Encephalitozoon cuniculi (EC) is a parasite that causes lesions (called "granulomas") in such organs as the kidneys, liver and brain. Even today, EC is still not well understood in the rabbit community. Theories and opinions regarding transmission, treatment and the severity of the problems caused by this parasite vary widely. To date, none of the theories have really been proven or not proven by a definitive study. Because EC is a protozoon, it is difficult to find this organism in the live rabbit, and often, the protozoan is not even found during a post mortem exam. ...
Some experts feel that the majority of rabbits who have been exposed to EC do not suffer any health consequences as a result of this exposure. Others feel that EC is responsible for a whole host of health problems, including renal failure, convulsions, torticollis (head tilt or wry neck), hind limb weakness or loss of balance, nystagmus (eye twitching) and incontinence. Rabbits affected by EC before they are born (via the placenta) may develop lesions around the eyes, which sometimes cause visible white spots in the eye.
I took the photo below at a rabbit show yesterday of a Holland Lop with a vertical white line down the center of each pupil (in the eye, not on the cornea). I sent it to Dr. Hreiz of ARBA and thought others might find his reply informative:
"E. cuniculi is your #1 differential for stromal masses such as this. The parasite migrates all over the spinal cord and even can be in the eye where they form abscesses such as this. Your other differential would be a stromal abscess of bacterial origin.
This individual should prophylactically treat this rabbit with fenbendazole (Safeguard or Panacur) at 30 mg/kg once a day for 28 days is the treatment. This organism is shed via urine so any rabbits in close proximity should be treated as well. If signs do not resolve, then an antibiotic would be good to give a try."
Recent studies by some rabbit experts are beginning to swing the pendulum away from this disease as either being a cause of head tilt and being that treatable. And testing for it seems almost pointless as they are little help (at this time at least... perhaps better tests are in the near future) in determining if a rabbit has this infection or not.