Plaque neutralization assays were carried out using the rhabdoviruses of pike fry (2 isolates), spring viraemia of carp and grass carp. When rabbit anti-pike fry rhabdovirus sera were used, no neutralizing activity was observed but plaque counts increased with heat-inactivated sera or sera that had been stored at -20 °C for prolonged periods. Antibody binding to the virus was demonstrated by gradient centrifugation experiments. The use of immune serum alone or in combination with anti-rabbit gamma globulin serum resulted in aggregation of radioactive virus. Aggregation was also observed when anti-rabbit gamma globulin serum was added under neutralization test conditions. Addition of non-inactivated guinea-pig serum to mixtures of pike fry rhabdovirus and rabbit antiserum to spring viraemia of carp virus resulted in strong neutralization, whereas this antiserum alone or in the presence of inactivated guinea-pig serum gave an increase in plaque numbers. The increase in plaque number and the complement-dependent neutralization phenomena were weak or absent in the homologous spring viraemia of carp virus system.

Immunization of pike with pike fry rhabdovirus resulted in an increase in virus neutralizing activity which could be attributed to the macroglobulin fraction of the serum. After immunizations over a period of 6 months with virus antigens, no further increase in neutralizing activity was observed. Pike sera showed 50% plaque reduction at concentrations between 0.017 and 0.3%. Using these sera it could be demonstrated that the grass carp virus and the V 75/94 isolate from pike fry were indistinguishable from the original pike fry rhabdovirus, whereas spring viraemia of carp virus was different.


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