Abstract
Background: The oomycete Lagenidium giganteum forma caninum is an uncommon cause of severe dermal and subcutaneous infections in dogs with possible vascular invasion and other fatal sequelae. Infection within the central nervous system of affected dogs has not been previously reported.
Case Description: A 6-year old spayed female mixed-breed dog was evaluated at a referral institution for a 2-month history of a suspected fungal infection in the region of the right mandibular lymph node that was refractory to surgical resection and empiric medical therapy. Physical examination identified a 6 cm fluctuant subcutaneous mass caudoventral to the ramus of the right mandible and a second firm mass in the region of the right caudal maxilla. Lesional punch biopsies were submitted for fungal culture and polymerase chain reaction (PCR), which subsequently identified Lagenidium giganteum forma caninum infection. Initial treatment consisted of anti-inflammatory doses of prednisone and hyperbaric oxygen therapy. Four weeks following initial evaluation, the patient was presented with progressive neurological signs consistent with a forebrain lesion. Magnetic resonance imaging revealed soft tissue, contrast-enhancing lesions ventral to the calvarium adjacent to the site of original surgical resection and throughout the brain. Humane euthanasia was elected, and post-mortem examination was consistent with extension of local disease from the right masseter muscle into right ventral calvarium. Post-mortem DNA sequencing confirmed the identity of the organism as Lagenidium giganteum forma caninum.
Conclusion: This is the first reported case of intracranial lagenidiosis in a dog. Gene sequencing distinguished this species from other Lagenidium species and from oomycetes of other genera, such as Pythium insidiosum and Paralagenidium karlingii, which have not been reported to cause vascular invasion in the dog. Regional extension of cutaneous lagenidiosis should therefore be considered in cases with concurrent or spontaneous neurologic disease.

Key words: Hyperbaric oxygen therapy, Lagenidium giganteum forma caninum, neurologic disease, oomycete.