**2. Brief characteristics of the etiological factor of Marek's disease**

The etiological agent of Marek's disease is the herpesvirus (MDV) associated with the host cell (herpes virus-associated cell) of the genus Marbivirus. According to the principles of the new taxonomy announced by the International Committee on the

Taxonomy of Viruses, the division into 3 MDV serotypes is now as follows: Gallid herpesvirus 1 (serotype 1), Gallid herpesvirus 3 (serotype 2) and Meleagrid herpesvirus 1 (serotype 3) [25]. They were also divided into pathotypes of serotype 1 according to the same committee is as follows:


Test the classification of individual strains into appropriate serotypes based on examining with typically specific monoclonal antibodies should be scored. The so-called patotyping of strains within serotype 1 was production based on studies (pathogenicity test) using vaccinated chickens against Marek's disease.

As previously identified with the widespread use of molecular biology, the MDV genome analysis did not define the classical markers of Marek's disease virus strains with the virulence of each of the patotypes in serotype 1 [5, 10, 26].

The MDV virion has hexagonal symmetry with a diameter of approximately 150–160 nm [4]. In turn, the nucleocapsid consists of 162 capsomers, and has a diameter of approximately 85–100 nm. The MDV genome density is approximately 1.706 g / ml (in the CsCl gradient) [4]. The differences between sequences of Marek's disease virus strains within serotypes have been presented in **Figure 1**.

The MDV genome is made of DNA and contains in its composition a unique long sequence (UL), a unique short sequence (US), which are limited by terminal repeat (TRL) sequence and internal repeat sequences (IRS) [27, 28]. Due to the high density, it is extremely difficult to divide the genetic material of the virus from the genetic material of the host. Nevertheless, the genes of the HVT FC126 strain are very often used as a vector in the production of vaccines intended for poultry, e.g. against avian influenza or infectious bursa disease.

**Figure 1.**

*Genome differences between sequences of Marek's disease virus strains within serotypes.*
