Short communication House-dust mites and summer cottages

House-dust mites Dermatophagoides pteronyssinus andD. farinae have their natural habitats in house dust andnests of birds and mammals (1). The fact that the highestconcentrations of mites in dust from houses actuallyoccur in dust from beds (2) has led to the assumption thathouse-dust mites were dependent on human proximity forsurvival and growth.House-dust mites causes allergic asthma in predisposedindividuals with a threshold limit value (TLV) of100 mites/g of dust. Exposure above this level impliesan increased risk of being sensitized and development ofmite-allergic asthma (3). Likewise the disease activity inmite asthma is dependent on exposure above this TLV of100 mites/g of dust. A reduction in exposure below thislevel imply a reduction in symptoms to near-cure asdescribed when moving patients to specially designedmite-free dwellings (4) and when moving mite-allergicpatients to mite-free mountain residences (5). The expo-sure reduction is promising as treatment of mite-allergicpatients although till now largely unsuccessful (6).Culture experiments with house-dust mites (7) haveidentified their optimal food as human skin scales. Theyhave an optimum temperature of 25 C and havecontinued growth with temperatures between 15 and32 C. They tolerate even subzero temperatures forseveral days. They have an optimum relative humidity(RH) for population growth of 60–70% RH, but theydo not tolerate RHs for prolonged periods of timeoutside the range of 55–85% RH either because ofdesiccation (8) or because of overgrowth of fungi at tohigh humidities.In contrast to house-dust mites, storage mites (mostlyLepidoglyphus destructor and Acarus siro) prefer (9)higher indoor air humidities, and have an optimum of90% RH. They desiccate and die with a RH below 60%.Storage mites produce their own allergens known tocause allergic asthma with only a limited allergenic cross-reactivity with house-dust mites (10, 11).The aim of this study was to investigate house-dustmite and storage mite population sizes in surroundings ofextreme indoor climate conditions with low temperatures,high RHs and no human proximity for at least 6 monthsa year in a group of winter-uninhabited Danish summercottages.Material and methods