Germlings of Rhizopus arrhizus, rather than sporangiospores, lead to rhino-orbito-cerebral mucormycosis acquired intranasally in a murine model of uncontrolled diabetes

Mucormycosis is a rapidly fatal, angioinvasive fungal infection. The disease presents as distinct clinical entities, with each being linked to specific risk factors and routes of transmission. Rhino-orbito-cerebral (ROC) manifestation remains the predominant form of mucormycosis in the developing world, especially in India amongst patients with uncontrolled diabetes. Limited literature is available on experimental animal models to study mucormycosis, and a murine model of ROC mucormycosis that recapitulates the risk factor (diabetes) and mode of acquisition (intranasal) of this disease is largely undocumented. In this study, we demonstrate that sporangiospores of Rhizopus arrhizus (1x10 ⁶ cfu) failed to establish infection in diabetic mice when administered intranasally although intracerebral and intrasinus (ethmoid) inoculations were successful. The diabetic mice instilled intranasally with R. arrhizus spores (1x10 ⁶ , and even 1x10 ⁷ cfu) showed 100% survival, monitored upto 30 days post-inoculation. Their internal organs exhibited normal gross morphologies; fungal microscopy and culture were negative. In contrast, intranasal administration of 1x10 ⁶ germlings of R. arrhizus to diabetic mice led to successful development of the infection, with signs typical of ROC mucormycosis. 50% of the exposed animals became morbid within 48 h and died by 4–6 days of exposure. The fungus was recovered both in microscopy (KOH mounts and histopathology) and culture. These findings reveal that R. arrhizus germlings, rather than sporangiospores, are mainly responsible for the natural acquisition of ROC mucormycosis intranasally by a diabetic host. Further, the work establishes a clinically-relevant murine model of ROC mucormycosis that can be utilized for future studies.