E cuniculi is an obligate intracellular microsporidian parasite that affects many mammalian species (including dogs, humans, and rabbits). Infected patients shed infective spores in their urine. Organisms may be found in the gut, brain, and kidneys within 2 weeks of infection.14
Once suitable host cells are penetrated, the parasite proliferates (merogony), differentiates, and matures (sporogony), causing eventual rupture of the host cell and release of spores to complete the life cycle, which initiates the granulomatous response commonly associated with this disease.
In immunocompetent rabbits, infection remains latent and subclinical while the parasite and host immune response remain in balance.15 An immune response sufficient to clear the host of the organism would cause more damage to the host than the presence of small numbers of microsporidia.
Chronic granulomatous inflammation in host organs is likely responsible for the clinical signs (eg, CNS disease, renal disease, ocular disease) attributed to E cuniculi. Chronic inflammation results in the development of granulomatous lesions in target organs, primarily the kidneys and brain, although the liver may also be involved. Clinical signs are primarily associated with granulomatous meningoencephalitis or interstitial nephritis and fibrosis, notably vestibular disease and chronic renal failure.
E cuniculi has a variable prevalence (50%-75%) in rabbits.16 Most seropositive rabbits do not develop clinical encephalitozoonosis. In the author’s experience, it can be difficult to determine whether E cuniculi or another disease process is the cause in seropositive rabbits with compatible clinical signs (eg, head tilt, nystagmus, rolling), largely because many rabbits are seropositive with no clinical signs.
The prognosis for rabbits presented with acute head tilt is variable. Inner ear disease generally requires surgery, which is technically challenging and can result in postoperative complications (eg, facial nerve paralysis). Rabbits with confirmed encephalitozoonosis have a better prognosis with prompt and appropriate treatment. Most patients regain enough function to lead a relatively normal life. After appropriate treatment, many cases of acute head tilt improve; however, the patient may continue to display a small degree of head tilt, especially with stress or comorbid illness.