Since the start of the Covid-19 pandemic, many of us have become more sensitive to the signs of sickness while in public. Despite my mask, I dread having to sneeze or cough while in the grocery store, for fear of the sideways glances it will inevitably elicit from people around me. While we know observing symptoms that convey sickness can cause us to change our behaviour, can it also trigger our bodies to mount protective changes in our physiology? Ashley Love and colleagues from around the USA worked to test this idea, to better understand whether visible signs of infection can lead to changes in immunity.
Love and colleagues exposed domestic canaries (Serinus canaria domestica) to the bacterial pathogen Mycoplasma gallisepticum, which causes the birds to reduce their activity levels, become skinnier and develop severe eye inflammation known as conjunctivitis. After being infected with the bacteria, each bird gained a buddy in the form of an otherwise healthy canary. The two birds saw each other during the 14 h of daylight each day over the course of a month, but stayed just far enough apart to prevent the buddy from picking up the infection. The scientists figured if the healthy birds could detect the infection building in their bacteria-exposed pal, this visual information about disease risk could create a proactive change in their immune system in preparation for a potential bacterial attack, especially at the peak of the infection's transmissibility.
The researchers first observed how the infection impacted the birds, recording that the animals became lethargic around 5–10 days after exposure, while their conjunctivitis also took hold, providing the most obvious warning signals that the birds were harbouring an infection. At the same time, Love and colleagues collected blood from the buddies, to search for any clues of changing immunity as the signs of infection changed through time.
Amazingly, the team detected some profound changes in the immune systems of the otherwise healthy buddies. In particular, a group of proteins that stimulate a protective inflammatory response (known as complement activity) and heterophil white blood cells (known to ‘eat’, or phagocytose, invading pathogens) spiked at 6–12 days after their visibly sick pals picked up the infection. This time point also coincides with when their own infection risk would be highest, based on when the disease symptoms peaked. Collectively, these physiological changes – triggered simply by observing another bird carrying the infection – conspire to make the birds more resilient to infection.
Surprisingly, however, Love and colleagues measured no concurrent changes in the immune systems’ ‘communicators’, known as cytokines, which spread the word around your body of damage or infection. As immune responses are super energetically costly to mobilise, these mixed changes to the birds’ immune response could indicate a tempered reaction that could prepare the body for a potential infection, but delays mounting a complete defensive attack until an actual challenge is detected, to avoid wasting energy unnecessarily.
These results suggest that simply seeing signs of infection can lead to immune responses in otherwise healthy individuals, suggesting that the buddies recognized the presence of an infection risk and mounted a proactive response in preparation for a potential immune challenge. Thus, these pre-warning social signals about infection risk can create protective changes in the physiology of animals that potentially reduce their susceptibility to disease. The next step in this research will be to understand if these physiological changes actually increase protection from infection, lower disease severity, and/or reduce the time to recover, information that could inform our own human response to outbreaks of disease.