Most of our homes are soaked in mouse urine. It’s at the core of our asthma epidemic—but it helps rodents stay connected.
UNTIL A FEW WEEKS AGO I didn’t have the slightest interest in mouse urine. But after some study I’ve concluded that it is covertly running and ruining the world, strangling small children, and driving the profits of Big Pharma.
I came to know mouse urine, the molecules of which are known as MUPs (Major Urinary Proteins), and specifically as Mus m 1, because the molecules were stubbornly clinging to the studs of a cabin that I recently bought. Though I didn’t yet know the molecular names or weights of my MUPs, I knew they were there. Mice had burrowed through the cabin’s fiberglass insulation, and it looked like a splendid and huge pink ant farm.
Mice sort their food; there were larders of pasta, lentils, acorns, and blue poison crystals in my walls. Looking at the elaborate networks in the walls, it’s easy to imagine that my new-old (built in 1939) cabin’s erstwhile owner was a rustic prisoner who suffered respiratory ailments. The mice operated a space-age metropolitan economy in the walls around him, communicating through sophisticated molecular signaling.
Once the mice and the insulation were gone, a ghastly smell remained. So I set to googling the hows and whys, until the basic crystalline structure of our problem (specifically) and the scourge of mouse urine (generally) became clear: field mice, especially males, secrete proteins in their urine to signal to other mice their sex, degree of male dominance, age, and genetic makeup.
Turns out, our walls are a giant social network, a Facebook of old mouse news. And just as Facebook has been designed by diabolical engineers, the molecule Mus m 1 has been designed to stick around, blasting out its code for a very long time. A picture of Mus m 1 looks like an exploded cartoon of a clown—colored curls and a helix. It is barrel-shaped so that it can hold volatile, signaling chemicals and release them slowly over time. MUPs are like graffiti spray-paint: lightweight so they’ll stay airborne, and sticky so they’ll remain on the walls.
You might think that mouse urine is more my problem than yours. Ha! Eighty-two percent of American homes have mups floating about in them. In the inner cities of the Northeast and Midwest, the majority of all homes have mouse-urine proteins, but some of them have concentrations a thousandfold higher than homes in the suburbs. In the Northeast and Midwest we all live in a house made of mice. And that’s a big—very big—problem: mouse-urine proteins can trigger allergies and asthma. (Mouse-urine proteins are clever; they look like other allergens such as ragweed, which, according to the authors of a recent study, may be a trick that has arisen through convergent evolution.)
Dr. Elizabeth Matsui, a pediatrician and an associate professor at Johns Hopkins Children’s Center, started studying mouse urine a decade ago, when she was trying to figure out why a quarter of the kids in some low-income Baltimore neighborhoods have severe asthma. (The national rate is closer to 6 percent.) “If you live in public housing or Section 8 housing or anywhere … you can’t control mice, cockroaches, or other pollutants," Matsui says. She found that higher exposure to mouse-urine proteins led kids to develop more problematic asthma.
Mice are obviously not the only trigger for asthma—auto pollution, dust, pets, and cockroaches are also on the list—but about half of Matsui’s asthmatic subjects were allergic to mice. (But the immune system is a hairball: Matsui also came across a curious paradoxical effect. Not everyone exposed to high levels of mups becomes allergic; some become immune. High exposure to mouse urine can, in other words, work a bit like allergy-immunotherapy shots, by reshaping the body’s immune response.)
In 2005, kids under 15 made 679,000 visits to emergency rooms for asthma. Currently, our medical remedy focuses on getting kids into long-term treatment with anti-asthma drugs of various sorts, but the mouse urine remains. Which brings me to the mouse-pharmaceutical complex. We currently spend about $50 billion a year in the U.S. on health care costs for asthma, and this is bound to rise as children grow older.
But we aren’t giving kids mouse-free houses.
Without a coordinated public-health response, Matsui says, trying to bring down asthma rates, or even asthma attacks in individual kids, is like playing Whac-A-Mole.