Actinobacillus suis: an update on genotyping and antibiotic susceptibility

Posted by Administrator on July 17 2009 15:09

Introduction

Actinobacillus suis has emerged as a new threat to swine production. Recent data on isolation of this pathogen from clinical samples submitted to the University of Minnesota Veterinary Diagnostic Laboratory has shown an increase of 0.4% or 13,547 cases during the fiscal year (FY) of 2005 compared with FY 2002. Although swine veterinarians have made progress in developing prevention and control strategies to reduce mortality caused by other respiratory bacterial pathogens such as Haemophilus parasuis, Actinobacillus pleuropneumoniae and Streptococcus suis, little is know about the epidemiology and control of A. suis.

The main questions are:

• What are the main clinical presentations and lesions observed in the field

• What is the technology available for accurate diagnosis of A. suis

• Which organisms should be included in the differential diagnosis of A. suis infection

• Are there vaccines available in the market

• Is strain variability an issue

• Is serotyping and serological tests available

• Are antibiotic treatments effective? What are the main drugs that can be used for prevention and treatment.

Although limited information is currently available for epidemiology and control of A. suis infections, interest by field veterinarians and researchers has simultaneously increased in the past few months. At the University of Minnesota Veterinary Diagnostic Laboratory, we are currently working on the development of new diagnostic tests for detection and characterization of A. suis. Preliminary results are promising, and somewhat surprising. Actinobacillus suis differs considerably from other colonizers of the upper respiratory tract in their clinical presentation, lesions, and molecular epidemiology. It is certainly a new challenge for the swine industry.

Etiology and epidemiology

Actinobacillus suis is a Gram-negative bacterium that colonizes the tonsils, nasal cavity, and vaginal mucosa of healthy pigs. This organism is killed within 15 minutes at a temperature of 60º C, it is sensitive to most disinfectants, and will die out within a few days in clinical specimens.

Affected animals include neonates, weaned, and finishing pigs. Historically, high morbidity and mortality due to A. suis infection has been observed in high-health herds, mainly associated with lack of immunity to this organism.

Pathogenesis and virulence factors

Actinobacillus suis is a colonizer of the upper respiratory tract, and disease has been reproduced by intranasal inoculation. Invasion through abrasions in the skin and mucous membranes are also likely. One particularity of the systemic spread of A. suis is the formation of septic emboli, which will reach various organs forming microcolonies surrounded by areas of hemorrhage and necrosis.

Potential virulence factors that have been reported for A. suis include the expression of capsule, specific lipopolysaccharides, and toxins. At least 2 O antigens (LPS) have been described for A. suis: O1 and O2. O2 has been found to be more prevalent among isolates recovered from lesions or submitted for production of autogenous vaccines compared with O1 strains. Actinobacillus suis produces 2 toxins, which are similar to those produced by Actinobacillus pleuropneumoniae: Apx I and Apx II. These toxins are not identical between the 2 bacterial species, but are very similar, resulting in the development of similar lesions caused by these microorganisms.

Clinical signs and lesions

Clinical signs and lesions are particularly variable for A. suis. This organism may affect animals from different ages, and clinical signs and lesions may resemble those caused by other organisms, such as Haemophilus parasuis, A. pleuropneumoniae, and Erysipelothrix rhusiopathiae, for example. Sudden death of suckling pigs may occur in pigs from 2 days to 3 weeks old. Lesions are usually characterized by cyanosis, petechial hemorrhages, fever (104 °F), and congestion of the extremities. Weaned pigs may develop anorexia, fever, respiratory distress, pneumonia, and round/ rhomboid erythematous skin lesions. Sudden death is also commonly observed. Actinobacillus suis can colonize the vaginal mucosa, and sows may develop metritis and abortion. Finishing pigs may develop pneumonia, arthritis, fatal septicemia, and skin lesions.

Petechial to ecchymotic hemorrhages may occur in the lung, kidneys, heart, liver, spleen, skin, and intestines. Pleurisy and pericarditis may also develop. Miliary abscesses in several organs may result from the systemic spread of septic emboli.

Diagnosis

Clinical signs and lesions caused by A. suis are usually non-specific, and differential diagnosis with other pathogens that can cause septicemia is required. Lung lesions caused by A. suis and A. pleuropneumoniae may be indistinguishable, considering that these organism produce similar toxins that will contribute to development of necrosis and hemorrhage. Actinobacillus suis may also cause fibrinous pleuritis and pericarditis, and differential diagnosis with H. parasuis should be considered. A recent survey of bacterial pathogens isolated from 300 tissue samples with fibrinous exudate on the surface demonstrated that Haemophilus parasuis was isolated from 37 samples, whereas A. suis was isolated from 21 samples. Other pathogens isolated from fibrinous exudate included: Actinobacillus. indolicus (1), Actinobacillus minor (1), A. pleuropneumoniae (8), Arcanobacterium pyogenes (31), Bordetella bronchiseptica (32), Escherichia coli (19), beta-hemolytic Streptococcus sp. (9), Pasteurella multocida (55), Streptococcus suis (64), and Salmonella sp. (5).

Besides being isolated from lungs with necrosis, hemorrhage, and fibrinous pleuritis, A. suis may also be isolated from systemic sites such as liver, spleen, kidneys, lymph nodes, uterus and intestines. Isolation from systemic sites is associated with septicemia. Isolation from uterus may be associated with abortion and metritis. Isolation from the intestines is more prevalent in neonatal pigs and it is usually associated with septicemia and presence of septic emboli. A. suis may also be isolated from the intestines of pigs with no lesions.

Diagnosis of A. suis infection has been traditionally performed by isolating the microorganism from clinical samples of affected pigs. Gross and histopathological lesions (presence of septic emboli in several organs, extensive hemorrhage, and necrosis) are somewhat characteristic of A. suis infection, however differential diagnosis with A. pleuropneumoniae, H. parasuis, and E. rhusiopathiae for example, should be pursued.

Isolation of these pathogens from clinical samples may be influenced by antibiotic treatments or poor handling of samples prior to submission to the diagnostic laboratory. In this case, PCR can be used as an alternative tool to help on the differential diagnosis of A. suis infection. We have recently standardized PCR tests for detection of A. pleuropneumoniae and H. parasuis in clinical samples at the University of Minnesota VDL. We have also developed new PCR tests for detection of A. suis and E. rhusiopathiae. These tests have been especially helpful in the differential diagnosis of these pathogens.

Serological tests are not available for diagnosis of A. suis infection. Antigens currently used for serotyping (O1/O2) produce cross-reactions with convalescent sera from SPF pigs infected with P. multocida, H. parasuis, and A. pleuropneumoniae serotypes 1, 5 and 7.

Molecular epidemiology

Genotyping of A. suis has been previously performed using restriction endonuclease fingerprinting (REF) analysis. All A. suis isolates were found to be remarkably similar, belonging to one single group. We have recently been working on the standardization and validation of a repetitive element-based PCR for genotyping of A. suis. Preliminary results are in agreement with previously reported genotyping features; however, we did find strain variation among A. suis field isolates and reference strains. Another interesting feature of A. suis molecular epidemiology is the variation observed in antibiotic resistance profiles among strains with similar genomic fingerprints. Genotyping results for A. suis field isolates will be presented at the conference.

Control and treatment

Commercial vaccines are not available for A. suis. Data on efficacy of autogenous vaccines is limited, although requests to manufacture herd-specific vaccines are common. Most pigs usually already have a relatively high titer against A. suis (which may or may not be specific) prior to vaccination, and response to vaccination will be greater in pigs that have lower titers at the time of vaccination. Antibiotics can be used as a preventive measure or to treat affected pigs. Recent data for A. suis antibiotic susceptibility profiles for the FI of 2005 (University of Minnesota VDL) is shown in Table 1. Actinobacillus suis isolates sharing similar genomic fingerprints may have completely different susceptibility profiles. One hypothesis is that antibiotic resistance genes may be carried in plasmids, and strains with similar genetic makeup may carry different plasmids. Genotyping will only characterize the genomic DNA. Plasmids are not characterized using this technique.

Table 1: Antibiotic susceptibility profiles for Actinobacillus suis isolated from clinical cases submitted to the University of Minnesota Veterinary Diagnostic Laboratory during the fiscal year of 2005.