Dust mites living in human lungs--the cause of asthma?

Asthma does not have a clear cause and may represent a cluster of diseases. We propose that asthma in house dust mite sensitive patients may be caused by recurrent inhalation of live dust mites which are able to live for some time in the bronchioles of the lung. To provide themselves with a food source, the mites may purposefully excrete proteolytic enzymes, including Der p1, which increase epithelial shedding by freeing cells from the basement membrane. The mites then feed on the shed respiratory epithelial cells. Consequent loss of an intact respiratory epithelium exposes underlying tissues to dust mite protein and other allergens triggering sensitisation to these proteins. Later, repeated infestation provokes an allergic response which manifests itself as asthma attacks. The evidence for this hypothesis was tested against the Bradford-Hill criteria for causation; consistency, strength, temporal association and dose response. Potential areas for further research were also identified. The association between asthmatic symptoms and pulmonary acariasis was consistent across a number of studies. Determining the strength of the association and any dose response requires more work which is dependent on the development of better tests for the detection of mites in sputum. There was tentative evidence of a temporal association in the published studies identified. Biological plausibility and experimental evidence was available for pulmonary acariasis in primates and arsenic treatment was effective in humans. Better tests for mites in sputum are needed, as is work to assess more modern anti-acaricidal drugs in dust mite sensitive asthmatics.