The histology of pseudorabies differs materially in various animal species. In the rabbit, subcutaneous, intradermal or intramuscular inoculation leads to local inflammation and necrosis. The infection ascends the peripheral nerve (possibly both interstitially and by the axis-cylinders) to the corresponding spinal ganglia and segments of the spinal cord, where primary degeneration of nerve and glial cells takes place. The nerve cell changes are probably responsible for the cardinal symptom of the disease, itching. Death ensues soon after virus reaches the medulla, before visible changes have been produced here. Intracerebral inoculation is followed by characteristic lesions in the meninges, in subpial glial cells and in superficially placed nerve cells. Morbid changes in the lungs are not necessarily related to the presence of virus, but specific lesions may be present. Intranuclear inclusions bearing some resemblance to those in herpetic encephalitis, yellow fever, etc., occur in cells derived from all embryonic layers.

The disease in the guinea pig resembles closely that in the rabbit and is modified only by the slightly greater resistance of the animal.

In the monkey after intracerebral inoculation, widespread degeneration and necrosis of cortical nerve cells are accompanied by the appearance of specific nuclear alterations in nerve and glial cells, but not in cells of mesodermal origin. No lesions are found in other viscera.

In the spontaneous disease in the cow lesions approximate more closely to those in the monkey than to those in the rabbit.

In the pig vascular and interstitial lesions predominate, nerve cell degeneration is relatively slight and typical inclusions are not observed. These differences probably explain the benign course of the malady following subcutaneous inoculation in this animal. The lymphatic system, too, participates in the reaction to the virus.

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