Harmful fungus in bat feces used to fertilize marijuana plants found to have killed two growers

A team of medical professionals and infectious disease researchers at the University of Rochester, in New York, has found that two men who were growing their own marijuana plants died after contracting fungal lung infections that led to pneumonia. In their paper published in the journal Open Forum Infectious Diseases, the group describes the symptoms in the two unrelated cases and their diagnosis and issues a warning to marijuana growers about using bat guano as a fertilizer.

In recent years, several states in the U.S. have legalized the recreational use of marijuana—some (such as New York) have also made it legal to grow the plant. Such legalization has led to a growing market for marijuana-associated products. One such product is bat guano, which has been touted as being the best kind of fertilizer for growing the plant because of its high levels of phosphorus and nitrogen.

In this new case, the team in Rochester found that both patients had used bat guano to fertilize their marijuana garden. Unbeknownst to them, their fertilizer was harboring a type of fungus called Histoplasma capsulatum.

Both of the patients were male, one aged 59, the other 64. One of them had collected some bat poop he had found in his attic, the other bought a bag online. Both apparently inhaled the spores as they were fertilizing their marijuana plants. Both developed similar symptoms; a bad cough, fever, weight loss and eventually respiratory failure due to the development of histoplasmosis, a kind of pneumonia caused by breathing in H. capsulatum spores.

The researchers note that once such spores enter the lungs, they grow into a mature yeast, which can spread to other parts of the body through the bloodstream. Such infections are not uncommon, most often occurring in places where people come into contact with bat feces. They also note that most people recover from such infections when given antifungal therapies.

The two men in the case study were not able to overcome their infections due to underlying conditions—both were older, one had emphysema, and both were also cigarette smokers.

The research team suggests that people avoid using bat guano as a fertilizer.

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