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Keep dogs out of water that looks like pea soup or paint

July to October

Some blue-green algae blooms make toxins, and dogs are at higher risk than people because they swim in and drink water we avoid and then lick it off their fur. Massachusetts posts advisories for blooms, which the state says are most common in summer and early fall.

Why this time of year

The state Department of Public Health says harmful algae blooms in Massachusetts are most common in summer and early fall, and can last from several days to several months. The state keeps an updated list of waters with algae bloom advisories.

How to notice it

  1. Before a dog swims or drinks at a pond or river in summer, look at the water: CDC says a bloom can change its color or look like foam, scum, mats, or paint on the surface.
  2. If the water looks or smells bad, or an advisory is posted, keep the dog out of the water and away from scum on the shore.
  3. If the dog does go in, rinse it right away with clean, fresh water, and do not let it lick its fur first.
  4. Call a veterinarian right away if the dog vomits, drools, staggers, or has tremors or seizures after time near the water.

Try this with kids: Pick the dog's own water bottle as the drink for the walk, and fill it before we leave.

Good to know: Blue-green algae (cyanobacteria) are tiny water organisms that can multiply fast into a bloom.

Safety

  • CDC: animals can get sick and die within hours after swallowing toxins made by harmful algae.
  • Public health officials: if you, your child, or your pet comes in contact with the water or algae on the shoreline, rinse off immediately.

Sources

  • boston.govBoston Public Health Commission advisory (municipal primary), issued 2026-08-20, lifted downstream of Massachusetts Ave. Bridge as of 2026-09-22, last updated 2026-09-27: blooms 'make the water look like pea soup or paint'; 'most common in the summer months'; 'Do not allow dogs to swim in or drink the water'; dog symptoms 'vomiting, staggering, drooling, or convulsions'; 'If you, your child, or your pet comes in contact with the water or algae on the shoreline, rinse off immediately.' read 2026-11-10
  • cambridgepublichealth.orgCambridge Public Health Department, 2025-10-06: advisory 'was first issued by the Massachusetts Department of Public Health (DPH) in late July, 2025'; avoid letting dogs swim in or drink the water; 'Animals can get very sick or even die within hours of coming into contact with harmful algae or cyanobacteria'; lifted after two rounds of samples a week apart. read 2026-11-10
  • cdc.govCDC, 2024-04-08: 'Cyanobacteria can change the color of the water or look like foam, scum, mats, or paint on the surface of water'; 'keeping them away from water that looks or smells bad'; 'immediately wash them off with clean water'; 'do not let them lick their fur afterward'; 'Animals can get sick and die within hours after swallowing toxins made by harmful algae.' read 2026-11-10
  • riney.vet.cornell.eduCornell Riney Canine Health Center, Dr. Aly Cohen, 2025-09-05: blooms form 'mats, streaks, or pea-soup-like discoloration'; most common 'roughly early July through mid-August' (not Massachusetts-specific); 'Dogs are particularly vulnerable because they tend to have fewer discriminating behaviors'; 'immediately rinse your dog thoroughly with fresh water, prevent them from licking their fur.' read 2026-11-10
  • web.archive.orgMassachusetts DPH fact sheet (state primary, Wayback copy 2026-10-11): 'HABs in Massachusetts are most common in summer and early fall. They can last from several days to several months.' 'resemble pea soup or paint'. 'Dogs can become very ill and even die from licking cyanobacteria off their fur.' 'WHEN IN DOUBT, STAY OUT!' read 2026-11-10

Checked against these sources on November 10, 2026.

In the guide for July, August, September, October

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