Diagnosis and control of porcine reproductive and respiratory syndrome

1. Overview of the history of porcine reproductive and respiratory syndrome:

Porcine Reproductive and Respiratory Syndrome (PRRS) is a new disease that emerged in the late 1980s. Many people believe that PRRS is currently the most economically important disease that threatens the pig industry worldwide. In the United States, this unexplained viral disease was first reported in 1987 and its symptoms were mainly miscarriage, premature birth, stillbirth, mummification, a sharp drop in the survival rate of piglets, and respiratory symptoms in bred pigs. Afterwards, Canada, Japan, France, Spain, the Netherlands, and Denmark also reported the occurrence of the disease. The devastating outbreak that occurred only in Europe between 1990 and 1991 caused the death of more than 100 000 pigs, posing a serious threat to the pig industry and trade in its products. At the time, due to the devastating outbreak of PRRS, the harm was great and the cause of the disease was unknown. In October l990, the American Animal Health Association called it Mystery of the Pig (MSD); in the United Kingdom, the greatest feature of the disease was the weakness of the born piglets. High mortality rate, ear and ventral skin developed due to lack of oxygen, in June 1991 the British government called the "blue ear disease", referred to as "blue ear disease" (BEPD); it was also referred to according to clinical symptoms as “Swine infertility and abortion syndrome (Swine infertility and respiratory syndrome).” Until 1991, the Central Veterinary Authority of the Netherlands isolated pathogens from pig alveolar macrophages (PIV). The factor, Lelystad Virus LV, is really the beginning of the study of this disease. By studying the morphology, structure, genomic composition and immunological properties of the pathogen, it was determined that PRRS is a new discovery. In 1991, the European Union named the disease as PRRS, which was approved by the OIE in 1992 and listed it as In the mid-1990s, PRRS broke out in the Philippines, Japan, South Korea and Taiwan provinces in Asia. In 1995, PRRS broke out in Beijing in China, and then PRRS-like diseases occurred in some pig farms in North China and East China. The existence of PRRS was confirmed serologically, and the pathogen was isolated from Harbin Institute of Veterinary Medicine in 1996. It was identified as PRRS virus, which confirmed the existence of PRRS in China.

2. The etiology of porcine reproductive and respiratory syndrome

PRRS is very similar to equine arteritis virus (EAV) mouse lactate dehydrogenase virus (LDV) and monkey hemorrhagic fever virus (SHFV). Therefore, at the 10th International Congress of Virology in 1996, the International Virus Classification Committee classified arteritis virus as an independent new virus group, Arteriviridae, and was assigned to Nidovirale. Since there are significant differences in the gene composition and antigenic characteristics of structural proteins between the European strain represented by the Lelystade strain and the American type strain represented by the ATTG-VR-2332 strain. Therefore, PRRS were divided into subgroup A (European type) and subgroup B (Americas type). More than a dozen strains of PRRSV, represented by GHLa strain, isolated in China by Guo Baoqing were identified as American type. The PRRS virus is an enveloped single-stranded positive-strand RNA virus. Icosahedron symmetry. PRRSV is more sensitive to organic solvents. PRRSV can be stored for a long time at -20°C to -70°C, but it is easily killed under heating, drying and general disinfectant conditions. Like other RNA viruses. The PRRS virus also has genetic diversity, so not all PRRS virus infections are the same.

3 Porcine reproductive and respiratory syndrome clinical symptoms

3.1 species of sow

At the onset of illness, the sows exhibited elevated body temperature, depression, drowsiness, anorexia, and vomiting. Individual sows even had a hunger strike for 3-7 days. With the spread of the virus in pigs. It influences the sows in the second trimester of pregnancy. After 70 days of pregnancy, the sows with PRRS have an abortion rate of 0.5% to 4%, a stillbirth rate of 2-4 times that of normal, and a mortality rate of more than 20%. The increase in the number of weaker ones leads to an increase in the mortality of pre-weaned piglets, which is usually 20% to 50%. The mummi rate rose from 2% to 4% to 15% to 20%. During the outbreak of PRRS, more than 4% of the sows in the stock generally died.

3.2 boars

Boars infected with the PRRS virus may appear as depressed due to increased body temperature, but often do not show clinical symptoms. In general, the response of boars to PRRS is similar to that of sows. The virus in the blood disappears about 7 days after infection, and PRRS antibodies appear and then rise. Some studies have pointed out that during the period of time after a boar has been infected with PRRS virus, there are viruses in the semen, and the longest is 92d (range 6-92d) with an average of 35d. It is not yet known what effect put PRRS virus into semen.

3.3 Piglets

Premature suckling pig swollen umbilical cord, bleeding, and died 24 hours after delivery. The piglets are mainly characterized by rapid breathing and sometimes abdominal breathing, which is much heavier than other bacterial and viral infections. Resistant pig growth is slow and takes more than 3 weeks to recover.

3.4 Finishing pigs

The fattening pigs and the adult pigs showed similar symptoms. Some pigs were anorexia due to elevated body temperature, depressed, coughed, and often died of secondary bacterial or viral infections. Compared with piglets, pigs infected with PRRS virus are less likely to have symptoms.

4 Diagnosis of porcine reproductive and respiratory syndrome

Diagnostic criteria: When a farm meets the following three diagnostic criteria, it should be diagnosed with PRRS.

1 The herd showed typical symptoms of PRRS and had a history of PRRS. 2 There were PRRS lesions in the pigs. 3 PRRS virus found in diseased pigs.

However, because it is sometimes difficult or impossible to obtain suitable samples in practice, it is impossible to diagnose according to the above three criteria. For example, when piglets develop clinical symptoms, lesions, and PRRS antibodies in the blood, PRRS can be diagnosed if the pigs are known to have not been immunized with PRRS.

4.1 Porcine reproductive and respiratory syndrome

Adult pigs usually have rare PRRS lesions, but growing pigs (especially non-weaned piglets) often present with typical PRRS lesions. The most prominent lesion is usually the lung. In sow herds, aborted fetuses and mummies are generally not suitable for the diagnosis of PRRS. In general, the tissues of early piglets that develop respiratory disease, including the lungs, are the best samples to examine whether PRRS lesions occur.

4.2 Detection of Porcine Reproductive and Respiratory Syndrome Virus

The PRRS virus can be isolated from the blood, lungs, or tonsils of early piglets that exhibit symptoms of PRRS. PRRS virus can sometimes be isolated in the blood or tissues of early sow days. Weak, sick pigs aged 3 to 4 days are the best samples, and blood samples from stillborn or weaker siblings can be used to isolate PRRS virus.

Indirect evidence - detection of PRRS antibodies. This method is fast and inexpensive, and as long as the blood sample is taken, the test antibody can know if it has PRRS in the past. However, according to one sampling, it is difficult to estimate whether the time of its illness was recent or several months ago. Therefore, it is difficult to know whether PRRS is rising or falling or remains unchanged. It is necessary to perform multiple sampling tests.

There are two commonly used test methods for PRRS antibodies, the ELISA test and the IFA test (abbreviated). This test is very accurate, generally divided into positive and negative, some laboratories are divided into positive, negative and suspicious. In addition, PCR can be used to detect PRRS virus in boar semen.

5. Spread of porcine reproductive and respiratory syndrome

The most common form of transmission of the PRRS virus is the infection of newly introduced pigs infected with detoxification and the original pigs on the site. Infected pigs excrete virus mainly through oral and nasal secretions and can also be excreted through excrement. After a piglet has been infected with a PRRS virus, it is usually detectable in the blood within 1 to 2 weeks, but it is also sometimes detected by 23 days. This shows that the PRRS infection is generally active for individuals. However, PRRS virus was detected by smear culture of the tonsils of infected pigs after 2 days of infection. This important finding indicates that PRRS virus may be present or “hidden” in certain parts of infected pigs. Although it is not known whether these viruses will be excreted in the future, previous studies have indicated that pigs infected with the PRRS virus can transmit PRRS. Infected boars can evacuate viruses through semen and spread PRRSV to susceptible sows.

There have been reports that PRRS virus may spread through the air, especially in regions where pig production is intensive. However, it has been proved through tests that it cannot pass through airborne media. There were several short-term trials (approximately 1 month) in which uninfected pigs and infected pigs were housed in separate rows without direct contact, but the air could convect with each other, and none of the trials showed transmission of the PRRS virus. Although these tests did not rule out the possibility of airborne transmission, they were at least considered to be extremely unlikely for airborne transmission.

There is some evidence that wildlife (birds, rodents) is an important vector because PRRS virus can be excreted through excreta, which is the source of infection. The PRRS virus is stable under freezing temperatures, so equipment contaminated in the winter can cause transmission. Correctly grasping the way PRRS spreads within the herd can develop economically effective prevention and control programs.

6. Prevention and treatment of porcine reproductive and respiratory syndrome

At present, there is no effective treatment for this virus. The key is prevention. In practice, the control measures depend on whether the herd is infected with the PRRS virus; the type of production unit: purebred/reserved or commercial pigs; management: herd size and risk tolerance; equipment conditions.

A variety of biosafety measures are particularly suitable for the prevention and treatment of PRRS. Pigs in pest-free areas must be protected from PRS as much as possible.

1 Do a good job of quarantine at the time of introduction. The introduced pigs should be strictly isolated and observed for 60 days and tested for PRRS antibodies. In order to demonstrate the test results, it is necessary to understand the vaccination status of the rabbit in the original farm.

2 Learn more about the PRRS status of pigs introduced into pig farms. Never purchase pigs from pig farms that exhibit acute PRRS symptoms to ensure that purchased pigs are negative at the end of arrival and isolation.

3 For comparison of boar or semen to be purchased with this farm, PRRS-negative farms can only purchase PRRS-negative boars or semen from PRRS-negative farms.

4 Strict implementation of the "all-in, all-out" management program. All pigs, especially conservation and production finishing pigs, should adopt an all-in/all-out system.

5 Explain the purpose of immunization and develop an immune system. Immunization injections to pigs infected with the PRRS virus can only reduce clinical symptoms, but PRRS cannot be eradicated. The immunization is effective for pigs suffering from PRRS and high-risk herds. Immunization does not guarantee that 100% of the disease does not occur and therefore cannot rely on immunization. A practical immune system should be established.

6 For those farms that have been diagnosed with PRRS and cause significant production and economic losses, PRRS vaccine is recommended for the prevention and control of PRRS virus infection. Appropriately reducing the size of the population or the size of the conservation group will help control the PRRS in the nursery groups that are suffering from PRRS disease. However, a strict set of measures must be taken to succeed.

7 Control the prevalence of PRRS virus in breeding farms, which can reduce the chronic effects of PRRS virus on pig performance.

There are still many questions to be answered about PRRS, but it is expected that there will be many new answers for this disease that are of economic importance to the world pig industry.