We know that odors can provide helpful insights such as clues about the health status of a person. In their new paper, researchers at the Monell Center in Philadelphia reveal a biological change that can occur in healthy animals due to mere exposure to the odors of sick animals.

The study titled "Sharing an environment with sick conspecifics alters odors of healthy animals" was published in the open-access journal Scientific Reports on Sept. 24.

Responses to prepare or protect may be triggered in healthy individuals when they are exposed to the odors of a social partner who is sick. This minimizes the risk of impending infection, according to Stephanie Gervasi, Ph.D., a Monell chemical ecologist and lead author of the study.

In other words, we have evolved to recognize body odor as a red flag indicating the possible presence of something contagious, especially if visual signs of infection or illness are not available. Previous research from Monell has shown how inflammation, as a result of sickness, causes bodily odor changes.

In the new study, the research team triggered similar inflammation in mice by injecting them with a non-infectious bacterial toxin called lipopolysaccharide or LPS. Next, these "sick" mice were placed in the same cages as healthy mice.

Next, the team used a group of "sniffer" mice, bred at the Monell Center to differentiate between odors. The findings showed that the healthy partners of the sick mice also experience changes in their bodily odors and take on aspects of the sick animals’ odors.

The experiment demonstrated that odors not only provide signals of potential disease but can also cause changes in the individuals exposed to them. "This is a remarkable transfer of information via olfaction that specifically alters physiology and could play a role in disease transfer among individuals in many species," said Monell behavioral biologist Gary Beauchamp, one of the senior authors of the study.

However, it may be suspected that the odors transferred as a result of physical contact since the mice lived in the same cage. To examine exposure without physical contact, the study was repeated with sick and healthy mice separated by a perforated partition. The results were the same even though the partitions prevented physical contact, only containing small 0.5 mm holes which allowed odors to circulate.

"This knowledge that healthy animals can emit odors associated with sickness may inform our efforts to use bodily odors to understand how pathogens are transmitted within a population of animals," said Bruce Kimball, a senior author of the study.

Kimball, who is a research chemist from the USDA National Wildlife Research Center Research (NWRC), added that these findings are particularly relevant to wildlife populations.