A Review on the Marek's Disease Outbreak and Its Virulence-Related meq Genovariation in Asia between 2011 and 2021

Abstract

Marek's disease is an infectious disease in poultry that usually appears in neural and visceral tumors. This disease is caused by Gallid alphaherpesvirus 2 infection in lymphocytes, and its meq gene is commonly used in virulent studies for coding the key protein functional in oncogenic transformation of the lymphocytes. Although vaccines have been introduced in many countries to control its spread and are proven to be efficient, recent records show a decline of such efficiency due to viral evolution. In this study, we reviewed the outbreak of Marek's disease in Asia for the last 10 years, together with associated meq sequences, finding a total of 36 studies recording outbreaks with 132 viral strains in 12 countries. The visceral type is the most common (13 in 16 studies) form of Marek's disease, but additional unobserved neural changes may exist. MD induces liver lymphoma most frequently (11 in 14 studies), and tumors were also found in spleen, kidney, heart, gizzard, skin, intestine, lung, and sciatic nerve. Twelve viral strains distributed in China have been reported to escape the CVI988 vaccine, reaching a mortality rate of more than 30%. Phylogenetic analyses show the internal connection between the Middle East (Turkey, Iraq, Iran, Saudi Arabia), South Asia (India, Indonesia), and East Asia (China and Japan), while external viral communications might occasionally occur. In 18 strains with both sequential and mortality data, amino acid alignment showed several point substitutions that may be related to its virulence. We suggest more behavioral monitoring in Marek's disease-endemic regions and further studies on strain virulence, together with its Meq protein structural changes.

Keywords: Asia; Marek’s disease; meq sequence; pathology; virus.

Figure 1

The study location and number of Marek’s disease outbreaks in Asia between 2011 and 2021.

Figure 2

The phylogenetic tree of the isolated viruses according to meq sequences. On the strain names, green labels the vaccine strain CVI988, and blue and red label the strain that bypassed protection by the CVI988 vaccine, reaching a mortality of more than 30% and 50%, respectively. On the branches, red, brown, and purple label the China groups 1, 2, and 3, respectively. The blue, green, and yellow branches label the Middle East, India group 1, and India group 2, respectively. The meq consists of 1020 bp, and therefore 0.001 evolutionary distance can represent 1 different point mutation.