Red tide is a thing of legend, lore, and modern day media. In the past week The Wall Street Journal has published two articles on this topic, likely spurred on by the recent release of data from the National Oceanic and Atmospheric Administration that the 2018 red tide in Florida caused the death of 174 dolphins.
Wait, wait, red tide kills dolphins? Does It kills fish too? Let’s dive into this.
What exactly is red tide? Red tide is caused by an overgrowth of a specific type of algae called Karenia brevis (K. brevis), which is a dinoflagellate species of algae. It is most commonly seen in the southeastern USA, Gulf of Mexico and the Caribbean. This overgrowth of algae occurs when the nutrients from rich, fertilized inland soil get washed out into the ocean, especially richly fertilized farm lands. The influx of nitrates and phosphorus from the fertilizer are prime food for the dinoflagellate algae leading to an outrageous burst in growth and color. Though dinoflagallates are the family of algae that is the major oxygen producer in the ocean, when the growth is out of control it actually consumes all the nutrients and oxygen in the water, causing an area of the ocean that cannot support life.
When K. brevis grows out of control it is called an algae bloom, and because of the red color of the algae it has earned the name “red tide” (or “crimson tide” in Alabama). Some blooms, however, never reach high enough concentration to see a frank red color in the water, although some of the destructive effects of a red tide can be seen with even slight discoloration of the water, or no red coloration at all.
So why is red tide so bad? In addition to consuming all the oxygen in the area and effectively suffocating the ocean, K. brevis produces brevetoxins. Brevetoxins are heat stable, meaning they are not destroyed by boiling or cooking and it negatively affects the blood (hemolytic) and nervous system (neurotoxic), causing the two human syndromes associated with a red tide; Neurotoxic Shellfish Poisoning (NSP) and Aerosolized Red Tide Respiratory Irritation (ARTRI).
Neurotoxic Shellfish Poisoning is the most serious of the red tide effects on mammals. Shrimp, mussels, oysters and other shellfish filter the water containing the algae, concentrating the toxin within their tissues. Larger animals up the food chain like fish, dolphins and humans eat these delicious organisms, and suffer the neurotoxic and hemolytic effects of the brevetoxin, leading to illness and death. The shellfish are unaffected by the toxin.
In general, most humans don’t go around eating dead fish, but they do like to dine on oysters. Brevetoxin gets concentrated in the oysters, humans eat them at a restaurant and 20-30 min later start experiencing nausea, vomiting, diarrhea, numbness and tingling (paresthesias), dizziness, muscle weakness, and muscle aching. These symptoms are caused by increasing sodium ion channel permeability. In severe cases, or in late presentations to the ER they may even have paralysis, coma or death.
The first step in successful treatment is to be suspicious that brevetoxin could be the cause of an individual patient’s vague symptoms. Unfortunately there is no antitoxin, and supportive care is the best course of therapy with fluid resuscitation, antiemetics and intubation/mechanical artificial ventilation (if needed) based on the degree of the neurotoxic effects and paralysis the patient is experiencing. With appropriate care, most people will improve but there can be long lasting effects, including prolonged muscle weakness, deceased exercise tolerance and paresthesias.
Aerosolized Red Tide Respiratory Irritation (ARTRI) is caused by the aerosolized dispersion of the algae and toxin. This causes tearing, eye irritation, itching, rash, and bronchospasm (cough and wheezing). Patients with a history of eczema, asthma or COPD (emphysema and chronic bronchitis) are especially susceptible to the pulmonary effects and can have more severe symptoms. On arrival in the ER, patients should have decontamination performed immediately to wash off any residual algae or toxin that could prolong or exacerbate the symptoms. The victims of ARTRI should be treated with albuterol, Benadryl, fluids, and for patients with respiratory symptoms, steroids should be considered. Most patients have resolution of their symptoms in 3 days or less.
So why aren’t doctors talking about this? Red tide is widely considered to be a regional problem, however seafood from the gulf of Mexico can make its way across the country, and not all red tides cause a noticeable change in water color. There is that old adage – “only eat oysters in months with an R” – that’s because thawing snow and spring rain, mixed with warm temperatures and warm water flowing out to the ocean make for prime algae conditions. So if I were you I would be particularly wary about eating warm water oysters any time of the year!