Head tilt is one syndrome that may be associated
with E cuniculi infection.

Encephalitozoon cuniculi, or E. cuniculi for short, is a protozoan parasite of rabbits, which can cause partial or completer paralysis and kidney disease. 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 to 80% of the house rabbits will test positive on serology (blood test measuring the body's immune response to the organism.)

So what does this mean to the house rabbit owner? 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. It is thought it then travels via the blood to various organs in the body including the kidneys, eyes and brains. Within the kidney, the protozoa reproduces and then is shed in the urine thus completing its life cycle and being 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. This can result in the rabbit showing increased thirst and urination, weight loss and kidney failure or the rabbit may be a-symptomatic-meaning he shows no signs of the illness at all. The degree of illness or lack 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 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 or if the protozoa reproduces in the brain it can 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 an immune response to the organism. Regardless, infected rabbits can have partial or complete rear limb paralysis, head tilts, 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 whether the exposed bunny becomes ill or not the protozoa completes its life cycle in 3-5 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 immediate clinical signs once exposed to E. cuniculi.

In healthy rabbits spore cysts may develop in the brain or muscle tissue and remain dormant for years. These rabbits may appear clinically healthy for years and then age, stress or any factor that causes suppression of the immune system can allow theses dormant cysts to become active and the rabbit can start showing clinical signs of disease (usually neurologic or ophthalmic problems) years after initial exposure. This explains why a bunny that has not been exposed to any other rabbits for years can suddenly develop illness from E cuniculi.

So how does one make a diagnosis of E. cuniculi in rabbits? Rabbits who 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. The greater the degree of active: the greater the antibody quantity that will be detected. However, it must be noted that a positive antibody level is not synonymous with disease. Research has shown that 40 to 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. Diagnosis can also be made on tissue samples submitted to a pathologist if the infected rabbit should die.

Can E. cuniculi be treated? In most rabbits with E cuniculi the treatment is two-fold. First treating with an anti-parasite drug called fenbendazole for up to 21 days. And secondly, supportive care: good nutrition and drugs to control dizziness for those rabbits with head tilts, eye drops or ointments for those rabbits with eye inflammation and providing a comfortable and loving environment for rabbits with rear limb weakness. These rabbits require extra time and attention but can go on to live additional happy years. As with any chronic illness, our goal is to provide quality of life. Regular cage or litter box cleaning, to limit exposure of rabbits to infected urine, helps prevent this disease form spreading to other rabbits in your household. Recently (11-15-03) several exotics veterinarians have suggested the use of Ponazuril (MarquisÒ- Bayer Pharmaceuticals) an equine product used to treat Equine Protozoal Myeloencephalitis (EPM) for the treatment of E cuniculi in rabbits. Use of this product is still in the "research" phase but looks hopeful.

In summary, rabbits infected with the E. cuniculi protozoa may experience asymptomatic infections (rabbits do not manifest disease but carry the E cuniculi cysts) or display evidence of organ dysfunction from lesions that develop in the kidney, eyes or brain. Serologic tests can be used to identify infected rabbits, and definitive diagnosis can be made by identifying spores in infected tissues on biopsy. 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.