Cerebrospinal Nematodiasis in Lagomorphs 

Disease Summary

Alternative Names (Synonyms)

• Visceral larva migrans 
• Cerebral larva migrans
• Aberrant ascarid infection
• Clinical neural larva migrans

Disease Type

Infectious/Non-Infectious Agent associated with the Disease

• Baylisascaris procyonis
  • This species is the ascarid that is most frequently associated with disease in rabbits. (B614.11.w11)
  • The definitive host is Procyon lotor - Common Raccoon. (B602.20.w20, B603.3.w3, B606.13.w13, B614.11.w11)
• Baylisascaris columnaris (Ascaris columnaris) (J495.25.w3)
  • The definitive host is the skunk. (B602.20.w20, B603.3.w3, B614.11.w11)
    • This parasite is also found in the stoat, weasel and Siberian polecat. (B603.3.w3)
  • Synonym: Ascaris columnaris. (B603.3.w3, J29.5.w1)
• Toxocara canis
  • The definitive host is the domestic dog. (B614.11.w11)

In Lagomorphs:

• Rabbits are not the definitive host to adult ascarids, they are the aberrant (or paratenic) hosts to ascarids that infect carnivores. The migrating larvae of several species of ascarids can cause disease in rabbits. (B601.11.w11, B614.11.w11)
• Life cycle of the ascarid worms:
  • Adult worms are found in the small intestine of the definitive host. The eggs are passed in the faeces and they then need thirty days outside the host for development to the infective stage. (B614.11.w11)
  • Eggs can remain infective for at least a year. (B602.20.w20, B606.13.w13)
  • Rabbits become infected by ingesting materials contaminated with infective eggs. Larvae, released in the intestine, start to migrate to various tissues. The larval migration is not completed and they never mature because the rabbit is an aberrant host. (B614.11.w11)
• Pathogenesis:
  • The larvae of these ascarids tend to have a predilection for the CNS where they cause encephalomalacia. There may be extensive destruction of brain tissue even from only a few larvae; the destruction is progressive as the larvae continue migrating through the brain. (B602.20.w20, B603.3.w3)
  • The larvae migrans may be found in the cerebellum, cerebrum, medulla and mid brain. (B606.13.w13)

Infective "Taxa"

• Baylisascaris procyonis (Ascarididae - (Family))
• Baylisascaris columnaris (Ascarididae - (Family))
• Toxocara canis (Toxocaridae - (Family))

Non-infective agents

Physical agents

References

Disease Author

Major References / Reviews

Code and Title List

Other References

Code and Title List

Detailed Clinical and Pathological Characteristics

Clinical Characteristics

• The clinical signs seen in this disease vary depending on which tissues have been damaged by the migrating larvae. 
• Typically, affected rabbits may show intermittent improvement followed by more severe signs. (B602.20.w20)
• Signs may be slowly progressive over several weeks. (J495.25.w3)

Clinical signs reported include:

• Initial signs that may be noticed include depression, lethargy or nervousness. (B208.11.w11)
• Tremors (B601.11.w11, B602.20.w20, B603.3.w3, B606.13.w13, J4.205.w5)
  • In the front paws (B208.11.w11)
• Head tilt/torticollis (B600.12.w12, B208.11.w11, B601.11.w11, B602.20.w20, B603.3.w3, B606.13.w13, B614.11.w11, J29.5.w1, J495.25.w3)
• Body tilt (B208.11.w11)
• Circling (B601.11.w11, B603.3.w3, B606.13.w13)
  • Circling or jumping when disturbed; this may progress to continuous circling. (B208.11.w11)
• Ataxia (B601.11.w11, B602.20.w20, B603.3.w3, B606.13.w13, B614.11.w11, J29.5.w1)
• Ear droop (B614.11.w11)
• Vertical nystagmus (B208.11.w11, B601.11.w11, B602.20.w20)
• Poor weight gain. (J495.25.w3)

In more advanced cases:

• Motor weakness and incoordination, loss of balance, falling if the fore feet were raised. (J495.25.w3)
  • Eventually unable to place themselves in position to eat or drink. (J495.25.w3)
• Leaning or falling over (B208.11.w11, B602.20.w20)
• Lateral recumbency (B208.11.w11, B602.20.w20)
  • There may be paddling movements while the animal is recumbent. (B208.11.w11)
• Rolling (B208.11.w11, B601.11.w11, B602.20.w20)
• Opisthotonos (B208.11.w11, B603.3.w3, B606.13.w13)
• Blindness (B208.11.w11)
• Posterior paresis/paralysis (B208.11.w11, B601.11.w11, B602.20.w20)
• Hypotonia or extensor rigidity (B208.11.w11)
• Seizures (B601.11.w11, B602.20.w20, B603.4.w4)
• Coma (B208.11.w11)
• Death (B208.11.w11)

In four individual Oryctolagus cuniculus domesticus - Domestic rabbits: (J4.205.w5)

• 1) Severe torticollis, ataxia, tremors, falling over backwards and difficulty in maintaining posture while eating. Later also unable to stand without falling to the left side, and left facial contraction. 
• 2) Mild tremors and slight torticollis, progressive, in one rabbit.
• 3) Sudden onset ataxia and torticollis, circling to the left, falling right/backwards, bilateral nystagmus, with retinal haemorrhage of the left eye and loss of consensual reflex in the right eye. Otherwise alert with good appetite, normal hydration stats and normal vital signs.
• 4) sudden onset ataxia, trticollis, falling and rolling to one side (right), facial paralysis, marked nystagmus and inability to drink and eat.

(J4.205.w5)

In Sylvilagus floridanus - Eastern cottontails: (J4.167.w4)

• Head tilt, torticollis, ataxia, circling, abnormal posture, periods of extensor rigidity. In three rabbits, unilateral ocular atrophy, opacity and blindness. (J4.167.w4)
• In an experimentally-infected juvenile: head tilt, torticollis and hypersensitivity to external stimuli. (J1.12.w17)

In young Dutch Belt rabbits (Oryctolagus cuniculus domesticus - Domestic rabbits) with experimental infection. (J13.36.w2)

• Coughing, dyspnoea and epistaxis, ataxia, backpeddling with draging of the forelimbs, loss of balance and inability to right themselves, frantic hindlimb peddling when collapsed on one side, lying with the hind legs abnormally far forward, resulting in falling backwards. Prostration and weakness. (J13.36.w2)

Note: Motor function is impaired but spinal reflexes remain intact. (B601.11.w11, J4.167.w4)

Incubation

• Neurological signs due to Baylisascaris procyonis are seen within six weeks after infection. (B614.11.w11)
• Signs 80 days post infection in one juvenile Sylvilagus floridanus - Eastern cottontail. (J1.12.w17)
• In experimentally infected Dutch Belt rabbits (Oryctolagus cuniculus domesticus - Domestic rabbits), anorexia and dullness in two to three days, neurological signs by eight days. (J13.36.w2)
• In an outbreak in a commercial rabbitry, signs were seen about two months after rabbits were brought onto the premises. (J495.25.w3)

Mortality / Morbidity

• Occasionally high morbidity and mortality - 80 rabbits out of a population of 77 plus offspring in a period of three months. (J495.25.w3)

Pathology

Gross pathology

• None. (J495.25.w3)
• In Sylvilagus floridanus - Eastern cottontails, none, except in one rabbit on the ventral surface of the brain stem, brown, mottled discolouration "suggestive of extreme congestion or previous hemorrhage. (J4.167.w4)
• In young Dutch Belt rabbits (Oryctolagus cuniculus domesticus - Domestic rabbits) with experimental infection. (J13.36.w2)
  • Lungs: congestion, areas of the pleura yellow-black. (J13.36.w2)
  • Liver, kidneys, gastrointestinal tract and myocardium: multiple white foci, 1.0 - 5.0 mm diameter. (J13.36.w2)
• In naturally-infected Oryctolagus cuniculus domesticus - Domestic rabbits. (J4.205.w5)
  • No gross lesions in three rabbits. In the fourth, subdural haemorrhage over the congested right cerebral cortex, also right-sided meningitis, vasculitis and haemorrhage. (J4.205.w5)

• Granulomas may be seen in the liver, heart, brain, or other tissues along with cross sections of the larvae. (B614.11.w11)
• In Sylvilagus floridanus - Eastern cottontails two to 12 weeks after capture: (J4.167.w4)
  • CNS: 
    • Cerebellar and midbrain lesions of malacia, perivascular cuffing (mild to moderate) with lymphocytes and eosinophils, gliosis, presence of phagocytic gitter cells and microscopic haemorrhages and cavitations. (J4.167.w4)
    • In two individuals, a single Ascaris columnaris larva (later re-identified as Baylisascaris procyonis) in the cerebellum, and in another individual a single larva in the spinal cord. (J1.12.w17, J4.167.w4)
• In an experimentally infected juvenile Sylvilagus floridanus - Eastern cottontail:
  • CNS: Brainstem lesions of malacia, perivascular cuffing (mild to moderate) with lymphocytes and eosinophils, gliosis, presence of phagocytic gitter cells and microscopic haemorrhages and cavitations. One ascarid larva found in the thalamus. (J1.12.w17)
  • Hepatic: liver sinuses engorged, prominent capsular fibrosis. (J1.12.w17)
• In young Dutch Belt rabbits (Oryctolagus cuniculus domesticus - Domestic rabbits) with experimental infection. (J13.36.w2)
  • CNS: In the cerebral hemispheres, diffuse malacia, gliosis and astrocytosis. In the brainstem, slight perivascular cuffing and demyelination. Larvae, 20 - 45 µm diameter, in normal and diseased tissues. (J13.36.w2)
  • Liver, kidneys, GIT and myocardium: parasitic granulomas. (J13.36.w2)
  • Lungs: interstitial pneumonia. Mild haemorrhage, oedema. (J13.36.w2)
  • Ocular: in the ciliary process, eosinophilic granuloma. Beneath the sclera, sections of larvae. (J13.36.w2)
• In commercial rabbits (Oryctolagus cuniculus domesticus - Domestic rabbits) : no lesions other than in the CNS. (J495.25.w3)
  • CNS: irregularly-distributed multifocal malacia and inflammatory infiltrates, sometimes well-defined granulomas. (J495.25.w3)
  • Cross-sections of nematode larvae in 6/9 rabbits in the brain. Minimal inflammatory reaction around some larvae but many inflammatory cells (lymphocytes, glial and gitter cells, few neutrophils) around others. Note: in many lesions, no larvae were found. (J495.25.w3)
  • Perivascular cuffing in parenchyma near to inflammatory foci only. (J495.25.w3)
• In naturally-infected Oryctolagus cuniculus domesticus - Domestic rabbits. (J4.205.w5)
  • Rabbit 1: mild lymphocytic meningitis, multifocal malacia, 0.1 - 1.0 mm, with accompanying encephalitis, throughout the brain but particularly in the white matter of the cerebellum, brain stem and midbrain. Also Encephalitozoon cuniculi.
  • Rabbit 2: Multifocal encephalitis, malacia and granulomas. 
  • Note: larvae not detected in haematoxylin and eosin stained sections from four rabbits, despite presence of tracts typical of larval migrans. (J4.205.w5)

Human Health Considerations

Disease has been reported in either the wild or in captivity in:

• USA
  • Wild and feral rabbits commonly present with signs of cerebral larva migrans. (B602.20.w20)
  • This disease is also seen in pet rabbits. (B601.11.w11)
  • Cerebral larva migrans caused by Baylisascaris species is common in rabbits. (B602.20.w20)
  • In a commercial rabbitry in the USA [Michigan]. [1975](J495.25.w3)
  • In three pet rabbits from one property and one from another property, in Seattle, Washington, USA. (J4.205.w5)
  • Seen in wild Sylvilagus floridanus - Eastern cottontail as well as Oryctolagus cuniculus domesticus - Domestic rabbit. (J29.5.w1)
  • Seen in 16 of about 60 Sylvilagus floridanus - Eastern cottontails from 12 hectares of woodlot in southwestern Virginia, USA. (J4.167.w4)
• Europe
  • Aberrant migration of Ascaris columnaris has been reported in Europe. (B603.3.w3)

Further information on Host species has only been incorporated for species groups for which a full Wildpro "Health and Management" module has been completed (i.e. for which a comprehensive literature review has been undertaken). Host species with further information available are listed below:

Host Species List

• Oryctolagus cuniculus domesticus - Domestic European rabbit
• Sylvilagus floridanus - Eastern cottontail

(List does not contain all other species groups affected by this disease)

Disease has been specifically reported in Free-ranging populations of:

• USA
  • Wild and feral rabbits commonly present with signs of cerebral larva migrans. (B602.20.w20)
  • Seen in wild Sylvilagus floridanus - Eastern cottontail as well as Oryctolagus cuniculus domesticus - Domestic rabbit. (J29.5.w1)
  • Seen in 16 of about 60 Sylvilagus floridanus - Eastern cottontails from 12 hectares of woodlot in southwestern Virginia, USA. (J4.167.w4)

Further information on Host species has only been incorporated for species groups for which a full Wildpro "Health and Management" module has been completed (i.e. for which a comprehensive literature review has been undertaken). Host species with further information available are listed below:

Host Species List

• Sylvilagus floridanus - Eastern cottontail

(List does not contain all other species groups affected by this disease)

General Information on Environmental Factors/Events and Seasonality

• --

Regions / Countries where the Infectious Agent or Disease has been recorded

• USA. (B601.11.w11, B602.20.w20, J4.205.w5, J495.25.w3)
• Aberrant migration of Ascaris columnaris has been reported in Europe. (B603.3.w3)

Regions / Countries where the Infectious Agent or Disease has been recorded in Free-ranging populations

• USA. (B602.20.w20)

General Information on Investigation / Diagnosis

• CNS Histopathology: identification of larvae. (B614.11.w11)
  • Note: there may be very few larvae found compared to the number of tracts through brain tissue. (J495.25.w3)
• Baermann technique on brain tissue: finely mince half the brain, place in 0.85% saline and allow to sediment for two hours at 37 °C. (J495.25.w3)
  • Note: this may not always recover larvae. From four rabbits in which larvae were seen by histopathological examination, larvae were collected by this technique only from two of the rabbits' brains. (J495.25.w3)
  • Larvae 60 - 71 µm diameter, 1350 - 1500 µm long, with evident lateral cuticular alae easily visible in histological sections and by microscopic examination of coiled larvae in saline. Identified as Ascarid (Baylisascaris) columnaris. (J495.25.w3)
• Brain digest. (J4.205.w5)
  • Mince half the brain plus the anterior third of the spinal cord using a scalpel blade.
  • Add to one litre 0.85% sodium chloride solution together with 20 g pepsin plus 5 mL hydrchloric acid. 
  • Stir for 18 hours at 37 C.
  • Place in a gradualted cylinder and allow to settle for 16-18 hours.
  • Remove supernatant.
  • Wash the sediment three times with 0.85% sodium chloride solution.
  • Add formalin to the final 200 mL of sediment to produce a 10% solution.
  • Place small quantities of suspended sediment into Petri dishes and examine at x10 and x30 magnification using transmitted light and a dissecting microscope.
  • Typical third-stage Baylisascaris sp. larvae are about 56 x 1,500 µm; larvae of ascarids of domestic animals do not reach this size. 

  (J4.205.w5)

• Physical Examination of Mammals
• Necropsy of Mammals
• Chapter 2 - B36 Field Manual of Wildlife Diseases - Specimen Collection and Preservation
• Chapter 3 - B36 Field Manual of Wildlife Diseases - Specimen shipment
• Appendix A - B36 Field Manual of Wildlife Diseases - Sample specimen history form

Similar Diseases (Differential Diagnosis)

Swaying and falling are often pronounced with this condition, whereas these clinical signs are seen less often in the following two diseases:

• Encephalitozoonosis
• Otitis interna - see: Bacterial Otitis Media / Interna in Lagomorphs

(B602.20.w20, J495.25.w3)

Specific Medical Treatment

• Mebendazole at 100 mg/kg for three consecutive days has been used in experimental studies of this disease in rabbits. (B614.11.w11)
• Oxybendazole at 60 mg/kg orally every 24 hours indefinitely (B601.11.w11, B602.20.w20). Some resolution of clinical signs have been seen using this drug. (B601.11.w11)
  • In treatment of presumptive cerebral larva migrans, use of this drug appeared to slow disease progression, but with recurrence and progression of signs on withdrawal of treatment. (B602.20.w20)
• Treatment with tetramisole (8 mg/kg) or thiabendazole (25 mg/kg) was ineffective. (J495.25.w3)

• --

General Nursing and Surgical Techniques

• Rabbits that are suffering from severe posterior paresis may be fitted with a cart to facilitate movement. (B602.20.w20)
• Euthanasia is advised in severe cases. (B602.20.w20)

• --

Preventative Measures

--

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Environmental and Population Control Measures

Lagomorphs

• Prevent rabbits from accessing potentially contaminated materials. (B601.11.w11, B602.20.w20)
• Note: ascarid eggs are very resistant to disinfectants and environmental conditions. (B614.11.w11)

• Environmental and Population Management - General