Do standard measures of immunocompetence reflect parasite resistance? The case of Greenfinch coccidiosis.

1. Much research in evolutionary animal ecology is currently focused on issues related to parasite-mediated selection. Because of difficulties in estimation of actual parasite resistance, researchers often rely on surrogate measures of immunocompetence, such as the magnitude of immune responses to artificial antigens, assuming that these measures correlate with individuals’ ability to resist disease. 2. This assumption was tested in the Greenfinch ( Carduelis chloris L.) coccidiosis model by measuring the association between parasite resistance and two standard assays of immune function – a swelling response to phytohaemagglutinin (PHA) and antibody production against sheep red blood cells (SRBC). 3. Before performing immune challenge assays, host resistance to the intestinal parasite Isospora lacazei (Labbe) was assessed on the basis of individual infection intensities and responses to challenge infection with alien parasite strains. 4. Increases in infection intensities after inoculation of birds with novel parasite strains correlated positively with magnitude of swelling response to PHA injection. Thus, intense parasite multiplication in the host's digestive tract resulted in the enhancement of the host's cell-mediated immune function. 5. Among birds that were vulnerable to novel infections, no correlations emerged between infection intensities, SRBC antibody titres and condition indices. 6. Among non-vulnerable hosts (whose infection intensities did not increase after experimental reinfection), SRBC antibody titres correlated negatively with infection intensities and positively with indices of nutritional condition. Hence, high SRBC antibody titres reflected good nutritional status and superior resistance to an important parasite species in a subsample of studied Greenfinches. 7. Altogether, our results exemplify the diversity of outcomes when the host's immune system is simultaneously challenged by natural enemies and artificial antigens, and call for the cautious interpretation of the results of standard immune tests in the context of parasite resistance. [ABSTRACT FROM AUTHOR]