Gastroenteritis may be caused by eating bony fish or shellfish. There are three common types of poisoning caused by eating bony fish:
(See also Shellfish poisoning.)
Ciguatera poisoning may result from eating any of the more than 400 species of fish from the tropical reefs of Florida, the West Indies, or the Pacific. The toxin is produced by certain dinoflagellates, microscopic sea organisms that the fish eat. The toxin accumulates in their flesh. Older fish and large ones (such as grouper, snapper, and kingfish) are more toxic than smaller, younger ones. The flavor of the fish is not affected. Current processing procedures, including cooking, cannot destroy the toxin.
The initial symptoms—abdominal cramps, nausea, vomiting, and diarrhea—may begin 2 to 8 hours after the person eats the fish and last 6 to 17 hours. Later symptoms may include
For months afterward, unusual sensations and nervousness may persist.
Doctors may treat affected people with intravenous mannitol (a drug that reduces swelling and pressure), but it is unclear whether this provides any benefit.
Tetrodotoxin poisoning is most common in Japan, due to eating the puffer fish (fugu), but more than 100 freshwater and saltwater species contain tetrodotoxin. Early symptoms include numbness in the face and limbs followed by increased salivation, nausea, vomiting, diarrhea, and abdominal pain. If a large amount of the toxin is eaten, muscles can become paralyzed and death may result from paralysis of the muscles that regulate breathing. The toxin cannot be destroyed by cooking or freezing.
After fish such as mackerel, tuna, bonito, skipjack, and blue dolphin (mahi mahi) have been caught, the tissues of the fish break down, eventually producing high levels of histamine. When ingested, histamine causes immediate facial flushing. It can also cause nausea, vomiting, stomach pain, and hives (urticaria) a few minutes after the fish is eaten. Symptoms, which are often mistaken for a seafood allergy, usually last less than 24 hours. The fish may taste peppery or bitter. Unlike other fish poisonings, this poisoning can be prevented by properly storing the fish after it is caught. Antihistamine drugs such as diphenhydramine may help the symptoms.
Shellfish poisoning can occur from June to October, especially on the Pacific and New England coasts. Shellfish such as mussels, clams, oysters, and scallops may ingest certain poisonous dinoflagellates at certain times when the water has a red cast, called the red tide.
The dinoflagellates produce a toxin that attacks nerves (such toxins are called neurotoxins). The toxin, saxitoxin, which causes paralytic shellfish poisoning, persists even after the food has been cooked.
The first symptom, a pins-and-needles sensation around the mouth, begins 5 to 30 minutes after eating. Nausea, vomiting, and abdominal cramps develop next, followed by muscle weakness. Occasionally, the weakness progresses to paralysis of the arms and legs. Weakness of the muscles needed for breathing may even be severe enough to cause death. Those who survive usually recover completely.