Symptoms and Prevention and Treatment Methods of Porcine Reproductive and Respiratory Syndrome
Porcine Reproductive and Respiratory Syndrome
I. Pathogen
The pathogen of this disease is an enveloped RNA virus of the family Arteriviridae and genus Arterivirus, oval in shape, with a diameter of 40-60 nm, and protrusions of about 5 nm on its surface.
The virus has relatively weak resistance to the external environment, and mutation is a very obvious characteristic of RNA viruses. The mutation of the PRRSV genome is one of the important reasons why this disease is difficult to control.
The virus has a strong immunosuppressive effect. During the obvious epidemic stage of this disease, virus-carrying pigs are in the viremic period, which is often one of the main reasons for the failure of swine fever immunization.
The isolated strains of porcine reproductive and respiratory syndrome virus in China all belong to the American type, and no European-type strain has been found so far. At the same time, the isolated strains in China also show mutation phenomena, and deletion mutant strains have now been found to exist.
II. Epidemiology
1. Susceptible animals
Pigs of all ages and types can be infected, but pregnant sows and piglets are the most susceptible and show typical clinical symptoms.
2. Source of infection
Sick pigs and virus-carrying pigs are the source of infection. Various secretions, nasal fluid, urine, feces and exhaled gas of infected pigs all contain the virus, and surviving pigs can shed the virus externally for a long time.
3. Routes of transmission
Airborne transmission, vertical transmission, etc.
4. Epidemic characteristics
Strong variability, fast transmission speed, severe contamination, and once it occurs it is difficult to purify. After an outbreak, repeated outbreaks may occur after several months or years.
III. Clinical symptoms
The incubation period of this disease varies greatly. After introduction of the infection, the incubation period for PRRS in susceptible herds is as short as 3 days and as long as 37 days. The clinical symptoms of this disease vary greatly and are affected by virus strain, immune status, feeding and management factors and environmental conditions. Pigs at different stages have different clinical manifestations.
1. Sows mainly show depression, reduced or absent appetite, fever, and different degrees of respiratory difficulty. In late gestation (105-107 days), sows experience abortion, premature birth, stillbirth, mummified fetuses and weak piglets. The abortion rate of sows can reach 50%-70%, the stillbirth rate can reach more than 35%, and mummification can reach 25%. Some newborn piglets show symptoms such as respiratory difficulty, ataxia and mild paralysis, and the mortality rate increases significantly within 1 week after birth (40%-80%). A small number of sows show postpartum agalactia, retained placenta and increased vaginal discharge.
2. One-month-old piglets show typical respiratory symptoms, respiratory difficulty, sometimes abdominal breathing, reduced or absent appetite, body temperature rising above 40°C, and diarrhea. The coat is rough and disordered, with ataxia, progressive emaciation and eyelid edema. A small number of piglets may show purpling of the ears and body surface skin. The mortality rate of pre-weaning piglets can reach 80%-100%. After weaning, the weight gain of piglets decreases, the average daily gain can decrease by 50%-75%, and the mortality rate increases (10%-25%). Surviving pigs grow slowly and are prone to secondary diseases.
3. Growing pigs and finishing pigs show mild clinical symptoms, with varying degrees of respiratory symptoms. A few cases may show coughing and dark purple skin on the back, edges, abdomen and tail of both ears. Infected pigs are prone to secondary infection and show corresponding symptoms.
4. The incidence rate in breeding boars is relatively low, mainly showing general clinical symptoms, but the semen quality of boars declines, sperm abnormalities appear, and the semen can carry the virus.
IV. Pathogenesis
The virus mainly attacks the macrophages of pigs, causing a decline in the body's immune function, and sick pigs are extremely prone to secondary infection with various diseases. The specific target cell of this virus is the pulmonary macrophage. Once infected, the immune function of the body's respiratory system is reduced, and secondary invasion of the lungs by microorganisms tropic to the respiratory mucosa causes inflammation.
V. Pathological changes
The main lesions appear in the respiratory system and lymphoid tissue. The lungs are reddish-brown and mottled, and do not collapse. Microscopic lesions mainly manifest as interstitial diffuse pneumonia, thickening of alveolar walls, widening of pulmonary interstitium, edema of alveoli and alveolar septa, and inflammatory cell infiltration. Lymph nodes are enlarged, and some lungs adhere to the pleura.
VI. Diagnosis
It is very difficult to make a judgment based only on clinical manifestations, epidemiological characteristics and necropsy lesions. Laboratory testing technologymust be relied upon for confirmation. Laboratory diagnosis mainly includes serological diagnosis, pathogen isolation and identification, RT-PCR detection and other methods. ⟦32 VII. Disease prevention and control
VII. Disease prevention and control
1. Adhere to the principle of self-breeding and self-raising, establish a stablebreeding pigherd, and do not introduce breeding stock easily. If introduction is necessary, first clarify the epidemic situation of the source farm. In addition, serological testing should be carried out, and only negative pigs should be introduced. The introduction of positive virus-carrying pigs must be strictly prohibited. After introduction, an appropriate isolation area must be established and monitoring work must be done well. Generally, isolation and quarantine are required for 4-5 weeks, and only healthy animals may be mixed into the herd for rearing.
2. Large-scale pig farms must fully implement all-in-all-out, and at least achieve all-in-all-out in the two stages of farrowing house and nursery.
3. Establish and improve the biosecurity system of large-scale pig farms, regularly disinfect pig houses and the environment, and maintain the cleanliness and hygiene of pig houses, feeding and management tools and the environment. On the one hand, this can prevent the introduction of external epidemic diseases. On the other hand, through strict sanitary disinfection measures, the contamination of pathogenic microorganisms in the pig farm can be reduced to a minimum, which can maximize the control and reduction of the incidence rate of PRRSV-infected herds and the chance of secondary infection.
4. Do a good job in herd feeding and management. In pig farms infected with porcine reproductive and respiratory syndrome virus, feeding and management of pigs at all stages should be done well, good feed should be used, and the nutritional level of the herd should be ensured, so as to improve the herd's resistance to other pathogenic microorganisms, thereby reducing the incidence rate of secondary infection and the losses caused thereby.
5. Do a good job in immunization against other epidemic diseases and control other diseases well, especially the control of swine fever, porcine pseudorabies and swine enzootic pneumonia. In pig farms infected with porcine reproductive and respiratory syndrome virus, maximum efforts should be made to control swine fever well, otherwise it will cause high mortality in the herd; at the same time, immunization with swine enzootic pneumonia vaccine should be vigorously promoted to reduce the damage of Mycoplasma hyopneumoniae to the lungs, thereby improving the resistance of the herd's lungs to infection by respiratory pathogens.
6. Regularly monitor the infection status of porcine reproductive and respiratory syndrome virus in the herd to understand the activity status of the disease on the pig farm. Generally speaking, monitoring is conducted once a quarter. Samples are taken from pigs at various stages for antibody monitoring. If there is no significant change in the antibody positive rate in 4 monitoring rounds, it indicates that the disease is stable on the pig farm. On the contrary, if the antibody positive rate increases in a certain quarter, it indicates that there are problems in management and sanitary disinfection on the pig farm, and corrections should be made.
7. Strict blockade should be implemented for affected pig farms; certain measures should also be taken for pig farms around affected farms to avoid the spread of disease. Aborted placentas, stillborn fetuses and dead pigs should all be properly disposed of harmlessly, and the farrowing house should be thoroughly disinfected; sick pigs should be isolated, symptomatictreatmentshould be provided, and feeding conditions should be improved, etc.
8. Regarding vaccination, generally speaking, there are currently no very effective immune prevention and control measures. Commercial PRRS attenuated vaccines and inactivated vaccines have been launched at home and abroad, and there are also officially approved inactivated vaccines in China. However, the phenomenon of increased virulence due to reversion of PRRS attenuated vaccines and safety issues have increasingly aroused people's concern. There have been multiple outbreaks of PRRS in pig herds caused by the use of attenuated vaccines at home and abroad. Therefore, live vaccines should be used with caution. (Source: Song Yongxiang, Veterinary Research Testing)