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Avian Encephalomyelitis

New England Disease, Epidemic Tremor

Overview


Avian encephalomyelitis (AE) is an increasingly more common viral disease found in backyard poultry and poultry breeder farms worldwide. A survey conducted on backyard poultry farms in Finland between 2012 and 2013 revealed 86% of one or more tested chickens had antibodies against AEV. The disease is characterized by incoordination and rapid tremors, especially of the head and neck. AE is often referred to as 'epidemic tremor', due to the intense trembling and shaking observed in affected chicks. The disease is caused by an enterovirus, avian encephalomyelitis virus (AEV). AEV can infect not only chickens, but also turkeys, pigeons, quail, and pheasants.

Often the first sign of infection in chicks is depression, soon followed by loss of coordination, drooping wings, rapid muscle tremors of the head and neck (less commonly, the wings and legs), sitting on hocks, and falling over sideways.
Signs of Avian encephalomyelitis
Chicks may also become paralyzed, and observed lying spread out on the ground in a severely weakened state. Without intervention with supportive care to ensure chick stays hydrated and eating, chicks will often die from starvation or from getting trampled to death by other flock members. Some chicks may only be mildly affected and recovery completely. Older chickens that are infected with AE often show no clinical signs of being infected, other than an occasional decrease in egg production in laying hens. Typically, about 5% of the flock is affected, although mortality may be 50% or higher.

Complications
Chicks that recover from AE may have ongoing coordination issues, become permanently blind in one or both eyes, and have an increased risk of developing cataracts. There have been many other incidences where chicks hatched from chickens that have recovered from AE go blind for no apparent reason.

Transmission
AE is transmitted to flocks through several routes:
  • Introducing a new, often older chicken to the flock that is either an infected subclinical carrier of the virus or has recovered from AE.
  • The breeding hen was infected with the virus, and passed it to the newly hatched chick from the egg, through vertical transmission. If eggs are incubated, about 25% or more die just prior to hatching. Hatched chicks that are infected with AE will generally develop signs of infection anywhere from one day to three weeks.
  • Chickens that are infected shed the virus in their droppings, which contaminate the environment and ingested by other flock members. The virus is shed in the droppings of infected chicks for up to 2 weeks.
Incubation period
Chicks infected by vertical egg transmission start to develop clinical signs anywhere from one day to three weeks after they hatch. When infected by direct contact with diseased birds, signs of infection may take at least 11 days to start to present.

Summary of Documented Cases of AE

AffectedHistoryClinical signsDiagnostic FindingsTests usedRef
One-day-old chickensNumerous one-day-old chicks in a hatchery were affected.WeaknessNecropsy and histopathology of the brain revealed multifocal lesions affecting the large nuclei in the brainstem and cerebellar Purkinje cells, with neuronal degeneration.Necropsy; histopathology; ELISAConference 10-2013 Case: 02 cerebrum, cerebellum, brainstem - Chicken. AskJPC.org
Three flocks of 11 - to 14-week-old commerical leghorn pulletsThe flocks were vaccinated against fowl pox and AE with a combined product in the wing-web. Two weeks following vaccination, clinical signs appeared in the birds.Birds 'down on their legs', unilateral recumbency, sitting on hocks, lethargy, reluctance to move, dehydration, unevenness in size, stunted growth, tremors of the head in a few birds, and mildly to moderately elevated mortality.Necropsy revealed lesions consistent with AE, including lymphocytic perivascular infiltration and neuronal central chromatolysis in the brain and spinal cord and gliosis in the cerebellar molecular layer.Necropsy; histopathology; reverse-transcriptase PCRC Senties-Cue et al., 2016
An outbreak in one-day-old chicks from multiple hatches on a game farmThe incident occurred on a game farm producing chicks from its own breeding flock. They had recently introduced new roosters to the farm to breed with the hens. Upon hatching and to 7.5 weeks of age, clinical signs appeared. At one-day-old the chicks were bright and alert, but by 3-days-old they were severely affected and appeared dull.Head tremors (that was made more noticeable when the chicks were upright), incoordination, falling on their backs and unable to right themselves easily, hock-sitting, curled toes, and would sit or lay with their legs at unusual angles. Chicks tended to move about with splayed legs and wings, often falling over. Some chicks went blind. Some chicks had pale grey opacity of one or both eyes by the time they were 3-weeks old.The histopathology exam of the brain of revealed mild to moderate acute encephalomyelitis, in the brainstem, but not all of the chicks were affected.Necropsy; blood test; histopathology; ELISA for AEV antibody; PCRWelchman, D et al., 2009
20 out of 300, 12-day-old chicksThe chicks were suspected to have been vertically infected with the virus, from adult breeders.Dullness, Ataxia, hock-sitting, and recumbency prior to death.Necropsy and histopathology revealed lesions consistent with AEV.Necropsy; histopathologyCAHFS Connection, October 2012

Clinical Signs

Head tremors
Progressive incoordination
Depression
Dullness
Hock-sitting
Recumbency
Laying down to one side
Weakness
Blindness
Death

Diagnosis

  • History
  • Clinical signs
  • Physical exam
  • ELISA

Treatment

Supportive care: Isolate the bird from the flock and place in a safe, comfortable, warm location (your own chicken "intensive care unit") with easy access to water and food. Limit stress. Call your veterinarian.

Prevention

Prognosis

Chickens which survive the disease have a higher chance of developing cataracts later in life.

Scientific References

Good Overviews

Age Range

The disease usually occurs when chicks are 1-3 weeks of age. .

Risk Factors

  • Introducing a new adult chicken to the flock, especially if they are intended to be used for breeding purposes.