With the onset of apnea, low pressure develops in the airspace of the lungs because more oxygen is absorbed than CO2 is released. With the airways closed or obstructed, this will lead to a gradual collapse of the lungs and suffocation. However, if the airways are open, any gas supplied to the upper airways will follow the pressure gradient and flow into the lungs to replace the oxygen consumed. If pure oxygen is supplied, this process will serve to replenish the oxygen stored in the lungs and resume sufficient ventilation. The uptake of oxygen into the blood will then remain at the usual level, and the normal functioning of the organs will not be affected. A detriment to this hyperoxygenation is the occurrence of nitrogen washout, which can lead to absorption atelectasis.However, no CO2 is removed during apnea. The partial pressure of CO2 in the airspace of the lungs will quickly equilibrate with that of the blood. As the blood is loaded with CO2 from the metabolism without a way to remove it, more and more CO2 will accumulate and eventually displace oxygen and other gases from the airspace. CO2 will also accumulate in the tissues of the body, resulting in respiratory acidosis. Under ideal conditions (i.e., if pure oxygen is breathed before onset of apnea to remove all nitrogen from the lungs, and pure supplemental oxygen is insufflated), apneic oxygenation could theoretically be sufficient to provide enough oxygen for survival of more than one hours duration in a healthy adult.
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.