- Alternaria alternata is the most common outdoor allergenic mold in temperate regions and a major trigger of asthma exacerbations, particularly in late summer and autumn.
- Sensitization to Alternaria is a significant independent risk factor for severe asthma — associated with emergency department visits, near-fatal attacks, and asthma mortality.
- Unlike most other mold genera, Alternaria is primarily an outdoor species that infiltrates indoors; indoor amplification occurs primarily in high-moisture areas such as windowsills, shower stalls, and food storage.
- Alternaria spore counts peak in late summer and early autumn in temperate climates, coinciding with the seasonal peak in asthma hospitalizations.
- The genus contains over 300 species; A. alternata is the dominant human-relevant species and one of the most prevalent airborne fungi globally.
Alternaria is a cosmopolitan genus of filamentous fungi found worldwide in soil, decaying organic matter, and as plant pathogens. Alternaria alternata is the most important species from a human health perspective — it is a leading cause of mold allergy and a recognized risk factor for severe asthma. Understanding its ecology, seasonal patterns, and health significance is essential for managing mold-related respiratory disease.
Taxonomy and Classification
Alternaria alternata (Fr.) Keissl. belongs to the family Pleosporaceae, order Pleosporales, class Dothideomycetes. The genus Alternaria was first described by Friedrich Nees von Esenbeck in 1816. It is a dematiaceous (darkly pigmented) hyphomycete — one of the “black fungi” — with melanin pigmentation in both hyphae and conidia. The genus contains approximately 300 species, though the taxonomy has been subject to significant revision with molecular tools revealing that many morphologically defined species are genetically heterogeneous.
Morphology and Identification
Alternaria is readily recognizable microscopically by its distinctive conidia:
- Conidia: Large, multicellular, club-shaped to obclavate spores with both transverse and longitudinal septa (muriform). Ranging from 20–200 μm in length. Brown to dark olive-brown pigmentation.
- Conidial chains: Conidia form in chains, with each spore bearing an apical protuberance (beak) to which the next spore in the chain attaches. This beaked chain structure is diagnostic.
- Colonies: Macroscopically, colonies are grey-green to olive-black with a felt-like or powdery surface. Reverse colony color is typically black.
Ecology and Distribution
A. alternata is primarily a saprobe (decomposer) and plant pathogen with worldwide distribution. Its principal ecological niche is decaying plant material in both field and storage settings. As a plant pathogen, it causes significant agricultural losses in fruit, vegetables, and grains — diseases include early blight of tomato and potato, brown spot of citrus, and storage rots of many commodities. Its airborne conidia are among the most numerically abundant fungal spores in outdoor air globally, with published studies documenting it as the dominant or co-dominant airborne fungus in temperate and semi-arid regions including North America, Europe, and Australia.
Seasonal and Diurnal Patterns
Outdoor Alternaria spore concentrations exhibit pronounced seasonal and daily patterns that are clinically significant:
- Seasonal peak: Late summer (July–September in the Northern Hemisphere) is the period of highest outdoor spore concentrations, coinciding with dry, warm conditions and mature plant material. In arid regions like the southwestern United States, this peak extends through autumn.
- Diurnal pattern: Outdoor concentrations typically peak in the afternoon during dry, warm conditions — wind dispersal of dry conidia is highest during low-humidity, turbulent afternoon air.
- Weather effects: Rain events transiently reduce airborne concentrations by washing spores from the air but can produce brief spore release peaks as drying follows precipitation. Thunderstorms in particular can cause rapid spore release events.
Indoor Occurrence
Unlike species such as Stachybotrys or Chaetomium that are characteristically indoor amplifiers, Alternaria is primarily an outdoor genus that infiltrates buildings through air exchange. Indoor Alternaria concentrations typically track outdoor levels. Indoor amplification — growth beyond what outdoor infiltration would explain — occurs in specific high-moisture microenvironments: shower stalls and bathroom walls, windowsills with condensation, soil of indoor plants, and food storage areas where fruits or vegetables are stored. In buildings with significant water damage, Alternaria may amplify on cellulosic substrates, though it is less characteristic of water damage than Stachybotrys or Chaetomium.
Health Significance
Allergic Disease
Alternaria sensitization is one of the most prevalent mold allergies globally, affecting an estimated 3–10% of the general population and a substantially higher proportion of atopic individuals. Key allergens include Alt a 1, a highly specific allergen used in diagnostic testing, along with Alt a 2 through Alt a 28. Alt a 1 is considered a major allergen — IgE reactivity to Alt a 1 is found in the majority of Alternaria-sensitized patients.
Asthma
The relationship between Alternaria sensitization and severe asthma is among the best-established associations in mold allergy research. Multiple studies across North America, Europe, and Australia have found that sensitization to Alternaria is an independent predictor of: more severe asthma, greater bronchial hyperresponsiveness, higher rates of emergency department visits and hospitalizations, and increased risk of near-fatal asthma attacks. Several studies have linked thunderstorm-associated asthma epidemic events — during which large numbers of people experience simultaneous acute asthma attacks — to mass Alternaria spore release.
Frequently Asked Questions
Is Alternaria a common household mold?
Alternaria is primarily an outdoor mold that enters buildings through normal air exchange. Indoor concentrations typically track outdoor seasonal patterns. Indoor amplification (growth beyond outdoor levels) occurs in specific high-moisture areas such as shower stalls, windowsills, and areas near houseplants. Compared to genera like Aspergillus or Penicillium, Alternaria is less commonly a primary indoor amplifier, though it appears in indoor air samples year-round.
Why does Alternaria cause such severe asthma?
The reasons for Alternaria‘s particular association with severe asthma are not fully understood but likely involve the potency of its allergens (particularly Alt a 1), the high outdoor concentrations achievable in late summer that can represent massive allergen challenges, and possible direct airway effects of fungal proteases and other metabolites. Some research suggests that Alternaria activates innate immune pathways in the airway epithelium in ways that amplify allergic inflammation beyond what simple IgE-mediated mechanisms would predict.
When is Alternaria season?
In temperate Northern Hemisphere climates, Alternaria spore counts peak from July through September, with the highest concentrations typically in August. Warm, dry conditions favor high airborne concentrations; counts drop sharply with cold temperatures and frost. In arid climates (such as the southwestern United States), the season may extend into October and November. Daily peaks occur in the afternoon during dry, windy weather.
Can Alternaria produce mycotoxins?
Yes. Alternaria alternata and related species produce several mycotoxins, including alternariol (AOH), alternariol monomethyl ether (AME), tenuazonic acid (TeA), and altertoxins. These compounds are of concern primarily in food contamination contexts — Alternaria causes storage rot in fruits and vegetables, and these toxins can accumulate in affected food. In indoor air quality contexts, the health significance of Alternaria is primarily via allergen inhalation rather than mycotoxin exposure.
How can Alternaria-sensitized patients reduce exposure?
Key strategies include: monitoring outdoor mold counts (many areas provide daily mold count reporting analogous to pollen counts) and limiting outdoor activity on high-count days; keeping windows closed during peak afternoon hours in late summer; using air conditioning with adequate filtration; addressing any indoor moisture problems that allow indoor Alternaria amplification; and discussing allergen immunotherapy (subcutaneous or sublingual) with an allergist — Alternaria immunotherapy has demonstrated efficacy in clinical trials for reducing asthma symptoms.
