1Department of Veterinary Pathology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Theni-625 534, Tamil Nadu, India.
2Department of Veterinary Pathology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Orathanadu-614 625, Tamil Nadu, India.
3Department of Veterinary Microbiology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Orathanadu-614 625, Tamil Nadu, India.
4Department of Veterinary Public Health and Epidemiology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Orathanadu-614 625, Tamil Nadu, India.
5Department of Veterinary Pathology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Tirunelveli-627 001, Tamil Nadu, India.
*Corresponding Author: R. Ravikumar, Department of Veterinary Pathology, Veterinary College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Theni-625 534, Tamil Nadu. India. Email: ravikumar1975path@gmail.com
Marek’s disease (MD) has re-emerged as a significant threat to Indian poultry despite widespread vaccination, causing annual losses of 40 million Indian rupees with 10-40% mortality in vaccinated flocks across Punjab, Assam, Uttar Pradesh and Andhra Pradesh. The emergence of virulent (vMDV) and very virulent (vvMDV) pathotypes challenges disease management. Indian field isolates are categorized into mild, virulent, very virulent and very virulent plus pathotypes based on serotype 1-specific gene sequencing, with virulence correlating to fewer PPPP (four prolines) repeat sequences in the Meq gene.
This study investigated 46 MD suspected eye cases from 25 commercial layer farms in Namakkal and Thalaivasal districts, Tamil Nadu. Ocular manifestations predominantly affected birds aged 36-64 weeks, presenting with swollen closed eyes, conjunctivitis and greyish-white iris discoloration. Histopathological (HP) examination revealed 18 positive cases (39.13%), confirmed by PCR.
A modified scoring system (0-5) assessed pleomorphic lymphoid cell infiltration severity: Score 1 showed multifocal mild perivascular infiltration in choroid; Score 2 demonstrated focal moderate choroidal/iris infiltration; Score 3 revealed severe choroidal infiltration with lens fibre vacuolation; Score 4 showed diffuse moderate iris infiltration with mixed inflammatory cells; Score 5 exhibited diffuse severe iris and choroidal infiltration. Enhanced surveillance, improved biosecurity and more effective vaccines against virulent strains are essential for controlling this re-emerging disease in Indian poultry.
Eye, Polymerase chain reaction, Histopathology, Layer chicken, Lesion score, Marek’s disease, Pleomorphic lymphoid Cell (PLC)
