Scombroid poisoning is a form of food poisoning. It happens when a person eats fish species containing high amounts of a chemical called histidine.

Histidine is an amino acid that naturally occurs in fish. When people do not keep the fish in a refrigerator, bacteria break down the histidine and turn it into histamine.

Histamine is responsible for the toxicity that causes scombroid poisoning, according to the American Academy of Allergy, Asthma & Immunology (AAAAI).

When a person consumes large quantities of fish with histamine, they experience symptoms that resemble an allergic reaction, such as hives. However, it is not a true allergy to a particular fish, according to the National Capital Poison Center.

The condition usually is not long-term or severe, according to the Food and Drug Administration (FDA). Mild cases often disappear on their own without treatment, but antihistamine medications may help some people.

A person experiencing severe symptoms, such as chest pain or breathing trouble, should go to an emergency room or call local emergency services.

Keep reading to learn more about the causes, symptoms, treatment, prevention, and outlook of scombroid poisoning.

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The cause of scombroid poisoning is eating certain varieties of fish that have spoiled. Most commonly, according to the Centers for Disease Control and Prevention (CDC), the fish species involved are:

  • tuna
  • mackerel
  • bluefish
  • mahi mahi, or dolphinfish
  • herring
  • sardine
  • amberjack
  • anchovies
  • marlin

These fish contain high amounts of the chemical histidine, per the AAAAI.

When people do not store the fish in a refrigerator, it results in an overgrowth of bacteria. The bacterial strains responsible for the condition produce enzymes that turn histidine into histamine.

These strains include those present in the skin and intestines of the fish, per 2012 research, such as:

  • E. coli
  • Morganella morganii
  • Pseudomonas aeruginosa
  • Klebsiella species

When a person eats the fish with high amounts of histamine, their symptoms resemble an allergic reaction. Due to this connection with histamine, scombroid poisoning is also called histamine toxicity.

According to the AAAI, individuals with low levels of the enzyme diamine oxidase are more likely to experience scombroid poisoning. This enzyme breaks down histamine from food. So, a person with low levels of the enzyme might not break down the histamine as well as a person with higher levels.

The poisoning is not due to an allergy, so it is safe to eat the fish again as long as it has undergone refrigeration.

Symptoms usually start within a few minutes to 2 hours after eating the fish, per the FDA. The initial ones resemble an allergic reaction, such as:

  • sweating
  • facial flushing
  • peppery taste around the throat and mouth
  • headache
  • dizziness
  • nausea

These initial symptoms may progress to:

  • facial rash
  • swelling
  • hives
  • short-term abdominal pain and diarrhea

The symptoms typically linger 4–6 hours and rarely last beyond 1–2 days, according to the FDA.

Doctors often base a diagnosis on the circumstances, according to the AAAAI. For example, a person’s symptoms may coincide with an outbreak that affected several people who ate fish bought from the same place.

Treatment depends on the severity of scombroid poisoning. Mild cases often disappear quickly without medication. Antihistamines, such as diphenhydramine (Benadryl), may help some people with the condition.

Severe cases require a trip to the emergency room. Here, treatment may include the administration of intravenous fluids, oxygen, and other medications.

Signs of a severe case include:

  • chest pain
  • trouble breathing
  • swelling of the tongue and mouth

If a person has severe symptoms that are not life threatening, it is best to see an allergist. They can help determine if the person is having an allergic reaction to the fish or if it is scombroid poisoning.

Because scombroid poisoning is not a true allergic reaction, it generally does not require epinephrine injections or corticosteroids. It is also not an infection, so antibiotics are not necessary.

When a person catches their own fish, the only reliable way to prevent scombroid poisoning is keeping the fish in a refrigerator set to 40°F (4°C) or lower. Cooking or freezing the fish will not get rid of the histamine that causes the symptoms.

If an individual eats fish in a restaurant, there is no way to tell if it is spoiled. It generally does not have a noticeable smell or taste. However, a salty, sharp, or peppery taste may act as red flags that indicate spoilage resulting from histamine, per the CDC.

Additionally, when someone gets scombroid poisoning from a restaurant or fish market, they should report it to the public health authorities for investigation. Removal of the fish from distribution will prevent further outbreaks from the source.

Scombroid poisoning results from eating certain varieties of fish, such as tuna and mackerel, that have undergone spoilage. Early symptoms may include facial flushing and sweating, while later symptoms may include hives and short-term diarrhea.

Although the symptoms resemble an allergy, the condition is not a true allergic reaction to a particular fish. It is safe to eat the fish again as long as it has not spoiled.

Treatment depends on the severity. Mild cases often resolve on their own. Antihistamines, such as diphenhydramine (Benadryl), may help some people.