01 Porcine reproductive and respiratory syndrome (PRRS)
According to clinical and epidemiological data, more than six tenths of PRRS pneumonia cases involve mixed infections. In most mixed-infection scenarios, the PRRS virus is not the “primary pathogen” but an “amplifying factor”—it first damages the pigs’ immune systems, then amplifies the pathogenicity of other pathogens.
In addition, the immune efficacy of currently available commercial vaccines is not ideal, and infection rates in pig herds remain high, making mixed infections the main form of harm caused by PRRS.
02 A set of proportion data: PRRS mixed infections
The proportion of co-infections involving different pathogens and PRRS varies significantly
Actinobacillus pleuropneumoniae + PRRS: approximately 60%
Porcine circovirus type 2 + PRRS: approximately 55%
Among PRRS-positive pig farms, approximately 50% of pigs are also infected with swine influenza
Pasteurella + PRRS: approximately 30%
Streptococcus + PRRS: approximately 20%
Haemophilus parasuis + PRRS: approximately 14%
Figure 1: Proportion of mixed infections involving different pathogens and PRRS (based on clinical and epidemiological data)
In terms of lesion characteristics, interstitial pneumonia and bronchopneumonia are the most common lung conditions in pigs with mixed infections, and both are often present at the same time on post-mortem examination.
Pay particular attention to Mycoplasma hyopneumoniae. The higher the Mycoplasma infection rate, the higher the incidence of swine influenza, contagious pleuropneumonia, and PRRS; the three are strongly associated. This is because Mycoplasma first damages the respiratory cilia and mucosal barrier, effectively “opening the door” in advance for PRRS and bacterial pathogens.
03 Why is PRRS a “mixed-infection amplifier”?
PRRS virus amplifies mixed infections mainly through three pathways.
First, it damages alveolar macrophages, clearing the way for bacteria. After PRRS virus enters a pig, it specifically infects and replicates in large numbers in alveolar macrophages, immune cells in the lungs. This reduces the number of pulmonary phagocytes and weakens their ability to engulf and kill bacteria. Take Haemophilus parasuis as an example: when macrophages cannot effectively engulf and kill it, the herd becomes more susceptible to H. parasuis. Relevant international studies have also confirmed that PRRS virus infection reduces the bactericidal activity of macrophages, thereby increasing the risk of secondary infection.
Second, it causes immunosuppression, leading to pathogen “spread + persistence.” When the immune system is damaged, pigs become less resistant to various bacteria and viruses. Pathogens spread more easily among pigs, and infections are more likely to become persistent, forming a cycle of “carrying the virus—shedding the virus—reinfection”—a key reason why positive farms often have difficulty achieving long-term stability.
Third, it aggravates symptoms and complicates the disease course. After PRRS infection, the disease manifestations of pathogens such as porcine contagious pleuropneumonia, H. parasuis, Streptococcus, Mycoplasma, swine influenza, and circovirus all become markedly more severe. Pigs develop fever and difficulty breathing, mortality and culling rates rise, and treatment costs increase.
04 Three common misconceptions to rule out before discussing prevention and control
Misconception one: “Once vaccinated, there won’t be any mixed infections.” PRRS vaccines can significantly reduce disease and virus shedding, but due to strain diversity, interference from maternal antibodies, and immunosuppression, they can hardly completely prevent infection. The less-than-ideal immune response is precisely one of the factors behind the high incidence of mixed infections. Vaccination is necessary, but one shot is not a cure-all.
Misconception two: “Mixed infections mean PRRS wasn’t controlled.” Mixed infections result from the combined effects of PRRS and multiple pathogens. Relying solely on “PRRS control” cannot address the whole picture; prevention and control measures need to be designed around the mixed-infection chain as a whole.
Misconception three: “It’s enough to suppress the respiratory symptoms.” Symptoms are just the result; the cause lies at the immune level. Symptomatic treatment alone can lead to recurrence after medication is stopped and accelerate bacterial resistance.
05 Comprehensive prevention and control: Manage PRRS as part of the “mixed-infection chain”
For controlling PRRS co-infections on large-scale pig farms, the following is recommended.
1. Biosecurity: prevent introduction and spread.
Give top priority to breeding-stock introduction management: source animals from negative-status farms, and strictly enforce quarantine and testing before introduction to prevent new strains from being introduced. The competent authorities also explicitly recommend that pig farms formulate targeted measures, such as immunization, elimination and acclimatization, based on assessments of viral infection status and strain types, and strengthen testing and quarantine for introduced breeding stock.
Implement batch production and all-in/all-out management, with no cross-contact between batches.
Strictly control the movement of people, vehicles, and items; disinfect regularly. Disinfectants such as glutaraldehyde, peracetic acid, and dichlorine have a good ability to kill PRRS virus.
2. Immunization strategy: tailor it to the actual conditions on this farm; do not change strains blindly.
First determine the infection status and circulating strains on the farm, then develop an immunization plan.
For the breeding herd, inactivated vaccines can be combined with universal vaccination and acclimatization to maintain stable herd immunity; schedule immunization for commercial pigs according to disease patterns.
Use antibody monitoring and production performance to assess immunization effectiveness. When effectiveness is poor, identify the cause rather than frequently switching vaccines.
3. Husbandry management: reduce stress and protect the respiratory barrier.
Pay close attention to ventilation, temperature control, stocking density, and nutrition to reduce irritation of the respiratory mucosa by harmful gases such as ammonia.
Pay particular attention to mycoplasma control—it is an important “synergistic factor” in the mixed-infection chain.
4. Monitoring and early warning: Use data to assess stability.
Conduct regular serum antibody monitoring (track changes in S/P values) and antigen testing, with sampling by stage for sows, nursery pigs, and finishing pigs.
Take samples from abnormal pigs promptly and send them for testing, turning “going by feel” into “looking at the data.”
5. Medication and symptomatic intervention: Control secondary infections; do not attempt to tough out the virus.
When mixed infections occur, first conduct pathogen diagnosis. Use antimicrobials appropriately once secondary bacterial infection has been confirmed, select drugs based on susceptibility test results, and strictly observe the withdrawal period.
Immune-enhancing products can be used as an adjunct, but they cannot replace biosecurity and immunization management.
For eradication goals, it is recommended to manage herds in tiers according to their status: maintain negative status in negative herds; for unstable positive herds, first aim for “stability,” then assess the eradication pathway, without blindly pursuing negative status.