Actinomyces, Trueperella and Actinobaculum species

Infections by these organisms tend to be endogenous and most of the Actinomyces species cause pyogranulomatous reactions in animal tissues. Specific virulence factors of Actinomyces species of veterinary importance have not been identified.

Actinomyces bovis gains access to the alveolar region of the jaw in cattle from the oral cavity, probably through trauma to the mucosa. It initiates a rarefying osteomyelitis and soft tissue reaction, the condition being referred to as ‘lumpy jaw’. Bacterial colonies form in the tissues with ‘clubs’ of mineralized calcium phosphate forming around them to create microscopic ‘club colonies’ or ‘rosettes’. Figure 10.2 shows a diagram of a ‘club colony’. The club formation is the result of phosphatase activity and is a host reaction to a chronic infection. Granulation, mononuclear infiltration and fibrosis occur in the lesions with sinus tracts leading to the outside. Exudate from the tracts contains pus with ‘sulphur granules’ that are about 1–2 mm in diameter, within which club colonies can be found if the granules are crushed and examined microscopically. Strains of Actinomyces similar to A. bovis have been isolated from sows with granulomatous lesions in the mammary glands. These porcine strains show minor biochemical and antigenic differences from the bovine strains. Actinomyces hyovaginalis is associated with abortions and purulent vaginal discharge in sows and with infections in various sites in pigs and in sheep (Collins et al. 1993, Storms et al. 2002, Foster et al. 2012). A. viscosus causes a clinical syndrome in dogs indistinguishable from that initiated by Nocardia species. Two syndromes can occur, either separately or together. One is a localized granulomatous lesion involving skin and subcutis, the other is a pyothorax with granulomas in thoracic tissue and often a large accumulation of sanguineopurulent pleural fluid containing soft white granules about 1 mm in diameter.

Trueperella pyogenes produces a range of virulence factors, the most significant of which is a haemolytic exotoxin, pyolysin (Table 10.3). This toxin is cytolytic for neutrophils and macrophages. Adhesion to host tissues is facilitated by neuraminidases and extracellular matrix-binding proteins. The organism also possesses fimbriae, produces proteases and can form biofilms but the exact role of these factors in disease production has not been definitively established.

Nocardia species

Nocardiae are difficult to definitively identify using phenotypic methods and many species have been reclassified in recent years based on molecular techniques. These aerobic, and essentially saprophytic, bacteria cause suppurative and pyogranulomatous reactions in immunosuppressed hosts or animals that have been exposed to large doses of the bacterium. The pathogenic nocardiae survive within phagocytic vacuoles by preventing phagolysosome formation. This is probably due to the surface lipids; Nocardia species have a cell wall similar to the mycobacteria. Genes encoding putative virulence factors have been identified in some Nocardia species but the molecular pathogenesis of nocardial infections in animals is currently unclear. Superoxide dismutases and catalases appear to be important in resistance to phagocytosis and cell lipids may provoke the characteristic granulomatous reaction. Exudates are sanguineopurulent and can sometimes contain soft granules consisting of bacteria, neutrophils and debris. They lack the microstructure of the sulphur granules produced by some of the Actinomyces species. Nocardia asteroides was previously considered to account for the majority of infections in animals but following their reclassification, a number of species are now known to be associated with animal disease (Table 10.2). Nocardiform placentitis associated with a number of different members of the Actinobacteria, in particular Amycolatopsis species and Crossiella equi, have been documented in the USA and sporadically elsewhere (Erol et al. 2012).

Dermatophilus congolensis

Dermatophilus congolensis causes skin infections that can affect many animal species and humans, but the condition is most commonly seen in cattle, sheep, goats and horses and polar bears in zoological collections. The infection is characterized by the formation of thick crusts which come away easily with a tuft of hair, leaving a moist, depressed area with bleeding points from capillaries. Virulence factors include an alkaline ceramidase and a number of proteases which facilitate invasion of the epidermis (García-Sánchez et al. 2004, Norris et al. 2008). Infections can be localized but have a tendency to spread over large areas of the body and the morbidity and mortality can be high, especially in tropical regions. The position of the lesions varies with the predisposing conditions. In periods of high rainfall the lesions tend to occur along the backs of animals. Where there is a heavy infestation with Amblyomma ticks the lesions are present in the predilection sites of the ticks: dewlap, axillae, udder, scrotum and escutcheon. In the dry season, in tropical regions, when feed is scarce the lesions are on the muzzle, head and lower limbs due to the animals foraging in thorn-covered scrub.

The pathogenic actinobacteria and the diseases that they cause are summarized in Table 10.2 and virulence factors of T. pyogenes and D. congolensis are given in Table 10.3.

Specimens

Specimens include pus, exudates, aspirates, tissue and scrapings from the walls of abscesses if they have been incised. A volume of fluid or pus should be collected and submitted, if possible, rather than just a small amount on a swab. Thin sections of granulomas in 10% formalin are useful for histopathology.

Direct microscopy

Sulphur granules are the best specimens for direct examination in infections caused by A. bovis or A. viscosus. The pus or exudate is placed in a Petri dish and washed carefully with a little distilled water to expose the yellowish sulphur granules of A. bovis (Fig. 10.3) or the softer greyish-white granules of A. viscosus. A granule is placed on a microscope slide in a drop of 10% KOH and gently crushed by applying pressure on the coverslip. The characteristic clubs can be seen if the preparation is examined under the low-power objective of a microscope. A view of a club colony, in situ, can be seen in stained histological sections (Fig. 10.4). If smears are made from the granules and stained with the Gram stain, delicate, Gram-positive, branching filaments (Fig. 10.5) can be observed. Occasionally short filaments or pleomorphic diphtheroidal forms may predominate.

Gram-stained smears of pus or mastitic milk samples in T. pyogenes infections usually reveal large numbers of small, highly pleomorphic, Gram-positive forms (Fig. 10.6). They tend to be a mixture of cocci, rods and pear-shaped cells. Occasionally short branching forms may be seen.

Isolation

The Actinomyces species grow well on sheep or ox blood agar. Actinomyces bovis requires anaerobic conditions with 5–10% CO₂ added (H₂ + CO₂ commercial envelope). Actinomyces viscosus and T. pyogenes will grow aerobically but 5–10% CO₂ will enhance their growth. They are all incubated at 37°C, A. bovis and A. viscosus usually require two to four days but the growth of T. pyogenes can usually be seen in 24 hours. The isolation of A. hordeovulneris is greatly enhanced by 10–20% foetal calf serum, although it will grow on blood agar at 37°C under 10% CO₂ with either aerobic or anaerobic conditions. Colonies are visible in about three days.