Diabetes linked to flu

The flu virus has another trick up its sleeve ? it may trigger diabetes. The good news is that this discovery may give us a way to prevent some forms of the disease.

In diabetes, cells do not take up sugar from the blood. This can happen because cells have lost sensitivity to the hormone insulin, leading to what is called type 2 diabetes. Linked to diet and lifestyle, this form of the disease is rapidly becoming more common worldwide. Another cause of diabetes happens when the immune system destroys the pancreatic cells that produce insulin. People inherit a genetic predisposition for this condition, called type 1 diabetes, but an environmental trigger is also needed for it to appear.

Since the 1970s, researchers have suspected that viruses may provide this trigger, as type 1 diabetes often sets in suddenly after an infection. Enteroviruses and rotaviruses were both implicated; something about these infections confuses the immune system enough to make it attack the pancreas. But the picture remained unclear.

Then Ilaria Capua, of the World Organisation for Animal Health reference lab for bird flu in Legnaro, Italy, and her team decided to infect turkeys with flu. They did this because they knew birds with flu often have an inflamed pancreas, even when they have strains of the virus that do not normally spread outside the lungs. The team found that many of the turkeys developed severe pancreatic damage and diabetes.

Next, the researchers infected human pancreatic tissue with two common flu viruses. Both "grew really well" in the tissue, including in insulin-producing cells, says Capua.

Inflammatory response

Crucially, the presence of flu in the pancreatic cells triggered production of a set of inflammatory chemicals that have been shown to be central to the autoimmune reactions that lead to type 1 diabetes. One theory is that immune cells present bits of the infected tissue to destructive T-cells, to teach them to recognise the virus. But in the process the T-cells also learn to recognise the cells that make insulin, and to destroy them.

Can flu reach the pancreas? In humans, the virus is normally restricted to the lungs and gut, but can sometimes get into the blood. The virus might also travel up the duct that links the small intestine to the pancreas, Capua suspects. "Either way, when it gets to the pancreas it finds a good place to replicate."

Capua is now testing the effects of flu on mouse models of type 1 diabetes. She is also looking for signs of recent flu infection in people with newly diagnosed diabetes. She suspects the H1N1 swine flu virus that caused the pandemic of 2009, and is still circulating, could be a particularly good trigger. Doctors in Japan and Italy have reported many newly diagnosed cases of type 1 diabetes in people who had recently had flu, and an upsurge in type 1 diabetes after the 2009 pandemic.

Real impact

"The great thing is that even if flu only causes a few per cent of type 1 diabetes cases, we can vaccinate and prevent flu in people who are genetically predisposed, and that can have a real impact," says Capua. There are 65,000 new cases of type 1 diabetes worldwide annually, and that figure is growing by 3 to 5 per cent each year.

The link between diabetes and flu adds to growing evidence that many diseases considered non-infectious are actually caused by infection ? and can therefore spread.

There is also new evidence that flu can cause heart attacks. Previously, this was suspected, because of the surge in heart attacks that regularly follows the annual flu season. But researchers at the University of Toronto, Canada, have now demonstrated the effect in individual patients. They reported this week that vaccinating adults for flu, whether they already have cardiac problems or not, makes them half as likely to have a heart attack or stroke in the following year (Canadian Journal of Cardiology, doi.org/jnr).

Journal reference: Journal of Virology, doi.org/jnp

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