Encephalitozoon cuniculi (E. cuniculi) is a microsporidian parasite that lives latently in many rabbits and causes neurological disease (head tilt, hind-leg weakness, seizures), eye disease (cataracts, phacoclastic uveitis), or kidney disease. Fenbendazole for 28 days is the standard treatment. Most rabbits improve, though residual deficits are common.
Last reviewed: May 2026
What E. cuniculi Actually Is
E. cuniculi is an obligate intracellular microsporidian — a single-celled organism more closely related to fungi than to typical parasites. It infects rabbits primarily through ingestion of spores in urine or in utero from an infected doe. Once inside, the spores travel to the brain, kidneys, and eyes, where they form chronic granulomatous lesions. Roughly 50 percent of pet rabbits in the US and EU have serologic evidence of exposure, but only a fraction develop clinical disease.
When clinical signs do appear, they reflect which organ system carries the highest parasite burden. Neurological involvement is the most recognizable: sudden head tilt (typically 30 to 90 degrees), nystagmus, ataxia (stumbling, falling), hind-limb paresis, urinary incontinence with secondary scald, and occasionally seizures. Eye involvement causes a white cataract that's often visible as a dense lens opacity, sometimes with uveitis. Renal involvement is usually subclinical but can progress to chronic kidney disease.
How Vets Diagnose It
Diagnosis is harder than it sounds because exposure is common but disease is less common. Serology measures IgG and IgM antibodies. A positive IgG alone just means exposure. A high or rising IgG titer in a rabbit with consistent clinical signs supports the diagnosis. IgM positivity suggests recent or active infection. A negative serology with classic signs argues against E. cuniculi.
PCR on urine or CSF can sometimes detect spores or DNA but sensitivity is limited because spore shedding is intermittent. Imaging (CT or MRI) of the head rules out otitis media-interna (the main differential for head tilt) and may show changes consistent with granulomatous encephalitis. Per AEMV Pet Care Guides, 2024, most cases are diagnosed presumptively based on consistent signs plus supportive serology in a rabbit where other causes have been excluded.
Treatment Protocol
The cornerstone is fenbendazole (Panacur) at 20 mg/kg by mouth once daily for 28 days. Fenbendazole crosses the blood-brain barrier and reduces spore burden. It does not eliminate infection — the parasite can persist in granulomas — but it stops new spore production and allows the immune system to control disease.
Supportive treatment depends on symptoms. Neurological cases get anti-inflammatories (meloxicam, 0.6 to 1 mg/kg twice daily) to reduce inflammation around the granulomas. Severe vestibular signs warrant midazolam or diazepam to reduce thrashing. Rolling or recumbent rabbits need deep padded bedding, hand feeding, and assistance urinating; treatment is intensive nursing for the first 1 to 2 weeks. Uveitis is treated with topical NSAIDs or steroids depending on whether the lens capsule has ruptured. Severe phacoclastic uveitis with lens rupture sometimes requires surgical lens removal by a veterinary ophthalmologist.
What Recovery Looks Like
About 75 percent of rabbits with vestibular signs improve substantially with treatment, but full resolution is the exception. Most have a residual head tilt for life that doesn't significantly affect quality of life. Hind-limb weakness improves in many cases over weeks. Cataracts do not reverse — they progress slowly and may eventually need surgical removal if uveitis develops.
Renal disease tends to be chronic and progressive. Repeat bloodwork at intervals tracks the trajectory. Rabbits with elevated BUN, creatinine, and a low USG benefit from subcutaneous fluids at home and a moderate-protein hay-based diet.
Can Other Pets or Humans Catch It?
E. cuniculi is zoonotic in immunocompromised humans — there are case reports in HIV patients and transplant recipients. The risk to healthy people is very low, but standard hygiene (handwashing after handling rabbits, gloves when cleaning litter from a positive rabbit) is recommended. Immunocompromised owners should discuss risk with their physician. The parasite can also infect other rabbits, dogs, cats, and guinea pigs, though clinical disease in non-rabbit hosts is rare per Oglesbee & Lord, 2010, JEPM.
When to See a Vet
A new head tilt in a rabbit is always urgent. The differential (E. cuniculi vs. otitis media-interna) matters because treatment differs and delays cost recovery.
Call your vet today if:
- New head tilt, eye flicking, or circling
- Hind-leg weakness or dragging
- Urinary incontinence or wet perineum
- White spot or opacity in the eye
- Increased thirst and urination without other cause
Go to the ER immediately if:
- Rolling repeatedly with inability to right itself
- Seizures
- Complete inability to eat or drink for more than 6 hours
- Severe respiratory distress
- Collapse with cold extremities
What's going on with your pet?
Describe symptoms or snap a photo. Voyage tells you urgency, home care, and whether you need a vet.
First, tell us about your pet
Breed and age make a real difference in how Voyage interprets symptoms.
Describe the symptoms
🏆 Outperforms ChatGPT & Gemini · 🩺 Vet-grounded · 🔒 Private
Love it? See everything Voyage can do
Frequently Asked Questions
How much does E. cuniculi treatment cost?
Initial workup with exam, bloodwork, serology, and skull imaging runs $400 to $900. Fenbendazole treatment for 28 days runs $30 to $60. Supportive care for moderate cases including meloxicam, syringe feeding supplies, and rechecks adds $200 to $400. Severe cases requiring intensive nursing and follow-up imaging can exceed $1,500.
Can my rabbit recover fully from E. cuniculi?
About 75 percent of rabbits with vestibular signs improve substantially with treatment, but full resolution is uncommon. Most have a residual head tilt for life that doesn't significantly affect quality of life. Hind-leg weakness improves in many cases over weeks. Cataracts and renal disease are typically chronic.
Is E. cuniculi contagious to my other pets or me?
It can infect other rabbits, dogs, cats, and guinea pigs through ingestion of spores in urine, though clinical disease in non-rabbit hosts is rare. It's zoonotic in immunocompromised humans (HIV, transplant patients) but the risk to healthy people is very low. Handwashing after handling and gloves when cleaning litter from a positive rabbit are sensible precautions.
How is E. cuniculi diagnosed if my rabbit can't have an MRI?
Diagnosis is presumptive in most cases. A rabbit with classic signs (sudden head tilt, hind-leg weakness, cataracts) plus elevated IgG serology, with otitis ruled out by skull radiographs or CT and dental disease excluded, is treated as E. cuniculi. Response to fenbendazole within 2 weeks supports the diagnosis retrospectively.
Still Not Sure if Your Rabbit Needs a Vet?
When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of the head tilt, eye position, and how your rabbit walks, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.