IMMUNOSUPPRESSION IN SHEEP EXPERIMENTALLY INFECTED WITH EHRLICHIA-PHAGOCYTOPHILA

The effect of tick-borne fever (TBP) on antibody formation and lymphoc yte proliferation in sheep was studied following experimental infectio n with Ehrlichia phagocytophila. All infected sheep developed fever wi thin three to four days. The sheep recovered clinically within eight d ays. Both infected and non-infected control sheep were immunised twice with different antigens, that is, on days 9 and 35 following the expe rimental infection. The levels of antibodies produced against tetanus toroid and influenza virus in the infected sheep were significantly lo wer than in the control animals. The findings indicated that a TBF-inf ection may impair both primary and secondary antibody responses for up to six weeks. Immunisation with Actinomyces pyogenes resulted in sign ificantly higher antibody titres in the TBF-infected group than in the control group, as measured by an enzyme-linked immunosorbent assay (E LISA). It is believed that TBF-induced neutropenia may lead to increas ed exposure to A pyogenes-antigens and thereby enhance antibody produc tion. Antibodies to E phagocytophila were measured by the indirect flu orescent antibody test and by an ELISA. The inoculated sheep responded with the formation of antibodies to E phagocytophila at one week (P<0 .025), and showed a peak response at four weeks (P<0.0005) after inocu lation. The antibody titre decreased between four and six weeks, but w as still high at six weeks (P<0.0005). The lymphocyte responses to phy tohaemagglutinin (PHA) concanavalin A (Con A) and pokeweed mitogen (PW M) were lower than in the control group and this difference was signif icant at most time points after infection. The lower responses to Con A and PWM were demonstrated within six weeks after the inoculation, wh ereas the lower response to PHA was shown within four weeks. Serum fro m one sheep, collected four weeks after infection, suppressed the in v itro lymphocyte response to Con A strongly, and the response to PHA an d PWM moderately. This indicates that unidentified serum factors may a t least be partly responsible for the suppressed mitogen responses.