Malassezia spp. Infection: Malasseziasis

Malassezia spp. (formerly Pityrosporum spp.) is rarely asso­ciated with disease in rabbits. Opportunistic overgrowth of Malassezia spp. yeast forms has been reported in an outbreak of sarcoptic mange involving 4% of rabbits in a rabbitry.

Pneumocystis oryctolagi sp. nov. Infection

Pneumocystis spp. are obligate extracellular yeast-like organisms that are now classified as members of the fungal kingdom. There is high genetic divergence among Pneumocystis species resulting from coevolution within their various mammalian hosts. Each species is host-specific, including P. jirovecii in humans, P. carinii and P. wakefieldae in rats, P. murina in mice, and P. oryctolagi in rabbits. The list of genetically distinct Pneumocystis species is likely to grow with genetic analy­sis of Pneumocystis from other host species.

Epizootiology and Pathogenesis

Pneumocystis oryctolagi organisms are typically restricted to the alveoli of the lung, and rabbits appear to become infected as neonates, presumably by inhalation or maternal grooming. The life cycle of Pneumocystis includes a trophic form, which replicates asexually by binary fission, and sexual replication through formation of an ascus (cyst) containing 8 ascospores, plus several intermediate forms. Aerosol transmission is mediated by ascospores. Unlike other yeasts, Pneumocystis does not undergo budding. Trophic forms attach to type I pneu- mocytes. Pneumocystis spp. generally exist within their immunocompetent hosts as commensal organisms with minimal consequence. Infection becomes clinically sig­nificant under conditions in which the host is immune- compromised.

Pathology

Based on studies in Europe, subclinical pulmonary dis­ease is very common in rabbits during the postweaning period.

Deep Mycoses

Aside from pulmonary aspergillosis, deep mycoses are quite rare in domestic rabbits. A single case of adiaspir- omycosis in a rabbit with pulmonary granulomata caused by Emmonsia spp. has been reported. Histoplas­mosis has also been reported in 2 domestic rabbits. Affected animals had nodular cutaneous lesions. One case had disseminated disease, with multiple periocular, nasal, perioral, and prepucial alopecic nodules upon clinical presentation. Necropsy revealed additional involvement of other cutaneous sites, lymph node, and small intestine. Lesions featured dense infiltration of macrophages, with fewer numbers of heterophils and multinucleated giant cells. Both intracellular and extrac­ellular yeast forms were apparent.