Encephalitozoonosis in Rabbits

This disease is caused by a microsporidian protozoan parasite, Encephalitozoon cuniculi. It is an intracellular parasite that preferentially colonises the kidneys, eyes and brain. Rabbits are infected by ingestion or inhalation of spores excreted via faeces and urine of infected animals. In some cases transplacental transmission has been recorded

What clinical signs should I look for?

As with many diseases the signs can be non-specific. However, the following have been noted in rabbits:

• Nervous signs. Head tilt, torticollis (neck twisting) and weakness/ paralysis. Behavioural changes may be noted. Eventually fits and coma may result.
• Eye disease. Infection of the lens may occur. Eventually this will burst causing a massive inflammatory reaction.
• Kidney disease. Increased drinking and urination - this may be a cause of urine scalding in rabbits. The kidneys become shrunken and pitted – these changes may even be visible on x-rays.
• Heart disease. Myocarditis (inflammation of heart muscle) may be a cause of sudden or anaesthetic-related death.
• Lung disease. This is rare but pneumonia has been reported.

It is also seen as a problem in poorly run-down rabbits with abscesses and/or dental disease. Whether it is an underlying problem weakening the rabbit or whether it surfaces because the rabbit is run-down is hard to prove.

How can a diagnosis of Encephalitozoonosis be made?

Clinical signs may make one suspicious but are not sufficient in themselves - many other diseases can cause the same signs, and these will often need to be ruled-out (e.g. middle ear infection, spinal disease, etc.).

The only definitive method to diagnose is histopathology – the visualisation of microscopic parasites and accompanying reaction in the tissues. Given the main tissues infected this generally means that encephalitozoonosis is a post-mortem diagnosis.

"The only definitive method to diagnose is histopathology – the visualisation of microscopic parasites and accompanying reaction in the tissues."

However, serology may be useful. This is the measurement of antibodies against the organism in the blood.

Simply finding antibodies is not enough – many healthy rabbits will have antibodies and no disease. This is true irrespective of the level of antibody seen. Repeated samples showing changes in level of antibody do not indicate exposure – these can alter normally over time!

The finding of no antibodies in a sick rabbit effectively rules-out encephalitozoonosis.

The findings of antibodies in a sick rabbit indicates encephalitozoonosis is a possibility. The findings of antibodies in a healthy rabbit is of no significance UNLESS you are screening rabbits prior to entering a disease-free colony.

Newer tests using PCR technology to find spores in the urine are accurate. Unfortunately, most rabbits will not be shedding spores at the time of clinical signs and therefore will test negative.

Are any drug therapies effective?

Few therapies are totally successful. In the acute case, tetracyclines have been shown to limit (though not remove) the parasite. Short-acting corticosteroids may give symptomatic relief but will compromise immunity.

Fenbendazole (at 20mg/ kg once daily for 28 days – see Pharmacy Series) has been shown to eradicate infection in some cases. Albendazole may also be effective but can be toxic to the rabbit at the suggested doses!

How can I prevent my rabbit developing Encephalitozoonosis?

Many rabbits carry infection or have encountered and removed the parasite. At the moment it is impossible to distinguish these.

If maintaining an E. cuniculi-free group it is important to screen all new rabbits entering the group. This should involve strict quarantine for at least 12 weeks and paired serology (at least 6 weeks apart) showing negative antibody status.

Rabbits leaving such a group should be only allowed to enter other E.cuniculi-free groups or they will be vulnerable to infection.

Recently in the UK fenbendazole has been licensed for use at 20mg/kg sid for 9 days to be used 3 times a year as a preventive measure.

This is based on published therapeutic success of the drug and anticipates no protozoal contamination of the environment.

It is therefore best targeted round likely risk periods (e.g. stays with rabbit sitters, etc) and where there is likely to be environmental contamination, 28 day courses should be used.

Valuable though drug prophylaxis is it will never directly replace more basic biosecurity and quarantine procedures.