Studies on Complement‐Fixation Reaction in Equine Infectious Anemia

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Abstract
Complement‐fixing (CF) and complement fixation‐inhibiting (CFI) antibodies were investigated sequentially in three horses infected with equine infectious anemia (EIA) virus. The CF activity was first demonstrated 6 or 8 days after onset of the first pyrexia. The CFI activity developed a short period later, and caused a decrease of apparent CF titer of the whole serum. However, both antibodies tended to increase with advance of the disease course in two persistently infected horses, whereas they became completely undetectable during the late‐stage in the remaining horse which showed no evidence for recurrence of pyrexia or persistence of viremia. The CF activities determined with varying dilutions of serum were distinctly different in pattern between the early‐stage serum having the CF activity alone and the late‐stage serum having both the CF and the CFI activities. The CF antibody was precipitated by 27–30% saturation with ammonium sulfate (SAS) while the CFI activity distributed in a wider range of precipitates formed by 26–32% SAS. The CFI activity was demonstrated to a higher titer when a relatively small amount of antigen was sensitized with CFI antibody prior to addition of reference CF antibody than when the three reagents were mixed at one time. The late‐stage serum with a strong CFI activity against EIA antigen had no effect on the CF reaction of other viral antigen–antibody systems such as equine influenza and equine rhinopneumonitis.