An herb called echinacea may be able to help your rattie friends fight off respiratory disease. Numerous scientific studies indicate that this herb might improve immune system function in your rats, and that this in turn could lead to fewer and less frequent respiratory problems.
All indications are that the chances of toxicity are low.
The best known representative of this species is Echinacea purpurea, or purple coneflower, a plant native to North America. He adds that echinacea causes an increase in the number of immune cells, stimulates production of such immune-system products as interferon, and combats inflammation, among other effects. Ironically, most of the research on echinacea seems to have been done using humans. However, there is evidence that echinacea could be equally as beneficial to rats as it is to us. Steve Carter, a research associate at the Veterans Administration Hospital in San Diego, California., told me that researchers there have completed but not yet published research showing that echinacea has a significant effect on several measures of the immune system in rats.