Encephalitozoon cuniculi
Encephalitozoon cuniculi (E.cuniculi) is an emerging disease in pet rabbits, although it has been recognised for some time in farm and laboratory rabbits. More recently, human infections with this parasite have been diagnosed. Much is still unknown about infection in pet rabbits, but this article will attempt to clarify some of what we do know about this disease.
Life Cycle and Transmission.
E.cuniculi is a tiny parasite, a single celled organism called a protozoan, which lives inside the cells of its host. Theparasite produces spores, which are passed out in the urine of the host. Transmission between rabbits occurs by eating these spores in contaminated food and water. The developing foetuses can also become infected across the placenta during pregnancy, and occasionally rabbits can become infected by the inhalation of spores, although these two routes occur less frequently.
Once the spores have entered the rabbit’s body they are carried in the blood to the target organs, such as the liver, kidneys, brain and spinal cord, where they get into the host’s cells. There the parasites multiply, causing the cells to enlarge and eventually rupture, releasing further spores, which, in turn, infect nearby cells and spread via the blood to other organs. Rupture of the host’s cells causes inflammation – primarily in the liver, kidney, brain and spinal cord – which results in the symptoms seen in the infected rabbit.
The life cycle lasts three to five weeks. However, the spores can survive in the environment in extreme heat or cold,although at average temperatures and in dry conditions, they typically survive for about 4 weeks.
Prevalence.
Research has indicated that over 50% of normal, healthy pet rabbits have been exposed to E. cuniculi and are positive on blood sampling to look at antibody levels. This suggests that infection in rabbits is not always associated with any clinical signs. Instead, the infection remains hidden and the rabbit can carry the parasite with the possibility of passing it on to in-contact rabbits (and people). This makes control of the disease in a multi-rabbit household more difficult.
Clinical Signs.
The clinical signs in the rabbit depend on which organs are primarily affected:
- Central Nervous System:
Uveitis in a rabbit with
E. cuniculiWhen the brain and spinal cord are affected, the rabbits can show a variety of symptoms, including a head tilt, unsteadiness, weakness in the hind limbs, neck spasm and urinary incontinence.
- Kidney:
Infections of the kidneys can lead to chronic kidney failure, leading to excessive drinking and consequently excessive urine production,which can lead to urinescalding around the rabbits back end.
- Eye:
If the kits are infected in the uterus then spores can cross into the lens of the eye.Later in the rabbit’s life the spores multiply and erupt, causing cataracts (a ‘white’ eye) and lens rupture leading to inflammation within the eye itself (uveitis) which can lead to blindness.
The rabbit-parasite relationship has yet to be fully understood. Establishment of infection and the development of clinical signs are thought to be dependent on several factors: the strain and quantity of the parasite, the route of infection, the age of the rabbit at the time of infection and the rabbit’s immune status. Commonly there is a balance between the infection and the rabbits’ immune system, and no clinical signs are seen. In immunodeficient rabbits, or young animals with immature immune systems, disease is more likely. In rabbits with a healthy immune system a clinically silent, chronic infection develops. In these animals clinical signs could develop at a later date associated with concurrent infection, stress or immunosuppression.
Diagnosis.
A major problem with this disease is the difficulty in diagnosing an active infection in live rabbits. Post mortem is often the only definitive way of testing for the parasite.
Antibody levels can be measured in the blood, although with over 50% of clinically normal rabbits testing positive this does not mean that any symptoms are due to E. cuniculi, but just that the rabbit has been exposed to the parasite at some point in the past. It is important to note that antibody levels can persist for years in rabbits without any clinical signs. A negative antibody test also does not rule out infection with the parasite (in a healthy animal), since it takes two weeks for the levels to rise after initial infection. Thus a healthy rabbit would need to be tested again four weeks later to rule out the possibility of early infection prior to antibody levels going up. In a rabbit with clinical signs, however, a negative result does rule out E. cuniculi as a cause of clinical disease.
Treatment.
In affected rabbits the inflammation and release of spores results in the clinical signs, particularly affecting the target organs (brain, spinal cord and kidney). Treatment is aimed at reducing inflammation, using anti-inflammatory drugs, and stopping further infection of host cells by the parasite, using benzimidazole drugs. Albendazole is successfully used to treat infections in humans, and has been used in rabbits. However, some adverse reactions have been seen in rabbits, so now the condition is treated using fenbendazole. This is safer, and has proved effective in reducing clinical signs in less advanced cases and, more importantly, prevented infection in exposed in-contact rabbits.
The response to treatment varies a lot, and depends on how advanced the disease is. Chronic cases usually have neurological signs associated with severe cell damage, and successful treatment may not be possible. In acute cases urine dribbling may resolve with fenbendazole, and with eye disease there is a better prognosis, although in some severe cases the eye may need to be removed.
Control and Prevention.
Research has shown this parasite to be present in 52% of healthy domestic rabbits, so it may not be possible to prevent this infection in your pet. It is possible to form an E. cuniculi free breeding colony, but this requires time and money as regular blood tests for all the animals are necessary.
To prevent spread between rabbits, prophylactic treatment with fenbendazole may reduce the risk of infection. Other measures include regular disinfection of the rabbit’s environment, including food bowls and water
containers, lifting food bowls off the floor to reduce the risk of urine contamination, using water bottles rather than bowls, again to reduce the risk of urine splashing, and housing animals in separate hutches rather than tiered systems where urine may drop onto the lower cages. Wild rabbits and rodents could also act as a potential source of infection, so contact with these should be avoided.
Zoonotic Risks.
Although E. cuniculi is primarily a rabbit infection, it can affect other animals as well, including rodents, guinea pigs, foxes, cats and dogs, and also humans. In people, infection is usually only found in immunocompromised individuals such as patients with HIV, or those who are on immunosuppressive drugs. However, close contact between owners and their rabbits could lead to an increased exposure to the parasite, so care should be taken when cleaning out cages etc.
