Debilitated fish have difficulty feeding and eventually starve or succumb to predators before they are old enough to reproduce. When a diseased fish dies, thousands of parasite spores are released into the water. The spores are highly resilient and can survive for up to 30 years in an aquatic environment. In the water, the spores are ingested by the tubifex worm, the alternate host of the parasite. Inside the worm, the spores hatch into the parasitic form that can once again infect young trout.
Fish can also become infected by eating other diseased fish.
According to the Whirling Disease Foundation, the parasite is not transmissible to humans, but large populations of native trout are being depleted, raising alarm among environmentalists and fly-fishing enthusiasts. Weissman is also an avid fly-fisherman. The rainbow trout used to be several thousand per mile, and they are now down to less than fifty per mile. But very little is known about the parasite or the disease that results from infection. Rainbow trout appear to fall prey to the disease most easily, although cutthroat trout and chinook salmon are also susceptible.