WE KNOW that the use of condoms can help to prevent un-wanted pregnancies
and the spread of sexually transmitted diseases. One experimental study
suggests to us that the frequent use of condoms in households inhabited
by asthmatics might have another useful consequence. The reason is that
household dust mites can cause asthma attacks, that the mites seem to flourish
on semen, and that mattresses contain a rich source of dried semen. Condoms
could help to break the last link in this causal chain.
Some 20 years ago, Reindert Voorhorst and his colleagues at the University
of Leiden showed why exposure to house dust can cause allergic responses,
such as asthma, dermatitis or an inflamed, runny nose. The culprits in the
dust are mites of the family Pyrogliphidae, which carry allergens – foreign
substances, usually proteins – that can invoke an immune response. These
tiny mites are virtually invisible, being about one-third of a millimetre
long. They can feed on human skin scales, but in the laboratory they can
also live off human beard shavings, supplied by the researchers’ colleagues.
Each species of mite can produce many allergens. For example, one species,
Dermatophagoides pteronyssinus, produces at least six different allergens,
each linked to various allergies. But all the allergens fall basically into
three groups: group I allergens are glycoproteins excreted in the animals’
faeces, while allergens in groups II and III are smaller proteins. Most
people allergic to mites produce antibodies to all groups of allergens.
The World Health Organization has established standard reference extracts
for the group I and II antigens, which enable assays carried out in different
laboratories to be standardised.
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Despite the development of new and more effective drugs, allergic asthma
remains a big problem in both developed and developing countries. In Britain,
for instance, about two million adults suffer from asthma, and roughly one
child in 10 is affected. Recent epidemiological studies, reviewed at a meeting
sponsored by the World Health Organization, support the link between asthma
and dust mites: between 45 and 85 per cent of asthmatics are allergic to
mites, compared with between 5 and 30 per cent of nonasthmatic people.
A variety of other studies have established that asthma is more prevalent
in households with large numbers of dust mites. Overall, current estimates
suggest that dust bearing between 100 and 500 mites per gram puts potential
asthmatics at risk. High humidity and ample food seem to foster the growth
of mites, so the bedroom is a favoured site for them. Within the bedroom,
bedclothing and mattresses seem to provide good culture conditions. When
blankets were introduced into one region of Papua New Guinea, the prevalence
of asthma among adults increased about fiftyfold.
People badly affected by mites have sometimes resorted to plastic mattress
covers, frequent hot washing of bedding and a number of other hygienic procedures
in an attempt to reduce the infestation. One study found that switching
on an electric blanket every day (for about eight hours) halved the number
of mites in mattresses within a month, presumably because it made the beds
hotter and drier. Other studies, in typical households, found no difference
between the abundance of mites in mattresses with and without electric blankets;
it seems that the blankets were switched on only for relatively brief periods.
More generally, there is evidence that newer houses, less than 10 years
old, contain fewer mites, possibly because efficient insulation and central
heating make the houses drier and warmer.
What is it about beds that mites find so attractive? Beds are inevitably
rich in sloughed off skin and lost hair. But skin scales are poor fodder
for house mites. Matthew Colloff of the University of Glasgow argues, instead,
that semen should be very nutritious in comparison with skin scales, and
its input into bedding could be high.
The calculations go like this. The mean human ejaculate is about 3.5
millilitres, representing about 280 milligrams dry weight. Married couples
copulate twice a week on average, and at least one half of the ejaculate
is lost into the bed by gravity (according to Martin Johnson and Barry Everitt
of the University of Cambridge in their book Human Reproduction). This would
mean that ‘the mean dry weight of semen into the bed would be of the order
of 300-500 milligrams per week; about one-seventh of the estimate of the
mean weekly input of skin scales into the bedroom,’ Colloff concludes. This
estimate excludes ‘involuntary nocturnal emission, or masturbation’ which,
says Colloff, Alfred Kinsey’s pioneering but idiosyncratic study ‘found
to be the method of ejaculation most frequently employed by adult American
³¾²¹±ô±ð²õ’.
If Colloff is correct, the remnants of human sperm might be expected
to be rather common in dust samples taken from mattresses. Colloff tested
his idea by analysing the dust from 66 Glaswegian mattresses; 52 of the
samples did indeed contain dried semen. Furthermore, when Colloff fed mites
on dust from mattresses free of semen, the number of eggs produced per day
by adult females showed a small but significant decline from 1.4 to 1.3
after 12 days. When he added air-dried semen to the samples of the same
dust, similar mites increased their output to 2.5 eggs per day over the
same period of time. Widows’ mites could be very scarce indeed.
* * *
I’m gonna wash those mites right out of my home
DUST mites have long resisted our efforts to get rid of them, and now
seem to be reaching epidemic proportions throughout much of the world. Modern
houses are probably no dustier than their forebears, but they are definitely
cosier, at least from a mite’s point of view. These mites thrive at temperatures
of between 13 and 24 Degree C (55 and 75 Degree F) and relative humidities
of between 60 and 70 per cent. In Britain, domestic mites achieve peak numbers
in autumn, but persist throughout the winter.
Their faeces persist too, flooding the environment with allergens just
waiting to create an allergy in a susceptible individual. About the size
of pollen grains, at between 10 and 40 micrometres in diameter, these resistant
particles accumulate, especially in carpets, padded chairs and sofas, beds
and bedding.
Bruce Mitchell, director of immunology at the Blackrock Clinic in Dublin,
has been looking for feasible ways of reducing levels of mites and their
faeces in people’s homes. Previous attempts have been discouraging: people
with asthma usually improved only when they took drastic measures, such
as removing the carpets, wrapping mattresses or even moving house.
‘The failure of routine cleaning measures to significantly alter these
levels may explain the lack of enthusiasm for this approach on the part
of both physician and patient in the past,’ says Mitchell. Many people experienced
some relief from allergic symptoms only in the mite-free environment of
the hospital, or at high altitudes.
In the search for better ways of waging war against mites, Mitchell
charted the infestation of 35 houses in Dublin, chosen at random and inhabited
by people who were not allergic to the creatures. He estimated the severity
of the acarid plague in samples of dust he hoovered up from the houses,
by using monoclonal antibodies responsive to the major allergen of the mite
Dermatophagoides pteronyssinus.
Mitchell found that although ‘ordinary cleaning agents were useless’,
intensive washing of carpets with a proprietary solution, known as Allerite,
did reduce levels of mite allergens. The cleaning solution, sold by the
manufacturers of Vax vacuum cleaners for use in its ‘wet’ mode, is a mixture
of solvents and surface wetting agents. It penetrates into carpet fibres
and is then extracted by the vacuum cleaner, apparently dislodging the mites’
faeces in the process.
Mitchell says he cannot divulge the ingredients, but that the solution
contains a ‘cocktail’ of chemicals with a proven safety record. In any case,
he says, 80 per cent of the solution is removed by the machine, so the build
up of residues should be less of a problem. In studies to date, no one has
shown signs of allergy to the solution, he says.
Asthmatics will also want to know whether this treatment reduces concentrations
of allergens to safe levels. People are unlikely to become sensitised to
mites in the first place, or suffer allergic symptoms when exposed to them,
if levels of allergen are below 2 micrograms per gram of fine dust. But
when a gram of dust contains 10 micrograms of allergen or more, many sensitised
people suffer asthma attacks.
Does the new cleaning solution do the trick, reducing levels of allergen
to the ‘safe’ threshhold? ‘Using Allerite, the degree of effort involved
in achieving an acceptable level appears to depend upon the starting values,’
says Mitchell. In his studies, two treatments of a sample of carpet reduced
levels of mites to acceptable levels in half of the houses. But the more
severely infested houses needed at least four treatments over two months.
Beds, sofas and armchairs may prove even more of a challenge, and Mitchell
emphasises that ‘it is necessary to treat all carpets and items of furnishing
at the same time, thus preventing transfer of allergen from one site to
another’. Bedding often comes out worst, says Mitchell: ‘Feather and down
is appalling – the mites like to get in there and multiply. But they can
live in foam too.’
Several other companies are now selling products that purport to destroy
domestic mites, says Mitchell. These include Acarosan, a German preparation
containing benzoate, and an Australian product containing tannic acid. He
stresses that such products, if effective, must also be shown to be ‘both
safe and acceptable to the domestic user’.
Researchers in Manchester have been exploring another approach. Stephen
Owen and his colleagues at Wythenshawe Hospital have covered mattresses,
pillows and duvets with a polyurethane coating, Ventflex, produced by Coverplus
in Hyde, Cheshire. This coating lets water vapour in but blocks mites and
their droppings. The clinicians have recently published their findings in
The Lancet (17 February).
For 12 weeks, 16 volunteers slept alone in either a treated single bed
or a conventional one. At the end of the study, the coated mattresses bore
just 1 per cent of the mite antigen present in the ordinary mattresses cleaned
by vacuuming.
‘Ventflex effectively separates the patient from the house dust mite
antigen contained within the mattress,’ Owen and his colleagues conclude.
Nor did mite allergens build up on the mattress cover during the three months
of the study, ‘presumably due to loss of food substrates for the house dust
mite in addition to the ease of cleaning,’ they say.
Paul Harvey and Robert May are in the Department of Zoology at the University
of Oxford.
Further reading: M. J. Colloff, in Progress in Acarology: Volume 1 edited
by G. P. Channabasavanna and C. A. Viraktamath, p 141, Oxford and IBH, New
Delhi, 1989.