E. cuniculi Series

Encephalitozoon cuniculi series by Megan Schommer, DVM.

Dr. Schommer shares high-quality, evidence-based information about exotic companion mammals on her personal blog page on Faceboook. Below is her four part series on E. cuniculi (EC), reposted with her direct permission.

Basic Info
Let's start with a few definitions and basic information about the disease so that we're all on the same page.

E. cuniculi (EC) is an obligate intracellular parasite from the class Microsporidia, which for the moment is considered a form of fungus. EC can infect all mammalian species, including humans, although rabbits, guinea pigs, and mice are considered the primary reservoirs. EC doesn't typically cause disease in humans unless they are immunocompromised, but this is important to keep in mind since that means infected rabbits who are shedding the organism have zoonotic potential to immunocompromised/immunosuppressed owners.

Animals become exposed to EC via vertical transmission (being infected in utero) or via ingestion or inhalation of spores that are excreted in the urine of an infected animal. Upon exposure, the infectious spores first infect organs with high blood flow (kidneys, liver, and lungs), and later move to the nervous system. As the organisms multiply within thick-walled spores, the spores eventually rupture, causing an inflammatory response in the surrounding tissues. It is suspected that the inflammatory response is more responsible for the signs of disease rather than the organism itself. The most common site affected by disease are the ocular, renal, and neurologic tissues. Infected rabbits shed spores in their urine beginning about 1 month after infection, and shed spores in high quantities for about 2.5 months. Rabbits can continue to shed intermittent, low quantities of spores as long as they remain infected. Eventually, the rabbit's body is able to wall off these organisms and the inflammatory response subsides even though the organisms are still present within the tissues (Quesenberry et al. calls this a "balanced host/parasite relationship"). The EC organisms continue to remain viable. Stress, disease, or anything that causes immunosuppression can disrupt this balance and allow the parasite to suddenly start causing signs of disease even if it was many years after the initial infection.

Based on serology (evidence of antibodies against EC), healthy rabbit populations can have upwards of 75% of individuals show exposure to EC. EC organisms are ubiquitous (found in the environment, soil, etc), and so exposure is very common. This does not mean that 75% of rabbits will develop disease associated with their exposure to the organism. I'll get more specific about serologic testing and the challenges associated with diagnosing EC infections in rabbits in a later post.

Resources used:


 * Carolyn Cray, Madisen P. Liebl, Kristopher Arheart, Roman Peschke, Frank Künzel, Anja Joachim, Comparison of enzyme-linked immunosorbent assay and immunofluorescence test for determination of anti-Encephalitozoon cuniculi antibodies in sera from rabbits with different clinical and histopathological presentations, Journal of Exotic Pet Medicine, Volume 32, 2020, Pages 39-42, ISSN 1557-5063, https://doi.org/10.1053/j.jepm.2019.11.003.


 * Peter G. Fisher, Frank Künzel, Helena Rylander, 18 - Neurologic and Musculoskeletal Diseases, Editor(s): Katherine E. Quesenberry, Connie J. Orcutt, Christoph Mans, James W. Carpenter, Ferrets, Rabbits, and Rodents (Fourth Edition), W.B. Saunders, 2020, Pages 233-249, ISBN 9780323484350, https://doi.org/10.1016/B978-0-323-48435-0.00018-6.

Clinical Signs
This post is the second in my series on E. cuniculi, and today we'll be reviewing the clinical signs associated with E. cuniculi infection. It's important to remember that not all rabbits who are exposed to E. cuniculi become infected, and not all infected rabbits will develop symptoms.

There are three main organ systems that are targeted by E. cuniculi: the eyes (specifically the lens of the eye), the kidneys, and the brain. We'll take these one at a time.

Ocular manifestations: Rabbits who develop eye disease due to E. cuniculi were exposed in utero. Cataracts can develop in one or both lenses, as well as leakage from or rupture of the lens. "Phacoclastic uveitis" is the most severe end stage of ocular E. cuniculi, in which the lens ruptures and causes severe inflammation within the eye. Rabbits who experience this can develop pain, glaucoma, and blindness unless the lens material is removed through a specialty procedure called phacoemulsification. The procedure needs to be performed early on in the course of uveitis to have the best chance of restoring comfort and keeping the eye visual. Without quick treatment, affected eyes become blind and surgical removal should be considered as these eyes are at risk of developing glaucoma. Rabbits with ocular lesions of E. cuniculi should be diligently monitored for the other types of diseases associated with E. cuniculi infection since the organism is often present throughout the body.

Kidneys: E. cuniculi in the kidneys causes an inflammatory nephritis, followed by fibrosis. In some animal, this doesn't cause enough damage to have any long-term effects, but if enough damage occurs, then the kidneys lose their ability to concentrate urine and sometimes their ability to filter toxins. The most common clinical signs of renal disease in rabbits include polyuria/polydipsia (oftentimes with secondary urine scald from urinary incontinence), weight loss, and general signs of malaise (unkempt haircoat, poor muscling, lethargy).

Brain: Signs of central nervous system dysfunction are the most well-known manifestation of E. cuniculi. Common clinical signs reported to be associated with E. cuniculi infection include vestibular disease (head tilt, torticollis, nystagmus), seizures, and hindlimb paralysis or paresis (although there is at least some debate about how commonly paralysis/paresis are due to E. cuniculi- Quesenberry states that one of the authors (Dr. Frank Kunzel) "believes that these neurologic signs are often falsely attributed to the organism"). Episodes of vestibular disease often follow stressful events or develop secondary to some other systemic illness, which supports the theory that immunosuppression allows E. cuniculi to come out of dormancy and start reproducing, causing inflammation which induces clinical signs. Important differentials for head tilt in rabbits include otitis interna, neoplasia, and dental disease. It's easy to see a head tilt and immediately blame E. cuniculi, but of course we see head tilts develop in many other species that don't have E. cuniculi infections, so don't forget to include other causes on your differential list. Quesenberry et al. suggests that unilateral facial dysfunction or spasm are more likely to be seen with otitis interna than E. cuniculi.

A recent study of three large rabbit farms in Egypt found that out of 16,400 rabbits, a whopping 13,242 rabbits had clinical signs suggestive of E. cuniculi (including head tilt/vestibular signs, cataracts, phacoclastic uveitis, and signs of PU/PD such as urine scald). 81.5% of these rabbits had neurologic signs, 10.5% had ocular lesions (cataracts, phacoclastic uveitis), and 2.7% had signs of pulyuria. I'll admit that this study is complex and I don't quite understand how many of the rabbits were confirmed to have E. cuniculi on necropsy, but if anyone wants to dig into the details, full text is available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7036226/.

Resources used:


 * Peter G. Fisher, Frank Künzel, Helena Rylander, 18 - Neurologic and Musculoskeletal Diseases, Editor(s): Katherine E. Quesenberry, Connie J. Orcutt, Christoph Mans, James W. Carpenter, Ferrets, Rabbits, and Rodents (Fourth Edition), W.B. Saunders, 2020, Pages 233-249, ISBN 9780323484350, https://doi.org/10.1016/B978-0-323-48435-0.00018-6.


 * Anna L. Meredith, Jenna Richardson, Neurological Diseases of Rabbits and Rodents, Journal of Exotic Pet Medicine, Volume 24, Issue 1, 2015, Pages 21-33, ISSN 1557-5063, https://doi.org/10.1053/j.jepm.2014.12.007.


 * Morsy EA, Salem HM, Khattab MS, Hamza DA, Abuowarda MM. Encephalitozoon cuniculi infection in farmed rabbits in Egypt. Acta Vet Scand. 2020;62(1):11. Published 2020 Feb 22. doi:10.1186/s13028-020-0509-6.https://dx.doi.org/10.1186%2Fs13028-020-0509-6

Diagnosis
So, you think your rabbit patient might have E. cuniculi. She's developed an acute head tilt, or has phacoclastic uveitis, or has primary renal azotemia. You'd like to do some testing to see if E. cuniculi might be the cause. What's the right diagnostic test to run? This is a bit of a loaded question because, unfortunately, the majority of the time we cannot make a definitive diagnosis. Rabbits can be EXPOSED to E. cuniculi, but not infected. Rabbits can be INFECTED with E. cuniculi, but not develop disease. And rabbits can have syndromes often associated with E. cuniculi, like those listed above, and have some other disease process occurring (even if they also happen to be infected by E. cuniculi).

The only way to definitively say that a rabbit has been infected by E. cuniculi is to find the organism within the tissues on a biopsy. This is rarely something that we manage to do on a live rabbit, since it would require collecting tissue from the eye, brain, or kidney (although, if a rabbit needs an eye removed due to phacoclastic uveitis, that's a great opportunity to submit tissues for biopsy). Early on in the course of disease, E. cuniculi spores are shed in the urine, but this only happens intermittently, and the shedding period can occur long before a rabbit shows any signs of disease (the lab at UF-Miami tried examining urine samples alongside blood samples to see if screening for urine spores was helpful, but unfortunately urine testing didn't provide any useful information).

So, what we are left with is an imperfect antemortem tool called titer testing. Titer testing involves collecting a blood sample and measuring the numbers of antibodies created against the E. cuniculi organism. Different types of antibodies respond to infection at different times. IgM antibodies tend to rise earlier on in the course of infection, and IgG antibodies tend to develop later. A paper by Cray et al (2020) references a study of experimentally-infected rabbits that developed IgM titers about a week post-infection and showed that they maintained a detectable IgM titer at 18 weeks post-infection; a second study referenced showed that IgM titers started to decline around 35 days post-infection. This is a clue that rabbits with high or rising paired IgM titers most likely have an acute infection or reactivation of a chronic infection.

The University of Miami is generally considered to be the go-to lab for sending E. cuniculi labwork, as Dr. Carolyn Cray in the Avian and Exotics Lab has special interest in researching E. cuniculi. Some labs will only provide you with a "positive" or "negative" result, but this information is less helpful than getting a quantitative titer, which tells you not just that a rabbit has made antibodies but also how many. There is some nuance involved in interpreting what a low titer means vs a high titer (i.e. having a high titer doesn't necessarily mean a rabbit has more severe disease), but having titer levels above a certain threshold is more likely to indicate active disease rather than historic exposure. Currently, the University of Miami recommends checking an IgM and IgG titer in addition to a level called C reactive protein (CRP), which is a protein that becomes quickly elevated in the face of inflammation. This is the most reliable panel of tests that we currently have for evaluating a rabbit for E. cuniculi infection. Abnormal results in all three of these values are quite indicative of an active E. cuniculi infection.

The major diagnostic challenge comes when only one or two of these three values are abnormal. Many, many clinically normal rabbits have positive titers and never show signs of disease (upwards of 50% of healthy rabbit populations can have positive titers). The most helpful result is finding a negative titer, as that is a pretty reliable indicator that you can cross E. cuniculi off your list of possible causes. The other helpful result is a high IgM titer, high IgG titer, and elevated CRP in a patient showing signs consistent with E. cuniculi infection- that all information helps you know that proceeding with treatment for E. cuniculi is the best next step. In patients who have suspicious clinical signs, but have a low IgM or IgG titer, or have a high IgG/low IgM, or low IgM/high IgG, high titers but low CRP, etc etc... all becomes much more confusing to interpret.

If you've run your E. cuniculi panel on a rabbit patient showing suspect signs, and you've also done good baseline labs to screen for concurrent issues that might be causing disease, you might find yourself in this conundrum where you still don't have a clear answer. Unfortunately, this is the nature of the disease at the moment. Dr. Cray is a wonderful resource and may be able to give you guidance about how to interpret your panel. And if you have an interesting E. cuniculi case to share, she is constantly researching this disease and may be interested in hearing about your experience. You can find her contact information, plus a great breakdown of interpretation of titers, at http://cpl.med.miami.edu/avian-and-wildlife/ecun/. I hope that this is helpful, although it is probably clear as mud. Sorry, all. Next up: treatment options.

Resources used:


 * Carolyn Cray, Madisen P. Liebl, Kristopher Arheart, Roman Peschke, Frank Künzel, Anja Joachim, Comparison of enzyme-linked immunosorbent assay and immunofluorescence test for determination of anti-Encephalitozoon cuniculi antibodies in sera from rabbits with different clinical and histopathological presentations, Journal of Exotic Pet Medicine, Volume 32, 2020, Pages 39-42, ISSN 1557-5063, https://doi.org/10.1053/j.jepm.2019.11.003. (http://www.sciencedirect.com/.../pii/S1557506319301892)


 * Peter G. Fisher, Frank Künzel, Helena Rylander, 18 - Neurologic and Musculoskeletal Diseases, Editor(s): Katherine E. Quesenberry, Connie J. Orcutt, Christoph Mans, James W. Carpenter, Ferrets, Rabbits, and Rodents (Fourth Edition), W.B. Saunders, 2020, Pages 233-249, ISBN 9780323484350, https://doi.org/10.1016/B978-0-323-48435-0.00018-6.


 * F.M Harcourt-Brown, Encephalitozoon cuniculi infection in rabbits, Seminars in Avian and Exotic Pet Medicine, Volume 13, Issue 2, 2004, Pages 86-93, ISSN 1055-937X, https://doi.org/10.1053/j.saep.2004.01.004. (http://www.sciencedirect.com/.../pii/S1055937X04000064)