Apart from African swine fever, PRRS is one of the most serious diseases affecting pig herds in China. It occurs in four statuses on pig farms in China.
PRRS outbreak farms
In this status, pigs at every production stage show symptoms of PRRS. Laboratory tests show that PRRS virus antigens and antibodies are positive at every stage. PRRS virus circulates among boars, sows, and commercial pigs. The sow abortion rate exceeds 8%, and stillborn and mummified fetuses account for more than 20% of the total number of piglets born. The mortality rate of newborn piglets within 7 days of age exceeds 25%. Respiratory diseases are severe in nursery and finishing pigs, and the effectiveness of classical swine fever and pseudorabies vaccination is very poor.
PRRS-positive unstable herd
Antibodies are positive at all stages, with no viral circulation among the breeding pigs, and PRRS virus nucleic acid tests are negative. PRRS virus nucleic acid tests of testicular tissue exudate from newborn piglets are negative. Virus nucleic acid tests of serum from weak piglets after weaning are positive, and respiratory problems are severe among nursery pigs. A few sows have had abortions and stillbirths.
PRRS virus-positive stable (or inactive) herd
PRRS virus nucleic acid tests are negative in pig groups at all stages, but antibody tests are positive. The breeding herd was previously infected with PRRS virus, but did not shed the virus to piglets. There is also no viral transmission within the breeding herd, and there is no PRRS virus circulation among weaned piglets. There are no clinical symptoms, and overall production is relatively stable and in a normal state.
PRRS-negative farm
Laboratory tests at every production stage are negative for both PRRS virus antibodies and antigens. Production is stable, and there are no symptoms of PRRS at any stage. However, if there is a biosecurity breach, the introduction of a new PRRS virus or the virus being brought in with newly introduced breeding stock could lead to widespread infection and illness across the farm.
For many livestock farmers, maintaining a stable PRRS-positive status on the farm without affecting production is currently the best state of PRRS control.