Red Tide & Shellfish Poisoning: What You Need to Know

Most Suncoast residents are familiar with the respiratory irritation and itchy eyes commonly experienced during red tide blooms. But did you know that eating red tide-contaminated shellfish can make you sick — possibly really sick?

What we call “red tide” is known by ocean scientists as a harmful Karenia brevis (K. brevis) algae bloom. Florida ride tides create toxins called “brevetoxins,” which at severe levels can kill fish, sicken marine mammals and impact our health. 

Brevetoxins can also contaminate filter-feeder shellfish like oysters, clams, mussels, conch and scallops. 

As they eat, shellfish pump and filter large amounts of water through their small bodies. In red tide-affected waters, they consume K. brevis algae along with that water, and the brevetoxins accumulate in their flesh. The toxins are not harmful to the shellfish, but they can be to people who eat the shellfish.

It’s easy to unknowingly eat red tide-poisoned shellfish. The toxins do not change their smell or flavor, and brevetoxins are not destroyed by cooking. So whether eaten raw or cooked, they can lead to brevetoxicity, also known as neurotoxic shellfish poisoning. 

Warning Signs & Symptoms 

Brevetoxicity can cause typical food-poisoning symptoms like nausea, vomiting and loose stools, as well as a number of less common, more concerning symptoms, like limb numbness and muscle weakness. 

Symptom onset can begin as soon as 15 minutes after eating contaminated shellfish or as late as 18 hours afterward, but symptoms usually start within about 3 hours. Typically, the symptoms resolve about 18 hours after they begin, but they can last up to 72 hours in some people.

Initial Symptom Onset 

• Nausea and/or vomiting
• Abdominal pain
• Loose stools
• Tingling sensation or “pins and needles” (paresthesia)

Symptom Progression

• Complete numbness (limbs, abdomen, chest and up to the face)
• Muscle weakness, mild to severe (Weakness to the point of being unable to lift arms / legs when lying down is possible, but paralysis does not typically occur.)
• Hot/cold temperature reversal (Also called “allodynia,” this effect is not harmful. An example: Drinking an iced beverage will feel hot, and drinking a hot beverage will feel cold.)

Other Possible Symptoms

• Headache
• Dilated pupils
• Tremor
• Difficulty swallowing
• Decreased reflexes
• Loss of coordination/balance
• Fast heart rate
• Vertigo
• Anxiety

While some of these symptoms seem scary and can be severe, brevetoxicity is not typically fatal, especially with medical care.

Care & Prevention

The best treatment for brevetoxicity is prevention: Know where your seafood comes from. Do not harvest or eat shellfish from waterways where there’s a red tide bloom. Only eat shellfish sourced from clean water or bought in a trusted seafood market, grocery store or restaurant.

Sarasota Memorial’s emergency care teams have treated a number of patients for brevetoxicity over the years, including recently and during the severe red tide bloom that hit our area in 2018. Some of these patients were well enough to be discharged from the ER and recover at home; some were admitted to the hospital for observation and supportive care; and others were admitted to the Intensive Care Unit for treatment of severe symptoms.

If you’re concerned you may have eaten red tide-poisoned oysters, clams, mussels, conch or scallops, but you don’t yet have symptoms or have mild symptoms, you can call the Florida Poison Information Center-Tampa (1-800-222-1222) for treatment guidance. However, if you’re experiencing any of the more serious neurotoxic symptoms described above, call your physician or seek immediate medical attention.  

As a Sarasota Memorial emergency care and toxicology clinical pharmacist, Jeremy Lund, PharmD, MS, BCCCP, BCPS, has treated numerous emergency cases of accidental poisonings and toxin exposures, including recent cases of brevetoxicity, in the SMH Emergency Department.