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The mere sight of the disease can start a canary’s immune system

For canaries, just seeing their friends get sick with feathers may be enough to preventively revitalize their immune system.

Healthy birds housed in the sight of companion birds infected with a common pathogen have elicited an immune response, despite not being infected themselves, researchers reported online on June 9 in Biology Letters.

“It’s fascinating that some kind of visual signal can alter immune function,” says Ashley Love, a disease ecologist at the University of Connecticut in Storrs. She follows, precisely how much these alterations actually protect the birds.

The immune systems are like sentinels, patrolling the body by invaders and calling the cavalry once a pathogen is detected. Traditionally, pathogens have to get into bodies to stimulate that type of response.

But some research has already hinted that perceived threats can whip up immune cells. For example, an experiment in humans found that a simple photo of a sick person increases the activity of inflammation-stimulating chemicals called cytokines. But no one had ever looked at whether being within sight of a really sick individual could force the immune system to take preventative action, Love says.

“Many wildlife diseases have these obvious physical symptoms,” he says. If wild animals can be prepared, immunologically, at the first sign they can become infected, they can be better equipped to fight the invader once he arrives.

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To test this idea, Love and colleagues infected 10 caged canaries (Serinus canaria domestica) with Mycoplasma gallisepticum or MG, a common bacterial pathogen that causes conjunctivitis and extreme lethargy. Sick birds look “pretty fluffy,” Love says.

Nine healthy birds were housed in direct view of their sick siblings, but far enough away to prevent infection, which requires direct contact. An identical configuration, but with all the healthy birds, was located in the same room but on the other side of an opaque divider. Over the course of a month, researchers collected blood samples from the birds, measured various aspects of immune activity, and tracked how bad the infected birds were.

When the healthy birds witnessed that the neighbors were visibly ill, their immune systems were shaken. A measure of the ability of birds to explode foreign cells, called CH50 complement activity, increased along with the disease that infected birds appeared. White blood cell count was also significantly different in birds exposed to sick, rather than healthy, individuals. Cytokine levels did not differ between the two groups.

Blood tests showed that no healthy birds captured MG during the experiment, which suggested that some type of external signal altered immune function. That clue was probably visual, Love says. The smells and sounds of the sick could reach all the birds in the experiment, but only the birds in direct view of the sick birds showed an immune response.

“This was a pretty compelling study,” says Dana Hawley, a Virginia Tech disease ecologist in Blacksburg who didn’t participate in the research. Many animals avoid infection by social distancing, she says. For example, lobsters are far from decks occupied by sick individuals and finches that Hawley studies avoid individuals that look sick.

But social distancing has its costs, especially for highly social species.

For species that seek strength together or depend on safety in quantities, increasing the immune response to the mere sight of the disease may confer some protection while allowing animals to approach in person.

"It's great to avoid a pathogen," says Hawley, "but if you can't find food or get carried away by a predator, it really doesn't matter."

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