Benefits of parasitic infection: a test using a ground squirrel – trypanosome system

Abstract

A proposal that laboratory rats and mice infected with laboratory strains of trypanosomes survived better and grew faster than uninfected controls was tested with a host and parasite newly isolated from a natural field system. Wild-caught young of the year Richardson's ground squirrels (Spermophilus richardsonii) were brought into the laboratory and subjected to three pairs of experimental treatments in a factorial design: (i) infection with Trypanosoma otospermophili or no infection, (ii) restriction to 85% of ad libitum amount of food or ad libitum availability of food, and (iii) a diet deficient in pyridoxine (vitamin B6) or a diet containing pyridoxine. The effect of these factors on mass gain of the squirrels during their first summer, a period of rapid growth, was measured. Only squirrels receiving a pyridoxine-deficient diet and able to feed ad libitum showed beneficial effects of parasitic infection; infected squirrels had a greater mass gain than their controls. Squirrels on other diets were, if anything, negatively affected. These results indicate that features observed in laboratory systems probably occur in natural systems as well and that environmental variables may be important in determining the outcome of a host–parasite interaction.