What Doctors Found in House Dust Mite Droppings: A Scientific Review of Allergens and Contents

Thirty-three years ago scientists investigated the contents of house dust mite droppings. They found two species of fungus along with an active digestive enzyme acting as a major mite allergen. By 2011 fourteen separate mite allergens had been identified along with DNA from bacteria and mites, chitin and guanine. This is a review of some of the investigations.

Key Findings in Chronological Order

1978: Fungi in the Digestive System
'Ecological relationships between xerophilic fungi and house dust mites (Acaria: Pyroglyphidae) B.v.d.Lustgraaf, 'Oecologia (Berl.)' 33: p351-359. 'Numerous conidia [fungi, 'Aspergillus penicilloides'] were also present on the outer surface of the mite and in the digestive system and faecal pellets.'

1986 (Study 1): Microorganisms and Bacteria on the Mite
'Microorganisms associated with the house dust mite, 'Dermatophagoides'. Oh H. et al 'Jpn.J.Sanit.Zool'. 37: 3; p229-235. 'It is unlikely that the house dust mite would serve as a vector of disease by biting, although they harbour microbes and might transmit disease.' The bacteria isolated from the house dust mite were 'Bacillus spp', 'Staphylococcus spp.' gram negative non-fermenting rods and gram-positive coryneform rods. As for fungi, 'Aspergillus spp', 'Penicillium spp.', 'Cladusporium spp,', 'Alternaria spp.', 'Acremonium spp.', 'Paecilomycis spp. and yeast, were isolated.' [In this study the whole wild mite's body was investigated, not just the gut or droppings.]

1986 (Study 2): Sterile Mites in Lab Conditions
'Ultrastructure of house dust mites, 'Dermatophagoides farinae' and 'D. pteronyssinus'.' Tongu Y.,Ishii A.& Oh H. 'Jpn.J.Sanit.Zool'. 37: 3; p237-244 ' Microorganism was not found in the digestive tract of the mite maintained in our laboratory. [The mites were fed on sterilised food in laboratory conditions. The researcher's aim was to map the mite's digestive contour and function.]

1990: Selective Fungus Ingestion
'The relationship between house dust mites and fungi' Hart B.J. & Douglas A.E. 'The Acari, Reproduction, Development and Life History Strategies', Chapman & Hall, London 24: p319-324. 'It is concluded that mites select 'Aspergillus penicillioides' and 'Wallemia sebi' and other fungi are not ingested or are killed by the digestive processes of the mite.'

1991: Fungus Presence and Mite Association
'Ecology of the House Dust Mite'. Hay David B, Linacre College, Oxford University, (British Library, DSC D. 170040) 'Aspergillus penicillioides', 'Eurotium repens', and 'Wallemia sebi' were all isolated from 'D. pteronyssinus' derived from dust samples in which the fungi were not detected. Reference 5.2.2.1

1992 (Study 1): Fungi Passed Undamaged
'How relevant are house dust mite-fungal interactions in laboratory culture to the natural dust system?' Hay D.B. et al, 'Exp.& Appl.Acar'.16: p37-47. 'Found intact fungal spores in all parts of the gut but not elsewhere in the body of 'Dermatophagoides pteronyssinus'. Fungi do not appear to be retained selectively by 'D pteronyssinus' and are passed through the gut with the ingesta. 'Both house dust and house dust mites 'D pteronyssinus' contain a wider range of fungi than laboratory mite cultures. In total, nine species of fungi were isolated from 'Dermatophagoides pteronyssinus' in house dust and these included three xerophilic species ('Eurotium amstelodami', 'Aspergillus penicillioides' and 'Wallemia sebi') commonly found in laboratory cultures of 'Dermatophagoides pteronyssinus'.

1992 (Study 2): Fungus Does Not Add Allergenicity
'Evidence refuting the contribution of the fungus 'Aspergillus penicillioides' to the allergenicity of the house dust mite 'Dermatophagoides pteronyssinus'. Hay D.B., Hart B.J. & Douglas A.E., 'Int. Arch. All. Immun.' 97:p86-88. 'Allergen profiles of experimentally derived fungus-free adult mites and mites re-fed the fungus 'Aspergillus penicillioides' are identical'

1999: Spores Pass Undamaged (Again)
'Interactions between domestic mites and fungi'. Van Asselt L. 'Indoor Built Environment' 8: p216-220. 'It is known that mites feed on fungi since spores of 'Aspergillus penicillioides' have been observed in all parts of the alimentary canal of 'Dermatophagoides pteronyssinus'. Some of them are found undamaged in faecel pellets'

2009: Guanine and Fungal Spore Dissemination
'Dust Mites', Matthew J. Colloff, CSIRO Publishing, ISBN 978-0-6430-6589-5 'The colonising mites feed on fungi and defecate. Other mites aggregate at the site in response to the detection of quanine, which indicates that other mites are present and there is an opportunity to mate. They mate and disperse, having fed on mouldy endosperm, reproduce, egest and disseminate the fungal spores in their guts. Thus new fungal cultures are propagated.' This quote refers to the storage mite Acarus siro, endosperm is the outer casing of wheat. On page 54 of this book are photos of fungi growing out from house dust mite (Dermatophagoides pteronyssinus) faecal pellets.

Contents of Mite Faecal Particles (2011)

From the research published in 'Immunological Reviews' (2011), the following biological and molecular contents were confirmed to be present in mite faecal particles:

• Der p1: an active cysteine protease; destructive, even to itself
• Der p2: mimics immune signalling indicating infections.
• Mite DNA - unmethylated, [potentially pro inflammatory], 'Infect Immun.' 2008; 76(5): 2123 - 2129
• Bacterial DNA - unmethylated [potentially pro inflammatory, as above]
• Endotoxin - outer casing of gram-negative bacteria
• Chitin - outer casing of fungi