in some large-scale pig farms in northern China and found that the virus carriage rate of breeding pigs was 5%-20%. In addition, clinical monitoring results of classical swine fever antigen in weak piglets and nursery pigs in 65% of pig farms were also positive [4]. This indicates that the phenomenon of sow virus carriage in Chinese pig farms is quite common, only varying in degree, so classical swine fever is still an important disease that must be taken seriously in large-scale pig farms at the current stage. 2.2 The Occurrence of Classical Swine Fever Is Characterized by Reproductive Disorders in Breeding Pigs and a Mild Form Due to low immune levels, the frequent presence of low-virulence classical swine fever virus, or other reasons, classical swine fever virus forms persistent infection in the breeding pig herd. The virus persists in the pig body for a long time, and both classical swine fever virus antibodies and antigens can be detected simultaneously. Only through laboratory methods can virus-carrying pigs be detected. Persistently infected breeding pigs manifest as gestating sow virus carriage syndrome and transmission through boar semen, which is the fundamental reason for the continued existence of classical swine fever outbreaks in pig herds. Clinically, apart from possible reproductive disorders, breeding pigs often show no obvious clinical symptoms. However, breeding sows may abort and produce stillborn fetuses, mummified fetuses, and weak piglets, and congenital infection in piglets often leads to immune tolerance [5]. Clinically, it is often found that some breeding pigs have extremely low levels of classical swine fever virus antibodies and antibodies to other pathogens, but antigen test results are positive. If virus-carrying replacement pigs enter the production herd, a vicious cycle chain of classical swine fever will form in the pig farm. Therefore, to control classical swine fever outbreaks in large-scale pig farms from the source, the breeding pig herd must be purified [2]. 2.3 Classical Swine Fever in Large-Scale Pig Farms Often Occurs as Mixed Infection with Other Diseases In recent years, with the development of the large-scale pig industry, some immunosuppressive infectious disease pathogens such as porcine reproductive and respiratory syndrome virus and porcine circovirus type 2 have brought great harm to pig production. Classical swine fever virus usually forms mixed infections with these pathogens, which on the one hand aggravates the harmfulness of classical swine fever, and on the other hand makes diagnosis difficult. This requires laboratory methods to identify the types of pathogens, conduct specific analysis of particular pig herds, objectively and scientifically analyze the outbreak, and while carrying out classical swine fever purification, not neglect the harm of other pathogens, adopting comprehensive prevention and control measures.3 Favorable Conditions for Purifying Classical Swine Fever 3.1 Convenient Live Sampling Technique Since virus-carrying breeding pigs are the fundamental reason for the persistent existence of classical swine fever virus in pig farms, adopting a "testing and culling" strategy for breeding pigs can purify classical swine fever from the source. At present, specialized live pig tonsil sampling instruments are available in China, making live sampling of breeding pigs very convenient (Figure 1). This method is simple and easy to learn. About 30 mg of tonsil sample is hooked from each pig, stored at low temperature, and then sent to a qualified laboratory for detection of classical swine fever virus antigen by immunofluorescence or PCR. It should be noted that the sampling site must be accurate, and after sampling each pig should be disinfected promptly. Generally, 2%-3% sodium hydroxide or phenolic disinfectants can be used. 3.2 Efficient and Economical Methods for Detecting Classical Swine Fever Antigen and Antibody It must be pointed out that at present some pig farms monitor classical swine fever virus solely based on antibody test results, and even diagnose classical swine fever according to the level of antibody titer. Laboratory and clinical practice show that antibody testing and diagnosis involve major misunderstandings, because so far existing serological methods cannot distinguish vaccine antibodies from wild virus antibodies. Therefore, diagnosing classical swine fever solely based on antibody titer levels often yields uncertain results. Thus, detection of persistently infected breeding pigs with classical swine fever virus must focus on detection of viral antigen [6]. Among these, the classical swine fever virus immunofluorescence method is a national standard method and is also an internationally used method. Fluorescent-labeled antibodies directly detect classical swine fever virus antigen, and the cytoplasm of tonsillar crypt epithelial cells of virus-carrying pigs shows specific fluorescence (Figure 2). Since the lapinized attenuated classical swine fever vaccine does not form persistent infection in pigs, the immunofluorescence method can quickly and effectively detect persistently virus-carrying pigs [6,7], and can also test samples in batches, with relatively low testing cost. Fluorescent antibodies can be ordered from specialized institutions, but interpretation of results requires some experience. Purification of classical swine fever by immunofluorescence has already achieved success in some pig farms, and classical swine fever outbreaks were effectively controlled after purification [8,9]. The principle of the ELISA method for antigen detection is the double antibody sandwich method: microtiter plates are coated with classical swine fever virus serum, bind with classical swine fever virus antigen in the test sample, and then form the upper layer of the sandwich with classical swine fever virus-specific monoclonal antibodies, detected with horseradish peroxidase. The absorbance of the substrate is proportional to the viral antigen content. Classical swine fever virus antibody detection methods include ELISA and indirect hemagglutination. Due to standardization of reagents and methods, ELISA results can be quantified, and samples can be tested at high throughput, so it has gradually been used for clinical monitoring. However, because most kits are imported, the cost is relatively high, and the sample processing procedure is also relatively complicated. It should be noted that positive and negative controls need to be strictly set up in the testing process, and the technical requirements for test personnel are also relatively high. At present, the most commonly used method for determining classical swine fever virus antibodies in China is the indirect hemagglutination method. This method uses classical swine fever virus-sensitized red blood cells to directly determine serum antibody titers, requires relatively simple equipment, and gives intuitive result interpretation, so it is suitable for grassroots application. Based on experience, 1:16 or 1:32 is generally used as the cutoff standard. It should be noted that the relationship between antibody titer and protective efficacy is still inconclusive, and like other serological methods, it cannot distinguish wild virus antibodies from vaccine antibodies. However, regular antibody monitoring can promptly reveal the immune status of the pig herd and evaluate the effect of vaccine immunization. Therefore, regular antibody monitoring is an indispensable and important part of comprehensive classical swine fever prevention and control measures in pig farms. 3.3 Qualified Classical Swine Fever Vaccines The application practice of the lapinized attenuated classical swine fever vaccine shows that the existing vaccine strain (C strain) still has strong protective power against field strains of different virulence, meaning there is no need to doubt the efficacy of the vaccine strain. However, an efficient vaccine strain does not represent an effective commercial vaccine. It should be pointed out that the reasons why classical swine fever still occurs in immunized pig herds need specific analysis; virus carriage in breeding pigs and congenital infection in piglets are important reasons. In addition, there may be certain differences in virus titer among vaccines from different manufacturers during production, and improper measures during vaccine transportation, storage, and use can affect vaccine efficacy. Unreasonable immunization programs are also an important factor, such as interference from maternal antibodies. In view of the current epidemic status of classical swine fever, pig farms purifying classical swine fever still need to use classical swine fever vaccines. Qualified and efficient lapinized attenuated classical swine fever vaccines can provide strong protection for purifying pig herds. 4 Summary and Discussion Practice shows that classical swine fever is still an infectious disease requiring priority prevention in current large-scale pig farms, and persistently virus-carrying pigs are the key factor in the occurrence of classical swine fever. Therefore, as long as the production herd is purified at the breeding population level and a "testing and culling" strategy is adopted for breeding pigs, the vicious cycle chain of classical swine fever in pig farms will be controlled from the source. Large-scale pig farms should fully recognize the new characteristics of classical swine fever epidemics, update concepts, focus on the long-term development of the pig farm, and give priority to purification of the breeding population. Through 2-3 rounds of purification of the breeding population and testing of each batch of replacement pigs, the positive rate of the breeding pig herd will be significantly reduced, thereby fundamentally controlling classical swine fever outbreaks. At the same time, it must be fully recognized that the prevention and control of classical swine fever is a systematic comprehensive project. Pig farms should adopt strict biosecurity measures, control mechanical transmission by personnel, vehicles, equipment, and treatment tools, and strengthen disinfection work. High-quality and efficient classical swine fever vaccines should be used, classical swine fever antibody levels should be monitored regularly, and a suitable immunization program for the farm should be established through regular monitoring and strictly implemented. Large-scale pig farms should also establish a regular epidemiological monitoring mechanism to promptly understand the latent infection status of diseases in the pig herd. While carrying out classical swine fever purification, prevention and control of other seriously harmful diseases such as porcine pseudorabies, porcine reproductive and respiratory syndrome, and circovirus disease should also be strengthened. When conditions permit, multiple diseases can be purified simultaneously, truly establishing a healthy and stable breeding herd, which will extremely effectively improve production efficiency and enhance brand image.This method is troublesome...
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Whose method is simpler? Tell me.
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Whose method is simpler? Tell me.