The two most common parasites causing neurological disease in the rabbit are Encephalitozoon cuniculi (also known as Nosema cuniculi) and Baylisascaris procyonis. In this part of the series we will discuss Encephalitozoon cuniculi.
Cause: There is a protosoal (one-celled) organism in the microsporidia family that can infect a number of species of animals, including humans. In humans infections are rare except in immunocompromised individuals such as AIDS patients or those suffering from tropical diseases. Some species, such as dogs and cats, either die from the disease or survive the infection and completely clear it from their bodies. In rabbits and mice however, the infection is persistent throughout their lives and may or may not cause obvious signs of disease. Even within species it appears that some genetic strains are more resistant to infection than other. E. cuniculi is passed from infected animals through the urine. The oral route is the most common way it is picked up by another rabbit, i.e. when a rabbit eats material contaminated with urine containing infective spores. The spores also can be inhaled and enter the body through the lungs. There is still controversy over whether or not the disease is transmitted through the placenta in the unborn young. The rate of infection in kits is highest when the dam is positive for this disease. Whether that is because the spores are in the environment from previous shedding by the mother or because there is transmission through the milk or placenta has not been definitively determined. Once the spores are in the body, they become active and spread throughout various tissues including kidneys, liver, lung, spinal cord and brain. Microsporidia that infect the kidney go on to actively produce more spores that are then passed in the urine in large numbers within 30 days. The spores are produced for up to 90 days post infection, at which time they stop and the rabbit can no longer infect others. E cuniculi infections are very common in pet rabbits in the United States. Up to 80% of clinically normal rabbits tested in some populations were positive for this disease. It can not only be transmitted from other affected pet rabbits, but also from wild rabbits and rodents.
Signs: Fortunately most rabbits affected with this parasite remain completely normal throughout their lives. However, some rabbits develop clinical disease and others do not, although as mentioned previously it might depend partially on a genetics. Certainly animals that are immunocompromised are at a higher risk. The clinical signs in rabbits usually involve disease of the kidney, brain or spinal cord. If severe kidney disease is present, a generalized weakness may be observed which could be initially mistaken for a neurological problem. In addition to weakness and depression, the rabbit may exhibit a poor appetite, increased water intake, increased urination, an ammonia odor to the breath and sudden death. Kidney disease is usually diagnosed by blood tests and occasionally a kidney biopsy. If the brain or spinal cord is infected, the signs may vary depending on what area is damaged. A rabbit may experience any one or combination of the following: unilateral or bilateral facial paralysis, weakness in only one limb, complete hind limb weakness or paralysis, all four limb weakness or paralysis, head tilt, loss of appetite, behavior changes, depression, seizures (mild to severe) and sudden death.
Diagnosis: There is a blood test that can detect the presence of E cuniculi in the rabbit. It detects antibodies to the parasite. The test was primarily designed to detect positive animals in a breeding colony or laboratory setting so they could be removed from the population. The test is of limited use in pet rabbits because it does not prove that this parasite is the cause of the clinical signs present. In other words, many rabbits have been exposed to this parasite and are infected, but will never show any sign of disease. Therefore, if we get a positive test, it only means that the rabbit has the parasite in its body, it does not prove that the parasite is responsible for any of the clinical signs we are currently seeing. The only way to diagnose if the parasite is actually causing the signs would be to take a biopsy of the brain or spinal cord, which is dangerous and not at all practical. Therefore, we can only make a tentative diagnosis of E cuniculi based on the rule out of other diseases along with the presence of a positive E. cuniculi test showing that the animal has the potential for having a problem with this parasite.
Treatment: Unfortunately there is no treatment for an active infection of E. cuniculi. Drugs such as ivermectin and other parasiticides have been tried without success. The parasite lives within the cells and it is very difficult to get medication into this protected area. In Europe there was a study published in 1994 which suggested that the drugs fumagillin and albendazole might be useful, but they are not available in the U.S. and have not been tried on live animals. Even if a drug is discovered that can clear an infection of E. cuniculi, it will not be able to reverse the effects of brain, spinal cord or kidney damage once it has occurred. Rabbits that have clinical signs of the disease should be supported with good nursing care. Antibiotics, analgesics and anti-inflammatory drugs may be used as needed for secondary problems that may develop. Most pet rabbits have probably contracted the infection from the mother. Rabbits that are obtained as adults and that are not used for breeding are not liely to shed the spores. Hoever, young rabbits under 4 months of age that may have contracted the disease from the mother could be actively shedding the spores in their urine. It would probably be a good idea to isolate rabbits under 5 months of age from other rabbits in the house. An E. cuniculi test could be run on these youngsters to determine if they are carrying the parasite. If they are negative, they could be put with the other rabbits earlier than 5 months of age. We have been unable to find a definitive source that indicates the best product with which to clean the environment and remove any spores. Any materials that are contaminated with urine, such as wood or carpet, that cannot be cleaned thoroughly should be destroyed. Other substances, such as plastic and metal should be scrubbed with a phenol disinfectant (Lysol is one example) or a strong bleach solution and then rinsed thoroughly. In part three of this series on causes of neurological disease we will discuss another parasite, Baylisascaris procyonis and strokes.