PRRS is considered the number one killer of pig farms. Many pig farmers are alarmed by “PRRS,” and many are actively exploring ways to prevent and control the disease. But early warning is more important than control. Experts say there are several warning signs before a PRRS outbreak: abortions or premature farrowings in sows exceed 8% within 14 days; stillborn piglets account for 20% of the litter; mortality among piglets exceeds 25% within 1 weeks of birth; a large-scale outbreak of Haemophilus parasuis; the percentage of pigs carrying the virus is above 30%; or the sow replacement rate exceeds 40%. Farmers can use these indicators to assess the likelihood of a PRRS outbreak on their farms and take preventive and control measures in advance.
Warning sign one: Sow abortions exceed 8%
Professor Yang Hanchun of China Agricultural University believes that before a PRRS outbreak, production performance on a pig farm will inevitably change, particularly the sow abortion rate and piglet survival rate. If 2 of the following clinical indicators occur on a pig farm within 14 days, it can be classified as a suspected PRRS case: 1, sow abortions or premature farrowings exceed 8%; 2, stillborn piglets account for 20% of the litter; 3, mortality among piglets exceeds 25% within 1 weeks of birth.
It is reported that the Ministry of Agriculture has issued the main clinical diagnostic criteria for highly pathogenic PRRS: the morbidity rate in piglets can reach 100%, the mortality rate is above 50%, the abortion rate in pregnant sows is above 30%, and adult pigs can also become ill and die. Confirmation requires serological testing and virus isolation and identification. PRRSV-specific IgG appears 10-14 days after viral infection. Antibody level changes are tested using paired serum samples (at the onset of illness and 2 weeks later). If antibody levels rise significantly without vaccination, this indicates viral infection.
Warning sign two: Increased sow returns to estrus and high body temperature
According to Professor Deng Xianbai of South China Agricultural University, before a PRRS outbreak on a pig farm, the first signs are premature farrowing in pregnant sows, followed later by increases in abortions, stillbirths, mummified fetuses, and weak piglets, as well as a higher return-to-estrus rate and elevated body temperature. Piglets develop impaired peripheral circulation, with bluish-purple discoloration of the ears, tail, abdomen, and vulva, and swollen eyelids. The younger the piglets, the higher the mortality rate. Sick pigs show obvious respiratory symptoms, including sneezing and abdominal breathing. If these symptoms are observed in the herd, the sick pigs should undergo further post-mortem examination.
Pathological changes at post-mortem examination: The lymph nodes are enlarged and rust-colored, especially the mediastinal lymph nodes, with large amounts of iron ions in the lymphatic sinuses. Interstitial pneumonia is present, with pinpoint hemorrhages on the surface and slightly widened interstitium; lymphocytes proliferate in the walls of the bronchioles and alveoli. The muscles are yellowish and edematous.
Sign three: The proportion of pigs carrying the virus is above 30%.
“When the proportion of pigs carrying the virus exceeds 30%, the likelihood of a PRRS outbreak at the pig farm is very high.” Fan Fuhao, director of the Ministry of Agriculture Breeding Pig (Guangzhou) Testing Center, told the reporter that when more than 30% of a herd carries the virus, the environment cannot reach equilibrium through self-purification; the balance between antigens and antibodies is disrupted, and the disease can easily spread through the herd.
It is understood that the most labor-intensive part of using this indicator to monitor PRRS is collecting blood to test for the virus. The most troublesome part is the blood collection: first, it requires a high level of technical skill from workers; second, it causes considerable stress to the pigs. Experts in the United States have pointed out through experiments that testing saliva samples for PRRS and classical swine fever antigens and antibodies can achieve results similar to testing blood.
According to Fan Fuhao, saliva collection is very simple. It uses pigs’ curiosity and tendency to chew: saliva is collected with a gauze ball. A gauze ball is made and secured to the end of a metal wire. The pigs will chew the gauze ball voluntarily, and saliva enters it through capillary action. The gauze ball is then squeezed to release the saliva into a sample bag. The saliva in the sample bag is then transferred to a centrifuge tube. “It is much simpler than blood collection, but this method has too many unstable factors and requires further research. Many pig farms abroad are trying it.”
Warning sign four: Haemophilus parasuis runs rampant
“To some extent, Haemophilus parasuis disease can be defined clinically as an indicator of PRRS,” said Zhang Guihong, a professor at South China Agricultural University. If Haemophilus parasuis breaks out on a large scale within a short time at a pig farm, it is necessary to determine whether it is a primary or secondary disease, because pigs may develop secondary Haemophilus parasuis infections before a PRRS outbreak.
According to Zhang Guihong, there are three main causes of PRRS:
1, unscientific PRRS vaccine immunization: one approach is not to vaccinate; another is to vaccinate unscientifically, such as immunizing only breeding pigs or piglets, using vaccines from different strains to immunize the same group, using multiple vaccines from different strains at the same time, administering an insufficient dose, or leaving too long an interval between immunizations.
2, improper handling after an outbreak of porcine epidemic diarrhea or Foot-and-Mouth Disease: including failure to promptly provide booster vaccinations for PRRS, circovirus, and classical swine fever (these two diseases affect the normal immune status of the herd; after the outbreak stabilizes, the relevant vaccines must be promptly administered as booster vaccinations); failure to vaccinate against PRRS in a timely manner according to the immunization program, resulting in missed vaccinations; and using external serum to control the outbreak, which also introduced an external PRRS virus.
3, Improper introduction: the health status of this farm does not match that of the breeding pig farm from which breeding stock is introduced; breeding stock is introduced from multiple breeding pig farms at the same time or successively; a large batch of breeding stock is introduced at one time (the number introduced exceeds 20% of the original number of sows on the farm), etc.
Sign five: The sow replacement rate exceeds 40%
“Farms with an excessively high proportion of first-parity sows are prone to outbreaks of PRRS.” According to analysis by Cai Xuehui, a researcher at the Harbin Veterinary Research Institute, first-parity sows are usually a group with unstable antigen and antibody levels, making them especially susceptible to PRRS virus. If first-parity sows and replacement gilts together account for more than 40% of the herd, then PRRS warrants close attention.