Persistence of transmission of porcine reproductive and respiratory syndrome (prrs) virus

Porcine reproductive and respiratory syndrome (PRRS) virus has a great impact on the economic benefits of the global industry. The disease increases mortality, reduces pig production performance and growth rate, and increases the cost of vaccines and drugs. Although many prevention and control measures have been put forward, their effects are still different after implementation, so these measures are not suitable for some pig farms.

PRRS virus has long-term pathogenic infectivity and can spread horizontally and longitudinally. Fetuses in the womb and piglets before weaning are infected with the virus late and can be transmitted to the entire piglet house through bedding contaminated with the virus.

Recently, a team from the University of Minnesota School of Veterinary Medicine studied how the PRRS virus survives in infected pigs and how it spreads among individual pigs. After analyzing the different results, they came to the following brief conclusions.

1. In the herd of endemic infection, the infection rate of breeding pigs is lower. A commercial pig farm of 1500 adult sows was selected to investigate the diagnosis of 60 adult pigs. Only 1 to 60 pigs carried the virus. PRRS virus was isolated from the lymph nodes of a sow by ELISAS/P test. In order to eliminate the PRRS virus, it is forbidden to transfer or replace the reserve sow herd within 9 months after the sow is confirmed to be infected. The virus has been proved to have strong infectivity and pathogenicity. It can have a clinical impact on sows, and the piglets produced have no clinical manifestations, but are infected.

2. Sows in the incubation period of PRRS can transmit the virus. Experimental PRRS virus positive sows can lead to a three-month infection period in sows. Within about 1.5 to 3 months after infection, small groups of pigs can also transmit the virus. Although infected sows and negative sows are separated by fences, nose-to-nose contact can still transmit to each other.

3. It is of little significance to detect the virus in tonsil tissue before animal death. It is reported that the detection of tonsil tissue in vivo can not effectively confirm whether the pig is infected. Because the virulence of the virus in other tissues of pigs is stronger than that of tonsils, and the collection of tonsils is more difficult. This testing process is labor-intensive on the one hand and harmful to animals on the other. Oropharyngeal swabs are a good method for collecting almond tissue, but studies have shown that not all PRRS viruses from infected sows are parasitic at this site.

4. Genetic diversity, different strains of PRRS virus co-exist in polluted pig farms. Scientists have detected three different virus strains in a herd of 1500 sows. These viruses have been confirmed to have a complete structure, not caused by genetic mutation and genetic recombination, and to detect viruses at all different stages. It is not clear whether the program for reserve sows can ensure the prevention of cross-infection and whether it is consistent with cross-immunization.

5. PRRS virus can be transmitted through contaminated overalls, work boots, needles and so on. In infected pig farms, it is very important to prevent mutual infection among pig individuals, and scientists have confirmed that work clothes and work boots contaminated by the virus are an important route of transmission. In addition, the virus can be spread through needles and syringes contaminated by the virus. It should be noted that the delivery room is also an important way to spread the virus.

Taking into account cost, labor and time, the principle for pig producers in the United States to change needles is that every ten sows, each litter, piglets in the same delivery room or adult pigs in the same column. It is also very important to change overalls and work shoes frequently. After the hands of workers come into contact with infected pigs, they must wash their hands with soap.

6. Through the implementation of detection and eradication program, PRRS virus can be successfully eliminated. Recent studies on five pig breeding farms have shown that the virus can be successfully eradicated. The antibody of PRRS was detected by ELISA method, and whether the breeder pig was infected with PRRS virus was detected by PCR method. To make a perfect plan, the pigs were sampled and tested by ELISA method for 12 months, and the serum test was carried out at the end of the 12th month. All the pigs were negative. The limitations of this study are the size of the breeding herd (20-1200) and the inability to detect breeder pigs immunized with PRRSV vaccine.

Problems to be solved:

1. How to put an end to the re-infection of PRRS virus to sows when breeding pigs are endemic?

2. In order to reduce the risk of infection in sows, how long is it most appropriate for infected breeding pigs to be isolated?

3. How can we confirm the way that PRRS virus enters the pig farm?

Choose better ways to eliminate the disease and protect pigs. At the Minnesota Center for Disease Eradication in the United States, researchers have begun to study the above problems, and are working with pig farm leaders and expert organizations to use effective resources to overcome these problems.