Do Dead Spores Cause Allergies?
By Dr. Harriet Burge, EMLab P&K's Director of Aerobiology
Do non-living fungal spores release allergens? A few years ago I would have said categorically
"YES." However, several recent publications have indicated that spores may have to
actually germinate before their allergens can be released. One reason for this is the presence
of a strongly hydrophobic outer wall layer on spores composed of small proteins called hydrophobins.
Note that hydrophobins are not associated with other allergens sources such as pollen, cat
dander, or dust mite feces.
Fungal spore germination is a complex process. Three of the steps involved are 1) wetting of
the spore, 2) initiation of respiration, and 3) detection of appropriate nutrients. When these
steps are satisfied the spore can thin the outer cell wall to allow absorption of nutrients, a
germ tube can be formed and the germ tube can release enzymes and other proteins (i.e., allergens)
into the environment and begin digesting complex food sources. Spores probably do not have to
proceed to the germ tube stage to release allergens. The principal factor appears to be the
disruption or thinning of the hydrophobin layer.
In order to release allergens that could lead to sensitization and subsequently symptoms, a
spore must land in the respiratory mucous, absorb enough water, sugar, and oxygen to initiate
respiration, thin out its cell wall, and begin to produce and release enzymes. This process
may take from a few minutes to many hours.
The environmental conditions that control each of these steps are different for different fungi.
Some fungi can initiate the germination process in nearly pure water (e.g., Stachybotrys)
and germination is inhibited when nutrients are abundant. Others require one or more soluble
sugars and probably other nutrients as well. A few (e.g., some basidiospores) require
pretreatment before they will germinate. In the "wild" some mushrooms are eaten by
snails, and the spores only germinate after passage through the snail! These spores, and
probably many others, do not germinate in the human respiratory tract, and therefore do not
release their allergens. This explains the many fungal spores that do not appear to be
These facts may explain some of the conundrums in the field of fungal allergy. First, not all
fungi can germinate in the human respiratory tract. Needless to say, all of the fungi that can
cause human infections are able to germinate, and most of the studies on fungal germination in
respiratory mucosa have been done on the opportunistic pathogens such as Aspergillus fumigatus.
Alternaria alternata must be able to germinate readily, since this is the most common
sensitizer among fungi in spite of the fact that concentrations in air are generally much lower
than for many other fungi, including Cladosporium. Cladosporium may fall into a
group for which the germination percentage in the respiratory tract is quite low. It is also
important to remember that many fungal allergens are digestive enzymes produced by a variety
of different fungi. Thus, a positive skin test to Cladosporium may be the result of
sensitization by Alternaria.
Obviously it would be lovely to know just which fungi are important with respect to this
germination ability, and that information may be eventually available. Meanwhile, undoubtedly,
a dead spore cannot germinate, and therefore is unlikely to release allergens. This means that
if you are trying to trace disease-causing exposures, cultural analysis, or some other approach
where viability is indicated is probably important. On the other hand, if you are trying to map
sources in an environment, which is the goal of most of us, then spore counting remains the
method of choice.
This article originally appeared in the March 2010 issue of Indoor Environment Connections.
Reprinted by permission.
About Dr. Harriet Burge
Dr. Harriet Burge is EMlab P&K's Director of Aerobiology and Chair of EMLab P&K's
Scientific Advisory Board. Widely considered the leading expert in indoor air quality (IAQ),
Dr. Burge pioneered the field more than 30 years ago. She has served as a member of three
National Academy of Sciences committees for IAQ, including as Vice-Chair of the Committee
on the Health Effects of Indoor Allergens. View Dr. Burge's Curriculum Vitae.
Read "Ask Dr. Burge" articles