17 Actinobacteria
The actinobacteria are a phylogenetically diverse group of Gram-positive bacteria which tend to grow slowly and produce branching filaments. The bacteria in this group which cause disease in domestic animals belong to the genera Actinomyces, Trueperella, Actinobaculum, Nocardia and Dermatophilus. The genera are classified within three different bacterial families. Morphological and cultural characteristics of actinobacteria are presented in Boxes 17.1 and 17.2. Comparative features of actinobacteria of veterinary importance are presented in Table 17.1.
Table 17.1 Comparative features of actinobacteria of veterinary importance.
| Feature | Actinomyces species | Trueperella pyogenes | Actinobaculum suis | Nocardia species | Dermatophilus congolensis |
| Atmospheric growth requirements | Anaerobic or facultatively anaerobic and capnophilic | Facultatively anaerobic and capnophilic | Anaerobic | Aerobic | Aerobic and capnophilic |
| Aerial filament production | – | – | – | + | – |
| Modified Ziehl–Neelsen staining | – | – | – | + | – |
| Growth on Sabouraud dextrose agar | – | – | – | + | – |
| Usual habitat | Nasopharyngeal and oral mucosae | Nasopharyngeal mucosa of cattle, sheep and pigs | Prepuce and preputial diverticulum of boars | Soil | Skin of carrier animals, scabs from lesions |
| Site of lesions | Many tissues including bone | Soft tissues | Urinary tract of sows | Thoracic cavity, skin and other tissues | Skin |
Actinomyces, Trueperella and Actinobaculum species
Species in these genera are non-motile, non-spore-forming, Gram-positive bacteria which require enriched media for growth. Trueperella pyogenes was formerly called Arcanobacterium pyogenes and Actinobaculum suis was formerly known by other names. The species of veterinary importance in the group are Trueperella pyogenes, Actinobaculum suis, Actinomyces bovis, Actinomyces viscosus and Actinomyces hordeovulneris. Apart from A. hordeovulneris, pathogenic members of these genera colonize mucous membranes of mammals. Differentiating features of the genera are presented in Table 17.2. Disease conditions produced by Actinomyces, Trueperella and Actinobaculum species in domestic animals are summarized in Table 17.3.
Table 17.2 Differentiation of Actinomyces, Trueperella and Actinobaculum species of veterinary importance.
| Characteristic | Actinomyces bovis | Actinomyces viscosus | Actinomyces hordeovulneris | Trueperella pyogenes | Actinobaculum suis |
| Morphology | Filamentous, branching, some short forms | Filamentous, branching, short forms | Filamentous, branching, short forms | Coryneform | Coryneform |
| Atmospheric requirements | Anaerobic + CO2 | 10% CO2 | 10% CO2 | Aerobic | Anaerobic |
| Haemolysis on sheep blood agar | ± | – | ± | + | ± |
| Catalase production | – | + | + | – | – |
| Pitting of Loeffler's serum slope | – | – | – | + | – |
| Granules in pus | ‘Sulphur granules’ | White granules | No granules | No granules | No granules |
Table 17.3 Disease conditions produced by Actinomyces, Trueperella and Actinobaculum species in domestic animals.
| Species | Hosts | Disease conditions |
| Trueperella pyogenes | Cattle, sheep, pigs | Abscessation, mastitis, suppurative pneumonia, endometritis, pyometra, arthritis, umbilical infections |
| Actinomyces hordeovulneris | Dogs | Cutaneous and visceral abscessation, pleuritis, peritonitis, arthritis |
| Actinomyces bovis | Cattle | Bovine actinomycosis (‘lumpy jaw’) |
| Actinomyces viscosus | Dogs | Canine actinomycosis:
|
| Horses | Cutaneous pustules | |
| Cattle | Abortion | |
| Actinomyces species (unclassified) | Pigs | Pyogranulomatous mastitis |
| Horses | Poll evil and fistulous withers | |
| Actinobaculum suis | Pigs | Cystitis, pyelonephritis |
Trueperella pyogenes is a common cause of suppurative lesions in many domestic species worldwide, especially cattle, pigs and sheep. The principal virulence factor produced by T. pyogenes is pyolysin which is cytolytic for several cell types including neutrophils and macrophages. This organism also produces proteases and a number of adhesins. Lymphadenitis, osteomyelitis, peritonitis and neural abscessation are commonly associated with tissue invasion by this pathogen. This organism is also implicated in pyometra, metritis and acute mastitis in dairy cows. In the acute bovine mastitis referred to as ‘summer mastitis’ in Britain and Ireland, the anaerobic bacterium Peptoniphilus indolicus is usually associated with T. pyogenes. In foot lesions in ruminants and in other mixed infections, T. pyogenes also occurs in association with anaerobes. Diagnosis is based on the typical pleomorphic cell morphology in Gram-stained smears from specimens, colonial characteristics and the ability of T. pyogenes to pit a Loeffler's serum slope.
Invasion of the mandible and, less commonly, the maxilla of cattle by A. bovis causes a chronic rarefying osteomyelitis referred to as ‘lumpy jaw’. The organsim is presumed to invade the tissues following trauma to the mucosa from rough feed or through dental alveoli during tooth eruption. A painless swelling of the affected bone enlarges over a period of several weeks. The swelling becomes painful and fistulous tracts, discharging exudate containing ‘sulphur granules’ with characteristic club colonies, develop. Spread to contiguous soft tissues may occur, but there is minimal involvement of regional lymph nodes. Surgery is the treatment of choice when lesions are small and circumscribed. In advanced cases, surgical treatment may be ineffective.
Porcine cystitis and pyelonephritis caused by Actinobaculum suis affects the urinary tract of pregnant sows. The pathogen, which is transmitted by carrier boars at coitus, causes a potentially fatal infection. Anorexia, arching of the back, dysuria and haematuria are prominent signs. If both kidneys are extensively damaged, death may result.
Actinomyces viscosus affects dogs and usually presents in one of two forms. It may cause subcutaneous pyogranulomatous lesions or extensive fibrovascular proliferation on the peritoneal or pleural surfaces with sanguinopurulent exudate in the affected cavity. The thoracic lesions closely resemble those of canine nocardiosis.
Nocardia species
Members of Nocardia species are Gram-positive, aerobic, saprophytic actinobacteria. In smears of exudate from infected tissue, they appear as long, slender, branching filaments with a tendency to fragment into rods and cocci. When cultured, these organisms produce aerial filaments which may form spores. Components of the cell wall render Nocardia species partially acid-fast (modified Ziehl–Neelsen positive). Nocardia asteroides is the pathogen of greatest significance in this genus, although a number of other pathogenic species have been identified following reclassification of nocardial organisms using molecular methods.
Nocardia species are saprophytes found in soil and decaying vegetation. Infection, which is opportunistic, is usually associated with immunosuppression or, alternatively, may follow a heavy challenge. The usual mode of infection is by inhalation but it may also occur through skin wounds or via the teat canal. Virulent strains of N. asteroides survive intracellularly. Cell-mediated immunity is essential for protection against infection by this facultative intracellular bacterium.
Nocardial infections in domestic animals are presented in Table 17.4. The most commonly encountered conditions are cutaneous and systemic infections in dogs and mastitis in dairy cattle. Outbreaks of nocardial mastitis are often associated with contaminated intramammary tubes. Canine nocardiosis, due to N. asteroides, is acquired by inhalation, through skin wounds or by ingestion. Thoracic, cutaneous and disseminated forms of the disease are recognized. The thoracic form is characterized by fever, anorexia and respiratory distress. Sanguinopurulent fluid accumulates in the thoracic cavity. The cutaneous form presents either as an indolent ulcer or as a granulomatous swelling with discharging fistulous tracts. Nocardia asteroides strains show marked variation in their susceptibility to antibiotics. A presumptive diagnosis of infection with N. asteroides is based on clinical findings and laboratory procedures. Smears of exudate should be stained by the Gram and modified Ziehl–Neelsen methods. Nocardia asteroides is modified Ziehl–Neelsen positive. When cultured aerobically on blood agar, colonies are usually visible after five days. They are white, powdery and firmly adherent to the agar.
Table 17.4 Disease conditions produced by Nocardia species in domestic animals.
| Species | Hosts | Disease conditions |
| Nocardia species | Dogs | Canine nocardiosis:
|
| Cattle | Chronic mastitis, abortion | |
| Pigs | Abortion | |
| Sheep, goats, horses | Wound infections, mastitis, pneumonia, other pyogranulomatous conditions | |
| Nocardia farcinica | Cattle | Bovine farcy |
Nocardia farcinica is implicated in bovine farcy and occurs only in tropical climates. Because a number of organisms, including N. farcinica, have been isolated from the lesions, the aetiology of the disease is uncertain. Chronic infection of superficial lymphatic vessels and lymph nodes occurs. Early lesions consist of small cutaneous nodules, often on the medial aspects of the legs and on the neck. These nodules enlarge slowly and coalesce, forming large swellings which rarely ulcerate. Lymphatic vessels become thickened and cord-like. Internal organs may be affected occasionally with lesions resembling tuberculosis.
Dermatophilus congolensis
This member of the actinobacteria is a Gram-positive, filamentous, branching organism. It is unusual because it produces motile zoospores about 1.5 μm in diameter. Mature zoospores produce germ tubes which develop into filaments. Within these filaments, transverse and longitudinal divisions form segments which ultimately develop into zoospores. Mature filaments may be more than 5 μm in width and contain columns of zoospores which impart a ‘tram-track’ appearance to the filaments. Skin infections caused by D. congolensis occur worldwide but are most prevalent in tropical and subtropical regions. The organism seems to persist in foci in the skin of many clinically normal animals, particularly in endemic areas. Although zoospore survival in the environment is usually limited, there may be extended survival in dry scabs.
Trauma and persistent wetting predispose to skin invasion. When activated, zoospores produce germ tubes and these develop into filaments, which invade the epidermis. Pathogenic factors include an alkaline ceramidase and a number of proteases which facilitate invasion of the epidermis. Invasion leads to an acute inflammatory response, characterized by large numbers of neutrophils which ultimately form microabscesses in the epidermis. Factors which depress specific immune responses, including intercurrent diseases and pregnancy, may increase host susceptibility to dermatophilosis. Infections with D. congolensis are usually confined to the epidermis. When skin of the lower limbs of sheep is involved, the condition is termed ‘strawberry footrot’.
Although the disease affects animals of all ages, it is more prevalent and often more severe in young animals. Zoospores are most often transmitted by direct contact with infected animals. A number of blood-sucking insects may be important in disease transmission in the tropics. Economic loss derives from damage to hides and fleece.
Lesion distribution usually correlates with those areas of skin predisposed to infection. Heavy prolonged rainfall in association with warm environmental temperatures can result in lesions predominantly affecting the dorsum of farm animals. Trauma to the face and limbs of animals grazing in thorny scrub can predispose to lesions in these sites. Early lesions present as papules and are often detectable only by palpation. As lesions progress, serous exudate causes matting of hairs, resulting in a tufted appearance. Lesions may coalesce, forming irregular elevated crusty scabs. Tufts of hair can be readily plucked from the lesions along with adherent scab material and underlying exudate. Scab formation tends to be more pronounced in cattle and sheep than in horses. In severe infections, lesions may be extensive and deaths may occur occasionally in calves and lambs. Diagnosis is based on clinical appearance of lesions and demonstration of D. congolensis in scabs. Isolation of the organisms is confirmatory.
The outcome of treatment is influenced by the severity and extent of lesions. Parenterally administered antibiotics such as long-acting oxytetracycline are usually effective. Control measures are based on minimizing the effects of predisposing factors and early treatment of clinical cases. Where feasible, grazing areas should be cleared of thorny scrub and tick infestation should be reduced by dipping or spraying with acaricides. Control of intercurrent disease reduces the severity of dermatophilosis.