Parasite: Black Spot Disease

Author: Dean Taylor

With the recent Oscar winner Parasite making waves on the internet, I figured I would take advantage of the search traffic and talk about a real parasite that I came across this summer.

Besides the water-borne bacteria that infected my Pike-bitten hand, the Black Spot (Neacus) disease was the most memorable parasitic discovery from this summer. I ran into this parasite multiple times throughout the season while guiding in northwestern Ontario, particularly when fishing for Northern Pike, when the peppering of black spots on the fish’s body began to concern both myself and my clients. After many weeks of throwing these fish back and writing it off as a simple abnormality, I decided to do some more research into what exactly these spots were and how they were affecting the fish we were catching.

This parasite, better known as Black Spot Disease, originates in fish-eating birds such as Herrons or Kingfishers who consume a parasitic flatworm from infected fish. While the chicken or the egg argument can be had around where the worm originated from, we will begin its origin story from when it enters the bird.

Once the bird ingests the infected fish, the worm larvae are released in the bird's stomach and grow to sexual maturity in the bird's intestine. After mingling in the intestine, the worms begin laying eggs that are passed through the bird's droppings where they hatch in the water and become free-swimming organisms known as Miracidia.

These free-swimming parasites quickly attach themselves to their next host, typically Snails, where they develop further. After a brief development period, the full-grown worms ,known as Cercairae, leave the comfort of the snail and seek out a larger home.

Now free of the snail, these Cercairae burrow themselves into the skin of larger fish, often bottom-dwelling and shallow-water species such as Northern Pike, to begin their final stage. Once the worm enters its final host, the fish responds by forming a hard cyst made from a black pigment to trap the worm, creating small black spots throughout the fish's body and temporarily pausing the spread of the parasite. These cysts remain closed until the fish is consumed, typically by birds, where the cycle starts over again.

Thankfully, this parasite cannot be spread to humans and infected fish can surprisingly still be eaten, despite the grainy texture the fillets might have due to the cysts. If this doesn't sound appetizing, the fish can be returned to the water without any real fear of spreading the disease or harming the fish. The only real preventative measure in limiting the spread of this disease is to bury dead fish or filets to keep them away from birds that will allow the worm to complete its cycle over again.