Characterisation of Two Australian Isolates of Marek's Disease Virus 'in Vivo' Including Neuropathotyping

Virulent Marek's disease virus (MDV) induces early (1-3 weeks after challenge) neurological signs in chickens free of maternal antibody directed against MDV. A system for neuropathotyping MDV on the basis of the timing and severity of these signs has been reported in the USA and we report its application as part of the evaluation of virulence of Australian MDV isolates MPF23 and MPF57 at 3 different challenge doses in specific pathogen free (SPF) chicks over a 56-day post challenge period. MPF23 was the more pathogenic of the two viruses as assessed by a range of measures including mortality rate (81% versus 62%) and incidence of gross Marek's disease (MD) lesions (95% versus 76%). This was reflected in a clearly different pattern of clinical illness over the 56-day period with the MPF23 group showing earlier, more sustained and more severe clinical signs in the period 26-56 day post challenge (dpc). However there were few differences during the 0-23 dpc period used for the USA neuropathotyping system. The observed pattern during this period classifies both viruses as neuropathotype B consistent with a vv pathotype under the pathotying classification of the Avian Diseases and Oncology Laboratory (ADOL) of the USDA.