Environmental enrichment, immunocompetence, and resistance to Babesia microti in male mice.

Groups of male CFLP mice housed in cages furnished with shelves and nestboxes showed increased aggression and reduced resistance to an experimental infection of Babesia microti when compared with groups in unfurnished cages. Both a bystander measure of immunocompetence (serum total IgG concentration) and resistance to B. microti decreased as the number of attacks received by mice increased, but increased with the number of times individuals were recorded on shelves or in nestboxes. Serum concentrations of testosterone and corticosterone were generally downregulated in furnished cages; the absence of hormone-related reduction in resistance may have been due partly to this, but partly also to the apparent modulation of hormone concentrations in relation to concurrent immunocompetence. Some welfare implications of the results are considered.