A new disease having a characteristic and well defined symptom complex is described as occurring in young chickens in four New England states. Tremor, principally of the head and neck, and progressive ataxia are the characteristic symptoms, either or both of which may be present in a single bird. Age at onset in field epidemics ranges from 3 days to 6 weeks, with a majority of cases reported at 3 weeks. Morbidity in commercial flocks ranges from 5 to 50 per cent; mortality in affected hatches may be 50 per cent. The disease may or may not recur in successive hatches, and in the same flock in successive years. Although birds may survive an attack of the disease, nervous symptoms persist in a majority of cases.
There is no evidence that nutritional factors are involved. Normal chickens have not contracted the disease by contact with affected birds. The disease has been reproduced in normal chickens by intracerebral inoculation of brain and spinal cord from affected birds. Twenty brain-to-brain passages have been made up to the present time. The incubation period in laboratory passages ranges from 6 to 44 days with symptoms appearing usually between 21 and 28 days. The proportion of inoculated birds developing symptoms has increased with successive passages.
The infective agent in the brain has survived in 50 per cent glycerine for 69 days. No organism has been cultivated. The disease has been reproduced after inoculation with bacteriologically sterile filtrates obtained with Seitz and Berkefeld N filters.
Attempts to demonstrate the presence of the infective agent in the chicken embryo have been inconclusive. Chicks hatched from eggs laid by birds which had survived the disease were not infected, nor were they immune to inoculation at 6 weeks of age.
The characteristic lesion of the disease consists of microscopic focal collections of glia cells, perivascular infiltration, degeneration of Purkinje's cells, and degeneration of nerve cells. Foci of infiltration are present throughout the brain and spinal cord. In the viscera of birds from field epidemics, microscopic focal infiltrations of cells of the lymphoid series are often found. Their presence is most notable in the pancreas and heart. No cell inclusions have been demonstrated.