Diseases and Pathogenesis

Actinomyces bovis causes actinomycosis, or lumpy jaw, in cattle (Figure 6-1), as well as occasional similar human infections. Bovine actinomycosis usually affects bony structures, and is most commonly seen in the mandible, or lower jaw. Pulmonary infections may occur in cattle and swine. The organism has also been associated with equine fistulous withers and similar conditions, sometimes in company with Brucella abortus or Brucella suis. Disease in other ruminants has also been reported, and dogs often develop actinomycotic spondylitis as a consequence of migration of foxtails from the upper respiratory tract. Actinomyces hordeovulneris is a newly described species that is more common than A. bovis in these canine infections. Actinomycosis can also occur concurrently with lung adenocarcinomas in dogs.

FIGURE 6-1 Bone damage in Actinomyces bovis infection (lumpy jaw).

(Courtesy J. Glenn Songer.)

Actinomyces bovis is a normal resident of bovine oropharyngeal and digestive tract tissues. Organisms gain access to deeper tissues of thejaw by way of the dental alveoli or paralveoli, or through mucosae damaged by rough feed or foreign bodies. Mandibular granulation and chronic rarefying osteomyelitis follow. Suppurative necrosis can occur in the esophagus and reticulum, and thick green to yellow pus moves to the surface by way of fistulous tracts. Palpable masses can be found in normally soft tissues. Macrocolonies of A. bovis up to 5 mm in diameter, often called sulfur granules, are found in pus.

Pathogenesis of A. bovis infection is poorly understood. The pyogranulomatous response is a hallmark of infection, but it occurs by unknown mechanism. Areas of suppuration are surrounded by granulation, fibrosis, and infiltration of mononuclear cells.

Bone honeycombed with pus-filled sinus tracts replaces normal bone (see Figure 6-1), and affected animals may lose teeth, become unable to chew, and develop mandibular fractures. Jaw lesions are often impressive in extent, but if vascular dissemination occurs, infection does not establish in other tissues. Circulating antibody developed as a result of infection has no impact on recovery, and it is likely that resistance is cell mediated.

Actinomyces viscosus infection in dogs manifests as cutaneous, noduloulcerative lymphangitis. The organism also causes periodontal disease in hamsters and other rodents. The normal habitat of A. viscosus is likely the same as that of A. bovis, but it invades wounds in dogs and produces abscesses and pedunculated cysts. Actinomyces viscosus is commonly associated with thoracic infections in dogs, but abdominal infections and osteomyelitis also occur.

Actinomyces suis is frequently recovered from opportunistic mammary infections in sows, probably from a source among the normal inhabitants of the porcine oropharynx. Infection may begin in superficial teat wounds caused by piglets during suckling. Formation of a mammary actinomycotic granuloma results in enlargement and induration of the gland, and the suppurativeexudate contains typical sulfur granules.

Actinomyces spp. are prominent in the human oral cavity and are important factors in plaque development. Extensive study has revealed unique attributes that favor colonization and persistence in this environment. Fimbrial adhesins are perhaps the most important of these in that they enable adherence to receptors on tooth and mucosalsurfaces and interaction with other plaquebacteria. Sialidase may amplify the adhesion process by exposing host cell receptors. Diverse enzymatic activities of Actinomyces spp. enhance its own lifestyle and that of other oral bacteria, possibly exerting a positive selective effecton the actinomycetes. Such information is not available for the Actinomyces spp. associated primarily with animal diseases, but the same selective pressures may apply. Application of similar genetic systems and methods of approach to animal pathogenic Actinomyces spp. provides many opportunities, perhaps leading to better methods for prevention.

Epidemiology

As noted, Actinomyces spp. are found on oral mucous membranes and in the gastrointestinal tract. Thus most actinomycoses have an endo-genous origin and are noncommunicable, except via bites.

Diagnosis

Actinomycetes can usually be demonstrated in the “sulfur granules” associated with draining lesions. Colonies surrounded by a border of clublike structures (composed of mineral and the detritus of inflammatory and immune responses) are called rosettes. The radiating, somewhat distended clublike filaments may be found more commonly in tissue sections than in pus. Some staphylococcal lesions, especially in horses, contain sulfur granulelike structures; these are usually much smaller than true actinomycotic granules, and definitive differentiation can usually be accomplished by examination of the gram reaction and morphology of bacteria comprising the granules. Granules (not pus) should be washed and crushed under coverslips on a clean slide.

In such preparations, A. bovis appears as a tangled mass of filaments, with acidophilic capsular material at the periphery. The filaments are gram positive and basophilic. It is important to note, however, that diverse forms can be observed in crushed granules; A. bovis can appear as cocci, pleomorphic and branched rods, filaments, diphtheroidal forms, and even spirals. In vitro, A. bovis usually takes the diphtheroidal forms, although extended incubation, especially in a CO₂ atmosphere, drive the morphology toward branching filaments and clubs.

Actinomyces bovis is facultatively anaerobic and when cultivated aerobically, subsurface colonies predominate; the organism prefers an atmosphere containing 10% to 15% CO₂. Like all animal Actinomyces spp., A. bovis requires rich media, preferably containing serum or blood. Growthon blood-containing media is evident after 48 to 72 hours of incubation, although maximum colonial size is usually reached only after 5 to 6 days’ incubation. The organism is nonhemolytic.

Lesion-associated granules in A. viscosus infection contain the typical filamentous masses and diphtheroidal forms, but the clubs that are characteristic of A. bovis are absent (Figure 6-2). It produces flat, smooth-to-granular “molar tooth” colonies (Figure 6-3), which, at approximately2 mm in diameter, are perhaps twice as large as those of A. bovis. Microcolonies, after 24-hour incubation in an atmosphere enriched for CO₂, have a dense center and filamentous fringe, and after 72 to 96 hours are circular, convex, smooth and white to cream. Actinomyces viscosus is catalase positive, unlike other members of the genus. The organism is relatively uncommon in purulent exudates, and cultural false negatives may result. Granules may be cultured in thioglycollate broth.

FIGURE 6-2 Gram stain of Actinomyces viscosus.

(Courtesy Public Health Image Library, PHIL #1256, William A. Clark, Atlanta, 1977, Centers for Disease Control and Prevention.)