Figure 1. Avian bornavirus protein demonstrated by immunohistochemical testing in the central nervous system of birds with proventricular dilatation disease (PDD). A) within nuclei, cytoplasm and dendrites of several Purkinje cells of the cerebellum, bar = 50 µm; B) negative control: no immunoreactivity of Purkinje cells in a PDD-negative bird, bar = 50 µm; C–F), different phenotypes of positive neurons: C) within neurons, viral protein is expressed within intranuclear inclusion bodies; D) diffusely within the nucleus accompanied by faint cytoplasmic staining; E) both, within the nucleus and cytoplasm, with more intense staining of intranuclear inclusion bodies; F) exclusively within the cytoplasm and the nucleus spared, bar = 12.5 µm; G) within an axon in the white matter of the medulla oblongata, bar = 25 µm; H, negative control: no immunoreactivity of a cerebral neuron in a PDD-negative bird, bar = 12.5 µm.