JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

March 2023

N-T12

 

Signalment (JPC #2136869): 6-month-old Wapiti (elk)

 

HISTORY: This animal had incoordination for two weeks before being found recumbent, depressed, and with evidence of diarrhea.

HISTOPATHOLOGIC DESCRIPTION: Cerebellum: Multifocally, there is marked spongiosis in the Purkinje cell layer and to a lesser extent in the granular layer. Approximately 30% of Purkinje cells are swollen with several small or one large clear cytoplasmic vacuole and dispersion of Nissl substance (chromatolysis, degeneration), rare Purkinje cells are shrunken with hypereosinophilic cytoplasm and nuclear pyknosis or karyolysis (neuronal necrosis), and there is multifocal Purkinje cell loss with occasional remaining large round clear spaces (“empty baskets”). Often Purkinje cells have swollen axons (“torpedoes”) that extend into the granular layer, and swollen dendrites that extend into the molecular layer. Purkinje cells are occasionally located in the internal granular layer (ectopic Purkinje cells), and there is a slight decrease in the number of granular cells with mild thinning of the granular cell layer. There is scattered mild spongiosis in the underlying cerebellar white matter.

MORPHOLOGIC DIAGNOSIS:  

1.  Cerebellum, Purkinje cells: Vacuolar degeneration, necrosis, and loss, multifocal, with torpedoes, Wapiti (Cervus elaphus mannitobensis), cervid.

2.  Cerebellum, Purkinje cells: Ectopia, multifocal, mild, with mild granular layer thinning.

 

ETIOLOGIC DIAGNOSIS: Tremorgenic neuromycotoxicoses (neuronal mycotoxicosis)

CAUSE: Tremorgenic mycotoxin (lolitrems produced by Neotyphodium lolii [formerly Acremonium lolii])

CONDITION: Perennial ryegrass staggers

GENERAL DISCUSSION: 

Perennial ryegrass staggers:

• Occurs in the summer and autumn on dry, short pastures of Lolium perenne
• Caused by ingestion of indolic lolitrems produced by endophytic fungus Neotyphodium (previously known as Acremonium) lolii 
• Sheep cattle, and horses may be affected; reported in goats, red deer, fallow deer, and wapiti
• Common mycotoxin in New Zealand, Australia, and Europe; occasionally reported in the US, and southern Africa
• The toxins are known as tremorgens; four tremorgenic neurotoxins have been identified:  Lolitrem A, B, C, & D (lolitrem B is the most common)

Annual rye-grass toxicosis:  

• Caused by corynetoxin which is found in a yellow slime produced by the bacterium Rathayibacter toxicus (Clavibacter toxicus) which can be carried on the cuticle of the nematode Anguina agrostis which infects seedheads of annual ryegrass (Lolium rigidium) to form a gall as the plant sprouts

• The nematode itself is harmless to the animal

• Corynetoxin is closely related to tunicamycin antibiotics
• Toxin accumulates due to lack of rumenal breakdown
• Parasitized annual rye grass (L. rigidium ) can cause significant livestock losses in Australia and South Africa during the summer  
• Peak toxin levels occur as the plant dries in late summer

PATHOGENESIS: 

Perennial ryegrass staggers:

Annual rye-grass toxicosis:  

• Corynetoxin, a glycolipid, blocks N-glycosylation of glycoproteins which compromises cell membrane integrity
• Corynetoxin causes microvascular damage and alters the blood-brain cerebrospinal (CSF) barrier which leads to neuronal injury

TYPICAL CLINICAL FINDINGS: 

Perennial ryegrass staggers:

• Fine head tremors, head nodding, weaving at rest, stiff-legged uncoordinated gait, collapse with tetanic spasms; recover quickly
• Clinical signs appear 5-10 days following exposure
• Total recovery will take place within 3 weeks of removal from toxic pasture

Annual rye-grass toxicosis: 

• Severe neurologic signs that include excitability, aggression in cattle, disturbances of gait, opisthotonos, convulsions
• High mortality
• Pregnant ewes may abort
• Clinical course may be <12 hours
• Morbidity and mortality rates vary but may reach 100%

TYPICAL GROSS FINDINGS: 

• Perennial ryegrass staggers: None 
• Annual ryegrass staggers: Pulmonary edema, pale swollen liver, occasionally hemorrhages in various organs (cerebellum, liver, lungs, and spleen), rarely multifocal greenish hue to cerebellum and cerebrum

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

Perennial rye-grass staggers:

vacuolar degeneration and loss of Purkinje cells

Annual rye-grass toxicosis:  

• CNS lesions are subtle (especially with routine fixation by immersion):
  • Disrupted vascular integrity and protein leakage (edema)
  • Neuropil degeneration
  • +/- Extravasation of red cells from capillaries in neuropil +/- fibrin
  • Swollen astrocytes with acidophilic cytoplasm (gemistocytes) 
  • Widespread oligodendroglial necrosis in the cerebral gray matter
  • Patchy loss of Purkinje cells 
  • Scattered small foci of necrosis

ADDITIONAL DIAGNOSTIC TESTS:

Perennial rye-grass staggers:

• Axonal lesions are best demonstrated by the use of silver staining techniques (e.g. Holmes method)
• Microscopic identification of Neotyphodium on the perennial ryegrass

Annual rye-grass toxicosis:  

• Perivascular lakes or droplets that stain strongly with PAS stain
• Tracer injections - Widespread alterations to cerebrovascular permeability in the brain and meninges, indicating endothelial damage and disruption of the blood-brain barrier
• Corynetoxin can be extracted with boiling alcohol

DIFFERENTIAL DIAGNOSIS:

Perennial rye-grass staggers:

• Penicillium crustosum, Aspergillus clavatus, Claviceps paspali (ergot of Paspalum):  No “torpedoes” reported in these tremorgenic neuromycotoxicosis

• Other conditions with “torpedoes”:

• Abiotrophy (N-M01) – mild gliosis, decreased granular layer neurons
• Bovine viral diarrhea virus (N-V01, pestivirus) – cerebellar folia atrophy, mild astrocytosis
• GM2 gangliosidosis (N-M13) – neurons swollen with abundant granular to foamy cytoplasm that displaces the nucleus
  
  

COMPARATIVE PATHOLOGY:

Perennial rye-grass staggers:

• Cattle, sheep, goats, horses, deer, alpacas can be affected 
• Tremorgenic mycotoxicoses in other animals:

• Claviceps paspali, the ergot of Paspalum, produces tremorgenic paspalitrems in cattle, occasionally in sheep, and rarely in horses
• Dogs:  Tremorgenic mycotoxicosis may result from penitrem A or roquefortine (produced by Penicillium) associated with moldy cream cheese and hamburger buns
• Horses:  Tremorgenic mycotoxicosis may result from the consumption of dallis grass  

Annual rye-grass toxicosis:  

• Occur in ruminants, horses, and pigs
• Tunicamycin-like (similar to Corynetoxin) metabolites have been detected in mycotoxicosis in pigs
• Sheep and cattle - similar neurologic syndrome has been in the United States in animals fed Festuca nigrescens (chewings fescue) infected with A. agrostis and Rathayibacter-like organism

Other toxicity associated with Lolium perenne (perennial ryegrass):

• Sporidesmin (mycotoxin) is produced by Pithomyces chartarum, a fungus that grows well in dead Lolium perenne; unconjugated toxin is secreted into the biliary tree and causes necrosis of the epithelium of large intrahepatic and extrahepatic biliary ducts with minimal inflammation, resulting in cholestasis and subsequent photosensitization (type III, hepatogenous) with skin lesions (facial eczema)
  1. Recent report of 4 cases of obstructive rhinopathy in 4 Latxa sheep in Spain caused by sporidesmin contained Pithomyces chartarum spores. Sheep had concurrent liver atrophy and fibrosis (De las Heras, Vet Pathol, 2022)

References:

1. Cantile C, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:257, 298, 309, 322. 
2. De las Heras M, Lacasta D, Reséndiz RA, et al. Chronic pithomycotoxicosis associated with obstructive rhinopathy in sheep. Vet Pathol. 2022;59(6):950-959.
3. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Vol 1.  St. Louis, MO: Elsevier; 2016: 580.
4. Riet-Correa F, Rivero R, Odriozola E, Adrien Mde L, Medeiros RM, Schild AL. Mycotoxicoses of ruminants and horses. J Vet Diagn Invest. 2013;25(6):692-708.
5. Van Wettere A, Brown D. Hepatobiliary System and Exocrine Pancreas. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Elsevier; 2022:522-523.

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