**4. Virus replication and the pathogenesis of Marek's disease**

Marek's disease virus is an infection via the respiratory system. Subsequently, lymphocytes B and macrophages which have been found in the lungs are activated. The virus then moves to the bird's main lymphatic organs, Bursa of Fabricius, thymus, and spleen [31–33]. After replication in lymphocytes B cells, the virus spreads to T cells, mainly CD4 + cells. However, only some of the lymphocytes T are transformed and they are then a source of lymphoma formation. They are located within internal organs, mainly the kidneys, spleen, liver, ovary or testes, and even in the gizzard. They are very rarely found in peripheral nerves, skin and muscles [34]. The virus then enters a latent phase in most transformed lymphocytes T cells. Only a very small percentage of the neoplastic cells approximately (<0.001%) contain viral particles detected by electron microscopy (TEM) [35]. It should also be added that MDV occurs with a latency state only in lymphocytes, and not in neurons, as is the case with other alphaherpesviruses [36]. In the active phase of infection, the virus particles are transferred to the skin cells. Then, along with the exfoliating epithelium, it is spreader into the environment and is a direct source of infection for other birds. Of course, vertical transmission of MDV has not been confirmed, although there are reports of MDV genetic material being found in experimentally infected chicken embryos and egg surfaces [37]. On the basis of research using the immunofluorescence method, it was found that the so-called follicles play the main role in the process of MDV release into the environment of the poultry house. It has been shown that follicles can develop complete and mature virus particles which are capable of infecting other birds. It was also found that the full infectious particles of MDV in the poultry house environment is up to 7–9 months at room temperature, and much longer at lower temperatures. This time is significantly extended if the virus in the poultry house is "surrounded" by biological material such as dust or chicken droppings. That is why it is so important to thoroughly sanitize and disinfect poultry houses, especially before introducing 1-day-old chickens [38]. The results of conducted studies characterizing viral genes and proteins associated with feather follicles were also performed and published. It was confirmed that many viral proteins are expressed at a much higher level in feather follicles compared to expression in cells of internal organs [39].

Some authors believe that the replication of the virus particles in the feather follicles begins as early as 7–10 days after MDV infection. Most likely, the infected cells transmit the virus to the epithelial layer of the epidermal, where the virus replication and infects the neighboring cells of the skin, including fibroblast cells and melanocyte precursors. So far, a marker responsible for virus replication in skin epithelial cells has not been described [40]. In infected birds, 2 types of lesions may occur in the skin: neoplastic lesions in the form of tumors and lesions other than tumors. Many authors suggest that the nature of these changes does not allow them to be called the so-called "skin leukemia". It has also been shown in some studies that tumor cells do not contain viral antigens, and the research method used was immunofluorescence. Intensive research is underway on this issue [41]. As previously indicated, the genome structure of each of the MDV regions is similar, but the existing differences are nevertheless crucial. The oncogenicity of serotype 1 of MDV strains is determined primarily by the presence of the meq gene, but also the pp38 gene, vIL8 and vTR gene. They influence the oncogenic transformation processes and activated T lymphocytes. In turn, the dimmer form of the meq gene influences the expression level of cellular apoptotic factors and virus transformation [42, 43].

In molecular studies, it was found that all MDV strains represented three serotypes reduce the expression level of the Major Histocompatibility Complex (MHC). This phenomenon seems to be useful in studies to generate lines of birds resistant to MDV infection, and in particular with regard to cellular immunity, both after natural infection and prophylactic vaccination [44].

There are several unique features in the pathogenesis of Marek's disease. In the early (cytolytic) phase of the infection, lasting approximately 7 to 10 days, the virus causes a "massive destruction" of lymphoid cells and macrophages, resulting in the phenomenon of immunosuppression [45].

After this time, Marek's disease virus enters a latent phase, which lasts until the end of life in CD4 + and CD8 + T cells. Virtually all genes of Marek's disease virus are expressed at a much lower level. This condition is mainly due to neoplastic conversion of CD4 + cells and the formation / development of multiple lymphomas in several internal organs in infected birds. It causes mortality among birds starting from 3 to 4 weeks post infection and which is sometimes not "associated" with Marek's disease by producers /veterinarians. The first nervous symptoms in the form of paralysis may already appear, which results from a significant degree of lymphatic infiltration in the peripheral nerves [46].

After approximately 10 days, the virus spreads through the bloodstream to the skin epithelial cells / feather follicle cells. This is another type of interaction between Marek's disease virus and the host cell [45, 46]. On the basis of its own and many other authors' observations, it causes various symptoms of this disease or even syndromes that can be divided into two groups: oncogenic and non-oncogenic. This division depends on whether or not the birds have maternal antibodies. It should be presumed that most hatched chickens have maternal antibodies which disappear after 3–4 weeks [unpublished data].

The natural route of infection with Marek's disease virus is through the respiratory system by aspiration of the dust containing cell-free virus particles [47].

The role of lung cells in the pathogenesis, however, is not fully understood despite the presence of viral antigen in lung cells. Phagocytes present in lung tissue are believed to "capture" the virus and transmit it to the lymphoid organs (bursa of Fabricius, thymus and spleen). Infection with the virus affects of epithelial cells in internal organs and epithelial cells in blood vessels. There are already primary foci of necrosis and symptoms of inflammation in internal organs. At this stage of the infection, the viral antigen can be detected by electron microscopy in all infected cells. This condition causes transitional immunosuppression [48].
