Encephalitozoon cuniculi is a microsporidial parasite that commonly infects domestic rabbits, causing disease primarily of the central nervous system and/or urinary tract, although ocular lesions can be seen depending on when the rabbit was exposed to the organism.

Although E cuniculi primarily infects domestic rabbits, a small number of other species have been found to be susceptible to disease. These include monkeys, sheep, guinea pigs, mice, cats and horses. It is also a potentially zoonotic disease, with cases having been diagnosed in humans, although direct transmission from a rabbit has not been reported.

The clinical signs associated with E cuniculi infection are varied and will depend on the organs that have been affected. This parasite will migrate primarily to the central nervous system (CNS) and/or the urinary tract. Clinical signs, and the severity of them, will depend on how long the parasite has been present in the animal, where it is residing, the number of parasites that have ingested, the age at which infection takes place and the hosts immune response to disease.

Signs associated with infection of the CNS can include:

  • Head tilt (torticollis) and nystagmus
  • Circling and disorientation
  • Paraplegia or quadriplegia
  • Rolling over
  • Seizures and tremors

Signs associated with infection of the urinary tract can include:

  • Urinary incontinence and scalding
  • Renal failure or insufficiency (drinking and urinating more frequently, weight loss, dehydration etc)

Ocular lesions are seen in rabbits that are infected via the placenta, with cataracts and lens induced uveitis common in these cases.

Most rabbits will present with clinical signs suggesting infection in either the CNS OR the urinary tract. However, there are some cases where infection in both these body systems is seen.

Infection can also lead to retarded growth, collapse and death, in severe cases.

Diagnosis isn’t always straightforward, and it can be frustrating.

Blood samples can be taken from a rabbit with clinical signs that are compatible with E cuniculi infection. These can be checked for the presence of IgM and IgG antibodies against E cuniculi which tell us whether the rabbit has been exposed to E cuniculi.

Determining the IgM and IgG titres in suspected cases of EC is an indicator of a humoral response of the host organism, with IgM increasing acutely, and IgG indicating a more chronic, or latent, infection. However, even though these antibodies may persist for the lifetime of the host, they do not offer protection against reinfection.

Paired blood samples, taken 4-6 weeks apart are recommended to see whether the disease process being observed is active (where an increase in the IgM titres is seen between the samples), if the rabbit is recovering from disease (the IgM titre will decrease in this time) or if the rabbit has had the disease in the past and has a chronic, or more latent, infection (a raised IgG titre alone will be observed in these cases).

A negative result is usually enough to conclude that clinical signs are not due to this disease. However, there are some cases where the blood sample is taken very early on in the course of disease and the animal has not had time to mount an immune response, or if the animal is severely immunosuppressed and is unable to make the necessary antibodies.

A PCR test is also available, and is run on faeces or urine, to look for presence of the parasite in the sample. A positive result is conclusive for infection in that animal. However, a negative result may be seen if the parasite isn’t being shed at the time of sampling, which can occur as it tends to be excreted intermittently. Collecting a pooled sample over 3-5 days can increase the chances of the organism being excreted and the test returning positive.

E cuniculi can be transmitted via either a horizontal route, most commonly through oral ingestion; however, vertical transmission through the placenta is also known to occur, with infections during the first trimester leading to spores being identified within the lens, which can cause either cataracts or uveitis in these individuals.

Treatment can be given if there is a high suspicion of E cuniculi, if an animal has high antibody titres or is PCR positive. A 28-day course of fenbendazole (panacur) at 20mg/kg once daily by mouth is the current treatment of choice. Fenbendazole will reduce spore formation and slow the rate of multiplication of E cuniculi within the body. Anti-inflammatories may also be prescribed, and possibly antibiotics if there is a risk, or suspicion, of secondary bacterial infection.

Other treatments are under trial and may be available in the future depending on their success rate.

Curing the disease is something that we’re not sure is possible, although there has been a study that showed that a 28-day course of panacur did eliminate spores from brain tissue. However, many people believe that, even following treatment, an animal is likely to be a carrier of disease for the rest of its life. It has been shown, however, that prophylactic administration with fenbendazole will prevent  infection experimentally. In a non-laboratory situation however, we are unaware of when the spores may be around as they are microscopic, so this is unlikely to be possible with our pet rabbits.

Treatment can be effective, however, there are cases that may be refractory to medications. This may be due to high numbers of the parasite being present, or as a result of chronic inflammatory changes that have occurred prior to clinical signs being evident. In these cases, eradication of the parasites may not change the clinical presentation of disease and improve the rabbit’s quality of life because the chronic inflammatory changes are too severe.

Treatment can improve clinical signs to a point where no further medication is ever needed, however, some cases may require ongoing, or intermittent, medical intervention to maintain their quality of life and reduce clinical signs. There are also cases where medical treatment is not successful, and quality of life is severely affected. In these cases, it may be that euthanasia is the most humane form of treatment.

Flare ups of disease is possible, and, although it is not known what causes these further episodes, it may be that stress, or a weakened immune system can play a part. Re-infection is also possible as the antibodies that are produced do not protect against further infection.

There is no vaccination against E cuniculi.

Re-infection of EC positive rabbits is common. Treatment with fenbendazole kills only the active parasites, with dormant organisms unable to be eliminated with current treatment options. Reoccurrence of disease is, therefore, relatively common, with the parasites being excreted intermittently at times both when clinical signs of disease are present, and in subclinical cases, where no overt signs of illness are apparent.

Re-infection can occur at any time when the parasites are excreted in the urine or faeces, whether that is in the initial post-treatment period or weeks, months or years later. As the most common route of transmission is via oral ingestion, keeping food off the floor where urine and faeces is passed is of utmost importance. Having a hayrack for any hay that is given for consumption rather than bedding, and keeping the feeding, sleeping and toileting areas completely separate are all useful ways to help prevent re-infection. Regular cleaning and disinfection are also important as the spores are environmentally resistant, however they are easily killed with only 30 seconds contact with a 1-10% bleach solution. Other disinfectants are also effective. Keeping a closed colony once E cuniculi has been diagnosed in a rabbit will help to prevent infection of naïve animals. Do not share hutches, runs and cage furniture between groups of animals. Prevention of contact with wildlife will minimise the risk of spread from wild rodents and rabbits to pet animals.

Although completely preventing your pet rabbit from contracting E cuniculi is not possible, as it is present in the environment and wild animals, using these measures to help minimise the risk of exposure will reduce the chances of transmission and subsequent disease.

E cuniculi infection has been observed in humans, however, there are no reported cases to date, of this disease being spread directly from a pet rabbit to its owner. However, as with most diseases, minimising the risk of spread is important, by using everyday precautions and personal hygiene when handling your rabbit, e.g. washing hands following handling, not eating or drinking whilst handling your rabbit, etc are important. Limiting contact with, and exercising strict hygiene, with immunosuppressed individuals is important if E cuniculi has been diagnosed or is suspected in your rabbit. Seeking medical advice if an in-contact person becomes ill following contact with a EC positive rabbit is of utmost importance.

The answer to this is that there isn’t one simple answer.

E cuniculi positive rabbits can live perfectly happy and normal lives, some without any clinical signs ever being observed.

Some individuals can live a near normal life, having recovered from an episode of E cuniculi, with no residual signs, or minor changes, e.g. a mild head tilt, that the animal learns to adapt to living with. Occasional medical intervention may be necessary in these cases if flare ups occur. The quicker the intervention, usually the recovery is quicker and more complete.

However, for some rabbits the clinical signs and chronic inflammatory changes are too severe to either recover from with medical treatment, or live with, and experience a good quality of life. In severe cases, euthanasia can sometimes be the kindest option for that rabbit.