Complex interactions between bacteria and haemosporidia in coinfected hosts: An experiment

Abstract

Hosts are typically coinfected by multiple parasite species whose interactions might be synergetic or antagonistic, producing unpredictable physiological and pathological impacts on the host. This study shows the interaction betweenPlasmodiumspp. andLeucocytozoonspp. in birds experimentally infected or not infected withMycoplasma gallisepticum. In 1994, the bacteriumMycoplasma gallisepticumjumped from poultry to wild birds in which it caused a major epidemic in North America. Birds infected withM. gallisepticumshow conjunctivitis as well as increased levels of corticosterone. Malaria and other haemosporidia are widespread in birds, and chronic infections become apparent with the detectable presence of the parasite in peripheral blood in response to elevated levels of natural or experimental corticosterone levels. Knowing the immunosuppressive effect of corticosterone on the avian immune system, we tested the hypothesis that chronic infections ofPlasmodiumspp. andLeucocytozoonspp. in house finches would respond to experimental inoculation withM. gallisepticumas corticosterone levels are known to increase following inoculation. Plasmodiumspp. infection intensity increased within days ofM. gallisepticuminoculation as shown both by the appearance of infected erythrocytes and by the increase in the number and the intensity of positive PCR tests. Leucocytozoonspp. infection intensity increased whenPlasmodiumspp. infection intensity increased, but not in response toM. gallisepticuminoculation.Leucocytozoonspp. andPlasmodiumspp. seemed to compete in the host as shown by a negative correlation between the changes in their PCR score when both pathogens were present in the same individual. Host responses to coinfection with multiple pathogens measured by the hematocrit and white blood cell count depended on the haemosporidian community composition. Host investment in the leukocyte response was higher in the single-haemosporidia-infected groups when birds were infected withM. gallisepticum. A trade-off was observed between the immune control of the chronic infection (Plasmodiumspp./Leucocytozoonspp.) and the immune response to the novel bacterial infection (M. gallisepticum).