Encephalitozoon cuniculi, or E. cuniculi for short, is a protozoan parasite of rabbits that can cause cataracts, kidney disease, or affect the nervous system and cause a head tilt or complete paralysis. How it infects rabbits and how much disease it is responsible for has been almost as difficult to determine as the name of the organism is to pronounce. Even the percentage of rabbits infected is controversial—rabbit researchers say anywhere from 40–80% of house rabbits will test positive on serology (blood test measuring the body's immune response to the organism.)
Let's start by explaining what can happen when a rabbit is exposed to the E. cuniculi organism. Rabbits pick up the organism via inhalation or ingestion of a spore form of the protozoa. This spore then travels via the bloodstream to various organs in the body, including the kidneys, eyes, and brain. Within the kidney, the protozoa reproduces and spores are shed in the urine, serving as a source of infection to other rabbits. While in the kidney it can destroy cells and thereby create scarring and pitting of the kidney tissue. If enough of the kidney is damaged, you may see increased thirst and urination, weight loss, and kidney failure. The degree of illness depends on the number of protozoa the rabbit is exposed to and the body's immune response to these protozoa.
If the organism migrates to the eyes, it can result in sudden onset of eye inflammation and eventually cataracts. If the organism travels to the brain, several possible scenarios can occur. The protozoa may just form cysts in the brain tissue and cause no damage. If the protozoa reproduces in the brain, it can cause rupture of cells resulting in a severe inflammatory reaction. It is not known if the damage to the nervous system is due to the E. cuniculi organism itself or from the body's immune/inflammatory response to the organism. Regardless, infected rabbits can have partial or complete rear limb paralysis, a head tilt, tremors, or convulsions.
Having said all of this, the good news is that most infected/exposed rabbits have sub-clinical infections, meaning they have the protozoa in non-harmful locations within their bodies or in such low numbers that they don't destroy enough tissue to cause illness. Regardless, the protozoa completes its life cycle in three to five weeks (the time from ingesting a spore to the time it is shed in the infected rabbit's urine). The infected rabbit itself is only contagious to other rabbits for a few days to a few weeks during this shedding period. Once shed in the urine, the protozoa can survive in the environment for up to one month. Those rabbits under stress (poor environment and nutrition) and with poor immune systems will be more likely to show clinical signs once exposed to E. cuniculi.
In healthy rabbits, spore cysts may remain dormant in the brain or muscle tissue for years. These rabbits may appear clinically healthy and then over time age, stress, or any factor that causes suppression of the immune system can allow theses dormant cysts to become active. The rabbit can start showing clinical signs of disease (usually neurologic or ophthalmic problems) years after initial exposure and explains why a bunny that has not been exposed to any other rabbits for years can suddenly develop illness from E. cuniculi.
Rabbits that show clinical signs of kidney disease, neurological lesions, or eye inflammation and cataracts can have a blood test run (serology). This measures antibodies that determine if the animal has been exposed and an immune response has been initiated. It must be noted that a positive antibody level is not synonymous with disease. Research has shown that 40–80% of healthy house rabbits in the United States will test positive for E. cuniculi. This means they have been exposed to the parasite, but does not mean an otherwise healthy rabbit will show symptoms of the disease any time in its life.
In most rabbits with E cuniculi, the treatment is two-fold: treating with an anti-parasite drug such as fenbendazole (Panacur®) for up to 28 days, along with nutritional and supportive care. Other needed therapies may include a drug to control dizziness (for those rabbits with head tilt), eye drops or ointments for those rabbits with eye inflammation, or creating a comfortable and loving environment for those animals with rear leg weakness or paralysis. These rabbits may require extra time and attention, and with any chronic illness our goal is to provide quality of life.
In summary, treatments are directed at providing supportive care and parasite control. Control of disease transmission can be accomplished by eliminating spores from the environment through disinfection and by removing or isolating infected rabbits from uninfected rabbits living in a group or colony.