Dust Mite Allergen: An
Indoor Problem
Introduction
Dust mites are microscopic creatures that are not visible to the naked eye. Female dust mites are generally
between 200 to 500µm in size, with males being smaller. They prefer warm, moist surroundings close to
sources of food. Since one of their food sources is human skin scales, they are most prevalent in high-use
areas such as living rooms, bedrooms, and areas with upholstered furnishings where shed skin scales can
collect and serve as food. It has been estimated that the skin one human sheds each day (~1 gm) could feed
several thousand mites for up to three months. Pillows and mattresses are also primary locations for dust
mites. These are also key areas for exposure since in these locations a person's face is in very close
proximity to the allergens, which are present in the dust mite feces and dried body fragments.
Classification
Dust mites belong to the kingdom of animals; phylum, Arthropoda; class, Arachnida; and group, Astigmata;
with three genera (Dermatophagoides, Euroglyphus, and Blomia) important for
humans indoors. The mites most commonly found in house-dust in homes worldwide are D. farinae,
D. pteronyssinus, E. maynei, and B. tropicalis. In the Unites States, all of
these dust mites may be found indoors, but D. farinae and D. pteronyssinus are found most
frequently and are most widely distributed geographically.
Ecology
Both food and water are critical for mite survival. Since adequate food (e.g., human skin flakes) is
generally available, it is the relative humidity of a place that determines mite prevalence in a location.
In humid regions, usually all homes and many other buildings contain breeding populations of dust mites. In
dry climates, fewer homes contain dust mites and they are usually at low levels. However, by raising indoor
humidity through the usage of evaporative coolers may alter the indoor environment that may lead to
conditions that facilitate mite survival. Ambient relative humidity may influence the rate at which feeding
mites produce allergens and its accumulation in dust. It has been shown that by lowering the relative
humidity in a place may significantly drop the production of fecal allergen even though mites may continue
to survive. However, lowering indoor humidity can reduce mite population density overtime because mites
gradually dehydrate and die below 50% relative humidity.
Health
Effects
Hypersensitivity diseases caused by allergens from mites that live indoors constitute a major health
problem in the U.S. and elsewhere. House dust mites are primarily a concern in human dwellings, but dust
mites and mite allergen have also been identified in office buildings in association with and without
health complaints. Dust mite allergens are considered to be the major biological agent to have sufficient
evidence for the casual relationship of their exposure to the development of asthma in susceptible
children. They not only aggravate the problem in susceptible individuals but also may cause susceptible
children to develop asthma. House dust mites are also known to play a major role in triggering asthmatic
attacks in susceptible individuals. House dust mite allergen is the inhaled substance that actually
triggers an attack by causing an allergic reaction. Dust mite allergens are proteins, which come from the
digestive tract of mites and are found at high levels in mite feces. A dust mite fecal pellet, containing
partially digested food and digestive enzymes, is ~10 to 35µm in diameter and contain allergens
(protein) called Der p 1, Der f 1 and mite group 2. These allergens (proteins), when
inhaled, attach to sensitized cells in the air passages causing hay fever and asthma, and aggravate atopic
dermatitis in people who are susceptible to this problem. Approximately 85% of asthmatics are allergic to
dust mite allergens.
Sampling
Volumetric sampling of the air has repeatedly shown that concentrations of mite allergens are generally
very low to undetectable. The International Association of Allergology and Clinical Immunology (IAACI) has
recommended sampling settled dust to evaluate exposure to dust mite allergens. The ACGIH has stated that it
is important to sample multiple indoor locations to obtain a reasonable determination of mite prevalence.
Dust samples for dust mite allergens can be collected using a dust cassette or by using a specially
designed dust trap attached to a vacuum cleaner. A filter or a 9 square inch bed linen can otherwise be
placed between the hose and the attachment of vacuum cleaner to collect the dust. Keep samples cool (place
in refrigerator until shipped to the lab) and ship the samples to the laboratory for allergen testing using
a cold pack or in a cooler. By not cooling the samples, the dust mites present in the sample may grow in
numbers and produce more fecal pellets than actually present at the time of sampling and so may produce
inaccurate test results.
Sample
Analysis
Detection and measurement of dust-mite antigen (allergen) is performed using an immunoassay. An Enzyme
Linked Immuno-Sorbant Assay (ELISA) is used to quantify mite allergen concentration in dust samples. Using
monoclonal antibodies specific for dust mite allergens Der p1, Der f1 and mite group 2,
the level of dust mite allergens present in the sample are analyzed. The level of dust mite allergen
present in the sample is reported in micrograms per gram (µg/g) of dust.
Data
Interpretation
The IAACI proposed dust-mite allergen limits for residential dwellings considers exposure to dust
containing 2µg/g of mite allergen to increase the risk of dust-mite sensitization as well as the
development of asthma and bronchial hyperreactivity in affected persons. Exposure to dust containing
10µg/g of mite allergen represents a higher risk level and an increased chance of acute asthma
attacks.
Prevention and
Control
A few suggestions on how to minimize mite reproduction and allergen release following confirmation that
dust mites are present indoors are as follows:
-
Where mites may breed,
maintain relative humidity below 50%.
-
Vacuum regularly and discard
vacuum bags immediately.
-
Wash bedding every week in
hot water (54°C).
-
Replace carpet with hard
flooring if possible.
-
Use allergen free mattress
and pillow covers to minimize contact with the dust mites.
-
Keep upholstered furniture to
a minimum.
-
Use HEPA air filters to keep
mite particles out of air. (Note that since mite particles settle rapidly due to their relatively large
size, this may be of limited value.)
-
Minimize the use of curtains
or drapes by using shades.
References
Bioaerosols: Assessment and Control, Janet Macher, Sc.D., M.P.H., Editor. 1999. ACGIH, 1330 Kemper
Meadow Drive, Cincinnati, OH 45240-1634.
Clinical Reviews in Allergy and Immunology, Martin D. Chapman, Editor. June 2000, Volume 18;
Number 3.
|
Fungus of the month:
Cladosporium species
Dr. Payam Fallah, Dr. Kamash Ramanathan
Cladosporium is
considered by many to be one of the most prevalent fungal genera in the world. Its Latin name is derived
from Greek, which means spores in branched chains. The genus contains about 60 or so species. Almost all
species are either plant pathogenic or saprophytic on plant debris or man-made plant product such as paper
based material. Only one known species so far, C. carrionii, has been known to be pathogenic in
humans, causing skin lesions.
Spores of Cladosporium can easily be airborne and travel long distances. The fungus is capable of
growth in a wide range of temperatures (18-28 C° and as low as -6 C°). It is one of the first fungi
to be found in indoor environments. Under ideal environmental conditions the fungus can grow and multiply
on variety of surfaces including metals. This is especially true when condensation occurs on metal surfaces
in electrical outlets. Once spores settle on a wet surface (if still viable) they can germinate and form a
network of hyphal fragments that keeps on growing, and if conditions are suitable it will sporulate.
Generally, there are three
species that are commonly encountered outdoors, C. herbarum, C. cladosporiodes, and
C. sphaerospermum. While all three species can be allergenic, C. herbarum is most
prevalent outdoors and considered, by many, to be the number one cause of asthma and hay fever (type I
allergy) in the western hemisphere. The other two species are mostly prevalent in indoor environments. Type
III hypersensitivity pneumonitis can also occur by Cladosporium species. Cladosporium
species can be found on a variety of substrata such as textiles, wood, moist windowsills as well as moist
wallboard papers just to name a few.
MoldRANGE™ of
Cladosporium by Month
The MoldRANGE™ chart of
Cladosporium by month show the levels of Cladosporium spores detected in the outside air
during the different months of the year in the United States. The numbers on the left of the chart (0.0
-1.0) correspond to the grey bar on the chart and indicate the frequency of occurrence of the
Cladosporium spores at different months of the year in the United States. It can be observed
(looking at the bars) that Cladosporium spores seem to occur more than 95% (0.95) of the time
during all the months of the year indicating their common presence in the outside air at all times of the
year.
The purple, green and red lines
on the chart correspondingly represent the 5, 50, and 95 percentile levels of spores/m3 value for the
Cladosporium spores when it is detected during the different months of the year in the United
States (correspond to the numbers (spores/m3) on the right of the chart). It can be observed that whenever
detected, 50% of the time the Cladosporium spore were present in levels above the detection limit
and approximately below 500 spores/m3 during all the months of the year. Based on the chart it
can also be inferred that the 95 percentile value (purple line) fluctuates from low levels of ~ 400
spores/m3 to high levels of ~ 6900 spores/m3 during the different months of the year, indicating
that the levels of Cladosporium spores in the outside air are at the lowest levels during the
winter time (December to April) and are at the highest levels during the Summer time (June to November)
through out the country.
So whenever high levels of
Cladosporium spores are observed indoors they must be carefully evaluated with the levels present
in the outside air (due to their common occurrence outside in high levels) before making a judgement of
their indoor source.
|
