You’re staring at the bottom of a glass of water or maybe the rim of your toilet bowl, and something moves. It’s small. It’s greyish-brown. It looks like a tiny, flattened shrimp or a bug that belongs under a damp log rather than in your plumbing.
That’s a water louse.
Most people panic. They think the "lice" part means they’re going to get an infestation in their hair or that the local reservoir has become a breeding ground for parasites. Honestly? It’s a bit of a misnomer. These things aren't lice at all. They won't make your head itchy, and they don't want to live on your skin. But seeing one wiggle around in your drinking water is still enough to make anyone lose their appetite.
Defining the beast: What is water lice exactly?
If we’re being scientifically accurate—which we should be—the "water louse" (specifically Asellus aquaticus) is an isopod. It’s a crustacean. Think of it as a distant, aquatic cousin to the pill bug or "roly-poly" you find in your garden. They are scavengers through and through. They spend their lives crawling through the mud and detritus at the bottom of ponds, lakes, and slow-moving rivers.
They’re actually pretty hardy. While a lot of aquatic life dies off the second the oxygen levels dip or the temperature spikes, the water louse just keeps on trucking. They’ve got these segmented bodies and seven pairs of legs, which makes them look way creepier than they actually are. They don't bite. They don't sting. They just eat decaying leaves and algae.
The confusion usually starts with the name. In North America, people often call them "scuds" or "sowbugs," while in the UK and parts of Europe, "water lice" is the standard term. Regardless of what you call them, finding one in your home usually indicates a very specific set of circumstances within the water distribution network.
How do they even get into the pipes?
You’d think a water treatment plant would be like Fort Knox for bugs. Most of the time, it is.
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However, water systems are massive, sprawling labyrinths of cast iron, plastic, and concrete. Even the most sophisticated filtration systems can have a bad day. Sometimes, tiny juveniles or eggs manage to slip through the filters at the treatment works. Once they’re in the dark, cool environment of the water mains, they find a surprisingly cozy home.
The inside of an older water pipe isn't a smooth, clean surface. Over decades, a layer of "biofilm" builds up. This is a thin, slimy coating of bacteria and organic matter. To a water louse, this is a five-star buffet. They settle in, they eat the biofilm, and they start a family.
Usually, they stay stuck to the walls of the pipes. You never see them. But then, something changes. A fire hydrant is opened for testing nearby. A water main bursts three streets over. Even a sudden change in water pressure can "scour" the inside of the pipes, knocking these little guys loose and sending them tumbling toward your kitchen faucet. It's a mechanical accident, not an invasion.
Are they dangerous to your health?
Let's address the big question: can they hurt you?
Strictly speaking, no. According to the World Health Organization (WHO) and various national water quality boards, water lice are not known to carry diseases that infect humans. If you accidentally swallowed one while brushing your teeth, the worst part would be the "ick" factor. Your stomach acid would handle the rest.
But—and there is always a "but"—their presence is what we call an "indicator."
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While the bugs themselves aren't pathogens, having a bunch of them in the system suggests that the water mains have a significant amount of sediment or biofilm buildup. This can sometimes interfere with chlorine levels. If the chlorine is busy reacting with the organic gunk the lice are living in, there’s less of it available to kill actual harmful bacteria like E. coli.
So, while the louse isn't the villain, it’s often a sign that the pipes need a good cleaning.
The "Sediment" Problem and Why They Love Your Toilet
Most people don't find water lice in their drinking water first. They find them in the toilet tank.
Why? Because the toilet tank is the ultimate "low-flow" zone. Water sits there for hours. It’s still. It’s quiet. Any sediment or "passengers" that come through the pipes tend to settle there. If you see one swimming in the bowl, it almost certainly came from the tank, not up through the sewer.
Remember, these are freshwater creatures. They can’t survive in the harsh, chemical-laden environment of a sewer for long, and they certainly don't swim up through the "S-trap" of your plumbing. If they’re in your house, they came in through the main supply line.
Why "Lice" is a terrible name for them
Calling them lice was a massive PR mistake.
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True lice (Phthiraptera) are insects. They are parasites. They need a host to survive. Water lice are free-living crustaceans. They are much closer to a shrimp or a crab than they are to a head louse.
- Size: They get up to about 15mm-20mm long. Much bigger than a hair louse.
- Movement: They crawl with a jerky, scurrying motion.
- Diet: They are detritivores. They want your old plumbing slime, not your blood.
Dealing with an "infestation" at home
If you find more than one, you shouldn't just ignore it. It usually means there’s a localized population in your service pipe or the main street pipe.
First, check with your neighbors. If they're seeing them too, it’s a city problem. The water utility company will likely need to "flush" the mains. This involves opening hydrants at high pressure to clear out the sediment and the organisms living in it.
If it’s just you, the problem might be your own plumbing. Old galvanized steel pipes are notorious for this. They rust from the inside out, creating "pockets" where sediment collects and water lice thrive.
Don't bother with pesticides. Seriously. Pouring bleach down the drain won't do anything because they aren't coming from the drain. And you definitely don't want to be putting bug spray into your drinking water system.
What to do if you spot one today
If you’ve just seen one of these prehistoric-looking stowaways, don't panic. Take a breath. Your water is likely still chemically safe, but you should take a few practical steps to clear things out.
- Run your cold taps. Start with the one closest to where the water enters the house (usually the kitchen sink). Let it run fast for at least 10 to 15 minutes. This helps clear out any loose sediment sitting in your immediate service line.
- Clean your aerators. Unscrew the little mesh screens at the end of your faucets. You’d be surprised what gets caught in there. If a louse got stuck and died, it can start to rot and attract bacteria. Give them a soak in vinegar and a scrub.
- Inspect the toilet tank. Lift the lid. If you see "dirt" or sand at the bottom, that’s where they’re hiding. You can actually turn off the water, flush the tank to empty it, and wipe out that sediment manually.
- Contact your water provider. This is the most important part. They need to know. Most utilities have a "Water Quality" department. Tell them you've found Asellus aquaticus. Using the scientific name usually gets you transferred to a technician faster than just saying "I have bugs in my water."
- Use a filter in the meantime. A standard charcoal pitcher filter (like a Brita) or a 1-micron sediment filter will easily catch these bugs and their eggs. It provides peace of mind while the utility company deals with the pipes.
It’s an unsettling experience, no doubt. But in the grand scheme of water quality issues, water lice are more of a nuisance than a health crisis. They are a sign of an aging infrastructure, a reminder that our water travels through a living, breathing environment before it hits our glass. Keep the taps flowing, keep the communication with your city open, and you'll be back to bug-free water pretty quickly.