Glasserella parasuis: The Causal Agent of Glasser’s Disease
Glässer’s disease is an important cause of post-weaning morbidity and mortality in swine populations worldwide. The disease was first described in 1910, but the etiological agent was not isolated until 1922. It is a gram-negative bacterium Glaesserella parasuis (G. parasuis), formerly known as Haemophilus parasuis, that belongs to the Pasteurellaceae family (Dickerman et al., 2019). G. parasuis is part of the normal microbiota of pigs and is an early colonizer of piglets. The bacterium can be detected in the trachea, nasal passages or tonsils of piglets as early as two days after birth. Pigs can be colonized by both virulent and non-virulent strains. Although it is normally found in the upper respiratory tract (URT) of pigs, upon disruption of pig’s immune system, it causes Glässer’s disease. The disease is normally observed in 4 through 8-week-old pigs (nursery pigs) but it can sporadically occur in older pigs (Aragon et al., 2019). Weaned piglets are more susceptible because of waning maternal antibodies. Proper diagnosis and typing of isolates is crucial to understand the molecular epidemiology of strains involved and to design better herd-specific autogenous vaccines to control the disease. Knowledge of the circulating strains within or between farms is crucial. Traditionally, serotyping has been the most common typing method, with 15 known serotypes (Kielstein and Rapp-Gabrielson, 1992).
- Describe the lesions and clinical signs of Glässer’s disease.
- Summarize recent knowledge on the serotypes associated with disease and epidemiology
- Describe the diagnosis, control, and prevention of the disease
Lesions and Clinical Signs
Clinical signs are characterized by fever (105.8-107ºF), depression, abdominal breathing (thumping), coughing, swollen joints with lameness, central nervous system (CNS) signs like paddling, and septicemia with acute death (Vahle, Haynes and Andrews, 1995, Brockmeier et al., 2014). Animals with the per acute form of the disease can die without any characteristic gross lesions, but some may have pinpoint hemorrhages in systemic tissues. In the acute phase of the disease, lesions include polyarthritis, polyserositis (inflammation in the thoracic and abdominal cavities) (Figure A below), and meningitis. Fibrinous exudation (Figure B) on the surfaces of the lungs (pleura), heart (pericardium), abdominal organs (peritoneum), synovia, and meninges are common findings. Chronic infections can cause fibrosis of the surface of the lungs, heart, and the abdominal cavity resulting in reduced growth rate.
G. parasuis epidemiology and disease
Most piglets are colonized with the bacterium within the first 3 days of life, through nose to nose contact with sows. It is hypothesized that piglets that are colonized in the farrowing house in the presence of maternal immunity may develop active immunity against virulent strains and be protected against systemic infection after weaning and commingling. Piglets that are not colonized prior to weaning may remain naïve and develop systemic infection when commingled with colonized pigs. Systemic infection usually happens around 4-to-6-weeks after weaning, when maternal immunity is no longer protective.
Source and triggers of outbreaks
In the last decade, G. parasuis systemic disease was associated with 15% of total systemic cases diagnosed at the Iowa State Veterinary Diagnostic Laboratory (Poeta, Silva et al., 2021). The bacterium is common worldwide, but environmental stressors (e.g., fluctuation of temperatures, high humidity, etc.), management (e.g., pig commingling), and viral co-infections such as porcine reproductive and respiratory syndrome virus (PRRS) can trigger outbreaks (Brockmeier et al., 2014). All of these factors disrupt the pigs’ immune system and predispose pigs to pathogenic strains. Nasal colonization with virulent strains is a risk factor, but colonization with avirulent strains might protect pigs from subsequent exposure with virulent strains. Nasal microbiota may also contribute to the development of the disease (Correa-Fiz et al., 2016). For instance, practices (e.g., antibiotic use) that disrupt the pigs’ normal nasal microbiota may predispose pigs to the infection. G. parasuis control requires timely detection, strategic use of antimicrobials, and effective vaccines. Serotype-specific autogenous vaccines may or may not be protective for other serotypes or even strains within the same serotype because of the heterogenous nature of G. parasuis (Aragon et al., 2019).
Typing isolates is crucial to understand the epidemiology of strains involved in disease and to design effective farm-specific vaccines. Traditionally, serotyping is the most common typing method, with 15 known serotypes (Kielstein and Rapp-Gabrielson, 1992). The major pitfall of serotyping is the low discriminatory power and large number of untypable isolates. Development of molecular serotyping methods using polymerase chain reaction (PCR) in recent years has reduced the number of untypable isolates (Lacouture et al., 2017; Howell et al., 2015). Based on current knowledge, the most frequent serotypes vary between continents, with serotypes 4 and 5/12 being commonly isolated in many countries (Oliveira, Blackall and Pijoan, 2003; Jia et al., 2017; 2012; Macedo et al., 2021). In North America, the dominant serotypes are 4, 13, 1, 7, 2, and 5/12, but variations between studies occur. Earlier studies indicated certain serotypes were virulent and others non-virulent or moderately virulent. This has been scrutinized in recent years as some serotypes formerly indicated to be avirulent are now commonly isolated from systemic cases. For example, recent studies have associated serotype 7 (previously an avirulent serotype) with systemic disease or virulence (Macedo et al., 2021; Schulwerk et al., 2020). Recent work revealed serotype 7 as the most prevalent serotype from clinical cases (Mugabi et al., 2021). This emphasizes the need to further characterize G. parasuis isolates, e.g. determination of presence or absence of putative or virulence-associated genes.
Differentiation of virulent and commensal strains
Discovery of the virulence-associated trimeric autotransporters (vtaA) genes over ten years ago (Pina et al., 2009) has shed more light on the pathogenic potential of G. parasuis strains. Group 1 vtaA genes for instance, have been associated with virulent G. parasuis isolates (Pina et al., 2009; Olvera et al., 2012). Notably, group 1 vtaAs were shown to play a role in G. parasuis phagocytosis resistance by alveolar macrophages (Costa-Hurtado et al., 2012). Recently, a multiplex PCR was developed to amplify the translocator sequence of these vtaAs. It was shown that Group 1 positive isolates were associated with disease cases, whereas the negative isolates were associated with isolates from health animals (Olvera et al., 2012). This has improved our knowledge of the clinical relevance of G. parasuis strains isolated from pigs.
Diagnosis is dependent on the clinical signs and lesions described above. However, this should be interpreted with caution as other infections can present similar clinical signs and lesions. Other pathogens that present similar clinical signs at G. parasuis include; Streptococcus suis, Erysipelothrix rhusiopathiae, Mycoplasma hyorhinis, and Salmonella choleraesuis.
Definitive diagnosis can be achieved by isolation of the bacterium from the lesions. Since G. parasuis is fastidious, the Veterinary Diagnostic Laboratories (VDLs) mainly use polymerase chain reaction (PCR) directly on clinical samples, alongside bacteriological culture, to determine presence of the bacterium. For the purposes of autogenous vaccine development, culture is required to isolate and further characterize the circulating strains on a given farm. G. parasuis is rarely isolated from dead animals, so samples should be collected from euthanized animals, and submitted fresh to the VDL under refrigerated conditions as soon as possible, since the agent is temperature sensitive.
Ideal samples with lesions include; brain, fibrin, spleen, pericardium (sac around the heart), lung surface (pleura), interior surface of the abdominal cavity (peritoneum), and joints. Lung samples should only be submitted in cases of pneumonia. Upper respiratory tract samples are of no diagnostic relevance since the agent is part of the normal microbiota. The samples can be plated on enriched chocolate agar or blood agar with Staphylococcus aureus nurse culture as a source of V factor. If isolation is achieved, further characterization may include serotyping, multiloccus sequence typing (MLST), virulence and antimicrobial resistance (AMR) gene profile of isolates. These improve the understanding of the circulating strains’ clinical relevance and allows for the refinement of prevention and control strategies. Whole-genome sequencing (WGS) is now affordable and some VDLs are offering it as a service to characterize G. parasuis. This is replacing the traditional genotyping techniques based on restriction enzymes. WGS not only gives the best strain resolution, but also the virulence and AMR gene repertoire of a given strain. This is key for autogenous vaccine strain selection given the heterogeneity of the bacterium.
Prevention and Control
The most important elements in the prevention and control of the disease include; good herd management, good biosecurity, minimizing of co-infections, vaccination targeting the prevalent virulent strains and strategic use of medication.
- Practices or conditions that cause stress such as early weaning, transportation, harsh environmental temperatures can be immunosuppressive and trigger an outbreak. Therefore, provision of adequate temperatures and minimizing stress can reduce outbreaks.
- Avoid the introduction of animals that might be carrying novel virulent strains that can cause an outbreak in a naïve herd. Thus, understanding the endemic profile of incoming replacement gilts is critical.
- Commercial and autogenous vaccines have been used to control the disease. The major pitfall is that many of these vaccines are not cross-protective against the different serotypes. Serotype-specific autogenous vaccines can also give varied results with different strains within the same serotype. So proper diagnosis, isolation, and characterization of the agent is very important to target the prevalent strain causing disease on the farm.
- Antibiotics, both oral and injectable are frequently used as a treatment in a sick animal or as a prevention strategy prophylactically at the population level. Antibiotics include amoxicillin, ampicillin, oxytetracycline (OTC), sulphonamides, enrofloxacin, penicillin and ceftiofur. These should be used judiciously, and timely treatment of sick animals is key to minimize losses. As mentioned in the epidemiology section, if piglets are not exposed to the bacterium during the presence of maternal immunity (e.g. due to the use of long acting antibiotics), this may hinder development of active immunity, risking disease when maternal antibodies wane during the nursery phase. Thus, IM antimicrobials are recommended closer to weaning combined with an oral antibiotic administered in the feed or water prior to peak disease occurrence.
Although we have known of Glässer’s disease for over a century, it is still a challenging disease in the swine industry worldwide. Its causative agent is part of the normal microbiota, and pigs can be colonized with virulent and nonvirulent strains early in life. This should be considered for proper diagnosis, control and prevention strategies. Stress, management practices and coinfections with viral pathogens can trigger an outbreak. Control of the disease requires a multifaceted approach that includes proper herd and pig flow management, accurate diagnosis, isolation, and characterization of the causative agent, and strategic use of antibiotics and vaccines. Advanced technologies like WGS that give the best strain resolution and more information on the strain’s virulence and AMR genes are being adopted in some VDLs and are highly recommended. This is key in development of autogenous vaccines that target virulent strains causing disease in the herd. Commercial vaccines seldomly provide cross-protection to many strains.
References & Citations
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