The nation's ongoing fungal meningitis outbreak has killed 30 and sickened 419 people so far, but the fungus responsible has never wrought such havoc before.
The fungus, Exserohilum rostratum, is a plant-eating generalist equipped with a spore-launching mechanism ideal for going airborne, is not an especially picky eater and, although it prefers grasses, will dine on many items—including humans.
But just how a pathogen typically associated with the great outdoors got into the three lots of injectable steroids prepared inside an admittedly filthy laboratory—and why only three lots—remains a puzzling mystery.
The errant fungus has been identified in lab samples from 52 of those affected and was similarly found growing in unopened vials of the steroid alleged to have caused the outbreak, according to the U.S. Centers for Disease Control and Prevention. A third recalled lot is still being tested. But E. rostratum is not a household name, even among mycologists.
Glenn Roberts, a retired medical mycologist, says that in his 40 years of experience at the Mayo Clinic in Rochester, Minn., he had seen only one case: a soft-tissue arm wound in an immunocompromised patient. He was shocked when he heard the identity of the pathogen in the epidemic that originated with the New England Compounding Center pharmacy in Framingham, Mass.
“I could hardly believe it because it’s just so uncommon,” he says.
And yet, outside in the air and on plants, E. rostratum is not so uncommon.
In press reports, it has been described as occurring "on grasses," but that is not the full story. The fungus, which seems to prefer tropical and subtropical environments, has turned up on a wide variety of plant species, says Kurt Leonard, an emeritus professor in the Department of Plant Pathology at the University of Minnesota who retired in 2001 from the U.S. Department of Agriculture's Cereal Disease Lab (then the Cereal Rust Lab).
Early in his career, Leonard untangled the taxonomic mess of similar-looking, but only distantly related, fungi with multicellular dark spores that were causing disease in grains such as corn. He named one new genus he had created—Exserohilum—for the prominent protuberances called hila (the belly buttons of the fungal and botanical world) on its spores.
The modus operandi of one species in this genus – E. rostratum -- was to infect a plant and in some cases precipitate tissue death. Plant defenses—which can include induced cell fortification, cell suicide, toxic chemicals, and defensive enzymes and proteins—typically were sufficient to keep the infection in check, but not strong enough to eliminate it. The payoff came when the plant died—the fungus was first in line to feed on its decaying remains. "I think it's just a general weak pathogen of plants," Leonard says, "something that can infect plants while alive and not really do much damage until the leaf senesces."
Leonard found E. rostratum on corn, sorghum and Johnsongrass fairly often, although it was not nearly as common as several more severe corn pathogens. It was an opportunist and would sometimes infect ears and stalks when insects drilled into the plant, creating a convenient landing pad of dying tissue for the fungus.
Most often the fungus shows up on grasses and other monocots—plants often distinguished by flower parts in threes and parallel leaf venation—such as pineapples, bananas and sugarcane, but it has also been found on non-monocots such as grapes and muskmelon. It's a fungus that is not, apparently, very picky about its food. "It's just a really common fungus in the environment that mostly lives on dead and dying plant tissue," Leonard says. There are many such others, and many of them can also occasionally infect animals or people.