Understanding the Pathogenesis of Glässer's Disease: Virulence, Colonization and Disease Development

Glässer's disease develops through a complex interaction between the host, Glaesserella parasuis and environmental factors. Although G. parasuis is a common inhabitant of the upper respiratory tract of healthy pigs, only virulent strains are capable of invading systemic tissues and causing disease. Understanding the mechanisms of colonization, bacterial virulence and host susceptibility enables veterinarians to better predict disease risk and implement targeted preventive strategies. 

Colonization: The First Step in Infection 

Piglets are naturally colonized by G. parasuis shortly after birth while receiving maternal antibodies through colostrum¹. In most cases, colonization of the upper respiratory tract occurs without clinical disease, indicating that the presence of the bacterium alone does not necessarily result in systemic infection. 

Early colonization has been suggested to play an important role in reducing the later occurrence of Glässer's disease within a herd². However, colonization patterns may differ between farms and can be influenced by maternal immunity, sow parity and herd management practices^3,4. 

From Colonization to Systemic Disease 

Virulent G. parasuis strains possess the ability to cross the respiratory mucosa and disseminate throughout the body. Systemic invasion is considered one of the principal characteristics distinguishing pathogenic strains from non-virulent isolates⁵. 

Disease generally develops when pigs are exposed to stressors such as weaning, transportation, mixing of animals or concurrent infections, particularly during the period when maternally derived antibodies are declining^1,6. These conditions favour bacterial invasion beyond the upper respiratory tract, resulting in fibrinous polyserositis, arthritis, meningitis and other manifestations of Glässer's disease. 

Virulence Factors and Pathogenic Diversity 

G. parasuis is a highly heterogeneous bacterial species, with strains differing considerably in their pathogenic potential. Consequently, identification of the organism alone is insufficient to determine whether it is responsible for clinical disease. 

Virulence-associated trimeric autotransporters (vtaA genes) are recognised as important markers associated with pathogenic strains. PCR assays targeting the leader sequences of vtaA genes have improved the ability to differentiate virulent from non-virulent isolates⁷. 

In addition, multiplex PCR assays detecting combinations of virulence-associated genes have demonstrated value in identifying pigs carrying virulent strains and estimating the risk of disease development within a herd⁸. 

The Importance of Serovar Diversity 

Considerable variation exists among G. parasuis serovars, and multiple strains may circulate simultaneously within a single herd. This diversity contributes to differences in pathogenicity and influences vaccine performance. 

Molecular serotyping has become an important diagnostic tool because it facilitates identification of circulating serovars and supports herd-specific control strategies, including vaccine selection⁵. 

Understanding the characteristics of circulating strains allows veterinarians to interpret laboratory findings more accurately and develop more targeted prevention programmes. 

Practical Clinical Insights 

Pathogenesis should be considered a dynamic interaction between bacterial virulence, host immunity and environmental stress. The presence of G. parasuis in the upper respiratory tract does not necessarily indicate disease, whereas isolation from systemic lesions strongly supports infection with a virulent strain. Diagnostic investigations should therefore combine clinical findings with molecular tools capable of identifying serovars and virulence-associated genes. Integrating knowledge of colonization patterns, maternal immunity and bacterial diversity into herd health programmes enables veterinarians to implement more effective prevention and control strategies for Glässer's disease. 

References 

1. Cerdà-Cuéllar M, Naranjo JF, Verge A, Nofrarías M, Cortey M, Olvera A, Segalés J, Aragon V. Sow vaccination modulates the colonization of piglets by Haemophilus parasuis. Veterinary microbiology. 2010 Oct 26;145(3-4):315-20. https://www.academia.edu/download/41719905/Sow_vaccination_modulates_the_colonizati20160129-32224-ad1e8i.pdf 

2. Correa-Fiz F, Fraile L, Aragon V. Piglet nasal microbiota at weaning may influence the development of Glässer’s disease during the rearing period. BMC genomics. 2016 May 26;17(1):404. https://link.springer.com/content/pdf/10.1186/s12864-016-2700-8.pdf 

3. Brean M, Abraham S, Hebart M, Kirkwood RN. Influence of parity of birth and suckled sows on piglet nasal mucosal colonization with Haemophilus parasuis. The Canadian Veterinary Journal. 2016 Dec;57(12):1281. https://pmc.ncbi.nlm.nih.gov/articles/PMC5109632/pdf/cvj_12_1281.pdf 

4. 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. Veterinary microbiology. 2020 Mar 1;242:108595. https://www.sciencedirect.com/science/article/pii/S0378113519314713 

5. Howell KJ, Peters SE, Wang J, Hernandez-Garcia J, Weinert LA, Luan SL, Chaudhuri RR, Angen Ø, Aragon V, Williamson SM, Parkhill J. Development of a multiplex PCR assay for rapid molecular serotyping of Haemophilus parasuis. Journal of clinical microbiology. 2015 Dec;53(12):3812-21. https://journals.asm.org/doi/pdf/10.1128/jcm.01991-15 

6. Aragon V, Segalés J, Tucker AW. Glässer's disease. Diseases of swine. 2019 Jun 3:844-53. https://repositori.irta.cat/bitstream/handle/20.500.12327/3798/Aragon_Glasser_Anaporc_2024.pdf?sequence=1&isAllowed=y 

7. Galofré-Milà N, Correa-Fiz F, Lacouture S, Gottschalk M, Strutzberg-Minder K, Bensaid A, Pina-Pedrero S, Aragon V. A robust PCR for the differentiation of potential virulent strains of Haemophilus parasuis. BMC veterinary research. 2017 May 8;13(1):124. https://link.springer.com/content/pdf/10.1186/s12917-017-1041-4.pdf 

8. Howell KJ, Weinert LA, Peters SE, Wang J, Hernandez-Garcia J, Chaudhuri RR, Luan SL, Angen Ø, Aragon V, Williamson SM, Langford PR. “Pathotyping” multiplex PCR assay for Haemophilus parasuis: a tool for prediction of virulence. Journal of Clinical Microbiology. 2017 Sep;55(9):2617-28. https://journals.asm.org/doi/pdf/10.1128/jcm.02464-16