Avian malaria and bird humoral immune response

Abstract

Background: Plasmodium parasites are known to impose fitness costs on their vertebrate hosts. Some of these costs are due to the activation of the immune response, which may divert resources away from self-maintenance. Plasmodium parasites may also immuno-deplete their hosts. Thus, infected individuals may be less able to mount an immune response to a new pathogen than uninfected ones. However, this has been poorly investigated.

Methods: The effect of Plasmodium infection on bird humoral immune response when encountering a novel antigen was tested. A laboratory experiment was conducted on canaries (Serinus canaria) experimentally infected with Plasmodium relictum (lineage SGS1) under controlled conditions. Birds were immune challenged with an intra-pectoral injection of a novel non-pathogenic antigen (keyhole limpet haemocyanin, KLH). One week later they were challenged again. The immune responses to the primary and to the secondary contacts were quantified as anti-KLH antibody production via enzyme-linked immunosorbent assay (ELISA).

Results: There was no significant difference in antibody production between uninfected and Plasmodium infected birds at both primary and secondary contact. However, Plasmodium parasite intensity in the blood increased after the primary contact with the antigen.

Conclusions: There was no effect of Plasmodium infection on the magnitude of the humoral immune response. However, there was a cost of mounting an immune response in infected individuals as parasitaemia increased after the immune challenge, suggesting a trade-off between current control of chronic Plasmodium infection and investment against a new immune challenge.

Keywords: Keyhole limpet haemocyanin; Plasmodium relictum; Serinus canaria.

Fig. 1

Humoral immune response after immune challenges. Anti-KLH antibody level (mg/mL) after primary and secondary contact to KLH antigen and in uninfected and infected individuals. The star indicates a significant difference

Fig. 2

Parasitaemia prior to and after immune challenges. Parasitaemia (arbitrary unit, log transformed) as a function of antigen contact in infected canaries prior to the immune challenge, after primary contact and after secondary contact. Different letters indicate significant differences

Fig. 3

Haematocrit (%) as a function of parasitaemia (arbitrary unit, log transformed) in infected individuals

Fig. 4

Body mass (g) as a function of antigen contact prior to and after immune challenges. Different letters indicate significant differences