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What Happens When Trout Get Parasites?

Creatures big and small are affected by a variety of parasites and fish are no exception. Parasites are a common problem among many fish species but are no cause for alarm. Trout, which spend part of their life in brackish freshwater, are often more likely to have parasites. While most parasites are not harmful to humans, properly cook or freeze your fish before you eat it. If you notice an unusual parasite, check with your local health authority to ensure your fish is safe to eat.

Parasite Transmission
- Parasites are transmitted to fish through a cycle involving mammals, birds, larvae and crustaceans. After a mammal or a bird eats an infected fish, the parasite usually grows to maturity inside them. When they defecate, the parasite's eggs end up in the waterway, where they are then eaten by crustaceans, snails and other small shrimp-like creatures. Trout then become infected with parasites after eating these creatures, thus continuing the cycle.
Parasites Effects on Trout Organs
- Parasites have been known to negatively effect rainbow trout's eyesight, altering their ability to properly match their color with their surroundings and therefore making them more vulnerable to predators. Studies have also shown that the effect on their eyesight makes them slower in reacting to movement. Some parasites can also cause intestinal inflammation, kidney and liver lesions and swelling of the gills.
Parasites Effect on Fish Behavior
- Some studies have found that fish infected by parasites often behave differently. While it is unclear how parasites alter a fish's behavior, fish with parasites tend to surface for air more often. This makes them more susceptible to predators, such as birds. In a study by Kevin Lafferty and Kimo Morris, researchers at the University of California, 54 percent of parasite-riddled fish had disappeared, due to predation, in 20 days. Meanwhile, only 8 percent of fish in the same environment that were not infected by parasites were predated upon. It has also been shown that females will deliberately avoid mating with infected male fish.
Visual Clues
- Many parasites leave marks on trout which can help consumers identify an infected fish. Black spot disease is one of these parasites and dots the trout it infects with black spots which resemble crushed pepper. This parasite is not harmful to humans. You may also notice thin, white worms when you cut into your trout, or boils along the trout's sides. Although unsightly, these are not harmful to humans, but you should properly cook or freeze your fish prior to eating. Dark boils should be cut out as they may be a bacteria infection which resulted from the parasite.

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