Acute Mareks disease is an epidemic in a previously uninfected or unvaccinated flock, causing depression, paralysis, and death in a large number of birds (up to 80%). The age of onset is much earlier than the classic form; birds are four to eight weeks old when affected. Infiltration into multiple organs/tissue is observed. Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), unequal size of the pupils, and blindness. Cutaneous Mareks disease causes round, firm lesions at the feather follicles. Atherosclerosis is induced in experimentally infected chickens. Immunosuppression is impairment of the T-lymphocytes resulting from Mareks disease, preventing a competent immunological response against pathogenic challenge, with the affected birds become more susceptible to disease conditions such as coccidiosis and Escherichia coli infection. Furthermore, without stimulation by cell-mediated immunity, the humoral immunity conferred by the B-cell lines from the Bursa of Fabricius also shuts down, thus resulting in birds that are totally immunocompromised. Diagnosis
Diagnosis of lymphoid tumors in poultry is complicated due to multiple etiological agents capable of causing very similar tumors. It is not uncommon that more than one avian tumor virus can be present in a chicken, thus one must consider both the diagnosis of the disease/tumors (pathological diagnosis) and of the virus (etiological diagnosis).
Key clinical signs as well as gross and microscopic features that are most useful for differentiating Mareks disease from lymphoid leukosis and reticuloendotheliosis include:

Age: Mareks disease can affect birds at any age, including <16 weeks of age;
Clinical signs: Frequent wing and leg paralysis;
Incidence: >5% in unvaccinated flocks;
Potential nerve enlargement;
Interfollicular tumors in the bursa of Fabricius;
CNS involvement;
Lymphoid proliferation in skin and feather follicles;
Pleomorphic lymphoid cells in nerves and tumors; and
T-cell lymphomas.In addition to gross pathology and histology, other advanced procedures used for a definitive diagnosis of Mareks disease include immunohistochemistry to identify cell type and virus-specific antigens, standard and quantitative PCR for identification of the virus, virus isolation to confirm infections, and serology to confirm/exclude infections. The World Organisation for Animal Health (OIE) reference laboratory for Mareks disease is Avian Viral Oncogenesis group (led by Professor Venugopal Nair OBE) at The Pirbright Institute, UK.PCR blood testing can also detect Mareks disease, and proper testing can differentiate between a vaccinated bird with antibodies and a true positive for Mareks disease. Mareks disease is not treatable, however, supportive care can help. It is recommended that all flocks positive for Mareks disease remain closed, with no bird being introduced or leaving the flock. Strict biosecurity and proper cleaning is essential, using products like Activated Oxine or Virkon S and reducing dander buildup in the environment. Proper diet, regular deworming and vitamin supplements can also help keep infected flocks healthier. Reducing stress is also a key component, as stress will often bring about illness in birds infected with Mareks disease.