How long does the transmission of porcine reproductive and respiratory syndrome virus persist?

The porcine reproductive and respiratory syndrome (PRRS) virus has an enormous impact on the economic benefits of the global @#@227@#@ industry. This disease increases mortality and reduces pig production performance and growth rate, and increases the cost of vaccines and medications. Although many prevention and control measures have been proposed, their effectiveness after implementation still varies, so these measures are not suitable for some pig farms.

The PRRS virus has long-lasting infectious pathogenicity and can spread both horizontally and vertically. Fetuses in the uterus and pre-weaning piglets are infected by this virus late, and it can spread throughout the entire piglet barn through virus-contaminated bedding.

Recently, a research team from the College of Veterinary Medicine at the University of Minnesota studied how the PRRS virus survives in infected pig herds and how it spreads between individual pigs. After analyzing different results, they reached the following brief conclusions.

1. In endemically infected pig herds, the infection rate among breeding pigs is relatively low. At a commercial pig farm with 1500 adult sows, a diagnostic survey of 60 adult pigs found that only 1/60 of the pigs carried the virus. Through the ELISAS/P test, PRRS virus was isolated from the lymph nodes of one sow. In order to eliminate the PRRS virus, within 9 months after confirming that a sow is infected, it is strictly forbidden to introduce or replace the replacement gilt herd. This virus has been proven to have very strong infectivity and pathogenicity. It can produce clinical effects in sows, and the piglets produced show no clinical signs but are infected.

2. Sows in the latent period of PRRS can already transmit the virus. Experimentally infecting PRRS virus-positive sows can lead to a three-month infectious period in the sow herd. Within approximately 1.5 to 3 months after infection, small groups of pigs can also transmit this virus. Although infected sows and negative sows are separated by pens, nose-to-nose contact can still cause mutual infection.

3. Testing the virus in tonsil biopsy tissue before the animal dies is of little value. It has been reported that testing tonsil biopsy tissue cannot effectively confirm whether a pig is infected. This is because the virus has stronger pathogenicity in other tissue sites of the pig than in the tonsils, and collecting tonsil tissue is relatively difficult. On the one hand, this testing process is labor-intensive, and on the other hand, it harms the animal. Oropharyngeal swabs are a better method for collecting tonsil tissue, but studies have shown that not all infected sows have PRRS virus residing in this site.

4. Due to genetic diversity, different strains of PRRS virus coexist in contaminated pig farms. Technicians detected three different virus strains in a herd of 1500 sows. These viruses have been confirmed to have complete structures and are not caused by gene mutation or genetic recombination, and viruses were detected at all different stages. Whether the program implemented for replacement gilts can ensure the prevention of cross-infection, and whether it is consistent with cross-immunity, is still not very clear.

5. PRRS virus can be transmitted through contaminated work clothes, work boots, needles, and so on. On infected pig farms, preventing mutual infection between individual pigs is very important. Technicians have confirmed that virus-contaminated work clothes and work boots are important transmission routes. In addition, the virus can be transmitted through virus-contaminated needles and syringes. In particular, attention should be paid to the fact that the farrowing house is also an important route of virus transmission.

In consideration of cost, labor, and time, the principle for U.S. pig producers when changing needles is: every ten sows, every litter of piglets, piglets in the same farrowing house, or adult pigs in the same pen. Frequently changing work clothes and work shoes is also very important. After workers' hands come into contact with virus-infected pigs, they must wash their hands with soap.

6. Through the implementation of testing and eradication programs, the PRRS virus can be successfully eliminated. Recent research on 5 breeding pig farms showed that the virus can be successfully eradicated. The ELISA method was used to detect PRRS antibodies, and the PCR method was used to detect whether breeding pigs had already been infected with the PRRS virus. A comprehensive program was formulated, using the ELISA method to conduct sampling testing of the pig herd continuously for 12 months. At the end of the 12th month, serum testing was conducted again, and all pig herds were negative. The limitations of this study are the size of the breeding pig herds (200-1200 head) and the inability to test breeding pig herds immunized with the PRRSV vaccine.

Issues to be resolved:

1. How can re-infection of sows by PRRS virus be minimized when the breeding pig herd is endemically infected?

2. In order to reduce the risk of sow infection, what is the most appropriate isolation period for infected breeding pigs?

3. How can the route by which PRRS virus enters the pig farm be confirmed?

Choosing better methods to eliminate the disease and protect the pig herd, at the Minnesota Disease Elimination Center in the United States, researchers have begun to study the above issues and are working with pig farm managers and expert organizations to use effective resources to overcome these problems.