Eating salmon, especially when consumed raw or undercooked, carries a potential risk of parasitic infection. Parasites are naturally occurring organisms in the aquatic environment, and various species use fish like salmon as hosts during their complex life cycles. Understanding which specific parasites pose a threat, how they are transmitted, and the safety measures that destroy them is key to safely enjoying salmon and mitigating the risk associated with raw or lightly preserved fish products.
Key Parasites Affecting Salmon and Humans
Two primary types of helminths, or parasitic worms, are of concern when consuming salmon: the roundworm responsible for Anisakiasis and the tapeworm causing Diphyllobothriasis. Anisakiasis is caused by the larvae of the Anisakis species, often referred to as herring worm or cod worm. These larvae appear as small, coiled, white or translucent worms found in the muscle tissue or on the visceral organs of marine fish like salmon. Ingesting these live larvae causes gastrointestinal symptoms in humans, such as severe abdominal pain, nausea, vomiting, and diarrhea shortly after consumption, as the parasite attempts to penetrate the stomach or intestinal wall.
Diphyllobothriasis is caused by species of the genus Diphyllobothrium. This parasite can grow into the largest tapeworm found in humans, sometimes reaching lengths of up to 30 feet within the intestines. Unlike Anisakiasis, Diphyllobothriasis often presents with mild or no symptoms, but it can lead to chronic issues such as abdominal discomfort, weight loss, and a vitamin B12 deficiency. This vitamin deficiency can result in a form of anemia called megaloblastic anemia.
Understanding Zoonotic Transmission
The presence of these parasites in salmon is a result of a multi-stage life cycle involving several different host species in the aquatic food web. The transmission to humans is considered zoonotic, meaning the disease is transmitted from animals to humans, with people serving as accidental hosts.
In the case of the broad fish tapeworm, the cycle begins when eggs are released into freshwater environments, where they hatch into larvae that are ingested by tiny crustaceans known as copepods. When salmon or other fish consume these infected copepods, the parasite develops into an infective larval stage, called a plerocercoid, which encysts in the fish’s muscle tissue.
The roundworm follows a similar complex pathway, starting with eggs released into seawater that hatch and are consumed by small crustaceans, such as euphausiids. These infected crustaceans are then eaten by marine fish, including salmon, where the roundworm larvae migrate and encyst in the fish’s organs or muscle. Humans become infected by consuming the raw or improperly prepared fish containing these live larvae.
Safety Standards and Consumer Preparation
Mitigating the risk of parasitic infection from salmon relies on rigorous processing standards and proper preparation methods before consumption. For fish that is intended to be eaten raw, such as in sushi or sashimi, commercial food safety standards mandate a parasite destruction step through freezing. This commercial process typically involves freezing the fish at an ambient temperature of -4°F (-20°C) or below for a minimum of seven days, or using ultra-low temperatures of -31°F (-35°C) for shorter durations.
Many aquacultured salmon are often exempt from these freezing requirements due to the significantly reduced hazard. However, wild-caught salmon and other species are subject to these time and temperature requirements to ensure the parasites are killed before the product reaches the consumer.
For home consumers, cooking is the most reliable method of parasite destruction. Fish should be cooked to an internal temperature of 145°F (63°C), which is sufficient to kill any live parasites. Using a food thermometer to confirm this temperature in the thickest part of the fillet ensures the entire piece has been adequately heated.
Home freezers can also be used, but not all models can reach the necessary temperatures to quickly destroy parasites, making commercial processing more dependable. If attempting to freeze at home, the fish must be held at an internal temperature of -4°F (-20°C) for at least 168 hours, or seven full days, to be effective.