The Minnesota Pollution Control Agency is asking swimmers to be aware of algae-ridden lakes and foul-smelling water this holiday weekend after toxins from the blue-green blooms hospitalized a child and killed two dogs near Alexandria, Minn., last month.

Although not all algae are harmful, poisonous blooms typically look like pea soup, green paint or floating mats of scum. Officials warn that the clusters often emit a strong stench like cow manure.

“With the intermittent periods of rain, followed by high temperatures, blue-green algal blooms will be common on many lakes throughout Minnesota for the remainder of this summer,” Steve Heiskary, an MPCA scientist, said in a statement.

A young boy was swimming in Alexandria’s Lake Henry in June when he was exposed to the noxious plant and hospitalized, said MPCA’s Detroit Lakes spokesman Dan Olson. Researchers took samples of the water a few days later, confirming the presence of the dangerous blue-green algae. The boy made a full recovery.

Before the Lake Henry beach opened for the season, “water advisory” signs were hung in the swimming area warning visitors to stay away if the water looks like spilled green paint.

Olson said it’s difficult to say whether there are more outbreaks of the blooms this summer compared to previous years because Minnesota lakes are not regularly tested for it.

Blue-green algae forms when extra nutrients like phosphorus seep into bodies of water — often as a result of agricultural practices. In most cases, the blooms are “self-limiting due to weather conditions and don’t persist,” Olson said, adding that algae regularly disperse in wind and lower temperatures. Conditions change so quickly that it’s difficult to track an outbreak, which is why officials urge vacationers to be wary.

Olson said the Lake Henry incident is the first case of human exposure he’s heard of.

The unpleasant odor and latex-like consistency of a blue-green algal bloom usually keep people away, but those who choose to swim, boat or bathe in contaminated areas can become ill.

People who are exposed to the toxins by swallowing, having skin contact with, or breathing in airborne droplets of water may develop symptoms such as vomiting, diarrhea, rash, eye irritation, cough, sore throat and headache.

Animals are at far greater risk than humans because they tend to wade in the shoreline areas where algae may accumulate and aren’t necessarily deterred by the bad smell.

Two dogs died after taking a dip in Red Rock Lake in Douglas County last month, the MPCA warned on its website. Animals could ingest the toxins if they drink the water or lick their coats after swimming. Experts recommend hosing dogs off immediately after exiting water with visible algae growth, before they have the chance to lick themselves clean.

In addition to vomiting and diarrhea, dogs can develop difficulty breathing, general weakness, liver failure and seizures within a very short time.

Any person or pet showing symptoms should immediately receive medical attention.

Harmful algae blooms may also appear grayish brown, so officials don’t recommend entering water that looks thick and opaque. But the hazards are limited to where those blooms are located and don’t regularly affect the entire lake.

“If you happen to be on the side of a lake where it smells fine and looks fine, you’re probably going to be fine,” Olson said. “The problem is going to occur where the algae is and where you see it and smell it — because that’s where the concentrations are high enough to really cause a problem.”

The rule of thumb, he said, is “When in doubt, stay out.”

Potential algae blooms should be reported to the MPCA at 651-757-2419.