In Vivo Characterisation of Two Australian Isolates of MDV including Pathology, Viral Load and Neuropathotyping, based on Clinical Signs

We evaluate the pathogenicity of Australian Marek's disease virus (MDV) isolate MPF23 (1985) against the reference strain MPF57 based on pathology, viral load and neurotpathotyping on the basis of clinical signs. Two MDV challenge isolates (MPF57 or MPF23) were administered to unvaccinated SPF layer chicks on day 5 after hatch at three challenge doses (500, 2000 or 8000 plaque forming units (pfu)/chick). Mortality, body weight, immune organ weights, MDV load in peripheral blood lymphocytes (PBL) and clinical signs were measured to 56 days post challenge (dpc). MPF23 was the more pathogenic of the two viruses inducing higher mortality (81% cf. 62%) and incidence of MD lesions (100% cf. 76%). MPF23 induced earlier, more sustained and more severe neurological signs in the period 26-56 days post challenge (dpc). However there were few differences during the period 0-23 dpc used in the neuropathotyping classification under test. The observed pattern during this earlier period classifies both viruses as neuropathotype B consistent with a very virulent pathotype. MDV load in PBL at 7 and 44 dpc did not differ between virus isolates but load at 7 dpc was significantly and negatively associated with time to euthanasia or death. MPF23 appears to be as, or more virulent than MDV strains isolated over the subsequent two decades. The neuropathotyping system developed in the USA did not clearly differentiate between the two isolates under test, however extension of the period of assessment of clinical signs beyond 26 dpc did reveal clear differences.