Understanding the Health Risks Posed by Algal Blooms

A UT Health Austin neurologist explains how toxins from algae, such as those found in Lady Bird Lake and Lake Austin this summer, can harm you and your pets

Reviewed by: David Paydarfar, MD
Written by: Lauren Schneider

A blue heeler mix dog is standing on a paddle board with its owner on Lady Bird Lake.

Update: This piece was edited on October 31 to reflect the most recent results of algae sampling conducted by the Watershed Protection Department.

While Barton Springs, Lake Austin, and Lady Bird Lake provide much-needed recreational opportunities during the dog days of summer, Austinites should protect their actual dogs from algae in these bodies of water.

The Watershed Protection Department first detected a toxin called dihydroanatoxin-a in a blue-green algae sample taken on May 30 from Red Bud Isle in Lady Bird Lake. Since then, toxins have been detected in algae samples from the following monitoring sites:

Lady Bird Lake

Red Bud Isle
  • Cylindrospermopsin in samples from September 2, September 15, September 30, and October 14
  • Dihydronatoxin-a in samples from May 30, June 14, July 27, August 8, and October 14
  • Homoanatoxin-a in samples from June 27, September 15, September 30, and October 14
Auditorium Shores
  • Anatoxin in samples from August 8, September 15, and September 30
  • Cylindrospermopsin in samples from September 15, September 30, and October 14
  • Dihydroanatoxin-a in samples from July 10, July 27, August 8, September 2, September 15, September 30, and October 14
  • Homoanatoxin-a in samples from June 27, September 2, September 15, September 30, and October 14
Festival Ramp
  • Anatoxin in a sample from August 8
  • Cyanotoxin in samples from September 15, September 30, and October 14
  • Dihydroanatoxin-a in a sample from October 14
  • Homoanatoxin-a in samples from June 27, August 8, September 2, September 15, and September 30

Lake Austin

Emma Long
  • Anatoxin in a sample from August 11
  • Homoanatoxin-a in samples from June 29, July 11, July 27, August 11, and September 1
  • Unspecified toxin in samples from September 28
Walsh Boat Landing
  • Anatoxin in a sample from August 11
  • Homoanatoxin-a in samples from August 11 and September 28
Jessica Hollis Park
  • Homoanatoxin in a sample from September 1

Barton Springs

Barking Springs
  • Dihidroanatoxin-a in a sample from July 11

Harmful algal blooms on the rise

Blue-green algae, also known as cyanobacteria, produce several classes of toxic compounds collectively known as cyanotoxins. “These cyanobacteria are amazing factories of small molecules,” says David Paydarfar, MD, a neurologist who serves as Director of the Mulva Clinic for the Neurosciences at UT Health Austin.

According to Dr. Paydarfar, a combination of factors induces cyanotoxin production in blue-green algae, particularly during the summer. “The heat, sunlight, and agricultural runoff lead to this major amplification in production.” These sharp increases in cyanobacteria levels and cyanotoxin production are known as harmful algal blooms, and they pose health risks to humans and animals who are exposed to the toxins, typically by ingestion.

Harmful algal blooms have been on the rise in Austin in the past few years. The city first took notice in 2019 when a harmful bloom of algae containing dihydroanatoxin in Lady Bird Lake was linked to the death of several dogs; the toxin has been detected in the lake every year since, including this year at Red Bud Isle.

The specific health risks associated with these algal blooms depend on which cyanotoxins are produced. Cyanobacteria can produce toxins that affect the nervous system (neurotoxins), liver (hepatotoxins), skin (dermal toxins), digestive system (gastro-intestinal inflammatory toxins), or multiple organ systems at once (general cytotoxins). For example, cylindrospermopsin, a cyanotoxin identified last fall at Sculpture Falls after reports of illness, is known to affect multiple organ systems by preventing cells from making new proteins. The City of Austin prohibited swimming at the site until toxin levels subsided later in the season.

Dihydroanatoxin-A and homoanatoxin-a, the two toxins found in algae samples this summer, belong to a family of neurotoxins known as anatoxins. Anatoxins act on both the central nervous system (the brain and spinal cord) and peripheral nervous system (all other nerves) by binding to receptors on cells called nicotinic acetylcholine receptors (nAChRs).

In normal conditions, a chemical produced by neurons called acetylcholine binds temporarily to these receptors, facilitating certain types of cellular communication. Anatoxins bind nAChRs “almost irreversibly,” says Dr. Paydarfar. “It just wreaks havoc.”

In addition to binding neuronal receptors, anatoxins bind to nAChRs on muscle cells, disrupting the process by which the nervous system gives instructions to muscles. This initially causes muscle contraction and fasciculations, tiny contractions of only the muscle fibers controlled by a single motor neuron. Because the toxin never leaves the receptor, affected muscle cells reach a point where they can no longer be activated, resulting in paralysis.

Few cases of anatoxin poisoning have been reported in humans, but Dr. Paydarfar notes that symptoms have included fasciculations, paralysis, cognitive effects like delirium and hallucinations, and respiratory failure due to impaired respiratory muscle functioning.

Warning signs of anatoxin poisoning in pets

According to Dr. Paydarfar, early signs of anatoxin poisoning in pets that would warrant emergency evaluation include:

  • Confusion
  • Behavior change
  • Ataxia (not walking straight) or imbalance
  • Convulsions

Symptoms of anatoxin poisoning may manifest very suddenly and require immediate attention. “It’s a relatively small molecule, so it gets into the system quickly,” says Dr. Paydarfar.

Keeping you and your pet safe

Because anatoxin and its derivatives are so fast-acting, you should call a veterinarian right away if your pet shows signs of ingestion.

The next step is to call 3-1-1 to report the incident, according to Watershed Department environmental scientist Brent Bellinger. “They have questionnaires to determine if it was a pet illness and if a human possibly was exposed, at which point it’ll be routed to myself and/or Austin Public Health for follow-up.”

Bellinger says you can prevent algae ingestion by rinsing and toweling your dog off after swimming and keeping yourself and your pet out of water with the following characteristics:

  • Scum or mats floating on the surface
  • Stagnant
  • Odd smell
  • Discolored

“It might not even be a toxin issue. In those stagnant waters, you could have other bacteria, amoebas, viruses, or fungus,” he says.


What Austinites should know

No cyanotoxins detected in drinking water supply

The Austin Water website states that “current tests are non-detect for cyanotoxins in raw or treated drinking water.” The department routinely tests both raw water from Lake Austin and Travis as well as water from the Handcox, Davis, and Ullrich Treatment Plants. Last fall, Austin Water acquired new equipment to allow in-house testing for cyanobacteria and cyanotoxins in drinking water samples.

Controlling the spread in recreational waters

On June 13, Watershed Protection initiated Phoslock treatment in Lady Bird Lake. Phoslock is a gritty clay substance named for its ability to “lock” away phosphorus, which the algae otherwise would use as a source of nutrition.

The Watershed Protection Department collects algae samples every two weeks from three locations on Lake Austin and another three sites on Lady Bird Lake. The most recent samples were collected the week of October 14. Data from these tests can be tracked on the department’s dashboards monitoring algae toxicity in Lady Bird Lake and Lake Austin.

About UT Health Austin

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