Route of challenge is critical in determining the clinical outcome of infection with a very virulent oncogenic herpesvirus, Marek's disease virus

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Abstract
The majority of experimental studies examining Marek's disease virus infection have used parenteral injection of cell-associated virus. The aim of this study was to examine whether the route of entry of virus was critical in determining the outcome of infection. Susceptible (L7) and resistant (L6) White Leghorn chickens were infected with a very virulent Marek's disease virus, RB1B, by either the intra-abdominal or intra-tracheal route. Birds infected by the intra-tracheal route had earlier, higher or more sustained blood, spleen and lung viral concentrations than those infected by the intra-abdominal route. L7 birds had higher viral loads than L6 birds infected by the same route. Clinical outcomes reflected these data. Resistant birds infected by the intra-tracheal route had an increased prevalence of tumours and shorter survival times compared with those infected by the intra-abdominal route. Susceptible birds infected by the intra-tracheal route became paralysed 10 days after infection. L7 birds had shorter survival times and increased prevalences of tumours than L6 birds. The pathology and viraemia seen with intra-tracheal infection could not be fully replicated by increasing the dose in intra-abdominal infections. We conclude that instillation of infective dust produces a more aggressive infection that depends on the route of entry and form of virus, and not just on the challenge dose.