JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

January 2023

N-F05

 

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 and occasionally perivascular variably sized aggregates 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). Rarely, within these foci are 30-40µm diameter intracellular sporophorous vacuoles that contain myriad 1x3µm, ovoid, refractile microsporidian spores. Adjacent to the aggregates there is mild gliosis. Virchow-Robin space and the leptomeninges are expanded by a similar inflammatory infiltrate and there is a small amount of hemorrhage within the leptomeninges. Vessels are lined by reactive endothelium.

 

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 µm, rod-shaped microsporidia within areas of inflammation and occasionally in areas with no cellular reaction (pseudocyst).  

 

MORPHOLOGIC DIAGNOSIS: Cerebrum and diencephalon: 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

 

GENERAL DISCUSSION: 

• An obligate intracellular microsporidian eukaryotic organism that commonly results in latent infection of laboratory rabbits; immunosuppression may lead to clinical disease with neurologic signs and histologic lesions primarily in the brain, eye, and kidney; segmental vasculitis responsible for lesions in many tissues
• Currently classified as a fungus within the phylum microsporidia
• 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

 

PATHOGENESIS:  

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

 

LIFE CYCLE:

• 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

 

TYPICAL CLINICAL FINDINGS:

• Infections usually subclinical; may present with a variety of neurologic signs including head tilt, vestibular signs (circling, nystagmus, rolling), ataxia, 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
• Recent report of intraocular chondrosarcoma with serological evidence of E. cuniculi infection, which may have been a predisposing factor to development of this neoplasm (J Comp Path 2020)

 

TYPICAL GROSS FINDINGS: 

• Usually absent in the CNS
• Eyes- Severe endophthalmitis and cataracts
• Renal lesions (U-P01) – 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

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• 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, astrocytosis, and perivascular lymphocytic infiltration; microsporidia within astroglial cells, macrophages, or extracellular within the granulomatous inflammation; meningeal vascular lesions resemble polyarteritis nodosa, become dominated by sclerotic changes
• Kidney (U-P01):  Focal to segmental nonsuppurative (heavy, almost pure interstitial infiltrates of plasma cells) to granulomatous interstitial nephritis; degeneration of affected epithelial cells at all levels of the renal tubule; fibrinoid necrosis affects some glomeruli; microsporidia within renal tubular epithelium, free within collecting tubules, or in vessel walls
• 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, periarteritis within the uvea and retina
• Liver:   Focal granulomatous inflammation, periportal lymphocytic infiltrates
• Heart:   Multifocal myocardial necrosis and nonsuppurative inflammation, 

vasculitis; microsporidia in endothelial cells and cardiomyocytes

• Placenta: Necrotizing placentitis, trophoblasts contain large numbers of intracytoplasmic vacuoles filled with microsporidia
• Lung and spleen:  Microsporidia may be seen in endothelial cells, hepatocytes, or Kupffer with associatednon-suppurative portal inflammation and hepatic necrosis
• Organisms are small (2.5 x 1.5 µm), rod shaped and can be difficult to identify in chronic infections; in acute active infections, microsporidia can be found free in a focus of inflammation; contained within a sporophorous vacuole in endothelial cells, epithelial cells, macrophages, or intracellularly with no associated inflammatory response (pseudocyst)

 

ULTRASTRUCTURAL FINDINGS:

• Organism is often within a sporophorous vacuole (previously termed “parasitophorous vacuole” until reclassified as a fungus)
• 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

 

ADDITIONAL DIAGNOSTIC TESTS:

• 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
• Can be highlighted with modified trichrome and calcofluor white M2R
• IHC
• Serology, carbon immunoassay test, indirect IFA, dot ELISA, Western blot
• Spores are excreted intermittently in the urine during acute disease

 

DIFFERENTIAL DIAGNOSIS: 

     TOXOPLASMA                                                   ENCEPHALITOZOON

     Cyst small (60um or less)                                 Pseudocyst large (up to 120um)

     Spores not acid fast                                           Spores are acid fast         

     Gram negative, PAS-                                         Gram positive, PAS+

     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, crescent-shaped 2‑6 um                     Smaller, rods 1.5 x 2.5 um

     Tend to invoke necrosis                                    Necrosis not a common finding                            

• Neospora caninum:  Difficult to differentiate from Toxoplasma sp.

• Clinical differentials, neurologic signs: Otitis interna, toxoplasmosis, Baylisascaris sp.

 

COMPARATIVE PATHOLOGY: 

intestinalis, E. hellem, E. bieneusi, and E. septatai

• E. cuniculi has been reported in a wide range of hosts including mice, rats, muskrats, guinea pigs, hamsters, ground shrews, black-footed ferrets, goats, sheep, pigs, horses, domestic dogs, wild and captive foxes, domestic cats, other carnivores, nonhuman primates,  fur seal pups and reptiles
• Dogs, farmed foxes, mink:  Subclinical/asymptomatic infection occurs with an unknown prevalence; serious mortalities occur in farmed foxes;   Disease is most severe in young animals and can result in death; often due to in utero infectionCats:  Few reports of E. cuniculi causing lenticular disease in cats 
• Bearded dragons:  Recent report of intrapericardial E. pogonae-associated arteritis with fatal hemopericardium (Jour Vet Diagn Invest 2019)
• Birds:  

• E. hellem; reported in a wide range of companion birds, but commonly infects budgerigars and lovebirds; most infections self-limiting and inapparent, but can result in clinical disease with immunosuppression or poor husbandry

• Pigeons and doves (Columbids) appear to be carriers
• Psittacine Beak and Feather Disease (I-V04) common associated infection in lovebirds 
• Budgerigar Fledgeling Disease (Polyomavirus) (I-V03, U-V08), cryptosporidia, giardia, Macrorhabdus ornithogaster, and Chlamydia reportedly associated in budgerigars
• Gross: Lesions primarily in eye, intestines (distended with fluid and/or gas, diarrhea), liver and kidney (irregular pale foci)
• Histo: Subacute necrotizing mononuclear hepatitis and lymphoplasmacytic interstitial nephritis; organisms cluster within cytoplasm of infected cells (renal epithelium, hepatocytes, intestinal epithelium, spleen, lamina propria of intestines, ocular conjunctiva), resulting in cytoplasmic expansion and rupture 

• E. bieneusi genotype D: Fatal infections of falcons in the Middle East. 

• Gross: Yellow, well demarcated, raised nodules in liver, kidneys, spleen, intestine
• Histo: Multifocal necrosis and pyogranulomatous inflammation in the liver, kidney, and spleen; transmural, necrotizing enterocolitis with diphtheritic membrane in the intestine
• Feces from ducks, cranes, and pigeons may be the source of infection

• NHPs: E. cuniculi and E. bieneusi; limited to New World monkeys, mostly documented in squirrel monkeys, titi monkeys, and tamarins; clinical disease is usually absent or non-specific, but can be multi-systemic, most seriously in the central nervous system; no specific gross lesions 

• E. cuniculi: Granulomatous inflammation and organisms reported in brain, meninges, kidney, lungs, liver, adrenal glands and placenta; vasculitis/perivasculitis is the predominant finding

• Infected cells in the CNS include endothelial cells, macrophages, and ependymal cells.

• E. bieneusi: Proliferative non-suppurative cholecystitis and cholangiohepatitis, enteropathy
• Transplacental transmission documented, resulting in stillbirths and abortions

• Horses:   Rare cause of necrotizing placentitis and abortion; organisms within intracytoplasmic vacuoles of trophoblasts
• Captive meerkats:  Report of fatal meningoencephalitis (Church, PWZA 2018)
• Guinea Pigs: Subclinical infection as well as multifocal nonsuppurative meningoencephalitis and interstitial nephritis; also subclinicial carriers of E. bienusi 
• Mice:   Common in pet and fancy mice; rare in contemporary mouse colonies; detected in wild mice (E. bieneusi and E. hellem); multi-systemic disease, ascites, chronic wasting
• Rats:   Rare, nonsuppurative focal lesions in the brain, kidneys, and liver

 

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