One of the earliest links between influenza and neural dysfunction was a correlation between the 1918 Spanish flu, caused by a subtype called H1N1, and an epidemic of Parkinson’s a few decades later. In the 1940s and early 1950s, diagnoses of the neurodegenerative disease appeared to increase abruptly, from 1–2 percent of the US population to 2.5–3 percent, then fell back down to 1–2 percent, [neurobiologist Richard] Smeyne says. “Basically, 50 percent more people in those years got Parkinson’s.”
The evidence to suggest that influenza infection caused the neurodegenerative disorder was tenuous, to say the least, but the correlation was enough for Smeyne to investigate further. With his colleagues, he shot nonlethal doses of H5N1 or H1N1 up the noses of six- to eight-week-old mice.
[W]hile the H1N1 flu strain couldn’t cross the blood-brain barrier, it still caused central nervous system immune cells called microglia to flow into the substantia nigra and the hippocampus, causing inflammation and cell death in the area.
“Viruses are often ignored in relation to neurodegenerative diseases,” Yale University neurobiologist Anthony van den Pol tells The Scientist. “That’s in part because there’s no clear sign that a virus causes a neurodegenerative disease. But it might.”
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