Avian Encephalomyelitis (AE):

Synonym: Epidemic tremor, New England Disease

• It is an infectious viral disease of young chicken characterized by muscular incoordination, rapid tremors; especially of head and neck.
• Disease mostly affects poultry but infection can also occur in turkey, quail and pheasants. Baby chicks of 1-2 weeks of age are most susceptible.

Etiology:

• Avian encephalomyelitis virus (AEV)
• Virus is non-enveloped, contains RNA as genetic material, Genus: Tremovirus, Family: Picornaviridae family
• Natural field strain is enterotropic and multiply in intestine
• Virus is relatively resistant to physical and chemical agents as well as heat and cold.

Epidemiology:

• Disease was first reported in 2-week-old Rhode Island Red chicks from commercial flock.
• It is now distributed globally.
• Morbidity and mortality rates may vary and depend on level of egg transmission and extent of immunity in flock.
• In severe outbreaks, morbidity and mortality rates may exceed 50%.

Transmission:

• Disease is observed in all seasons.
• Vertical transmission through eggs.
• Through infected chicks (horizontal) to healthy ones during hatching.
• Virus is excreted in droppings for long period. Through infected litters, infection spread.
• Contaminated fomites may transmit the virus.
• Ingestion is the usual route of entry.

Pathogenesis:

• Virus enters tissue from infected egg, when infection is already present in eggs laid.
• After hatching, virus enters the mouth (oral route).
• There is rapid viremia and virus is deposited in pancreas, liver, heart, spleen, kidney and finally CNS.
• Virus multiplies in Purkinje cell and molecular layer of cerebellum.
• This results in clinical signs.
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Clinical Findings:

• Young chicks are primarily affected with high morbidity and mortality.
• Nervous signs are characterized by:
  • Dullness
  • Depression
  • Ataxia, incoordination
  • Sitting on haunches
  • Inability to walk
  • Paralysis
  • Birds show tremor of head and opacity of lens.
• Birds die due to trembling and inability to take food.
• In adult birds, diarrhea may be evident.
• 10-20% drop in egg production.
• Birds usually remains drowsy but any sound is made, they will get up and walk and finally go to tremors.
• Very mildly affected birds recover completely.

PM findings:

• No gross lesion is observed. Only lesion observed is whitish areas in muscles of gizzard and proventriculus.
• Histopathological examination of pons, medulla and spinal cord shows eccentricity and chromatolysis of nucleus.
• Perivascular infiltration may occur in these areas.
• Gliosis occur in molecular layer of cerebellum.

Diagnosis:

• Based on age of bird and typical clinical signs
• Based on histopathological changes
• Isolation of virus on experimental production of disease
• Virus neutralization test (VNT), FAT
• AGPT, ELISA

Differential Diagnosis:

Avian encephalomyelitis should be differentiated from following disease/affections:

1. Ranikhet Disease:

• Respiratory signs; coughing, sneezing is observed.
• Greenish diarrhea is evident in this disease.
• Hemorrhagic lesion in proventriculus.

2. Marek’s Disease:

• Asymmetric paralysis is characteristic feature.
• Grey eye
• Tumors in nerves and visceral organs
• It usually affects older birds (>6 weeks)

3. Vitamin E/Selenium deficiency:

• No virus isolation
• Disease is often related to diet.
• Cerebellar hemorrhage and necrosis is seen grossly.

4. Mycotoxicosis:

• Liver damage
• Feeding history of mouldy grain
• Multiple age group are affected.

5. Arsenical involvement:

• History of exposure to arsenic
• Diarrhea, respiratory distress

6. Exudative diathesis:

• Edema, subcutaneous hemorrhage
• Greenish-blue discoloration of skin
• Gelatinous subcutaneous exudate is seen grossly.
• Birds of any group can be affected.

Treatment:

• There is no effective treatment for the infected birds.

Control Measures:

• In case of outbreaks, affected birds are to be segregated immediately and advisable to destroy the affected birds.
• Sanitation and hygienic measures are to be adopted.
• Strict hygienic measures are to be adopted in the hatchery.
• Birds should be purchased from disease-free stock.
• Vaccination via drinking water or by spraying at 10-16 weeks of age, prior to egg laying.