Indoor Dust Mite Fecal Pellets
This is a tapelift of a dark deposit on the underside of plywood roofing collected in the
attic of a home in western Washington State. There is a common sequence in the attic flora and fauna of the marine
Northwest. First mold growth from condensation, then mites grazing on the mold, then preditors eating the mites, and finally,
summer heat that kills them all or drives them to other parts of the attic space or home.
Transmitted Off Crossed Circular Polarized Light Illumination
Dust mite and many other mite fecal pellets (mite frass) are rounded elipsoids containing birefringent nitrogen containing chemical
crystals (bright patches in the pellets). These may be found in tapelifts or surface dust samples. Fragments of these
particles may be found in air samples. These particles also contain the allergen for the mites. The main nitrogen
containing compound is guanine.
Mites or their debris are found in indoor environments frequently. They all tend to be small ranging from about 40
micrometers to about a millimeter in largest dimension. They lay eggs and then develop through a number of intermediate
stages. The adult mite has 8 legs but the intermediate stages may have 2 to 8 legs, depending on the species and the stage.
All mites require a relative humidity above 50% but they will often occupy cooler parts of a home where the relative
humidity is naturally higher. They will also inhabit bedding and clothing where the relative humdity is elevated due to
the presence of the human body. Their prefered food varies by species. Many of them will survive on skin flakes.
Significance in the Environment:
There are over 124 different types of mites found in homes. Most of these are associated with allergies or asthma. The
standard tests for mite allergen can only detect 2 of the 124 mites that may be present. The collection efficiency of
a vacuum for even the 2 mites that could be detected is estimated at about 1%. Minor flucuation in that efficiency result
in variations of a factor of 20X (2000%). Part of this variation is due to the fact that the mite allergen is often
concentrated in a few large particles. The allergen of a few fragments would be swamped by the allergen associated with
one whole mite. Tapelifts of settled dusts in homes have been much more consistent in detecting mite problems.
These particles tend to be round to oval in shape and contain numerous birefringent crystals of guanine.
Generally some mite fragments can be found in samples containing significant numbers of these particles.
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