Pneumonia in pigs is driven by a shifting mix of bacteria, viruses, and environmental stressors that rarely act alone. The most economically damaging bacterial agents are Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae, while Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and swine influenza virus rank among the top viral triggers. What makes the disease so persistent on commercial farms is that these pathogens reinforce each other, a phenomenon veterinarians call the Porcine Respiratory Disease Complex, and the barn environment itself can tip the balance from manageable exposure to full-blown outbreak.
The Major Bacterial Causes
Mycoplasma hyopneumoniae is the single most widespread bacterial cause of pig pneumonia worldwide. It works by attaching directly to the ciliated cells lining the pig’s airways and destroying those cilia, the tiny hair-like structures that sweep mucus and debris out of the lungs. Research using porcine tracheal organ cultures showed that when the organism was physically separated from the airway surface by a fine membrane, or when immune serum was added, the damage stopped, confirming that close contact with the airway lining is essential for the pathogen to do its work.1PubMed Central. Ciliostasis and loss of cilia induced by Mycoplasma hyopneumoniae in porcine tracheal organ cultures Once those cilia are gone, the pig’s natural defense against secondary invaders collapses, which is why M. hyopneumoniae infections so often lead to more serious bacterial pneumonia on top of the original disease.
Actinobacillus pleuropneumoniae (APP) causes a far more dramatic illness. While M. hyopneumoniae typically produces a chronic, slow-burning cough, APP can kill pigs within hours of the first symptoms. The bacterium produces a family of toxins known as ApxI, ApxII, and ApxIII, and the severity of disease depends on which combination a particular strain carries. Experimental work has shown that ApxI and ApxIII are the most damaging, while ApxII contributes to a lesser extent.2PubMed Central. Endobronchial inoculation with Apx toxins of Actinobacillus pleuropneumoniae leads to pleuropneumonia in pigs These toxins punch holes in lung cells and white blood cells, producing the severe hemorrhagic lung lesions that characterize pleuropneumonia.3PubMed Central. JMM Profile: Actinobacillus pleuropneumoniae: a major cause of lung disease in pigs but difficult to control and eradicate
A third bacterium worth knowing is Glaesserella parasuis (formerly Haemophilus parasuis), the cause of Glässer’s disease. It tends to hit younger pigs, especially shortly after weaning, and produces a distinctive pattern of inflammation in the membranes surrounding the brain, joints, and organs rather than purely in the lungs.4PubMed Central. Transcriptional profile of Glaesserella parasuis in swine serosal and joint fluids In field investigations of polyserositis in weaned piglets, G. parasuis was consistently found alongside other pathogens rather than on its own, underscoring how rarely respiratory disease in pigs is a single-agent affair.5PubMed Central. Effect of Vaccination against Glässer’s Disease in a Farm Suffering from Polyserositis in Weaned Pigs Other bacteria that commonly enter the picture include Pasteurella multocida, which often acts as a secondary invader after mycoplasma has weakened the airways, and Bordetella bronchiseptica, another agent capable of damaging respiratory cilia.
Viral Drivers of Lung Disease
PRRSV is arguably the most economically devastating swine pathogen in the world, and its effect on the lungs is both direct and indirect. The virus targets alveolar macrophages, the immune cells that sit inside the lung’s air sacs and serve as the first line of defense against inhaled bacteria. Infected macrophages ramp up the production of molecules that suppress the pig’s broader immune response, including markers associated with T-cell exhaustion.6PubMed Central. Porcine Reproductive and Respiratory Syndrome Virus Infection Upregulates Negative Immune Regulators and T-Cell Exhaustion Markers In practical terms, a PRRSV-infected herd is a herd whose lungs are wide open to secondary bacterial invasion. Pigs that might have shrugged off a mild M. hyopneumoniae exposure instead develop severe, compounding pneumonia.
Swine influenza A virus (swIAV) takes a different approach. It replicates aggressively in the epithelial cells lining the airways, causing prominent tissue death in the bronchi and bronchioles along with variable inflammation deeper in the lung.7PubMed. Influenza A virus infections in swine: pathogenesis and diagnosis The damage comes from two directions: the virus kills cells directly, and the pig’s own inflammatory response amplifies the destruction. Studies in pigs have documented strong innate and adaptive immune cytokine responses in the lungs of infected animals.8PubMed Central. Swine influenza H1N1 virus induces acute inflammatory immune responses in pig lungs: a potential animal model for human H1N1 influenza virus Not all influenza strains hit equally hard; work with pig airway cell cultures showed that an H3N2 strain known to cause severe clinical disease produced significantly more barrier damage than H1N2 or H1N1 strains associated with milder illness.9PubMed Central. Porcine Airway Organoid-Derived Well-Differentiated Epithelial Cultures as a Tool for the Characterization of Swine Influenza a Virus Strains
Why Coinfections Are the Real Problem
Single-pathogen pneumonia happens in textbooks more than it happens in barns. On commercial farms, the pattern veterinarians encounter most often is the Porcine Respiratory Disease Complex (PRDC), a term that describes the overlapping infection of two or more respiratory pathogens. The classic recipe involves viruses like swIAV, PRRSV, and Porcine Circovirus type 2 (PCV2) alongside bacteria like APP, M. hyopneumoniae, and Bordetella bronchiseptica.10PubMed Central. Coinfections and their molecular consequences in the porcine respiratory tract The viruses typically arrive first, weakening mucosal barriers and immune function, which lets bacteria establish infections they would not have been able to initiate in a healthy lung. This layered assault produces worse clinical signs, more extensive lung damage, and higher mortality than any single agent could cause alone.
PRDC is one of the main reasons blanket treatment protocols sometimes fail: you may clear the bacterial component with antibiotics, but if an underlying viral infection is keeping the immune system suppressed, the same or different bacteria simply recolonize within days. Effective management therefore demands understanding which pathogens are circulating in a given herd at a given time.
Environmental and Management Risk Factors
Even when a herd carries the usual respiratory pathogens at low levels, the barn environment determines whether those pathogens stay under control or flare into clinical disease. Air quality is at the top of the list. Ammonia, produced by the breakdown of urine and manure, irritates and weakens the respiratory lining. Sensor-based monitoring in pig barns has recorded ammonia levels ranging from 1 to 30 parts per million, and elevated concentrations pose clear respiratory health risks.11PubMed Central. Virtual MOS Sensor Array Design for Ammonia Monitoring in Pig Barns Dust is another concern, as it carries bacteria, endotoxins, and fungal spores directly into the lungs with every breath.
Stocking density matters in a straightforward way: more pigs per square meter means more pathogen shedding, more ammonia, and more airborne particulates. Research comparing high-density housing with standard stocking found that crowded pigs had a significantly higher coughing index throughout the study period.12PubMed Central. High stocking density triggers stress-induced physiological changes and alters nasal and fecal microbiota in finishing pigs Stress from overcrowding also shifts the composition of nasal and gut bacteria, potentially favoring pathogenic strains.
Ventilation design ties many of these factors together. A comparison between naturally and mechanically ventilated pig barns found that pigs housed in naturally ventilated buildings experienced higher ammonia and COâ‚‚ levels, a higher daily prevalence of respiratory disease cases, and poorer overall welfare scores. The odds of testing positive for H1N1 influenza were roughly three times higher in the naturally ventilated unit.13PubMed. Environment-, health-, performance- and welfare-related parameters in pig barns with natural and mechanical ventilation Mechanically ventilated barns gave producers more control over temperature, humidity, and gas concentrations, and pigs in those barns showed better feed conversion and daily weight gain.
Diagnosing and Monitoring Pneumonia
Clinical diagnosis on the farm is usually obvious when pigs present with cough, fever, labored breathing, and reduced feed intake. The harder challenge is catching subclinical disease, the kind that silently erodes growth rates without pigs ever looking dramatically ill. Slaughterhouse lung scoring is one of the most practical tools for this. By scoring lungs at slaughter for the extent and type of lesions, veterinarians can trace disease patterns back to specific farms, buildings, or production stages. A study of over 10,000 pigs from farms in Northern Italy used a standardized scoring method to quantify pneumonia lesion severity, identify risk factors, and evaluate whether lesion presence correlated with lower carcass quality.14PubMed. Pneumonia disease assessment using a slaughterhouse lung-scoring method
A more recent comparison of two scoring approaches, one based on visual inspection only and another combining visual and hands-on palpation, found that the visual-only method tended to classify more lungs as completely healthy while underidentifying lungs with minor or moderate lesions compared to the palpation-based method.15PubMed Central. Comparing Visual-Only and Visual-Palpation Post-Mortem Lung Scoring Systems in Slaughtering Pigs For producers relying on slaughter checks to monitor respiratory health, this means a visual-only approach may underestimate the true burden of disease on the farm.
On the pathogen identification side, oral fluid sampling has gained popularity as a low-stress, herd-level surveillance tool. A rope is hung in the pen, pigs chew on it, and the collected saliva is tested for viral DNA or antibodies. This method was first proposed for PRRSV and PCV2 monitoring in 2008 and has since expanded to cover the detection of more than 23 swine viral pathogens, including large-scale implementation in herds of over 12,000 pigs.16PubMed Central. Guidelines for oral fluid-based surveillance of viral pathogens in swine It is not a replacement for individual animal diagnostics in acute cases, but for routine herd monitoring it is far more practical than bleeding individual pigs.
Antibiotic Treatment and the Resistance Problem
When bacterial pneumonia breaks out, antibiotics remain the frontline intervention. For M. hyopneumoniae infections, the drug classes with documented activity include tetracyclines, macrolides, lincosamides, pleuromutilins, amphenicols, aminoglycosides, aminocyclitols, and fluoroquinolones. Treatment can be given through feed or water (for whole-group medication) or by injection (for individual animals). Most of these classes have been shown to improve clinical signs and reduce lung lesion severity in both experimental and field conditions.17PubMed. Antimicrobial treatment of Mycoplasma hyopneumoniae infections A critical caveat, though, is that antibiotics may not prevent infection or fully clear M. hyopneumoniae from the respiratory tract, which means treated animals can remain carriers.
Susceptibility varies substantially across pathogens. Testing of respiratory bacteria from Spanish pig farms found that APP and Pasteurella multocida were highly susceptible to ceftiofur, florfenicol, and the macrolides tilmicosin, tildipirosin, and tulathromycin, with susceptibility at or above 90%. However, susceptibility to doxycycline was low for both organisms. Bordetella bronchiseptica was the hardest to treat: only tildipirosin and tulathromycin maintained full effectiveness, while susceptibility to florfenicol hovered near 50% and dropped below 30% for most other drug families.18PubMed Central. Antimicrobial Susceptibility Pattern of Porcine Respiratory Bacteria in Spain These patterns are region-specific and shift over time, which is why culture and susceptibility testing before choosing an antibiotic is increasingly considered best practice rather than a luxury.
M. hyopneumoniae is inherently resistant to beta-lactam antibiotics (penicillins, cephalosporins), sulfonamides, and trimethoprim, so these classes are off the table for mycoplasma-driven disease from the start.17PubMed. Antimicrobial treatment of Mycoplasma hyopneumoniae infections A few reports of acquired resistance to other drug classes have surfaced, and the absence of defined clinical breakpoints for this organism makes it difficult to predict whether a given antibiotic that looks active in a lab test will actually work in a living pig.
Anti-Inflammatory Support During Outbreaks
Because a large part of the lung damage in pneumonia comes from the pig’s own inflammatory response rather than direct pathogen destruction, anti-inflammatory drugs have a logical place in outbreak management. A study of ketoprofen, a non-steroidal anti-inflammatory drug (NSAID), administered orally to finishing pigs during an acute respiratory disease outbreak found that treated pigs showed reduced sickness behavior and lower rectal temperatures compared to a placebo group. However, ketoprofen did not significantly improve clinical signs overall, feed intake, or blood parameters, and it was associated with somewhat reduced weight gain over a 30-day follow-up.19PubMed Central. Effect of oral KETOPROFEN treatment in acute respiratory disease outbreaks in finishing pigs NSAIDs can improve animal comfort and reduce fever, but the evidence for them as standalone performance-boosters during respiratory outbreaks is thin. They are best viewed as a welfare tool used alongside targeted antimicrobial therapy, not a replacement for it.
Vaccination Strategies
Vaccination is the backbone of pneumonia prevention on most commercial pig farms, though expectations need to be realistic. Vaccines against M. hyopneumoniae and PCV2 are the most widely used. A field trial comparing two combined vaccine protocols (both targeting M. hyopneumoniae and PCV2 simultaneously) found that neither scheme improved overall wean-to-slaughter growth rates, but both significantly reduced lung lesions, lung fissures, and pleurisy at slaughter. Vaccinated pigs also showed strong seroconversion against both pathogens and had lower circulating PCV2 viral loads.20PubMed Central. Effectiveness of two intramuscular combined vaccines for the control of Mycoplasma hyopneumoniae and porcine circovirus type 2 in growing pigs: a randomized field trial The takeaway is that these vaccines limit tissue damage and pathogen circulation rather than delivering a visible jump in daily gain, though the long-term economic value of fewer lung lesions and lower disease pressure should not be underestimated.
Timing matters. A separate trial on a farm struggling with respiratory disease compared a one-shot M. hyopneumoniae vaccine given at weaning to a two-shot program with a booster before weaning. At slaughter, the group vaccinated earlier showed a lower prevalence of bronchopneumonia (roughly 63% versus 71%) and significantly less extensive enzootic pneumonia lesions.21PubMed Central. Clinical efficacy of two vaccination strategies against Mycoplasma hyopneumoniae in a pig herd suffering from respiratory disease In herds where pigs encounter M. hyopneumoniae early, beginning vaccination before weaning can provide measurable protection by the time pigs enter the high-risk nursery and finishing phases.
Vaccines against APP exist but face more challenges. The Apx toxins are leading candidates for cross-protective vaccine antigens because they are shared across multiple APP serovars.22The Thai Journal of Veterinary Medicine. Protective Immunity of the Pore-Forming Domains of Actinobacillus pleuropneumoniae Apx Toxins in a Mouse Model However, APP is notoriously difficult to fully control or eradicate through vaccination alone, and many producers still rely on targeted medication, strict all-in/all-out management, and herd closure strategies to manage it.
Air Filtration and Biosecurity
For PRRSV in particular, airborne transmission between farms in pig-dense regions is a major route of introduction. Air filtration has emerged as one of the most impactful biosecurity investments a breeding herd can make. A 16-year longitudinal study found that farms using year-round air filtration had roughly half the PRRS incidence rate of non-filtered farms, and farms using positive-pressure filtration systems fared even better, at about 0.42 times the incidence rate. The protective effect grew stronger over time.23PubMed. Sixteen-year longitudinal study assessing the effects of air filtration on the occurrence of porcine reproductive and respiratory syndrome in breeding herds
An earlier study covering breeding herds before and after filtration installation was equally striking: the odds of a new PRRSV introduction were about eight times higher in the pre-filtration period, and the median time between outbreaks nearly tripled, going from 11 months without filters to 30 months with them.24PubMed Central. Evaluation of the long-term effect of air filtration on the occurrence of new PRRSV infections in large breeding herds in swine-dense regions On the finishing side, a commercial farm trial comparing different filtration setups found that recirculating air filtration produced the lowest total dust concentration and the best lung health among all groups tested.25PubMed Central. Impact of different supply air and recirculating air filtration systems on stable climate, animal health, and performance of fattening pigs in a commercial pig farm Filtration is expensive to install and maintain, but for breeding herds in regions where PRRSV circulates heavily, the return on investment in avoided outbreak costs can be substantial.
The Production Cost of Lung Disease
Pneumonia’s economic toll extends well beyond treatment costs. Research has documented a clear negative relationship between pneumonia severity (measured both through radiographic imaging during life and lesion scoring at slaughter) and growth performance during the finishing period.26Preventive Veterinary Medicine. Effect of pneumonia on growth rate and feed efficiency of minimal disease pigs exposed to Actinobacillus pleuropneumoniae and Mycoplasma hyopneumoniae More severe lungs at slaughter correspond to slower daily gains and worse feed efficiency throughout finishing, which means producers pay more in feed for every kilogram of pork produced. This also explains why subclinical disease, where pigs cough occasionally but never look dramatically sick, can quietly cost a farm more money than a short, sharp outbreak that gets treated quickly.
Feed Additives and Alternatives to Antibiotics
With regulatory pressure to reduce antimicrobial use in livestock growing across the world, the search for non-antibiotic tools to support respiratory health has intensified. Essential oils derived from plants like oregano, thyme, and cinnamon have drawn interest for their antimicrobial and anti-inflammatory properties. Reviews of the evidence have concluded that essential oils show good potential as antibiotic alternatives in swine feed, though much of the data comes from growth performance and gut health studies rather than specific respiratory disease trials.27PubMed Central. Essential oils as alternatives to antibiotics in swine production Other strategies being explored include probiotics, acidifiers, and immunomodulatory compounds. None of these are ready to replace antibiotics in an active pneumonia outbreak, but they may help reduce the frequency with which antibiotics are needed by supporting overall immune function and gut integrity.
Breeding for Respiratory Resistance
A less obvious but increasingly promising approach involves genetics. Not all pigs respond to the same respiratory challenge in the same way, and some of that variation is heritable. Genome-wide studies comparing pigs with healthy lungs to those with respiratory disease lesions have identified over a hundred candidate genes in regions of the genome associated with disease resistance or susceptibility. Many of these genes are involved in immune signaling, fat metabolism, and growth factor pathways. Several highlighted genes, including TRAF6, CD44, and TGFB1, play central regulatory roles in immune responses and show measurably different expression between healthy and diseased lung tissue.28PubMed Central. New insights into host adaptation to swine respiratory disease revealed by genetic differentiation and RNA sequencing analyses Breeding programs that select for these genetic markers could, over time, produce lines of pigs with naturally stronger respiratory defenses. This is a slow-burn strategy, years from widespread commercial impact, but it represents one of the few tools that could reduce pneumonia pressure without relying on either drugs or vaccines.