Dogs and cats can look perfectly fine while hosting a whole microscopic horror show. Parasites don’t always announce themselves with dramatic symptoms—sometimes it’s just “weird breath,” random itching, mood changes, or stomach issues you keep chalking up to stress. The problem is, parasites don’t just irritate your pet—they can drain nutrients, trigger inflammation, wreck the gut, and in some cases, spread to humans. If your pet’s been “off” lately, these are the parasites vets worry people miss the most.
1. Roundworms
Roundworms are one of those parasites that sound old-school, but they’re still everywhere—especially in puppies, kittens, and pets that spend time outdoors. They live in the intestines, steal nutrients, and can cause a pot-bellied look, weight loss, diarrhea, or vomiting that seems to come out of nowhere. The scary part is how easy they spread through contaminated soil or feces, and how casually pets can pick them up. Some pets have them with almost no obvious symptoms until things get worse.
Roundworms also matter because they’re zoonotic, meaning people can be infected too, particularly kids. The CDC warns that roundworm eggs can persist in soil and can cause serious illness in humans if accidentally ingested. If your pet has inconsistent poop, a bloated belly, or sudden appetite changes, this is one of the first culprits vets consider. It’s gross, but it’s also common—and treatable if you catch it early.
2. Hookworms
Hookworms are tiny, but they fight dirty. They attach to the intestinal wall and feed on your pet’s blood, which can lead to anemia, weakness, pale gums, and lethargy that looks like “my pet is just tired lately.” In puppies and small dogs, hookworms can become serious fast because their bodies can’t compensate as easily. You might also see dark, tarry stools if there’s bleeding in the gut.
The tricky part is that pets can get hookworms through contaminated soil, licking paws, or even skin contact in some cases. If your dog loves the dog park, digs in the yard, or hangs out where other animals poop, the risk goes up. Hookworms can also cause skin irritation where larvae penetrate, so “mystery itching” can be a clue. A fecal test and deworming can change the entire trajectory here.
3. Tapeworms
Tapeworms are the parasite that loves drama without symptoms. Your pet may seem totally normal, then you notice rice-like segments near their butt, in their bedding, or stuck to fur like a cursed sprinkle. Tapeworms usually come from swallowing infected fleas or, less commonly, eating prey animals. So if your pet has fleas—even “a few”—tapeworms are absolutely on the table.
What owners miss is the flea connection, because they think they’d “definitely notice fleas.” The Companion Animal Parasite Council (CAPC) notes that tapeworm infection commonly occurs when pets ingest fleas during grooming, which means flea control is part of tapeworm prevention. Tapeworms don’t always cause dramatic illness, but they can cause irritation, scooting, and weight changes over time. Treating the worms without addressing fleas is basically cleaning up water while the pipe is still leaking.
4. Giardia
Giardia is the parasite that turns your pet’s gut into a chaotic mess, especially after boarding, daycare, hikes, or lake days. It often causes soft, greasy, foul-smelling diarrhea that comes and goes—so owners assume it’s food sensitivity or “he ate something weird.” Pets can pick it up from contaminated water, surfaces, or poop particles in shared environments. It’s one of those infections that thrives where dogs socialize.
The reason it’s so annoying is that it can be stubborn and reinfect easily if you don’t clean properly. Even after treatment, pets can pick it up again from the same park, the same puddle, or the same daycare run. If your pet has recurring diarrhea that doesn’t fully resolve, Giardia is a prime suspect. A vet test matters here, because guessing can keep you in the loop for months.
5. Coccidia
Coccidia is common in puppies, kittens, shelters, rescues, and any environment where animals are in close quarters. It can cause watery diarrhea, mucus in stool, dehydration, and a “decline” vibe that can look like stress or food transition. Some pets carry it with minimal symptoms, then flare when their immune system is strained. If your new rescue has diarrhea that won’t quit, this is high on the list.
The Merck Veterinary Manual notes that coccidiosis is especially significant in young animals and can be triggered by stress and crowding, which is why it’s so common after adoption or travel. Owners often blame the new food or anxiety, and sometimes that’s part of it—but coccidia can be the underlying engine. Treatment is usually straightforward, but it’s not the same as routine deworming. Catching it early can prevent a scary dehydration spiral.
6. Fleas
Fleas aren’t just “itchy.” They can trigger allergic dermatitis, hot spots, hair loss, secondary skin infections, and relentless scratching that ruins sleep for everyone in the house. Some pets are so allergic that one bite causes a full-body reaction, while others quietly suffer with mild but constant irritation. Fleas also contribute to anemia in small pets and, casually, they can transmit tapeworms.
Owners miss fleas because they don’t always see them. Fleas can hide, jump off, or be present in low numbers while still causing chaos. If your pet is chewing at the base of the tail, suddenly “dandruffy,” or you’re finding tiny black specks in fur, don’t assume it’s dry skin. Flea control is health care, not a luxury.
7. Ticks
Ticks are the parasite version of a slow-burn threat. The bite itself may look harmless, but ticks can transmit serious diseases that cause fever, lethargy, joint pain, appetite loss, and neurological symptoms depending on the pathogen. Some pets just seem “off” weeks later, and owners don’t connect it to that one hike or yard day. Ticks are also getting more common in more places as climates shift.
The CDC emphasizes that ticks can spread multiple diseases and that prevention plus prompt removal reduces risk. If your dog suddenly seems stiff, tired, or feverish after outdoor time, it’s worth bringing up tick exposure even if you didn’t find one. Tick prevention is one of those things that feels optional until it absolutely isn’t. It’s not just about the tick—it’s about what the tick brings with it.
8. Heartworms
Heartworms are the parasite that breaks hearts because they’re preventable, but devastating once established. They’re spread by mosquitoes and can cause coughing, fatigue, reduced stamina, weight loss, and heart and lung damage in dogs. Cats can also be affected, sometimes with sudden respiratory issues that look like asthma. The early stages can be subtle, which is why people miss it.
Many owners assume heartworm is only a “southern” problem, but mosquitoes don’t respect your assumptions. Prevention is much easier than treatment, and treatment can be intense and expensive. If your pet seems oddly winded on normal walks, don’t brush it off as “aging.” It’s worth testing and talking prevention seriously.
9. Ear Mites
Ear mites are a special kind of maddening because they make pets miserable fast. You’ll see head shaking, ear scratching, irritation, and that dark, coffee-ground gunk in the ear canal. Cats get them more often, but dogs can too—especially if they’ve been around other animals. It can look like “just dirty ears” until it becomes inflamed and painful.
Left untreated, ear mites can lead to secondary infections and damage from constant scratching. Owners sometimes try to clean it at home and accidentally make it worse. A vet can confirm mites and prescribe targeted treatment that actually ends the cycle. If your pet suddenly hates having their ears touched, that’s information.
10. Mange Mites
Mange mites can cause intense itching, hair loss, scabs, and skin thickening that looks like a severe allergy or “mystery dermatitis.” There are different types, including sarcoptic mange, which is highly contagious and can even affect humans briefly. Dogs may become frantic with itchiness, chewing and scratching until they create wounds. Owners often treat it like dry skin for way too long.
Demodectic mange is different and can flare when the immune system is compromised. The result is still skin damage, patchy hair loss, and infections that spiral. Either way, mange doesn’t usually fix itself. If your pet’s skin looks raw, patchy, or suddenly “different,” mites are worth ruling out.
11. Lungworms
Lungworms are less talked about, which is exactly why they sneak through. Pets can get them from eating snails, slugs, or prey animals, or from environments where larvae are present. Symptoms can include coughing, breathing changes, lethargy, and sometimes vomiting—so it’s easy to mistake for kennel cough or allergies. This is especially relevant for outdoor pets and hunters.
Owners often focus on the lungs and forget parasites can be the cause. If a cough lingers or keeps returning after the usual treatments, it’s worth asking about lungworm testing. The solution can be surprisingly straightforward once you identify it. The danger is ignoring it until inflammation becomes chronic.
12. Whipworms
Whipworms are intestinal parasites that can cause chronic diarrhea, weight loss, and abdominal discomfort that looks like a sensitive stomach. They’re notorious because eggs can survive in the environment for a long time, creating reinfection cycles in yards and shared outdoor spaces. Symptoms may be intermittent, which tricks owners into thinking the problem is “over.” Meanwhile, the gut is still inflamed.
They’re also not always caught on a single fecal test, depending on shedding patterns. So you can get a “normal” test and still have a lingering issue. If your dog has recurring diarrhea that doesn’t fit a pattern, whipworms are a candidate. Treatment plus environmental control is often the key.
13. Lice
Yes, pets can get lice, and it can look like relentless itching, a rough coat, and visible tiny insects or eggs on hair shafts. It’s more common in pets with poor grooming, crowded living situations, or compromised health, but it can happen to any animal. People often confuse lice with dandruff until they see movement. And by then, everyone is stressed.
The good news is lice are species-specific, so your pet’s lice aren’t the same as human lice. The bad news is they spread easily between animals. Treatment usually involves topical medications and thorough cleaning of bedding and grooming tools. If your pet suddenly looks scruffy and itchy at the same time, don’t rule it out.