**5. Clinical symptoms and anatomopathological changes in the course of Marek's disease is most common in poultry**

Laying hens, broiler chickens, turkeys and quails [47]. Recent years have also confirmed that it can also occur in geese. Several cases have already been recorded in Poland, both in the flocks of reproductive geese, but also in the flocks of geese intended for fattening [49]. In the case of breeding geese, Marek's disease is found in birds in the 2nd and 3rd laying season, while in geese intended for fattening at the age of 6–7 weeks, these changes are most often found during post-mortem examination [unpublished data].

A common feature of infections in geese is the fact that previously laying hens or broiler chickens have been reased in the same buildings as geese in which clinical Marek's disease was confirmed (unpublished data). Based on our own observations, Marek's disease conducted with rapid course of the disease in flock at the beginning of the infection, with a large number of dead birds. The infection then gradually "silences" [unpublished data]. In turn, the frequency of Marek's disease also depends on many predisposing factors, including: transport stress, vaccination stress, too high density of birds, sex or the content of undesirable substances in feed and water [47]. In the course of Marek's disease, clinical symptoms are closely related to the location of neoplastic lesions in the internal organs of the birds [47]. General apathy of the birds, stunted growth and even diarrhea are observed (it can be bloody if it becomes infected with coccidia). The paralysis and twist of the neck and head in the case of neurological disorders in the nervous system. One sided paralysis may be present. In the case of changes in the nerves of the eye and inflammation of intraocular structures, we may be dealing with blindness or pathological changes in the eye [50]. Generally, it can be assumed that the pathological changes can be classified into 3 groups: changes in internal organs, skin and in peripheral nerves [47].

Pathological changes in internal organs can be nodular, diffuse or both. If the lesions are diffuse, then the internal organs are significantly enlarged (rarely of normal size) with white or gray discoloration. When the lesions are nodular, the lymphomas appear singly or in small clusters of white or gray color [51].

As previously mentioned, skin lesions are associated with feather follicles and are best seen in featherless carcasses. The folios are greatly enlarged and in the form of diffuse lesions. Changes can also be seen on combs and bells. Red lesions and cavities on the skin of the thighs are called "Alabama Red Leg" syndrome. Such changes were described in Poland in 2005 in a flock of broiler chickens [52]. Peripheral nerve enlargement can be called the "golden sign" which we can observe during the anatomopathologiocal examinations [53]. Nerve changes can take the form of unilateral or bilateral nerves dysfunction. The altered peripheral nerves can be 2–3 times enlarged, lose their physiological shine and transverse striations, gray or yellow in color, sometimes swollen or with the presence of streaked hyperaemia [54]. In general, anatomopathological changes in Marek's disease occur most frequently in internal organs such as the liver and spleen. There are the changes described above. Of course, some changes can also occur in other organs, but much less frequently. The following can be mentioned here: the reproductive system (especially the ovary and testicle), kidneys (most often manifested significantly enlarged with a marked structure of renal

tubules), and glandular stomach (significant thickening of the wall). In addition, there may be complete atrophy of the thymus and bursa of Fabricius [47].

In the course of Mare's disease, in its non-oncogenic form, there are usually three forms: lymphodegenerative syndrome, transient paralysis and panophthalmitis (also known as "gray eye") [55]. Lymphodegenerative syndrome is observed only in unvaccinated birds against Marek's disease and in which no maternal antibodies are present. On the other hand, lymphatic organs such as the bursa of Fabricius and the thymus undergo very quick atrophy, already around 6–8 days after infection, and turn yellowgreen. It has been proven that they are more atrophied after infection with Marek's disease virus of low pathogenicity in case of classical strains. If the birds are subsequently infected with highly virulent train expressing higher pathogenicity, atrophy does not progress. Additionally, necrotic foci may appear [47]. Transient paralysis is most common in birds vaccinated against Marek's disease, mostly in broiler chickens around 40 days of age. It mainly affects the neck muscles in the initial period, and then the paralysis gradually progresses in other parts of the muscles [56].
