Differentiating pasteurellosis from encephalitozoonosis as a cause of vestibular disease is challenging and is most often
determined by using the diagnostic techniques outlined in Table 2. Rabbits with suppurative otitis media caused by P. multocida often show no clinical evidence of infection. The otitis may extend into the inner ear, resulting in torticollis. If the
infection is unilateral, the rabbit's head will tilt down on the affected side. Occasionally, nystagmus may be present. Affected
middle ears are characterized by tympanic bullae filled with thick, yellow pus. Tympanic membrane rupture is possible. Radiographic
or computed tomographic examination of the skull may show soft tissue density in the bullae, indicating pus-filled bullae.
Table 2. Differentiating Pasteurellosis from Encephalitozoonosis
The head tilt in these rabbits is generally irreversible, but its worsening can be prevented with appropriate antibiotic therapy.
Some clinicians report a lower incidence of residual head tilt after successful therapy. Chloramphenicol, enrofloxacin, ciprofloxacin,
and trimethoprim-sulfa have been reported to be successful antibiotic therapy.4 The prognosis is favorable if a positive response is exhibited within the first week after therapy; therapy is continued
for one week after clinical signs resolve.
Treatment of otitis media and externa should be long-term (four weeks or longer). Antimicrobials commonly used as initial
therapy while waiting for results of culture and sensitivity include chloramphenicol (50 mg/kg subcutaneously or orally every
12 hours) or enrofloxacin (5 to 10 mg/kg orally every 12 hours). Also for otitis externa, clean and flush the ear canals,
and apply topical 0.5% enrofloxacin–1% silver sulfadiazine otic solution (Baytril Otic—Bayer).5 In cases of severe, painful otitis externa, medical treatment may need to precede ear cleaning. Another treatment option
when tympanic bullae are involved is tympanic bulla osteotomy, although complications—including facial nerve palsy, fistula
formation, vestibular disease, and hypoglossal nerve dysfunction—have been associated with this surgery.
Encephalitozoon cuniculi, an obligate, intracellular, microsporidian parasite, is often associated with neurologic disease in pet rabbits. Although
many rabbits infected with E. cuniculi are asymptomatic, signs of neurologic disease caused by E. cuniculi include behavioral changes, torticollis, nystagmus, ataxia, rolling, or seizures and often follow a stressful event in the
rabbit's life. Encephalitozoonosis in pet rabbits is more likely to be the cause of vestibular signs than is pasteurellosis.6 Other neurologic signs may include urinary incontinence, a stiff rear gait, and posterior paresis. Stressed or immunocompromised
animals appear to be more susceptible. Transmission is generally by ingestion or by oral inoculation of infective spores shed
in the urine, although transplacental transmission may also occur.
Although it has been reported that encephalitozoonosis can be presumptively diagnosed based on signs of neurologic disease
together with demonstration of high concentrations of serum antibodies (paired titers are preferable),5 this diagnostic approach is not always reliable. While rabbits with encephalitozoonosis are seropositive, many seropositive
rabbits are apparently healthy and do not show signs of disease.3 In addition, a high antibody titer in a rabbit does not necessarily indicate it has an active or recent infection. However,
the absence of antibodies (seronegativity) for E. cuniculi indicates that other differential diagnoses must be considered in sick rabbits.3 Enzyme-linked immunosorbent assays (ELISAs), indirect immunofluorescence assays, and carbon immunoassays are all suitable
for detecting antibodies.5 So although a serologic evaluation can be helpful, it is far from diagnostic, and a definitive diagnosis of E. cuniculi as the cause of disease in a live rabbit is difficult6 and requires histologic identification of the organism.7,8
Table 2 shows how to differentiate between encephalitozoonosis and pasteurellosis as the cause of neurologic disease.