Does Specific Fungal Allergen Really Matter?

Exposure and sensitization to allergens can promote development and exacerbation of allergic disease. Various indoor allergens, including dust mites, animal dander, and mold have been reported to associate with allergic disease. Fungal spores and particles are widely distributed in outdoor and indoor environments and exposure to fungi has been shown to induce allergic disease and exacerbate symptoms in individuals sensitized to fungi.1 However, it is still unclear whether the total fungal spore count or specific species is more relevant to allergic responses. It should be investigated that the contribution of fungi to allergic disease is species-specific, with specific fungal species leading to allergic disease exacerbations.

Many studies examining the relationships between fungal exposure and allergic disease have focused on the total fungal spore count instead of specific fungal species as an indicator of exposure.23 In an elementary school with indoor mold count ranging 6,000 to 50,000 spores/m3, 85% of the students and teachers reported significant allergy symptoms.2 The author concluded that the elevations of the total spore count can have clinical implications, and a building is considered unhealthy at a total spore count of >1,000 spores/m3.2 Another observational study which is conducted in a large urban population demonstrated a positive correlation between daily concentrations of fungal spores and the frequency of visits to the emergency department.4 Therefore, changes in total fungal spore concentrations appear to be associated with an increase in allergy exacerbations. A more recent study5 on fungal spores in 173 asthmatic homes and 85 non-asthmatic homes reported that the total fungal spore and specific fungal species were higher in asthmatic homes than in non-asthmatic homes. However, that study did not confirm that the differences in the total fungal spore count or the distribution of specific fungi can cause asthma symptoms.

In contrast, case-control studies conducted with patients with asthma6 or allergic rhinitis7 showed no significant differences in the total spore count between cases and controls. The authors of these studies documented a potential relationship between genus-species and allergic diseases because some allergenic fungi were significantly higher in the allergy patients' homes, suggesting the type of mold is more important than the total spore count. In a large study of Taiwanese schoolchildren, there was no significant correlation between asthma and the total fungal spore in classrooms; however, Aspergillus/Penicillium and basidiospores spp. significantly correlated with current asthma.8 In a birth cohort study, exposure to specific fungal species in water-damaged buildings during infancy was associated with childhood asthma at 7 years.9 Thus, it could be possible that specific fungal species are more important in developing allergy symptoms rather than the total fungal spore count. This indicates that fungal species may play a different role in allergic disease. There is accumulating evidence that precise recognition of species-specific sensitization to fungal allergens is important in the management of allergic disease.

In this issue of the AAIR Journal, Lin et al.10 report the importance of specific allergenic species rather than the total fungal spore count for the exacerbation of allergy symptoms in fungus sensitized patients. They measured total and specific spore concentrations in 12 allergic patients' homes at active and inactive symptom stages. They could not find any significant association between exacerbations of allergy symptoms and the total or viable fungal spore count. Instead, they revealed that specific fungal species were more frequently found during the active stage. They also concluded the importance of specific species rather than the total fungal spore count for allergy exacerbations. However, an extreme small sample size (10 fungus-sensitized patients), heterogeneity of allergic disease, and no investigation into the effect of house dust mite (HDM) concentrations, the most important risk factor for allergy exacerbation in HDM-sensitized patients, should be considered in the interpretation of their results.

Meanwhile, recent studies1112 have demonstrated the clinical implications of a high concentration of small fungal fragments in asthma severity. Submicron fungal fragments, such as fungal hyphae and fragmented conidia, are more common than airborne spores, and these fragments may function as allergen sources. These authors found that the presence of submicron fungal fragments is positively associated with a higher prevalence of reported asthma symptoms.1112 Researchers emphasized the importance of submicron fungal fragments rather than fungal spores as a potential risk factor for allergic disease exacerbations, because they can penetrate into the lungs and be deposited therein.12 Recent progress in gene technology has greatly contributed to the identification of species-specific allergic molecules from different allergenic fungal sources.13 However, data verifying the allergenic reactivity of fungal allergens are insufficient. Studies regarding the species-specific impact of indoor fungi on the allergen sensitized population are warranted.

Identification of exact contributing factors for allergy exacerbations is crucial to evaluating the role of fungal species in fungal allergic patients.1314 Identification of the spectrum of fungal species in allergic patients' homes will provide information about potential intervention targets for promoting allergic disease and improving understanding of possible mechanisms involved in fungi associated allergy. It is also necessary to assess not only specific fungal species but also submicron fragments for better understanding of exact mechanisms for allergic exacerbations in fungal allergic patients.