In horses, as compared with other animals, actinomycosis has been thought to be a relatively uncommon disease. In recent years, the rapid identification of Actinomyces species has become possible. Actinomyces bovis and Actinomyces viscosus have been reported as causative agents of actinomycosis in horses, while the most common species isolated from the abscesses of equine oral cavity is Actinomyces denticolens. Actinomycotic clumps are often found in the tonsils in human beings. Although Actinomyces colonisation in the tonsils does not indicate an active infection in human beings, it can cause obstructive tonsillitis.
Strangles is a condition characterised by swelling and pus formation in the submandibular and retropharyngeal lymph nodes in horses. This contagious disease was assumed to be caused by Streptococcus equi and not by actinomycosis. In addition, when Actinomyces-like bacteria were isolated from the pus-filled lesions it might have been difficult to accurately identify the species in a general diagnostic laboratory due to the complexity of the bacteriological procedures or misdiagnosis. However, today's genetic analysis technology has solved these problems. As a result, abscesses have been reported as being caused by Actinomyces denticolens infections in the cervicofacial region. It is therefore unlikely that equine actinomycosis is actually a rare disease in horses.1
Corynebacterium pseudotuberculosis is a gram‐positive pleomorphic intracellular bacterium that causes external abscesses, internal abscesses, and ulcerative lymphangitis in horses. It has been implicated in internal or systemic disease conditions including pneumonia, pleuritis, pericarditis, purpura hemorrhagica, abortion, and panniculitis. The most common sites of external infection in horses are the abdomen and breast region, consequently the disease is often termed "pigeon fever." Successful treatment of horses with these diseases requires early diagnosis and treatment with a long course of antimicrobials (typically 30 days or longer). More recently, a mortality rate of 30% was reported in horses treated for internal Corynebacterium pseudotuberculosis infection.
Klebsiella species are ubiquitous in the environment and are part of the normal urogenital and intestinal microflora of the horse. They are also a common cause of bacterial pneumonia with abscesses. Complications associated with pneumonia or antimicrobial treatment occur in about 50 percent of affected horses, with thrombophlebitis and laminitis occurring most frequently.
Intracellular pathogen Rhodococcus equi is an important cause of disease and death in foals. For approximately 30 years, the combination of a macrolide antibiotic and rifampin has been the treatment of choice. Because a vaccine effective for protecting against R. equi pneumonia is not available, early detection and treatment of pulmonary abscesses has become common practice at many large breeding farms.