March 2017



Signalment (JPC #1902063):  Young, female New Zealand white rabbit


HISTORY:  This rabbit developed diarrhea and CNS signs.


HISTOPATHOLOGIC DESCRIPTION:  Slide A:  Cerebrum, level of hippocampus and thalamus:  Scattered throughout the gray and white matter are random, occasionally perivascular, variably sized aggregates of moderate numbers of epithelioid macrophages, activated microglia (rod cells), fewer lymphocytes, plasma cells, and rare heterophils admixed with small amounts of pyknotic and karyorrhectic cellular debris (necrosis).  Subjacent to the aggregates, there is mild gliosis.  Virchow-Robin space and the leptomeninges are expanded by a similar inflammatory infiltrate.  Vessels are lined by reactive endothelium and there is a small amount of hemorrhage within the leptomeninges.


Slide B: Brown and Brenn"s Gram stain: Cerebrum, level of hippocampus and thalamus: Multifocally there are moderate numbers of intracellular and rarely extracellular gram positive 1 x 3 um rod-shaped microsporidia within areas of inflammation and occasionally in areas with no cellular reaction (pseudocyst).  


MORPHOLOGIC DIAGNOSIS:  Cerebrum:  Meningoencephalitis, lymphohistiocytic, multifocal and perivascular, moderate, with gram-positive microsporidian spores, New Zealand white rabbit (Oryctolagus cuniculus), lagomorph.


ETIOLOGIC DIAGNOSIS:  Cerebral encephalitozoonosis


CAUSE:  Encephalitozoon cuniculi


ETIOLOGY SYNONYM:  Originally known as Nosema cuniculi



·         An obligate intracellular microsporidian eukaryotic organism that commonly results in latent infection of laboratory rabbits; recently reclassified as a fungus within the phylum microsporidia

·         Reported to infect a wide range of hosts, including rodents, carnivores (especially farmed foxes), and NHPs

·         Immunosuppression may lead to clinical disease with neurologic signs and histologic lesions primarily in the brain and kidney

·         Large domestic rabbits often have subclinical infections with incidental renal lesions; Dwarf rabbits are especially susceptible and often have clinical signs including torticollis, uveitis with cataractous change, and disseminated disease

·        Currently there are three known strains of E. cuniculi:  Genotype I (‘rabbit strain’); genotype II (‘mouse strain’); genotype III (‘dog strain’)



·         Infection is acquired through ingestion or inhalation, transplacentally, or through iatrogenic transmission via contaminated needles; zoonotic

·         Carnivores can be infected by consuming infected rabbit carcasses

·         Ingested spores release the sporoplasm, which invades endothelial cells via extrusion of the polar filament which penetrates the host cell membrane

·         Initial target organs are those of high blood flow, especially lung, liver, and kidney with lesions evident in the brain at three months



·         Obligate intracellular organism with a simple and direct life cycle

·         Spores are shed in urine from infected renal epithelial cells and complete the life

cycle in 3‑5 days

·         Spores (infective stage) enter host via digestive tract > sporoplasm is released through polar filament and enters susceptible cells (predominantly endothelial cells) via mechanical forces exerted by the extruded polar filament or due to an active migratory process by the sporoplasm > asexual replication occurs to form a parasitophorous vacuole > sporoblasts develop into mature spores until the cell ruptures > release of spores into extracellular spaces > spores infect adjacent cells, enter the vascular system, or the renal tubular lumen, may also pass in infected feces



·         Infections are usually subclinical but rabbits may present with a variety of neurologic signs, including head, tilt, ataxia, vestibular signs (circling, nystagmus, rolling), and occasionally behavioral changes

·         Uveitis and cataracts from lens capsule penetration may be present in young rabbits

·         Rarely, placentitis and abortion in a wide range of animals, including horses has been reported



·         Usually absent in the CNS

·         Renal lesions – focal, irregular, depressed areas (“pitted appearance”); on cut section, there are indistinct linear, pale, gray-white areas

·         Focal hepatic necrosis and focal myocardial necrosis occasionally present



·         Granulomatous and nonsuppurative inflammation in target organs; tissue changes are most prominent in kidney and brain but organism selectively parasitizes vascular endothelium à segmental vasculitis

·         CNS:  Lesions do not normally occur until 30 days postexposure and most frequently arise in cerebrum; widespread nonsuppurative meningoencephalomyelitis, astrogliosis, and perivascular lymphocytic infiltration; organisms within astroglial cells, macrophages, or extracellularly within the granulomatous inflammation

·         Kidney:  Focal to segmental granulomatous interstitial nephritis; degeneration of affected epithelial cells at all levels of the renal tubule; minimal involvement of the glomeruli; organisms within renal tubular epithelium or free within collecting tubules

·         Eye: Phacoclastic uveitis and cataract formation which is believed to follow transplacental infection is common in dwarf rabbits; keratitis, rupture of lens capsule, heterophilic and granulomatous inflammation

·         Liver:  Focal granulomatous inflammatory response, periportal lymphocytic


·         Heart:  Multifocal myocardial necrosis and nonsuppurative inflammation,

vasculitis; organisms in endothelial cells and cardiomyocytes

·         Placenta: Necrotizing placentitis and trophoblasts containing large numbers of intracytoplasmic vacuoles filled with organisms

·         Lung and spleen:  Organisms may be seen in endothelial cells with associated


·         Organisms are small (2.5 x 1.5 um), rod shaped and can be difficult to identify in chronic infections; in acute active infections, organisms can be found free in a focus of inflammation; contained within a parasitophorous vacuole in endothelial cells, epithelial cells, macrophages, or intracellularly with no associated inflammatory response (pseudocyst)



·         Organism is often within a parasitophorous vacuole

·         Thick outer coat enclosing a coiled polar filament (polar tube) and one or two nuclei; one end of the filament is attached to the spore wall and the other end is coiled within the spore from 4 to over 30 times depending on the species

·         Corrugated, proteinaceous electron dense exospore (spore wall); chitinous

radiolucent endospore; anchoring disc at anterior pole; electron lucent posterior vacuole



·         Spores are acid fast, gram positive and contain a PAS positive polar granule

·         Carbol fuchsin (an acid fast stain) will stain the organisms a distinct purple color

·         Modified trichrome stain with light microscopy and calcofluor white stain with ultraviolet light microscopy both recently described as effective in identifying organism

·         Serology, carbon immunoassay test, indirect IFA, dot ELISA, Western blot

·         Spores are excreted intermittently in the urine during acute disease



·         Primary differential is Toxoplasma gondii:

     TOXOPLASMA                                                   ENCEPHALITOZOON

     Cyst small (60 um)                                              Pseudocyst large (up to120 um)

     Spores not acid fast                                            Spores are acid fast        

     Gram negative                                                     Gram positive

     Do not stain with carbol fuchsin                      Stain with carbol fuchsin

     Giemsa:  Cytoplasm is granulated                   Giemsa:  Cytoplasm is light blue

     Stains well with H&E                                         Stains poorly with H&E

     Larger organism 2‑6 um                                    Smaller organism 1.5 x 2.5 um

     Tend to invoke necrosis                                                Necrosis not a common finding                           

·         Neospora caninum:  Difficult to differentiate from Toxoplasma sp.

·         Differentials of rabbits with neurologic signs: Otitis interna, toxoplasmosis, and Baylisascaris



·         Recently identified Encephalitozoon species of pathogenic importance include E.

intestinalis, E. hellem, E. bieneus, andE. septatai

·         E. cuniculi has been documented in rabbits, mice, rats, muskrats, guinea pigs, hamsters, ground shrews, goats, sheep, pigs, horses, domestic dogs, wild and captive foxes, domestic cats, a variety of carnivores, and nonhuman primates

·         Dogs and blue foxes:  Disease is most severe in young animals and can result in death; often due to in utero infection; blue fox are one of the most susceptible species

·         Cats:  Symptoms are variable and can include muscle spasms, depression, paralysis and death 

·         NHPs: limited to New World monkeys, mostly documented in squirrel monkeys; clinical disease is usually absent or non-specific; no specific gross lesions have been reported. 

·         Granulomatous inflammation and organisms reported in brain, kidney, lungs, liver, and placenta; vasculitis/perivasculitis is the predominant finding

·         Primary lesions in Goeldi’s monkey is arteritis and aortitis

·         Transplacental transmission documented

·         Horses:  Rare cause of necrotizing placentitis and abortion; organisms within chorionic epithelial cells

·         Recently reported in South American fur seal pups and reptiles



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7.     Rodriguez-Tovar LE, et al. Histochemical study of Encephalitozoon cuniculi spores in the kidneys of naturally infected New Zealand rabbits. J Vet Diagn Invest. 2017 Feb 1:1040638716668559. doi: 10.1177/1040638716668559 (Epub ahead of print).

8.     Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:418.

9.     Seguel M, et al. Encephatitozoonosis in 2 South American fur seal (Arctocephalus australis) pups. Vet Pathol. 2015;52(4):720-723.

10.  Simmons J, Gibson S. Bacterial and mycotic diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardiff S, Morris T, ed. Nonhuman Primates in Biomedical Research: Diseases. Vol 2. 2nd ed. London: Academic Press; 2012:149-150.

11.  Snowden KF, Lewis BC, Hoffman J, Mansell J. Encephalitozoon cuniculi infections in dogs: a case series. J Am Anim Hosp Assoc. 2009;45(5):225-231.

12.  Summers BA, Cummings JF, De Lahunta A. Veterinary Neuropathology. St. Louis, MO: Mosby-Year Book, Inc; 1995:373-375.

13.  Wasson K, Peper RL. Mammalian microsporidosis. Vet Pathol. 2000;37(2):113-128.

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