Avian encephalomyelitis (AE) is a common viral infection which causes tremors of the head and neck, and progressive ataxia in young chicks, between 1 to 3 weeks of age. The disease is caused by an enterovirus, called avian encephalomyelitis virus (AEV).
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.
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.
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.
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.
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
|Affected||History||Clinical signs||Diagnostic Findings||Tests used||Ref|
|One-day-old chickens||Numerous one-day-old chicks in a hatchery were affected.||Weakness||Necropsy and histopathology of the brain revealed multifocal lesions affecting the large nuclei in the brainstem and cerebellar Purkinje cells, with neuronal degeneration.||Necropsy; histopathology; ELISA||Conference 10-2013 Case: 02 cerebrum, cerebellum, brainstem - Chicken. AskJPC.org|
|Three flocks of 11 - to 14-week-old commerical leghorn pullets||The 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 PCR||C Senties-Cue et al., 2016|
|An outbreak in one-day-old chicks from multiple hatches on a game farm||The 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; PCR||Welchman, D et al., 2009|
|20 out of 300, 12-day-old chicks||The 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; histopathology||CAHFS Connection, October 2012|
- Clinical signs
- Physical exam
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.
Live vaccination: breeder pullets at 10-15 weeks old to prevent vertical transmission of the virus to their eggs and to provide their offspring with maternal immunity against the disease
Chickens which survive the disease have a higher chance of developing cataracts later in life.
- Gabriel Sentíes-Cué, C., Gallardo, R. A., Reimers, N., Bickford, A. A., Charlton, B. R., & Shivaprasad, H. L. . Avian Encephalomyelitis in Layer Pullets Associated with Vaccination Avian Diseases (2016)
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- Cooperman, J. M., et al.. The influence of thiamine on the susceptibility of chicks to avian encephalomyelitis ournal of bacteriology (1946)