JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

March 2023

N-V01

 

Signalment (JPC #1492905): Full-term calf

 

HISTORY: This calf was unable to stand and nurse. A section of cerebellum from a normal calf is on the slide for comparison.

 

HISTOPATHOLOGIC DESCRIPTION: Cerebellum: There is diffuse cerebellar hypoplasia characterized by shortened and blunted cerebellar folia with blending of the grey matter layers, a marked decrease in cellularity of the granular cell layer with few individual necrotic external granular cells, a paucity of Purkinje cells, and a markedly thin molecular layer that is approximately one-third normal size. Multifocally, remaining Purkinje cells are often disorganized, located within both the internal granular layer and molecular layer (ectopic Purkinje cells), and are often either swollen with vacuolated cytoplasm (degenerate), angular with deeply eosinophilic cytoplasm and nuclear pyknosis (necrotic), or absent, with only a clear space remaining (empty basket). Multifocally, the meninges are expanded by few lymphocytes, macrophages, and fibroblasts.  

 

MORPHOLOGIC DIAGNOSIS: Cerebellum, cortex: Hypoplasia, diffuse, severe, with multifocal ectopic Purkinje cells, and Purkinje cell degeneration, necrosis, and loss, breed unspecified, bovine.

 

ETIOLOGIC DIAGNOSIS: Pestiviral cerebellar hypoplasia

 

CAUSE: In utero infection with bovine pestivirus (bovine viral diarrhea virus - BVDV)

 

DISEASE: Bovine viral diarrhea

 

GENERAL DISCUSSION: 

• Worldwide, economically important disease of cattle caused by an enveloped ssRNA virus (family Flaviviridae, genus Pestivirus)
• Two virus species have been recognized (BVDV type 1 and BVDV type 2), each of which contains distinct subtypes with genetic and antigenic variation; BVDV type 2 is more often isolated from severe cases 
  1. Further classified into cytopathic (cp) or noncytopathic (ncp) based on in vitro cell culture characteristics; cytopathogenicity in vitro is not directly related to virulence in vivo
  2. Hobi-like virus or BVDV-3 is a recently discovered pestivirus species and causes similar disease; it has been identified in Brazil, Southeast Asia and Europe
  3. Wild-type BVDV1 and BVDV 2 strains (subgenotypes) exist (23 for BVDV1 and 4 for BVDV2) (Fritzen, J Vet Diagn Invest, 2020)
• A diverse and complex range of diseases occur depending on the age and immune status of the host and the strain and virulence of the virus

 

PATHOGENESIS:

• Inhalation or ingestion via direct contact or fomite exposure à replication in monocytic cells (e.g., tonsils) à Leukocyte trafficking and/or viremia
• Outcome viremia determined by viral genotype and virulence, host immune status, and host pregnancy status and stage of gestation (see below)
• Viral attachment:
  1. Virus outer membrane proteins E1 and E2 may act as attachment proteins
  2. CD46 is a likely cellular receptor required for attachment to host cells, and clathrin-mediated endocytosis is required for internalization
• As an RNA virus, BVDV is highly mutable
  1. Recombination of ncp to cp results from splitting of the nonstructural protein NS2-3 protein to NS3, a viral protease which is used by cp strains to induce host apoptosis; NS3 is a marker for cp BVDV
•  In utero infections may cause, according to gestation day:
  1. Prior to day 100: Abortion, fetal resorption, or mummification
  2. 90-120 days gestation: Cerebellar hypoplasia (and other teratogenic lesions)
     • Local vasculitis and ischemia -> Cerebellar folial edema -> Depending on severity, folial edema may result in focal lesions, cavitation, or total folial destruction
     • The high mitotic activity of germinative cells of the external granular layer make them especially vulnerable to teratogenic agents
       • Folial edema compounds this and can lead to necrosis of external granular layer 
  3. <125 days gestation (ncp strain): Possible persistent infection (PI)
  4. After 170 days: Unlikely to be teratogenic/abortogenic, but can cause intrauterine growth retardation and atrophy of the thymus and lymphoid tissues
• A key to the pathogenesis of BVDV is persistently infected (PI) animals which serve as the farm reservoir and source of infection
  1. If fetus survives infection by ncp strain prior to 125 days gestation, it may develop immunotolerance and persistent infection (PI) (bovine fetuses become immunocompetent at 150-200 days of gestation)
  2. PI animals are immunotolerant (seronegative) for the virus because they were infected as a fetus early in gestation by a ncp strain before development of the humoral immune system
  3. Mucosal disease (MD, see D-V09) is the result of a PI calf infected with a cp strain
• Acute transient postnatal infections result from either cp or ncp strains; exposure often occurs from viral shedding of PI animals

 

TYPICAL CLINICAL FINDINGS:

• Calf with cerebellar hypoplasia: Clinical signs usually present at birth; Wide-based stance, ataxia, falling backward, opisthotonos, hypermetria, hyperreflexia
  1. May appear blind: No menace response, dilated pupils unresponsive to light
• Reproductive disease: Oophoritis, fertilization failure, embryonic death, birth of small calves or with congenital defects
• Acute infections: 
  1. Diarrhea, depression, oculonasal discharge, anorexia, decreased milk production, oral ulcerations, and pyrexia
  2. Leukopenia characterized by lymphopenia and neutropenia
  3. Thrombocytopenic syndrome: Thrombocytopenia with bloody diarrhea, epistaxis, petechial hemorrhages, and bleeding from injection sites; results from type 2 strains with increased virulence

 

TYPICAL GROSS FINDINGS:

• Fetal (teratogenic) lesions: 
  1. CNS lesions: Cerebellar hypoplasia (folial atrophy and agenesis accompanied by cavitation, folial edema, and focal hemorrhage); porencephaly, hydranencephaly, microencephaly, hydrocephalus, hypomyelinogenesis, myelination defects
  2. Ocular lesions: Microphthalmia, retinal dysplasia and atrophy, cataracts, optic neuritis, and persistent pupillary membrane
  3. Musculoskeletal lesions: Brachygnathism or brachycephaly; arthrogryposis (failure of proper limb development) is uncommon
     • Segmental enamel hypoplasia due to enamel formation during the period of virus infection 
     • Similarly, growth retardation with growth-arrest lines in long bones
  4. Other lesions: Thymic aplasia; enlargement, nodularity, and mottling of the liver; splenomegaly or lymphadenomegaly; pulmonary hypoplasia; hypotrichosis with partial alopecia that spares the tail, the lower portion of the limbs, and the head (points of initial hair growth during fetal development)
• Acute infections:
  1. GI: Ulceration of the alimentary tract, fibrinohemorrhagic exudate overlying Peyer’s patches and/or typhlocolitis 
  2. Respiratory: +/- ulceration on muzzle and nares as well as tracheitis, bronchitis, and bronchiolitis
     • May progress to secondary bacterial pneumonia (BDV is part of the Bovine Respiratory Disease Complex)
     • BVD infection can play an important role in pathogenesis of respiratory disease (Ridpath, J Vet Diagn Invest, 2020) 
  3. Musculoskeletal: Growth retardation lattices of long bones (sclerotic metaphyseal bone), 
  4. Other: Cataracts, retinitis, coronitis with multifocal dermatitis

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Fetal (teratogenic) lesions: 
  1. Necrosis and loss of the external granular layer
  2. Vasculitis, especially of the white matter (characterized by endothelial proliferation and perivascular leukocytic infiltration) with resulting focal cortical hemorrhages may accompany folial edema
  3. Features of cerebellar atrophy: Depletion of granular cells, ectopic Purkinje cells, swollen Purkinje cell axons (torpedoes)
  4. Other CNS lesions: Hypomyelination (white and grey matter appear blended), lymphoplasmacytic leptomeningitis, occasionally with fibroplasia, may cause adhesions between adjacent cerebellar folia and focal obliteration of the subarachnoid space
  5. Other fetal lesions: Mononuclear, predominantly macrophage, infiltration of hepatic portal areas, myocardium, spleen, and lymph nodes; nonsuppurative retinitis +/- microphthalmia and optic neuritis
• Acute infection:
  1. Multifocal epithelial necrosis; hyaline degeneration and fibrinoid necrosis of submucosal and mesenteric arterioles; lymphoid (especially GALT) necrosis

 

ADDITIONAL DIAGNOSTIC TESTS:

• Reverse transcription polymerase chain reaction (RT-PCR) 
  1. Ear notch samples were used with very high specificity and sensitivity at different days to identify PI calves (compared ELISA to RT-PCR) (McDougal, J Vet Diagn Invest, 2021)
  2. Pan-bovine pestivirus primers recently developed (Monteiro, J Vet Diagn Invest, 2021)
  3. Demonstrated to be found within stable flies (as a possible surveillance tool) (Carlson, J Vet Diagn Invest, 2020)
• Virus isolation or antigen capture ELISA (ACE) 
  1. Ear notch samples were used with very high specificity and sensitivity at different days to identify PI calves (compared ELISA to RT-PCR) (McDougal, J Vet Diagn Invest, 2021)
• Virus isolation, virus neutralization (VN) assays, and direct buffy coat 5'-UTR sequencing (Peddireddi, J Vet Diagn Invest, 2018)
• Immunoperoxidase or immunofluorescence
• Immunohistochemistry (skin biopsies in PI animals)

 

DIFFERENTIAL DIAGNOSIS:

• Cerebellar hypoplasia in cattle:
  1. Akabane virus (Bunyaviridae): Can lead to porencephaly, hydranencephaly, ventral horn neuronal degeneration in spinal cord of ruminants; Japan, Australia, and Israel
  2. Bluetongue virus (Orbivirus, N-V02): With in utero infection (at time of development) can lead to hydranencephaly and retinodysplasia in calves, but more common in lambs
  3. Wesselsbron virus (Flaviviridae) and Rift Valley Fever (Phlebovirus): Hydranencephaly of lambs in Africa
  4. Genetically linked cerebellar lesions in Hereford, Angus, shorthorn
  5. Dandy-Walker syndrome: Atrophy or absence of cerebellar vermis seen in numerous species
  6. Schmallenberg virus: Neurologic signs and/or head, spine, or limb malformations in lambs, kids, and calves 
• Hereditary neuraxial edema (N-M05): Autosomal recessive branched-chain α-ketoacid decarboxylase deficiency (BCKD) in neonatal Hereford or cross calves; results in dull or recumbent calves -> death; cerebrum may be swollen with flattened cerebral gyri with status spongiosus

 

COMPARATIVE PATHOLOGY:

• Cerebellar atrophy/abiotrophy: (N-M01)
  1. Dogs: Cerebellar abiotrophy in Kerry blue terrier, Gordon setter, & rough-coated collie among other breeds
  2. Horses: Abiotrophy in Arab, Arab-cross foals, and Gotland ponies
• Cerebellar hypoplasia:
  1. Cats: Feline panleukopenia virus (feline parvovirus) destroys the actively proliferating cells in the cerebellar external germinal layer
  2. Dogs: 
     • Cerebellar hypoplasia in Chow Chows
     • Canine parvovirus DNA has been found in lesions of cerebellar hypoplasia, but there is no definitive proof that parvovirus is causative
  3. Sheep: Border disease virus (ovine pestivirus) 
  4. Swine: Classical swine fever virus (porcine pestivirus) 
  5. Rat: Rat virus (RV) or Kilham’s rat virus (Parvoviridae): Cerebellar hypoplasia may uncommonly be seen in neonatal or infant rats; the most notable lesion is peritesticular hemorrhage and fibrosis in adults; intranuclear inclusions may be present; infertility, fetal resorption, and abortion may occur in pregnant females
  6. Mice: Minute virus of mice (MVM) (Parvoviridae): Experimental exposure of infant mice may target the cerebellar outer granular layer, leading to cytolosysis, resulting in cerebellar hypoplasia; intranuclear inclusions may be present.
• BVDV infection in other species: 
  1. BVDV can infect most even-toed ungulates: demonstrated in numerous species, including various farmed and free-range wildlife (e.g., different species of deer and camelids)
     • Reproductive failure and PI BVDV infection documented in camelids (alpacas and crias), mouse deer, white-tailed deer, mountain goats, sheep and goats
     • Virus distribution among tissues in PI sheep, white-tailed deer, and mountain goats similar to cattle
  2. Rocky Mountain bighorn sheep: Presented with necroulcerative-to-necrohemorrhagic gastrointestinal lesions due to contaminated modified-live bluetongue virus vaccine (Fox, J Vet Diagn Invest, 2018)
  3. Camelids: A new substrain of BVDV (BVDV-1q) was first identified in Bactrian camels in China -> Later identified in dairy cattle and swine 
  4. Swine: Increased incidence of natural BVDV infection; may complicate the diagnosis of classical swine fever (another pestivirus); animals are usually asymptomatic but may have anemia, tremors, or diarrhea
     • Virus may circulate in backyard pigs in Brazil (Mósena, J Vet Diagn Invest, 2020)
  5. Swine and goats: Stillbirth and neonatal death reported

 

References:

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2. Barthold SW, Griffey SM, Percy DH. Mouse. In: Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley Blackwell; 2016:17-19.
3. Barthold SW, Griffey SM, Percy DH. Rat. In: Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley Blackwell; 2016:122-124.
4. Carlson JM, Vander Ley BL, et al. Detection of bovine viral diarrhea virus in stable flies following consumption of blood from persistently infected cattle. J Vet Diagn Invest. 2020 Jan;32(1):108-111. 
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7. Fox KA, Kopanke JH, Lee JS, Wolfe LL, Pabilonia KL, Mayo CE. Bovine viral diarrhea in captive Rocky Mountain bighorn sheep associated with administration of a contaminated modified-live bluetongue virus vaccine. J Vet Diagn Invest. 2019 Jan;31(1):107-112. 
8. Fritzen JTT, Morettin AB, et al. Bovine viral diarrhea virus subgenotype 1a in a mummified fetus from a Brazilian dairy cattle herd. J Vet Diagn Invest. 2021 Sep;33(5):966-968. 
9. Howerth EW, Nemeth NM, Ryser-Degiorgis MP. Cervidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:176. 
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13. Mósena ACS, Weber MN, Cibulski SP, Silva MS, Paim WP, Silva GS, Medeiros AA, Viana NA, Baumbach LF, Puhl DE, Silveira S, Corbellini LG, Canal CW. Survey for pestiviruses in backyard pigs in southern Brazil. J Vet Diagn Invest. 2020 Jan;32(1):136-141. 
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