Our Weird World: Parasite and Fungus Odd Couple

You already know that a parasite takes advantage of its host. But today I’m going to talk about a parasite that goes a step further and takes advantage of a fellow traveller...

Meet the cow lungworm (Dictyocaulus viviparous). As its name suggests, adult lungworms live and breed inside of cow lungs. Like a lot of other roundworms, lungworms have a life cycle that involves a torturous grand tour of their host’s tissues. Larval lungworms live in pastures, where they climb up blades of grass and wait for a cow to eat them. If a young worm manages to get swallowed, it digs through the cow’s intestinal wall, migrates to its lymph nodes, takes a quick break to molt, then dives into the cow’s bloodstream and gets swept to its lungs. The worm then molts one last time to become an adult, and joins the lungworm orgy rocking on in the bronchi. There’s plenty of lungworm baby-making and egg-laying going on in an infected cow. But if the young worms are going to infect new hosts, they have to get back out into the grass. So the worms (somehow) make the cows cough. Their eggs are coughed up out of the lungs and get swallowed. They hatch in the cow’s intestines, and wind up in the inevitable final product of that particular place: a cow pattie.

So they’re outside. But compared to some other parasites, lungworm larvae aren’t very mobile. And cows don’t graze near their own poop. So if a young lungworm is going to grab a new host, it needs to move away from the cow chip. It can sit in the dung and wait for a good rainstorm to wash it further out into the grass. But if it’s lucky, it’s sharing the cow poop with another organism taking a trip through the cow ecosystem: the fungus Pilobolus.

Pilobolus essentially eats dung. And it uses the most expedient method possible to find fresh food: its spores only grow after they’ve passed through an animal’s digestive system. Spores stick to grass, get eaten, and ride through the gut undigested until they get deposited in their own private restaurant. But Pilobolus has the same problem as the lungworm – cows don’t graze where there are lots of cow patties. If its spores are going to get into a cow, they have to leave their natal pattie. Fungi don’t have muscles, so they can’t crawl away. But Pilobolus has evolved another, more remarkable solution: its spore packet grows on top of a stalk that fills with pressurized water until it explodes and shoots the spores up to 4 feet away. What does this have to do with worms? If a larval lungworm happens to be in a cow pattie that’s growing some Pilobolus, the worm crawls up the fungal stalk, curls up on top of the spores, and waits for the explosion.