Human infections are dead ends for valley fever fungus

By John Upton

People infected with two closely-related species of fungi are dying in growing numbers in the American southwest. The Coccidioides spores are blown with dust into lungs, where they can trigger a painful and sometimes-deadly condition known as valley fever.

But any cocci that ends up in a human has hit a dead end. It will not reproduce to spawn a new generation.

That’s because of the lifecycle adopted by these varieties of cocci after evolving with the rodents that share their desert home. The coccis’ ancient ancestors lived and dined on plants. Then they evolved to feast instead on the rotting flesh of dead animals. Now they have evolved to live inside a living mammal, sometimes waiting for years for the host to die so they can pounce and quickly consume the fresh kill.

Illustrated by Perry Shirley.
Illustrated by Perry Shirley.

Mammals whose immune systems can’t control the fungus may die quickly. But as I explain in Vice’s Motherboard blog, most animals that are infected with cocci develop few symptoms — and those symptoms are normally short-lived:

Normally, [the Cocci] eek out lives as filaments called hyphae. The hyphae live in the soil and produce spores, a lucky few of which get sniffed into the lungs of desert rodents. The spores balloon in size inside the host, forming spherules. The mammal immune system kicks quickly into gear at this point, building walls around the spherules, containing them and developing immunity against further attacks.

It’s when the immune system fails to contain these spherules that the fungus can propagate throughout its victim, sometimes with deadly consequences. As an infected rodent dies, collapsing into the desert, the cocci burst out of suspended animation and unleash streamers of hyphae that eat the rotting meat. As the fungus feasts, hyphae and spores slip back into the soil, ready to start the cycle all over again.

Humans don’t slip into the desert sands when we die. We are embalmed or cremated, making any infection a waste of time for the fungus and, in some cases, a waste of life for humanity. “If a cocci spore gets into a human, it has made a big mistake,” John Taylor, a University of California at Berkeley mycologist, told me. “It’s unlikely to ever become adapted to living in humans.”