Persistent parapoxvirus infection in cattle.

The possibility of persistent parapoxvirus (PPV) infection was investigated by serologically and genetically using cattle infected with the virus experimentally and naturally. Three cattle were inoculated with the virus subcutaneously at several spots in the lips and abdominal regions. Small papules developed in the inoculated regions, and antibodies to the virus developed and continued persistently. One animal, from which one PPV had been previously isolated, was also subjected to serological and viral detection tests as a naturally infected case. Two of these four cattle were injected with dexamethasone (DM), and one was injected with interferon-gamma (IFN-gamma). The viral genome was rarely detected from the peripheral blood leukocytes in the ordinary condition, but frequently when the animals were injected with IFN-gamma. The viral genome was also detected from the lymph nodes as these PPV infected animals were euthanized. These results indicated that cattle were infected with PPV subclinically and persistently, and the virus was activated in stressed or immunosuppressed animals. The virus would be harbored in the lymphotic tissues of the animals when they show no clinical symptoms.