With the help of the National Zoo’s pathologist, Dr. Tim Walsh, we solved the mystery of the yellow bump-covered darters! Official diagnosis? Grubs. That’s right, the good ol’ encysting fluke worm also known as the yellow grub (Clinostomum). This is a common parasite that starts its life in a snail, ends up in a bird, and in the middle lives in freshwater fish like darters, dace, shiners, and chubs. The worms hatch in the water, but have only hours to find a snail host or else face certain death. Once the larval grubs leave the snail, they enter the fish and set up shop in its flesh, making a swollen lump called a metacercaria. Are they harmful to fish? Sometimes. Severe infestations can be fatal. But then again many fish can get by just fine with a more modest number of the yellow, wriggling parasites. Are they harmful to humans? Well… It seems like the jury is still out on that one. Traditional thinking seems to be that, like other parasites of birds, this grub won’t infect people. But in 1995 a Korean man went to the doctor with pain and a “foreign body sensation” in his throat. Yup, you guessed it – he got grubs. Since then many other infections in humans have been reported, always involving individuals who eat uncooked fish. So it’s always a good idea to cook fish thoroughly.
For the time being we’ll keep an eye on this grub-infested fish population. It will be interesting to see how they cope with the parasites. Why did these grubs suddenly appear in a new fish population? We may never know, but we’re still looking for clues.
You can find out more information about yellow grubs and other fish parasites at the PA Fish & Boat Commission website.