January, 2012
Inspection News and Views from the American Society of Home Inspectors

Mouse Matters


Mice are fuzzy and some folks even think they're cute. (We had a client who actually fed rodents in her yard, not  realizing that they were living in her crawl space!) But a mouse infestation can have health, as well as IAQ, implications.

A recent paper in the medical literature noted that home occupants suffer significantly more asthma symptoms in homes with elevated levels of mouse allergens. More recent findings shed light on something that I have been curious about for years. In homes with long-standing mouse infestations, I often have wondered why there were reddish-purple stains in the paths of well-trodden mouse trails. You often see these patchy-colored trails, dotted with droppings, on the tops of foundation walls or on the floors at the floor-wall intersections in basements and garages. The trails are also visible in precarious places where you might not expect them, such as on the tops of PVC pipes and electric cables, or even on vertical concrete surfaces in basements, particularly under the main beam (in a "U" shape) or at concrete corners.

A mouse-urine trail into a basement closet.
Photo © Jeffrey C. May.

A mouse-urine trail on a drain pipe,
Photo © Jeffrey C. May.

A mouse-urine trail around a main beam.
Photo © Jeffrey C. May.

Researchers in Liverpool, England, have been studying these mouse-urine trails and have discovered that mice use their urine as an important part of their communication system. Each mouse excretes urine with a characteristic mixture of odors. Members of a "clan" can distinguish each other, as well as enemies, by the odors in their urine trails. Females indicate when they are ready to mate, and the dominant mouse lets other members of the community know who he is and where he's going.

There is a reason why homes that have endured long-term mouse infestations have a characteristic unpleasant animal odor, usually strongest in the basement and/or attic. The odors are somewhat volatile, but they are so important in "mouse talk" that there is a whole set of characteristic proteins (called major urine proteins or MUPS) that mice secrete with their urine to trap the odor molecules and make them persist longer than they would if simply excreted onto a surface. MUPS work like the chemical (cyclodextrin) in Febreze, which is a large molecule with a cavity in the middle; odor molecules are "sucked" into the hole and reduced in strength so they evaporate more slowly and the odor lasts longer.

There is one more interesting note to add to this lovely picture. My curiosity was piqued by the MUPS revelations, so I took a tape sample from a mouse trail on top of a foundation 44 wall in a garage. Under the microscope, I was amazed to see that the trail was full of Aspergillus mold growing on the urine-dampened dust. This adds another allergenic note to the symphony of problems due to mice. As mice scurry about, they disturb this mold growth. Aspergillus spores are very readily aerosolized and responsible for many of the allergy and other symptoms that people experience in sick buildings (and we haven't even discussed the droppings, which get moldy when damp and can carry Hanta virus!).

Finally, an even sourer note. In the last few years, for some unknown ecological reason, shrews have begun to infest homes and even schools and commercial buildings. Shrews are rodent-like creatures, about the size of mice, that have a powerful body musk akin to skunk odor (one client described the odor as "burnt Tootsie Roll"). Shrews are strictly meat eaters (some species kill their prey with a neurotoxin-containing bite), and a favorite food of shrews is mice. So, if you let mice in, shrews may follow.

Speaking of entries, some folks think that blocking a foundation or framing hole with fiberglass will keep out rodents. Unfortunately, putting fiberglass in a hole is like stuffing it with cotton candy because fiberglass is a favorite nesting material for mice. Fiberglass is a "Motel 6" invitation to infestation.

Most pest control operators (PCOs) are happy to return every few months to place (or replace) their mouse-bait stations, but the best solution is to find the mouse entry points and seal them with masonry or with foam (with embedded hardware cloth), as appropriate. Then let the PCO do his work. The most common entry points in homes are gaps under or at the side of the overhead garage door and then into the house through an opening around a pipe or wire or through gaps in a bulkhead door and then into the basement through openings around the framing of the interior door (if there is one). Other favorite entry points are at concealed framing, such as the band sill behind a stoop or a sill plate covered by siding, that is decayed usually due to roof-water splash.

If you include pest inspections in your services, be sure to look for the telltale reddish-purple stains (a mirror on a telescoping extension is a handy aid). Even if you do not include pest inspections, if you see signs of a mouse infestation, encourage your client to get a pest inspection. His or her health may depend on it.

This article is from the May Indoor Air Investigations Newsletter, December 2011/January 2012. Reprinted with permission of Jeffery C. May. © 2011 Jeffrey C. May. Visit www.mayindoorair.com.