Glässer disease is a disease of pigs caused by the bacterium Glaesserella parasuis. The most common form is characterized by fibrinous polyserositis and polyarthritis; septicemia with sudden death and bronchopneumonia also can occur. Diagnosis is based on clinical signs and bacterial identification by isolation or by detection with PCR assay. Antimicrobial treatment is the first option to control a disease outbreak; however, commercial or autogenous vaccines might be useful for prevention.
Glässer disease is a severe, infectious, systemic disease of pigs characterized by severe systemic disease involving fibrinous polyserositis, polyarthritis, and/or meningitis.
Etiology of Glässer Disease
The causative agent of Glässer disease, Glaesserella parasuis (formerly Haemophilus parasuis), is a small, gram-negative pleomorphic bacterium of the family Pasteurellaceae.
G parasuis colonizes pigs during the first days of lactation, and although it is mainly a commensal organism of the upper respiratory tract, stressful events such as weaning, environmental changes, or coinfections with other agents can trigger disease.
In the laboratory, G parasuis grows on enriched chocolate agar; it can also be cultured in blood agar with a staphylococcal nurse streak. However, G parasuis is fastidious, and its isolation in pure culture from diseased animals is usually difficult and frequently complicated because of antimicrobial treatments. G parasuis requires supplementation with V factor (nicotinamide adenine dinucleotide, or NAD) but not X factor (hemin) for growth.
Fifteen serovars of G parasuis have been reported, but many of the evaluated isolates cannot be typed. Wide differences in serovar virulence have been described. Serovars 1, 2, 4, 5, 12, 13, 14, and some isolates that cannot be typed are usually isolated from pigs with systemic Glässer disease. Serovars 3, 6, 7, 9, and other nontyped isolates are frequently isolated from the upper respiratory tract. The correlation between serovar and virulence is not clear, and strains belonging to the same serovar might vary in virulence.
Serotyping also has been used as the basis to establish criteria for vaccination against G parasuis; however, cross-protection between different serovars is variable and difficult to predict. Therefore, current methods of G parasuis identification and characterization are primarily genotyping (fingerprinting or sequencing methods). Genotyping methods have confirmed a much higher heterogeneity in G parasuis than that provided by serotyping.
The identification of virulence genes in G parasuis has received increased attention because of the possibility of differentiating strains with pathogenic potential via PCR assay, as well as of developing vaccines. Additional genetic studies have been carried out to determine not only virulence, but also antimicrobial resistance, among G parasuis serovars (1, 2).
G parasuis induces adhesins that enable colonizing and form biofilms in the upper respiratory tract. Virulent strains are subsequently able to invade epithelial cells, attach to extracellular proteins, induce apoptosis, and release proinflammatory cytokines. These strains can delay the activation of alveolar macrophages, survive in the lungs, and invade systemic organs, causing inflammation and the consequent characteristic lesions of Glässer disease.
In contrast, avirulent G parasuis strains are cleared from the lungs by alveolar macrophages and remain in the upper respiratory tract.
Epidemiology of Glässer Disease
Glässer disease is associated with high rates of morbidity and mortality and occurs worldwide. Primarily young animals (4–8 weeks old) are affected; however, the age of affected animals can vary, depending on the level of acquired maternal immunity. Sporadic disease occurs in adults (eg, as a result of introduction of a naive adult to a healthy herd).
The susceptibility of pigs to Glässer disease is modulated by concomitant environmental stressors, as well as by viral infections affecting the immune system. The incidence of Glässer disease has increased since the emergence of porcine reproductive and respiratory syndrome and porcine circovirus 2 systemic disease.
Clinical Findings of Glässer Disease
Glässer disease has a sudden onset and a short course.
The typical clinical signs of acute Glässer disease can include the following:
high fever (41.5°C [106.7°F])
abdominal breathing
swollen joints
CNS signs (eg, lateral decubitus, paddling, and trembling)
Peracute disease can result in sudden death without the presence of characteristic gross lesions; petechiae can occur in some organs in these cases, indicating septicemia.
Animals with chronic Glässer disease might have a decreased growth rate as a result of severe fibrosis in the thoracic and peritoneal cavities, which can result in carcass condemnation at slaughter.
Glaesserella parasuis can be isolated from the lungs of pigs with suppurative bronchopneumonia; however, the causality is not clear, because this lesion (which causes coughing and dyspnea) is not consistently reproduced under experimental conditions. G parasuis is not considered an important cause of coughing.
Lesions of Glässer Disease
Histopathological lesions of Glässer disease include septicemia-like microscopic lesions such as DIC and microhemorrhages. An increase of fluid in the thoracic and abdominal cavities, without the presence of fibrin, can also occur in peracute cases.
Peracute Glässer disease can cause petechiae in some tissues without gross lesions.
Acute systemic G parasuis infection is characterized by the development of fibrinous polyserositis, polyarthritis, and meningitis (see polyserositis and perihepatitis and perisplenitis images). The fibrinous exudate can occur on the pleura, pericardium, peritoneum, synovia, and meninges and is usually accompanied by an increase in the amount of fluid. Fibrinous pleuritis can be accompanied by cranioventral pulmonary consolidation (suppurative bronchopneumonia). Lack of characteristic gross lesions is also common in swine showing CNS signs.
Courtesy of Dr. Joaquim Segalés.
Courtesy of Dr. Joaquim Segalés.
Chronically affected pigs usually have severe fibrosis of the pericardium, pleura, and/or peritoneum.
Diagnosis of Glässer Disease
Clinical signs and lesions
Detection of the bacterium
Diagnosis of Glässer disease is based on observation of compatible clinical signs and gross lesions, in association with detection of Glaesserella parasuis in affected swine via isolation or via molecular methods such as PCR assay.
Most current diagnostic methods do not differentiate virulent from nonvirulent G parasuis isolates, so it is important to sample only from systemically affected sites such as pleura, pericardium, peritoneum, joints, and brain. Samples collected from clinically affected, untreated animals that are euthanized increase the chances of isolation.
A multiplex PCR technique able to differentiate between virulent and nonvirulent G parasuis isolates has been developed. This technique might be used to prevent introduction of pigs carrying potentially virulent strains onto farms that are free of clinical Glässer disease.
Because G parasuis is a common commensal organism, isolation from the upper and/or lower respiratory tract has no relevance in the diagnosis of systemic infection.
Pearls & Pitfalls
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Differential diagnoses for Glässer disease include infections by the following:
septicemic Escherichia coli
Treatment and Control of Glässer Disease
Antimicrobials
Vaccination
Glässer disease is treated primarily with antimicrobials. Antimicrobial susceptibility for Glaesserella parasuis is assessed via antibiogram analysis. PCR testing is used for pathotyping and serotyping, and even to detect genes associated with resistance (3).
G parasuis is one of the few gram-negative organisms that can be successfully treated with penicillin; however, resistance to penicillin has been reported in G parasuis worldwide (4). Other antimicrobials used to treat G parasuis infection include ceftiofur, ampicillin, enrofloxacin, erythromycin, tiamulin, tilmicosin, florfenicol, and potentiated sulfonamides. Individual treatments must be administered parenterally to have any substantial effect, and all pigs in affected groups (not just those showing clinical signs) eventually require treatment.
Antimicrobial withdrawal times and restrictions per local law must be followed. To avoid the development of antimicrobial resistance, preventive treatments should be discouraged.
Either commercial or autogenous vaccines can be used to control G parasuis infection; however, their efficacy varies. The broad range of potentially pathogenic serovars and genotypes has impaired the development of a universal vaccine against G parasuis.
Homologous protection against isolates from the same G parasuis serovar group is relatively satisfactory; heterologous protection is restricted to a few serovars. Several “universal” (serovar-independent) vaccine prototypes have been experimentally developed but are not commercially available.
Key Points
G parasuis is a commensal bacterium of the upper respiratory tract of pigs. Under stressful conditions, it can cause systemic disease.
Glässer disease is characterized by sudden death in its peracute form; however, the most usual form is acute disease, characterized by wasting and dyspnea, as well as fibrinous polyserositis, polyarthritis, and meningitis.
Diagnosis is based on clinical signs and detection of the bacterium from systemic sites.
Control is based on antimicrobial treatment of affected pigs; commercial and autogenous vaccines can be used to prevent the disease.
For More Information
Gross images of Glässer disease are available at the Veterinary Pathology Image Database.
Glässers Disease (Haemophilus parasuis). The Pig Site.
Glasserella parasuis: The Causal Agent of Glasser's Disease. Pork Information Gateway.
References
Alvarez-Vega L, De La Cruz-Monroy KSM, Garcia-Barraza A, et al. Genomic insights into diversity, antimicrobial resistance and virulence of Glaesserella parasuis from diseased swine in Peru. Front Microbiol. 2025;16:1678153. doi:10.3389/fmicb.2025.1678153
Gong X, Cui Q, Zhang W, et al. Genomic insight into the diversity of Glaesserella parasuis isolates from 19 countries. mSphere. 2024;9(9):e00231-24. doi:10.1128/msphere.00231-24
Rao J, Wei X, Li H, et al. Novel multiplex PCR assay and its application in detecting prevalence and antibiotic susceptibility of porcine respiratory bacterial pathogens in Guangxi, China. Microbiol Spectr. 2023;11(2):e03971-22. doi:10.1128/spectrum.03971-22
Costa-Hurtado M, Barba-Vidal E, Maldonado J, Aragon V. Update on Glässer’s disease: how to control the disease under restrictive use of antimicrobials. Vet Microbiol. 2020;242:108595. doi:10.1016/j.vetmic.2020.108595
