Laboratory Diagnosis of E cuniculi Disease
E cuniculi is a very difficult disease to definitively diagnose ante mortem, and even at post mortem the histopathological results may suggest E cuniculi, but unless organisms are found on histopathology this is still a ‘consistent with…’ diagnosis. Research is still ongoing to answer many of the questions posed by E cuniculi in the pet rabbit population with natural infections/disease in contrast to laboratory rabbits where much of the work has been carried out.
Much is made of the use of titres to E cuniculi as opposed to a positive/negative result, but some of the points below are still being investigated:
- Accuracy and precision of the titres produced by the individual laboratories
- Methodology of the test producing the titre
- Correlation of the reported titres to disease state – titres indicate exposure, not disease
- Titres produced in each individual rabbit are not of the same level and are not produced at the same time after infection (not linear)? So how can you interpret titres?
- Will titres change with treatment – evidence suggests generally no
Recent studies in the UK published in the Veterinary Record in 2003/2006 and elsewhere have shown a high seroprevalence of E cuniculi and this is after routine screening of ‘healthy’ animals. So a titre does not indicate disease only exposure. Rising titres would be expected during seroconversion, but this alone will not differentiate exposure, and an appropriate immune response, from clinical disease. It is suggested that after exposure and infection a silent chronic infection may develop which will be activated after a period of immunosuppression, concurrent infection or stress. Will titres increase again at this time? Are the organisms present in the blood stream at this time?
Infection does not necessarily produce disease.
Image Many of the signs of E cuniculi relate to the organs infected and the response to infection and any secondary infections also present. These signs are not specific to E cuniculi and other differentials must also be taken into consideration.
The parasites are ingested and move via the blood stream into the kidneys and other organs, particularly liver, brain and spinal cord. The parasites replicate in the kidneys and are shed intermittently in the urine. This infectious period lasts for several weeks. Organisms take some time to migrate into the brain tissue probably due to the blood brain barrier, and kidney infection may well be resolved at this point leaving only the classical pitting and scarring lesions. Other organs react by producing microgranulomas, but the organisms are often not present in these granulomas. These lesions may be the host response to infection and indicate a chronic response with the organism no longer present.
PCR tests are available to detect the organism in the urine when it is being shed. Previous studies on laboratory experimentally infected rabbits showed intermittent excretion of E cuniculi in the urine.
However, when clinical signs are present due to CNS disease any shedding and parasitaemia may not be present, so a negative result will not rule out E cuniculi associated disease. A German thesis showed that the use of PCR for detection of E cuniculi in CSF in rabbits with clinical signs was of no help in the diagnosis of disease. PCR tests may be useful to determine the incidence of infection in a rabbit population where you are trying to remove infection, with new animals being isolated and repeatedly tested (note intermittent excretion). However antibody levels are said to increase before the organism is excreted in the urine, so routine screening with antibody levels would be an easier procedure than repeatedly collecting and testing rabbit urine.