House Dust Mite Experiments
Experiments have been carried
out in two major areas to answer
the following questions
1. Do mites move in response to
hygrothermal conditions within
the mattress?
2. How many mites can you get in a
mattress?
or
What is the Carrying Capacity of
the mattress?
1. Do mites move in response to
hygrothermal conditions within
the mattress?
Horizontal variation of RH
within a mattress
65-70
65-70
65-70
60-65
60-65
60-65
55-60
55-60
55-60
50-55
50-55
50-55
45-50
45-50
45-50
40-45
40-45
40-45
65-70
65-70
60-65
60-65
55-60
55-60
50-55
50-55
45-50
45-50
40-45
35-40
30-35
40-45
35-40
30-35
1
Vertical variation of RH within a
mattress
80
Bed occupied
75
Horizontal variation of
temperature within a mattress
35-39
35-39
35-39
31-35
31-35
31-35
27-31
27-31
27-31
23-27
23-27
23-27
19-23
19-23
19-23
15-19
15-19
15-19
Relative Humidity (%)
70
65
60
55
50
45
35-39
35-39
31-35
31-35
27-31
27-31
23-27
23-27
19-23
19-23
15-19
15-19
40
35
30
1
2
3
4
5
6
7
8
9
10
11
12
13
Time (Hours)
Surface
1cm from top
Middle
1cm from bottom
Room
Vertical variation of RH within a
mattress
How would mite movement
affect the models predictions?
40
Bed occupied
Temperature (C)
35
30
25
20
15
1
2
3
4
5
6
7
8
9
10
11
12
13
Time (hours)
Surface
1cm from top
Middle
1cm from bottom
Room
How would mite movement
affect the models predictions
• Hygrothermal conditions within the mattress
transient and heterogenous.
How would mite movement
affect the models predictions
• Hygrothermal conditions within the mattress
transient and heterogenous.
• As a result some areas of the mattress become
more favourable to mites and others become less
favourable.
2
How would mite movement
affect the models predictions?
• Hygrothermal conditions within the mattress
transient and heterogenous.
• As a result some areas of the mattress become
more favourable to mites and others become less
favourable.
Mite movement
• Techniques such as heat extraction indicate
that mites can detect and move in response
to extremes of environmental conditions.
• How do they respond to conditions similar
to those found within the mattress?
• If mites can sense more favourable conditions and
move into them they will increase their ability to
survive and their reproductive rate.
The mite movement box
The addition of wadding
Mites Added
RH & Temp
Gradient
Mite extraction
Analysis of results
Zone 1
Hot plate
Hot Plate
Zone 2
Zone 4
Humid
end
Zone A
Sticky tape
Zone 3
Hot plate
Dry
end
Dry
end
Zone B
Humid
end
3
RH and temperature gradients
within the experimental chamber
Mite movement along a temp and RH gradient for
different periods of time
100%
T em p *C an d % R H
T e m p *C a n d % R H
100
80
60
RH
40
Temp
20
80
60
2
3
4
Temp
20
0
1
RH
40
0
1
5
2
3
4
P e rc e n ta g e
RH only Gradient
RH and temp gradient
Zone 4 (Humid)
60%
Zone 3
40%
Zone 2
20%
Zone 1 (Dry)
0%
5
Zones
Zones
80%
1 hr
4 hr
24 hr
Time period
Mite movement along a RH gradient for different
periods of time
100%
100%
80%
80%
Zone 4 (Humid)
60%
Zone 3
40%
Zone 2
20%
Zone 1 (Dry)
60%
Zone B (Humid)
40%
Zone A (Dry)
20%
0%
1hr
4hr
0%
24hr
1hr
Time period
2 hr
4hr
24hr
Time period
Mite movement along an RH gradient for different
periods of time
100%
P e r c e n ta g e
P e rc e n ta g e
P e rc e n ta g e
Mite movement between two zones along a temp and
RH gradient for different periods of time
Conclusions
Mites move towards more humid conditions along a
RH & temp gradient and an RH gradient albeit slowly.
80%
60%
Zone B (Humid)
40%
Zone A (Dry)
20%
0%
1hr
2 hr
4 hr
Time period
This indicates that mites are able to sense both RH and
temperature. The movement in response to RH and
temperature appears to be more pronounced than RH
alone, but this needs further investigation.
24 hr
This movement will affect the model’s predictions.
4
2. What is the carrying capacity
(k) of the mattress?
What is the carrying capacity (k)
of the mattress?
Carrying Capacity is the maximum number of
organisms which a particular habitat can
normally support.
Why is carrying capacity (k)
important?
Long term carrying capacity
experiments
• The mattress represents a large but finite
environment.
Tube lid with holes in lid
“Mite proof” vapour
permeable material
• Even under beneficial conditions house dust
mites would be unable to grow indefinitely.
Tube containing mites and
food
• Knowing the carrying capacity allows us to
set a maximum population for the model.
Long term carrying capacity
experiment
Is space or food the limiting
factor ?
Control
Space
Food and
space
39
0
34
8
30
6
26
3
21
7
16
9
12
6
Food added
83
0
16000
14000
12000
10000
8000
6000
4000
2000
0
42
Mean number of Mites
Population growth of Dp in 1.5 grams of food (mpg)
Time (days)
5
Is space or food the limiting
factor?
Density dependent migration
Av number of mites in entire tube
Mite tube
Sticky tape
Mite Movement
House dust mites
trapped by the tape
Sticky tape
35000
Number of mites
30000
25000
20000
Verm
Food
15000
Control
10000
5000
0
1
2
3
4
5
6
7
Time
Density dependant migration at
25*C
Density dependent migration (25*C) mpc
% m igration
3
20000
2.5
15000
2
1.5
*1.6
1
10000
*8
5000
0.5
0
Num ber of m ites
25000
3.5
% pop
migration
Total no of
mites in tube
0
1
2
3
4
5
6
Time wks
7
8
Conclusions
• Mite populations are unable to grow indefinitely.
• Carrying capacity is determined by the amount of
food and space available.
• When carrying capacity is reached mites react by
migrating, other behavioural changes are also
likely to occur.
• Other factors may influence carrying capacity
(more experiments required).
• So how many mites can you get in a mattress?
Back of envelope calculation
Humans can shed 1.5g of skin per day
In bed for 8 out of 24hrs= 0.5 grams of skin per day
Half of that removed by washing bed sheets, vacuuming etc
= 0.25 grams of skin (food) per day.
A 3 year old single mattress will therefore contain 0.25*1095=275grams of skin
Assuming k is 12 000 mites per gram and the mattress has not been infested prior
to this the maximum number of mites you will get in this particular bed is
approximately
3 million Mites
6