Neural form: Symptoms vary depending on which nerves are affected. In general, clonic spasms and paralysis occur, sometimes leaving infected birds unable to move, eventually leading to death by starvation.

—Eye shape: Typically affecting the iris, in which case the pupil appears elongated, asymmetric, and unable to react to the light pulses.

If the injury affects the eyes, it can lead to blindness, which prevents the animal from finding food.

Visceral form: When a bird has tumors in its internal organs, it appears lethargic and dazed, with ruffled feathers, suffers from severe loss of appetite, and eventually dies.

Marek's disease presents a number of injuries, very characteristics that affect various nerves and tissues:

Peripheral nerves:

The disease presents in chronological order edema, phenomena and degenerative appearance of lymphocytic infiltrates accompanied by plasma cells and macrophages.

Within this lesion fundamental, the nerves can be divided into three types:

—Type A: The appearance of Marek's cells, which are large basophilic cells with vacuolated cytoplasm and a large nucleus.

Can be blastemal cells that have undergone degenerative processes.

Lymphocytic infiltrate abundant, demyelination and proliferation of Schwann cells.

—Type B: Diffuse infiltration of plasma cells and lymphocytes, small. Edema, sometimes demyelination and proliferation of Schwann cells.

—Type C: The nerve appears normal and is only observed a slight diffuse infiltration by plasma cells and small lymphocytes.

These three types of injuries are the evolutionary phases of the same process, that is to say, that an injury of type C would be the time to be kind To, to achieve then an intermediate form between A and B and the result after a lesion of type B.

The rate of change varies from bird to bird and is believed to depend on individual susceptibility or resistance to the disease.

Muscle injuries:

It are only observable when the process of Marek's disease lasts a long time.

Macroscopically characterized by:

Muscle atrophy, loss of color, and sometimes whitish nodules.

Microscopically, they are divided into two categories:

— Atrophy trope neurogenic: 

Swelling of nuclei and the sarcolemma. Prominence and elongation of the nucleus. Proliferation of nuclei.

—Metastatic nerve lesions:

Lymphocytic infiltrate between the muscle fibers, and even accumulations' lymphocyte subsets that result macroscopically in the appearance of whitish nodes, and this results in the degeneration of the muscle fiber.