Summary The immunoprophylactic capacity of specific immune plasma was evaluated in pony foals infected experimentally with Rhodococcus equi. Immune plasma, produced by repeated parenteral administration of viable R. equi to adult horses, was harvested and frozen. Group I (six control foals) and Group II (six principal foals) received lactated Ringers solution and immune plasma respectively at three and five days of age. R. equi were aerosolised into a caudal lung lobe of all foals at seven days of age. Clinical signs, haematological alterations, immune responses, thoracic radiographs and technetium99m pulmonary perfusion scans were monitored. All foals were destroyed and complete post mortem examinations performed. All foals developed pneumonia as evidenced by clinical, radiographic and perfusion alterations, but the survival rate of principal foals was significantly (P < 0.01) greater than that of control foals. Five control foals developed terminal disease, whereas all principal foals recovered. There was no significant (P>0.05) difference in temperature response, or peripheral blood leucocyte, neutrophil or fibrinogen concentrations between groups. ELISA values for R. equi antibody were significantly (P<0.001) greater in principal foals following treatment, but there was no significant (P>0.05) difference in IgG or IgM concentrations between groups. Results of the haemolysis inhibition assay indicated that equi factor neutralising antibodies were transferred by immune plasma to the principal foals. Post mortem examinations of five control foals destroyed at approximately three weeks post infection because of terminal disease, revealed severe pyogranulomatous pneumonia. One control and all principal foals were either free of lesions or had resolving lesions and/or minimal scar formation at three months post infection. The results of this investigation document the importance of humoral factors in controlling the disease process, and the capacity of humoral immunoprophylaxis to alter the clinical progression of R. equi pneumonia.

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A cohort study was undertaken on a dairy experiencing endemic Neospora caninum abortions, to characterize dam serologic variations during pregnancy, and to determine if dam N. caninum antibody levels during gestation predicted congenital infection or abortion. Blood samples were collected monthly during pregnancies of 254 cows and precolostrally from 87 of their calves. Antibody levels, as measured by an enzyme-linked immunosorbent assay, indicated 60.6% of cows were seropositive at some time during pregnancy and 87.4% of seropositive cows were seropositive throughout pregnancy. The rate of seroconversion was 8.5/100 cows/yr. The risk of abortion for seropositive cows at the time of pregnancy diagnosis and during gestation was twice that for seronegative cows (P = 0.025, P = 0.006). Calves born to seropositive cows were more likely to be seropositive at birth if the dam had high antibody levels at 240 days of gestation (P = 0.04) and an increase in antibody levels between 90 and 240 days (P = 0.08) than if the respective values of the dam were low or decreasing. Seropositive cows with high antibody levels at 180 and 210 days of gestation were less likely to abort than cows with low antibody levels at those times (P = 0.05, P = 0.03). Results support a causal effect between exposure to N. caninum and abortion, indicate that acquisition of infection during pregnancy is not necessary for congenital infection or abortion to occur, and suggest that maternal immune response influences congenitial infection and abortion.