This Case Report describes a periarticular abscess and a suppurative arthritis of the atlanto-occipital joint in an adult horse. The horse showed a painful swelling localised on the atlanto-occipital region and ataxia in all 4 limbs. During hospitalisation, the horse developed recumbency, dysphagia, facial paralysis and seizures. Ultrasonography of the atlanto-occipital region enabled a tentative diagnosis to be made, which was later confirmed at autopsy. Culture of the abscess revealed a Staphylococcus aureus. However, the horse had a history of recurrent fever, hypertrophy and abscessation of the submandibular lymph nodes, neck pain and dyspnoea for 2 months, which was suggestive of strangles.