Many different bacteria can play a role in cervicofacial and upper respiratory tract infections in horses. In this respect, strangles is an important disease for horses and their owners. This highly contagious disease is caused by the bacterium Streptococcus equi subspecies equi (S. equi), which, in addition to respiratory symptoms, quite often causes lymph node abscesses. However, occasionally other bacteria are found to cause abscessation in the cervicofacial region. For example, Actinomyces denticolens, a species that has been sporadically described in recent years as the causative agent in case reports of such chronic inflammations (1–4). However, to date, these reports are scarce, and in horses, actinomycosis is considered to be a rare condition.
Actinomyces spp. are gram-positive, non-spore-forming, facultative anaerobic rods that are part of the normal microbiota of the oral cavity and respiratory tract of humans and several animal species, including horses. They are recognized as possible causative agents of orally associated infections. As opportunistic pathogens, they incidentally invade the tissue and cause infection when mucosal barriers are disrupted (6, 9–11). In cattle, for example, Actinomyces bovis is typically introduced from the oral cavity into the gingiva and jawbone via mucosal injuries, usually as a result of dental disease or penetrating/migrating foreign bodies. The most common resulting clinical picture is mandibular osteomyelitis, known as “lumpy jaw syndrome”. “Lumpy jaw syndrome” is also the most common clinical manifestation seen in human actinomycosis, following a mucosal lesion due to, for example, gingivitis, dental extraction, or neoplastic conditions. The Actinomyces species predominantly found in this context are A. israelii and A. gerencseriae.
A. denticolens was described for the first time in 1984 after isolation from dental plaque in healthy cattle. Over time, it was also isolated from the oral cavity and lower respiratory tract of healthy horses and donkeys. Recently, a study in slaughter horses identified A. denticolens as part of the common oral flora that colonizes the equine tonsil crypts.
Clinical signs in horses due to an infection with A. denticolens can be both local and generalized. These may include fever, anorexia, depression, painful swelling, and abscessation in lymph nodes or other soft tissues, followed by rupture and drainage of purulent material. In the cases reported so far, the infection was mainly localized in the submandibular lymph nodes. Taken together, this may result in a clinical picture very similar to that of strangles.
Laboratory confirmation of A. denticolens may be difficult because of a general lack of familiarity with this pathogen and a low success rate in culturing as a result of its slow-growing and fastidious nature. Furthermore, identification on the species level is demanding for a routine diagnostic laboratory because it requires additional phenotypic or genotypic diagnostic technologies such as MALDI-TOF MS or 16SrRNA sequencing. Communication between a clinician and microbiologist is of added value in this respect because the chances of laboratory confirmation may significantly improve when a targeted search for A. denticolens is carried out.
A recent study by GD experts Linda van den Wollenberg, Kees van Maanen, Rianne Buter and E. van Engelen, P. Janszen (Equine Clinic De Raaphorst) and F. Rey (Veterinary Clinic Winsum) shows the identification and whole genome sequencing of three A. denticolens isolates originating from submandibular abscesses of unrelated horses. In addition, high genetic variability between the different isolates and the low presence of known virulence and antibiotic resistance genes were demonstrated.
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