Animals, like plants, can respond in different ways to parasite infections: they can mount a full-scale immune response or they can tolerate the invader. Andrew Read and colleagues from the University of Edinburgh, UK, introduced rodent malaria into five strains of laboratory mice and monitored parasite load and animal health, as measured by anaemia and body mass. In some strains of mice, parasite load increased and the mice stayed healthy, which indicated that the mice could tolerate the parasite. In other mice strains, however, parasite load decreased, which indicated that the mice were resisting infection. The authors suggest that these findings could have important implications for pathogen evolution: pathogens might not be pushed to evolve increased virulence if they are tolerated rather than destroyed. However, tolerated pathogens are also more likely to spread.
In a somewhat related paper, Lynn Martin, from the University of South Florida, and colleagues report that fast-living strains of mice develop high fevers in response to simulated infection, but slow-living strains do not. The authors propose that the 'live fast, die young' mice tolerate the harm that fever inflicts on bodily tissues, as they already have short lifespans. Slow-living mice, however, have more to lose and may therefore adopt targeted strategies, such as antibody production. Science/Funct. Ecol.
Nature Reviews Microbiology 6, 9-10 (January 2008)