Fish frequently host parasites, commonly referred to as worms, which is a natural biological occurrence in both marine and freshwater environments. While this fact can be alarming, the presence of these organisms is not an automatic sign of poor quality fish. The concern for human health is manageable through proper food handling and preparation techniques, allowing consumers to safely enjoy seafood.
Identifying the Common Parasites
Two significant parasitic worms found in fish that can affect human health are the roundworm Anisakis and the tapeworm Diphyllobothrium. Anisakis simplex, often called the herring worm or cod worm, is a nematode whose larvae are small, thread-like organisms. They are typically found coiled in the muscle tissue or on the visceral organs of marine fish like cod, herring, and salmon.
Diphyllobothrium species, known as the fish tapeworm, is the largest parasite that can infect humans. This segmented cestode is primarily carried by freshwater fish, including trout and salmon.
How Infection Occurs
Humans contract these parasites when they consume fish containing viable, live larvae that have not been destroyed. A live larva, once ingested, attempts to penetrate the wall of the human gastrointestinal tract, leading to symptoms like abdominal pain and nausea.
The consumption of raw or minimally treated fish preparations carries the highest risk of infection. Dishes such as sushi, sashimi, ceviche, and lomi lomi rely on little to no heat and are a concern if the fish has not been adequately processed. Similarly, lightly preserved products like cold-smoked salmon or gravlax are often processed below the temperature required to eliminate the threat.
Eliminating Risk Through Preparation
The most effective way for consumers to ensure fish safety is by applying specific temperature treatments to kill any potential parasites. This process is centered on either heat or extreme cold, both of which render the larvae harmless. For cooking, all fish should reach a minimum internal temperature of 145°F (63°C) and maintain that temperature for at least 15 seconds. This heat requirement is sufficient to destroy all common parasitic worms.
Freezing Standards for Raw Consumption
For fish intended for raw or undercooked consumption, such as in sushi, freezing is the required method for parasite destruction. Retailers and suppliers must adhere to specific commercial freezing standards to ensure safety.
These standards include:
- Holding the fish at -4°F (-20°C) or below for a continuous period of seven days.
- Reaching and maintaining a temperature of -31°F (-35°C) for 15 hours.
- Freezing to -31°F (-35°C) and then holding at -4°F (-20°C) for 24 hours.
Home freezers typically operate between 0°F and 10°F and are often not cold enough to reliably meet these destruction standards. Therefore, it is important to source raw-consumed fish from suppliers who can confirm this pre-treatment has occurred. Processors also perform visual inspections, sometimes using a light-table method called candling, to remove visible worms, but this does not completely eliminate the risk.