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Actinomycosis, Nocardiosis, and Mycobacterial Infections
1
Joanna Whitney, BSc, BVSc, PhD, MVetStud, MACVS (SMAnimMed, ECC) and Vanessa R. Barrs, BVSc (Hons),
MVetClinStud, MACVSc (Small Animal), FACVSc (Feline), GradDertEd (Higher Ed) 2
1 Small Animal Specialist Hospital, North Ryde, NSW, Australia
2 University of Sydney, Sydney, Australia

Actinomyces, Nocardia, and Mycobacteria belong to the Actinomyces form histopathologic lesions characterized
order Actinomycetales. All are bacilli that form branching by pyogranulomatous inflammation with areas of central
filaments and cause pyogranulomatous infections. However, necrosis surrounded by macrophages, neutrophils, and
differences in antimicrobial susceptibility patterns, zoonotic plasma cells. Subacute and chronic infections result in
implications, and prognoses make an understanding of granulation tissue formation, extensive fibrosis, and sinus
how the organisms are differentiated clinically important. tracts. Exudates and effusions are often malodorous.
Actinomyces may form bacterial colonies in infected body
fluids, commonly referred to as “sulphur granules,” that
Actinomycosis grossly appear tan or white.

Etiology/Pathophysiology and Epidemiology Signalment
Actinomyces spp. are facultatively anaerobic or microaero- Young, male, large‐breed outdoor dogs are overrepre-
philic, Gram‐positive, nonacid‐fast, nonsporulating, and sented. Hunting and working animals are often affected,
nonmotile bacilli. These opportunistic pathogens are com- particularly with soft tissue abscesses or pyothorax asso-
mensals of the mucous membranes, especially the oral cavity ciated with plant material foreign bodies. Young, male
but also the gastrointestinal and genitourinary tracts. cats are also at increased risk from aggressive interac-
Actinomyces are often involved in polymicrobial infections tions involving scratches and bite wounds resulting in
that enhance their pathogenicity. A. viscosus, A. hordeovuln­ inoculation of oral bacteria.
eris, A. bowdenii, A. canis, A. catuli, A. turicensis, A. weissii,
and A. odontolyticus have been isolated from canine infec- History and Clinical Signs
tions. A. viscosus, A. meyeri, A. odontolyticus, A. hordeovuln­
eris, and A. bowdenii have been isolated from infections in In both dogs and cats actinomycosis is most commonly
cats. Actinomyces pyogenes was recently reclassified based associated with subcutaneous and soft tissue abscesses
on 16S rRNA gene sequencing to be included in the genus and intrathoracic disease. Clinical findings may include
Arcanobacterium. This organism has been reported to pro- pneumonia, lymphadenopathy, intra‐ or extrapulmonary
duce actinomycosis‐like infections in both dogs and cats. masses or pyothorax. Central nervous system (CNS),
Actinomyces cause disease following inoculation of periodontal, ocular, peritoneal, urinary bladder, cardiac,
body tissues, frequently in conjunction with other bacte- and orthopedic infections have also been reported in
ria. Portals of entry include oral mucosa damaged by dogs. In cats, less common presentations include abdom-
chronic gingivitis and periodontal disease, foreign body inal granulomas, intracranial/spinal empyema, and
migration, subcutaneous inoculation via bite wounds or cholecystitis.
plant material, and aspiration of oropharyngeal material Cervicofacial actinomycosis occurs in both cats and
or grass awns. Infections occur primarily in immuno- dogs associated with extension of oral bacteria into the
competent individuals. soft tissues of the head and neck secondary to periodontal

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical