SHIC-Funded Study Develops Diagnostic Tools for Glaesserella australis

Glaesserella australis, a newly recognized Gram-negative bacterium species, was isolated from the lung lesions of pigs in Australia in 2018 and in two swine herds in Ontario, Canada in 2023. In response, the Swine Health Information Center funded a study led by Dr. Nubia Macedo, Iowa State University, to investigate the detection and characterization of this potential emerging swine pathogen in US clinical samples. Diagnostic tools developed through this project were used to screen historic isolates and current clinical samples. All US swine samples tested to date have been negative for this emerging bacterium. The tools developed herein will contribute to accurate detection of G. australis and help inform understanding of this potential pathogen.

G. australis causes clinical signs and pathologic lesions that appear very similar to that caused by Actinobacillus pleuropneumoniae. Because no methods were available to detect G. australis in swine samples at US VDLs, ISU investigators hypothesized that infections may have been misdiagnosed as A. pleuropneumoniae. The overall goals of this study were to develop and optimize diagnostic tools to detect the presence and prevalence of G. australis in the US. Aligned with SHIC’s mission to protect the health of the US swine herd, surveillance and discovery of emerging diseases such as G. australis is critical.

In the affected Australian farms, most pigs had no signs of respiratory disease but there was increased pleurisy and pulmonary abscesses at the abattoir. On one farm, three affected pigs, aged 12, 16, and 20 weeks old, presented with cough and reduced growth rates followed by sudden death with cyanosis of extremities. Pulmonary lesions in these pigs affected up to 50% of lungs, characterized by multifocal necrotizing and fibrinosuppurative bronchopneumonia. In these cases, G. australis was isolated in pure culture. Other farms positive for G. australis had co-infections with other pathogens, including PCV2, A. pleuropneumoniae, Pasteurella multocida, Glaesserella parasuis, Streptococcus suis and Mycoplasma hyopneumoniae.

To date, there are no reports of detecting G. australis in US swine samples. However, G. australis was detected for the first time in two swine herds in Ontario, Canada, in 2023 when the bacterium was isolated from the pericardium and lung of one pig in each herd. Co-infections in the Canadian swine were present and included Streptococcus suis, Pasteurella multocida, Actinobacillus porcitonsillarum, IAV, and PRRSV. 

In the US, the identification of a potential G. australis case could have been missed by VDLs for two reasons, including similarities to other pathogens, such as Glaesserella parasuis and Actinobacillus pleuropneumoniae, and a lack of diagnostic tools to accurately identify G. australis. Researchers sought to develop accurate diagnostic methods and increase awareness among producers, veterinarians, and diagnosticians on this potential pathogen threat.

Specifically, the objectives of the study were to 1) phenotypically characterize historical ISU VDL Actinobacillus sp. isolates using traditional biochemical methods and whole genome sequencing (WGS) and 2) develop and optimize diagnostic tools for G. australis detection within US swine herds including ISH and real-time PCR.

First, sequencing of historical isolates with similar characteristics was attempted at the ISU Veterinary Diagnostic Lab, but no US sample sequence was found to be a match for G. australis. Twenty-one isolates from the ISU VDL inventory were selected for further identification with WGS revealing that 19 isolates were closely related to Actinobacillus minor or A. porsitonsillarum. Final identification of the remaining two isolates in underway. For testing of future submissions, the G. australis reference strain was added to ISU VDL’s sequencing platform and to its MALDI-TOF database for real-time screening of clinical samples.

Second, a PCR test was developed, validated and is now available for further screening and diagnostic purposes. Using the TaqMan-MGB-based primer/probe sequences, the RT-PCR proved to be highly specific and sensitive. An ISH assay was developed and is currently undergoing validation to detect this organism in tissues associated with lesions. Once the ISH assay is available, it will allow the screening of G. australis directly in affected tissues, as well as detect concurrent pathogens such as G. parasuis, A. pleuropneumoniae, and P. multocida.

Since January 2024, ISU VDL staff have screened for G. australis through MALDI-TOF and all samples have been confirmed as negative for this new swine pathogen. Nevertheless, the tools described here will contribute to the G. australis screening process in the US and help improve understanding of its prevalence and pathogenesis. In addition to the ongoing MALDI-ToF screening, the RT-PCR is available for surveillance and a sequencing pipeline is in place to quickly and accurately confirm any suspect isolates identified. 

The emergence of G. australis highlights the importance of ongoing disease surveillance and the development of accurate diagnostic tools. Producers should be aware of this potential new pathogen and consult with their veterinarians if they observe respiratory clinical signs in their pigs. The availability of these new diagnostic tests will improve the ability to detect and manage G. australis if present in the US swine population, protecting herd health and productivity.

The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at mniederwerder@swinehealth.org or Dr. Lisa Becton at lbecton@swinehealth.org.