Systematic Veterinary Virology

PATHOTYPES

• Infection with the extremely virulent viruses may result in sudden, high mortality with comparatively few clinical signs. The virus is grouped into five "pathotypes" on the basis of the predominant signs in affected chickens. They are as follows;
  • Viscerotropic velogenic (Doyles form): viruses responsible for disease characterised by acute lethal infections, usually with haemorrhagic lesions in the intestines of dead birds. The infection caused by this pathotype is also called as Asiatic or exotic ND.
  • Neurotropic velogenic (Beach’s form): viruses causing disease characterised by high mortality, which follows respiratory and neurological disease, but where gut lesions are usually absent.
  • Mesogenic (Beaudettes form): viruses causing clinical signs consisting of respiratory and neurological signs, with low mortality.
  • Lentogenic (Hitchner’s form): viruses causing mild infections of the respiratory tract.
  • Asymptomatic enteric: viruses causing avirulent infections in which replication appears to be primarily in the gut.
• Some workers differentiate NDV into three pathotypes:
• Lentogenic,
• Mesogenic, and
• Velogenic, reflecting increasing levels of virulence. The most virulent (velogenic) isolates are further subdivided into neurotropic and viscerotropic types.