title: In vivo characterisation of two Australian isolates of Marek's disease virus including pathology, viral load and neuropathotyping based on clinical signs.
abstract: OBJECTIVE
  To evaluate the pathogenicity of Australian Marek's disease virus (MDV) isolate MPF23 (1985) against the reference strain MPF57 based on pathology, viral load and neuropathotyping on the basis of clinical signs.
  
  
  METHODS
  Two MDV challenge isolates (MPF57 or MPF23) were administered to unvaccinated specific-pathogen free (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).
  
  
  RESULTS
  MPF23 was the more pathogenic of the two viruses, inducing higher mortality (81% vs 62%) and incidence of MD lesions (100% vs 76%). MPF23 induced earlier, more sustained and more severe neurological signs in the period 26-56 dpc. However, there were few differences during the 0-23 dpc used in the neuropathotyping classification under test. The observed pattern during this earlier period classified 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 the load at 7 dpc was significantly and negatively associated with time to euthanasia or death.
  
  
  CONCLUSIONS
  MPF23 appears to be as, or more, virulent than the 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.
authors.affiliation: Animal Science, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia.
authors.forename: S J
authors.initials: SJ
authors.lastname: Wajid
authors.affiliation: Animal Science, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia.
authors.forename: S W
authors.initials: SW
authors.lastname: Walkden-Brown
authors.affiliation: NSW Department of Primary Industries, Beef Industry Centre, University of New England, Armidale, NSW, Australia.
authors.forename: B A
authors.initials: BA
authors.lastname: Vanselow
authors.affiliation: Animal Science, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia.
authors.forename: A F M F
authors.initials: AF
authors.lastname: Islam
authors.affiliation: Animal Science, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia.
authors.forename: K G
authors.initials: KG
authors.lastname: Renz
mesh_terms: D000818:Animals; D001315:Australia; D002645:Chickens / Q000821:virology; D004195:Disease Models, Animal; D053208:Kaplan-Meier Estimate; D025922:Mardivirus / Q000145:classification / Q000302:isolation & purification / Q000472:pathogenicity*; D008380:Marek Disease / Q000097:blood / Q000401:mortality / Q000473:pathology / Q000503:physiopathology* / Q000821:virology*; D016133:Polymerase Chain Reaction / Q000662:veterinary; D011201:Poultry Diseases / Q000097:blood / Q000401:mortality / Q000473:pathology / Q000503:physiopathology / Q000821:virology*; D019562:Viral Load
keywords: Marek's disease virus; immunosuppression; neuropathotyping; pathotyping; viral quantification virulence