Living with chronic infection: Persistent immunomodulation during avirulent haemoparasitic infection in a wild rodent

Abstract

Apicomplexans are a protozoan phylum of obligate parasites which may be highly viru-lent during acute infections, but may also persist as chronic infections which appear tohave little fitness cost. Babesia microti is an apicomplexan haemoparasite that, in im-munocompromised individuals, can cause severe, potentially fatal disease. However,in its natural host, wild field voles (Microtus agrestis), it exhibits chronic infections thathave no detectable impact on survival or female fecundity. How is damage minimized,and what is the impact on the host's immune state and health? We examine the dif-ferences in immune state (here represented by expression of immune-related genesin multiple tissues) associated with several common chronic infections in a populationof wild field voles. While some infections show little impact on immune state, wefind strong associations between immune state and B. microti. These include indi-cations of clearance of infected erythrocytes (increased macrophage activity in thespleen) and activity likely associated with minimizing damage from the infection (anti-inflammatory and antioxidant activity in the blood). By analysing gene expressionfrom the same individuals at multiple time points, we show that the observed changesare a response to infection, rather than a risk factor. Our results point towards con-tinual investment to minimize the damage caused by the infection. Thus, we shed lighton how wild animals can tolerate some chronic infections, but emphasize that thistolerance does not come without a cost.