Aspergillus fumigatus is a thermotolerant, saprophytic fungal species that is ubiquitous in the environment. Mold was nominated to the National Toxicology Program (NTP) in response to public concern regarding suspected adverse health effects associated with personal exposure in indoor and occupational settings. A. fumigatus is of particular concern in the biowaste industry as the species can contaminate self-heating compost piles. Because of this potential for personal and occupational exposure and the lack of available toxicity data, toxicity studies were conducted in which male and female B6C3F1/N mice were exposed to A. fumigatus conidia (spores) two times a week for 3 months. All in-life procedures, including inhalation exposure, test article preparation, and hematology analysis, were completed by the National Institute for Occupational Safety and Health (NIOSH, Morgantown, WV). Battelle (Columbus, OH) conducted terminal necropsies, measured terminal body and organ weights, and evaluated gross lesions on‑site at NIOSH. Tissue processing and histopathology were completed at Battelle. Grocott’s methenamine silver (GMS) staining was performed at NIOSH. Genetic toxicology studies on mouse peripheral blood erythrocytes were conducted by Integrated Laboratory Systems, LLC (Research Triangle Park, NC).
Groups of 10 male and 10 female mice were exposed via nose-only inhalation to 1 × 10e+5 A. fumigatus viable spores (viable A. fumigatus), 1 × 10e+5 nonviable spores (heat-inactivated particle control), or to an air control. All male mice survived to the end of the study, whereas two female mice, one in the air control group and one in the heat-inactivated particle control group, died during the study. There was no effect of exposure on body weights. Gross lesions, observed at study termination, consisted of enlarged, gray bronchial lymph nodes in 5 out of 10 females exposed to viable A. fumigatus. Mean absolute and relative lung weights were significantly increased in male (43% and 47%, respectively) and female (68% and 75%, respectively) viable A. fumigatus-exposed mice compared to the air control groups.
Nonneoplastic lesions were observed in the larynx, lung, and bronchial lymph nodes. In the larynx, lesions presented primarily as epithelial squamous metaplasia at the base of the epiglottis in both males and females exposed to viable A. fumigatus; exposure to heat-inactivated control spores did not affect the larynx. The increased lung weights in the viable A. fumigatus-exposed groups correlated histologically with chronic active inflammation and hyperplasia of the bronchus‑associated lymphoid tissue (BALT) and bronchiolar epithelium in a majority of males and females. Mice in the heat-inactivated particle control groups also showed BALT hyperplasia but at lower incidences as compared to viable A. fumigatus-exposed groups. The lungs of all mice exposed to viable A. fumigatus spores also showed medial hypertrophy in small- to medium-sized pulmonary arteries. GMS-stained lung sections of viable A. fumigatus-exposed mice revealed spores within the alveoli and alveolar macrophages. Hyperplasia, either of lymphocytes or plasma cells, was additionally observed in the bronchial lymph nodes of viable A. fumigatus-exposed mice and corresponded to the grossly enlarged bronchial lymph nodes in females.
No increases in the frequencies of micronucleated erythrocytes were observed in peripheral blood samples from male and female mice obtained after 3 months of inhalation exposure to viable A. fumigatus spores, indicating no chromosomal damage was induced in progenitor cells in the bone marrow of these mice.
Under the conditions of this 3-month study, target organs identified in B6C3F1/N mice following inhalation exposure to A. fumigatus spores were the larynx, lung, and bronchial lymph nodes. Significant differences were observed between viable A. fumigatus exposure and both air control and heat-inactivated particle control exposures. These results build on initial NIOSH pulmonary immunology studies using the same exposure parameters and demonstrate that the immunological responses and histopathology could be enhanced by the viability of the A. fumigatus spores.
National Toxicology Program (NTP). 2021. NTP technical report on the toxicity studies of Aspergillus fumigatus administered by inhalation to B6C3F1/N mice. Research Triangle Park, NC: National Toxicology Program. Toxicity Report 100. https://doi.org/10.22427/NTP-TOX-100