Histopathological changes and sequence analysis of field strains of MDV associated with tumors in commercial layer farms in 2015 to 2019 [electronic resource].

Includes references.

Marek’s disease is considered one of the major economic problems in all poultry sectors
especially commercial layers despite of intensive vaccination. This study reports the
existence of MDV serotype1 very virulent plus (VV+) in commercial layer farms aged 16-
40 weeks during 2015 to 2019 in eight Egyptian governorates. The study includes 31
affected layer farms demonstrating characteristic tumors with clear typical picture of
Marek’s disease (high mortality and paralysis). Tumor samples (Liver and spleen) were
collected for detection of MDV viral genome by (PCR) using oligonucleotide primers to
amplify 132 bp tandem repeat of specific DNA sequence to differentiate between the field
strain (434 bp) or vaccine strain (multiple repeats of 132 bp) of MDV. Results revealed
that 4 samples out of 31 were positive (two field and two vaccinal strains of MDV). The
two field isolates of MDV were from Qalioubia flocks at 15 weeks old, and from Sharkia
at 21weeks old, whereas the other two positive vaccinal strains were from Qalioubia at 22
weeks old and Gharbia at 16 weeks old. Sequencing of the amplified fragments of MDV
genome was carried out and the obtained sequences were submitted to gene bank with
accession numbers (OK040950 MDV1/AHRI-1/Egypt and OK040951 MDV1/AHRI 2/Egypt).
Sequence analysis of the 2 isolates showed that both viruses are identical.
Indeed, the study reports the circulation of VV+ MDV in commercial layer farms.
Whether the existence of field strains due to a vaccination failure or emerging
of new strains of MDV need further investigation.
Keywords: Marek’s disease virus (MDV), Polymerase Chain Reaction (PCR), Very Virulent Plus (VV+)

Summary in Arabic.