Dogs can carry parasites without obvious symptoms. External parasites live on the skin, coat, or in the ears; internal parasites live inside the body. Fleas, ticks, mites, lice, heartworms, intestinal worms, Giardia, and coccidia all require different prevention and diagnostic approaches—and ringworm is a fungal look-alike, not a worm.
Dogs can carry parasites without obvious symptoms. The main distinction is between ectoparasites, which live on the skin, coat, or in the ears, and endoparasites, which live inside the body. Fleas, ticks, mites, and lice are external parasites; heartworms, roundworms, hookworms, whipworms, tapeworms, Giardia, and coccidia are internal parasites or parasite-like organisms that can affect dogs.
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Not every “bug,” itchy patch, ear discharge, or rice-like object is a parasite. Ringworm, for example, is a fungal infection—not a worm. Proper identification usually requires a veterinary examination and, in many cases, skin, stool, blood, or ear testing.
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External parasites can cause itching and skin damage, but some also transmit infections or internal parasites. A dog may have only a few visible signs—or none at all—depending on the parasite, the dog’s sensitivity, and how long the infestation has been present.
Fleas
The principal flea found on dogs in the United States is usually the cat flea, Ctenocephalides felis, although flea species vary by geography. Fleas feed on blood and can cause:
- Persistent scratching, licking, or chewing
- Small red bumps or papules, especially around the lower back, tail base, abdomen, or inner thighs
- Hair loss, scabs, hot spots, and secondary skin infections
- Flea-allergy dermatitis, in which a small number of bites cause intense itching
- Anemia in puppies, debilitated dogs, or dogs with a heavy infestation
Fleas can also transmit Dipylidium caninum, a tapeworm acquired when a dog swallows an infected flea while grooming. Seeing a flea or flea dirt does not necessarily indicate how large the infestation is: much of the flea life cycle occurs in the environment rather than on the dog.
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Fleas can survive indoors year-round, including during winter. Occasional bathing may remove some adult fleas temporarily, but it does not address eggs and immature stages in the home. Effective control generally requires treating every susceptible pet as directed, reducing environmental contamination, and maintaining a veterinarian-selected preventive.
For inspection, a fine-tooth dog flea comb can help collect adult fleas or dark flea dirt from the coat. Comb over a white surface or damp paper so dark particles can be examined; flea dirt may leave a reddish-brown stain when wet. A comb is an inspection and grooming tool, not a complete treatment for an infestation. Dispose of what you collect safely and ask a veterinarian how to control the dog, other pets, and living environment.
Ticks
Ticks attach to the skin and feed on blood. Important U.S. examples include the blacklegged tick, American dog tick, brown dog tick, lone star tick, and Gulf Coast tick. The species present—and the diseases they can transmit—depend heavily on geography and exposure.
Ticks may transmit Lyme disease, ehrlichiosis, anaplasmosis, Rocky Mountain spotted fever, babesiosis, and other infections. The brown dog tick deserves particular attention because it can infest indoor environments such as homes and kennels.
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A tick bite may be followed by no immediate visible illness. After possible exposure, watch for changes in appetite, behavior, energy, gait, or temperature, and contact a veterinarian if your dog becomes unwell. Daily checks are especially useful after hiking, hunting, camping, boarding, or walking through tall grass and brush. Examine between the toes, inside and around the ears, under the collar, around the eyelids, in the armpits, and around the groin as well as across the rest of the body.
If you find an attached tick, use a suitable tick-removal tool for dogs and remove it with steady traction close to the skin. Do not rely on burning, petroleum jelly, alcohol, or other substances applied to the tick. Clean the bite area afterward, wash your hands, and record the date and location of exposure if your dog later develops symptoms. A removal tool does not prevent tick-borne disease and does not replace veterinary evaluation or an appropriate tick-control plan.
Mites
Mites are microscopic or nearly microscopic and cannot reliably be identified by looking at a dog’s skin. Several different mite conditions are possible, and they are not interchangeable.
Sarcoptic mites: canine scabies
Sarcoptic mites cause canine scabies, an intensely itchy and contagious skin disease. Affected dogs may develop severe scratching, redness, crusts, scabs, and hair loss, often beginning around the ear margins, elbows, hocks, chest, or abdomen. The condition can spread to other animals and can cause a temporary itchy rash in people.
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Because sarcoptic mange is contagious and can resemble allergies or other skin diseases, suspected cases need veterinary diagnosis. Household animals, bedding, and the environment may need attention according to veterinary instructions.
Demodex mites: demodicosis
Demodex mites normally live in small numbers in many dogs. Disease develops when mite numbers increase, producing localized or generalized demodicosis. Age, genetics, immune status, or another illness can contribute. Demodicosis is not the same as sarcoptic mange and is not usually spread between healthy adult dogs in the same way.
Localized disease may involve a few areas of hair loss, while generalized disease can cause widespread hair loss, redness, crusting, and secondary bacterial infection. The extent and underlying cause determine how seriously it should be treated.
Ear mites
Ear mites are more common in cats but can affect dogs, especially those with close animal contact. They may cause head shaking, ear scratching, inflamed ear canals, and dark debris. However, ear infections have many possible causes, including bacteria, yeast, allergies, foreign material, and other inflammation. Dark discharge alone is not enough to diagnose ear mites, and putting an unapproved substance into an inflamed ear can make the problem worse.
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Lice
Dogs may have chewing lice or sucking lice. Dog lice are generally host-specific, meaning they remain associated with dogs rather than becoming a persistent human infestation. Heavy infestations can cause itching, coat deterioration, skin irritation, restlessness, and anemia, particularly in puppies or otherwise vulnerable dogs.
Diagnosis usually involves finding adult lice or nits during a coat examination. Treatment should follow veterinary or product-label directions. Do not assume that a flea product, household insecticide, or treatment intended for another species is safe or effective for dog lice.
Internal parasites found inside dogs
Internal parasites may be acquired through mosquitoes, contaminated soil or feces, nursing, skin contact with infective larvae, infected prey, raw animal tissues, fleas, or contaminated water. Some cause obvious diarrhea or weight loss; others can damage organs before signs become noticeable.
Heartworms
Heartworm disease is caused by Dirofilaria immitis and spread by mosquitoes—not by ordinary contact with an infected dog. Developing worms eventually inhabit the pulmonary arteries and heart. Over time, infection can cause lung disease, coughing, exercise intolerance, vascular damage, heart failure, and death.
Heartworm disease can remain clinically silent for a long time. A dog may appear healthy while the infection is developing, which is why visible symptoms are not a safe basis for deciding whether prevention or testing is needed.
Major veterinary parasite organizations and the U.S. Food and Drug Administration support year-round heartworm prevention, annual testing, and veterinary oversight. Annual testing commonly includes heartworm antigen testing and microfilaria testing; additional testing may be appropriate in areas or situations with greater exposure.
Heartworm preventives target developing stages. They do not simply remove established adult worms. If prevention has been missed, interrupted, or is unknown, a dog should be tested before starting or restarting medication according to veterinary direction. Suspected or confirmed heartworm disease is not a do-it-yourself treatment situation.
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Roundworms
Roundworms, especially Toxocara canis, are common intestinal parasites. Puppies can acquire them before birth or through nursing. Dogs of any age may ingest infective eggs from contaminated soil, objects, or feces.
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- A pot-bellied appearance
- Slow growth or poor body condition
- Vomiting or diarrhea
- Weight loss
- Coughing during larval migration through the body
Heavy infections can be dangerous in puppies. Roundworm eggs can contaminate soil for long periods and may infect people, particularly children. Prompt feces removal, handwashing, routine veterinary care, and an age-appropriate deworming and prevention plan reduce the risk.
Hookworms
Hookworms such as Ancylostoma caninum attach to the intestinal lining and consume blood. In dogs—especially puppies—heavy infections can cause anemia, weakness, poor growth, dark or tarry stool, and potentially fatal disease.
Dogs can acquire hookworms by ingesting larvae, through skin penetration, and through nursing in some species. Some dog hookworms can infect people when larvae penetrate skin exposed to contaminated soil or sand. This produces itchy, winding skin lesions known as cutaneous larva migrans.
Do not allow children or adults to walk barefoot where dog feces may have contaminated soil or sand. Pick up stool promptly, wash hands after handling feces or soil, and follow the veterinarian’s testing and treatment plan.
Whipworms
Trichuris vulpis lives in the large intestine. Some infected dogs have no signs, while others develop intermittent diarrhea, mucus or blood in the stool, dehydration, weight loss, or poor body condition.
Whipworm eggs can persist in the environment for long periods. This makes reinfection and contamination of yards, kennels, and other shared areas difficult to control. Fecal testing and veterinarian-selected repeated treatment may be needed; one negative test or one treatment is not always the end of the problem.
Tapeworms
The most familiar dog tapeworm is Dipylidium caninum. Dogs acquire it by swallowing an infected flea or, occasionally, an infected louse. Owners may notice rice-like segments around the anus, in bedding, or in the stool, although many infections cause few other signs.
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Dogs can also acquire other tapeworms by eating infected prey or raw animal tissues. The exact parasite matters because treatment and human-health implications differ. If you see a suspected segment, take a clear photograph or place a sample in a clean container for your veterinarian rather than assuming every rice-like object is a tapeworm.
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Giardia
Giardia is a microscopic protozoan, not a worm. Dogs acquire it by swallowing cysts from contaminated feces, soil, surfaces, or water. Infection may cause soft or watery diarrhea, gas, abdominal discomfort, nausea, or vomiting. Some dogs have no signs, while young animals are more likely to become ill.
People are considered unlikely to acquire giardiasis from dogs or cats because the types that commonly infect people are usually different. Fecal hygiene remains important, particularly in households with young children, older adults, or immunocompromised people. Persistent diarrhea should be evaluated rather than treated based on stool appearance alone.
Coccidia and other protozoa
Coccidia, including Cystoisospora species, are intestinal protozoa that particularly affect puppies and dogs living in crowded or stressful conditions. They may cause watery or bloody diarrhea, dehydration, poor appetite, and poor growth.
Diagnosis requires fecal testing, and treatment is parasite-specific. Routine “dewormers” do not necessarily cover coccidia. Cryptosporidium and other protozoa may also be detected in dogs, but their clinical importance depends on the species, the dog’s immune status, other infections, and whether diarrhea or dehydration is present.
Quick comparison: what might be affecting a dog?
| Parasite or condition | Where it is found | Common clues | Important point |
|---|---|---|---|
| Fleas | Coat and environment | Itching, flea dirt, papules, hot spots | Can trigger flea allergy, anemia, and flea-transmitted tapeworms |
| Ticks | Attached to skin | Visible attached parasite or later changes in energy, appetite, gait, or temperature | Species and disease risks vary by location |
| Sarcoptic mites | Skin | Severe contagious itching, crusts, redness, hair loss | Can affect people and other animals |
| Demodex mites | Hair follicles and skin | Localized or widespread hair loss and skin inflammation | Different from sarcoptic mange; underlying factors may matter |
| Ear mites | Ear canals | Head shaking, ear scratching, dark debris | Dark debris has several possible causes |
| Lice | Coat and skin | Itching, poor coat, visible lice or nits | Dog lice are generally host-specific |
| Heartworms | Pulmonary arteries and heart | May be silent; later cough, exercise intolerance, breathing problems | Spread by mosquitoes and requires prevention and testing |
| Roundworms | Intestines and, during migration, other tissues | Pot belly, poor growth, vomiting, diarrhea, weight loss | Eggs can contaminate soil and infect people |
| Hookworms | Intestinal lining | Anemia, weakness, poor growth, dark stool | Can cause serious puppy disease and skin infection in people |
| Whipworms | Large intestine | Intermittent diarrhea, mucus or blood, weight loss | Eggs persist in the environment |
| Tapeworms | Intestines | Rice-like segments | Different species have different sources and treatment |
| Giardia and coccidia | Intestinal tract | Diarrhea, gas, dehydration, poor growth—or no signs | They are protozoa, and treatment is organism-specific |
Signs that mean a dog should see a veterinarian
Arrange veterinary care for persistent itching, recurring ear problems, unexplained weight loss, chronic or intermittent diarrhea, visible worms or segments, or an unknown or interrupted prevention history. A veterinarian should also evaluate a dog after significant tick exposure if illness develops.
Seek prompt or emergency veterinary care for:
- Pale or white gums, weakness, collapse, or suspected anemia
- Repeated vomiting, severe or persistent diarrhea, or bloody or black stool
- Breathing difficulty, marked coughing, or severe exercise intolerance
- Abdominal swelling, especially with weakness or breathing changes
- Seizures, tremors, loss of coordination, or other serious neurologic signs
- Severe itching that has caused open wounds or extensive skin damage
- A heavy flea burden in a puppy or debilitated dog
- A suspected reaction after applying or administering a flea, tick, or heartworm product
Do not wait for visible worms or severe symptoms before addressing prevention. Heartworms and some intestinal parasites can be present while a dog appears normal.
How veterinarians diagnose parasites
No single test detects every parasite. Depending on the symptoms and exposure history, diagnosis may include:
- Physical examination and inspection of the skin and coat
- Flea combing and examination for fleas, flea dirt, lice, or nits
- Ear examination and microscopic evaluation of ear material
- Skin scraping or other dermatologic testing for mites and skin disease
- Fecal flotation, preferably using a centrifugal method, for many intestinal parasites
- Fecal-antigen testing for selected parasites that release detectable antigens
- Targeted Giardia or protozoan testing
- Blood tests, including heartworm antigen and microfilaria testing
- Imaging or additional laboratory work when organ disease, anemia, or another condition is suspected
A negative result does not always exclude infection. Parasites may be shed intermittently, be present at a stage a particular test does not detect, or require a different sample or test method. Puppies, dogs with ongoing diarrhea, dogs with heavy exposure, and dogs with an incomplete prevention history may need repeat or additional testing.
Bring a fresh stool sample when a fecal test is planned. Tell the veterinarian about travel, hunting, raw-food exposure, dog parks, boarding, swimming, wildlife contact, mosquito exposure, known tick exposure, preventive medications, and missed doses. These details help determine which tests are useful.
Testing frequency should reflect age, lifestyle, geography, prevention, and clinical signs. Life-stage guidance commonly places fecal examinations somewhere between once and four times per year, depending on those factors, rather than treating every dog as having the same risk.
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Prevention that works across the household
A practical parasite-control plan usually combines several measures:
- Use veterinarian-selected prevention year-round. A broad-spectrum plan may address several risks, but the correct product depends on the dog’s age, weight, health, geography, exposure, current medications, diagnosis, and the product label. Year-round protection is especially important for heartworms because mosquitoes and indoor flea exposure are not limited to one season.
- Test for heartworms annually. Testing remains important even when prevention has been used, and it is particularly important after missed or uncertain doses. Follow the veterinarian’s schedule for additional testing.
- Use risk-based tick protection and testing. Local tick species, travel, hiking, hunting, boarding, and wildlife exposure should influence the plan. Daily tick checks help find attached ticks promptly.
- Have feces removed quickly. Prompt pickup limits contamination from roundworms, hookworms, Giardia, and other intestinal organisms. It also helps reduce reinfection and protects people and other animals.
- Provide safe drinking water. Discourage drinking from puddles, stagnant water, or areas contaminated with feces.
- Control fleas on every susceptible pet. Treating only the itchy dog may fail when other pets or the environment maintain the flea life cycle. Coordinate treatment with a veterinarian.
- Reduce contact with wildlife feces, contaminated soil, infected prey, and raw animal tissues. Hunting, scavenging, and raw-food exposure can change the parasites a dog is likely to encounter.
- Clean bedding and high-contact areas. Wash bedding as appropriate and follow veterinary instructions for contagious skin disease or an established flea infestation. For routine feces control, keep dog waste bags for daily cleanup available where the dog walks and eliminates.
Using flea, tick, and other pesticide products safely
Flea and tick products come as oral medications, topical applications, collars, sprays, shampoos, and other formulations. Use only a product labeled for the dog’s species, age, and weight. Read the entire label, follow the interval exactly, and do not give an extra or early dose because you found a flea or tick.
- Ask a veterinarian before treating a puppy, senior dog, sick dog, pregnant or nursing dog, or dog taking other medication.
- Never use a dog product on a cat. Some ingredients that are appropriate for dogs can be dangerous to cats.
- Do not combine products unless a veterinarian or the product labels specifically support doing so.
- Keep products away from children and store them in their original packaging.
- Buy from a reputable source and be cautious of counterfeit or illegally imported pesticide products, which may have unreliable contents, missing warnings, poor dosing information, or inadequate packaging.
FDA safety information states that isoxazoline flea and tick products are considered safe and effective for most dogs and cats but have been associated with neurologic adverse events, including tremors, ataxia, and seizures, in some animals. That does not mean every dog should avoid the class, but a dog’s individual history should be considered when selecting prevention. Contact a veterinarian promptly if neurologic signs or another suspected product reaction occurs.
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Natural remedies, essential oils, and household pesticides are not automatically safe alternatives. A product being labeled “natural” does not establish that it is safe, effective, correctly dosed, or appropriate for a dog, cat, child, or household surface.
Can dog parasites affect people?
Some can, but the risk depends on the organism and the route of exposure. The most relevant precautions include:
- Roundworms: Eggs from contaminated feces or soil can infect people, with children at particular risk because of hand-to-mouth behavior.
- Hookworms: Larvae in contaminated soil or sand can penetrate human skin and cause cutaneous larva migrans.
- Sarcoptic mites: Canine scabies can cause an itchy, usually temporary skin reaction in people and can spread to other animals.
- Flea-associated tapeworms: People can acquire Dipylidium by swallowing an infected flea, although this is uncommon and flea control reduces the risk.
- Giardia: People are unlikely to acquire giardiasis from dogs or cats because the types that commonly infect people are usually different, but careful fecal hygiene is still appropriate.
Everyone should wash hands after handling feces, cleaning an elimination area, gardening in soil accessible to dogs, or handling a dog with suspected contagious skin disease. Take extra care in homes with young children, older adults, pregnant people, or immunocompromised household members. Ask a physician about human symptoms after a relevant exposure.
What not to assume
- “My dog is indoors, so parasites are impossible.” Fleas can live indoors, mosquitoes can enter homes, and parasites can arrive on people, other animals, soil, prey, or contaminated objects.
- “No scratching means no parasite.” Heartworms and intestinal parasites may cause few or no visible signs.
- “Every itchy dog has mange.” Allergies, bacterial or yeast infections, contact irritation, parasites, and other diseases can look similar.
- “Dark ear debris proves ear mites.” Ear disease has multiple causes and needs an examination.
- “Every rice-like object is a tapeworm.” A photograph or sample can help a veterinarian identify what was seen.
- “A routine dewormer covers everything.” Different worms and protozoa require different tests and treatments; routine dewormers do not necessarily cover coccidia.
- “A bath solved the flea problem.” Bathing may remove some adult fleas but generally does not end the environmental life cycle.
Bottom line
Dogs can be affected by external parasites—including fleas, ticks, mites, and lice—and internal parasites or protozoa—including heartworms, roundworms, hookworms, whipworms, tapeworms, Giardia, and coccidia. Their signs overlap with allergies, infections, dietary illness, and other medical problems, so appearance alone is not a dependable diagnosis.
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Year-round, veterinarian-selected prevention; annual heartworm testing; risk-based fecal and tick-borne disease testing; daily tick checks; prompt stool pickup; handwashing; and careful product-label use provide the strongest general protection. A dog with pale gums, collapse, breathing problems, severe gastrointestinal signs, neurologic symptoms, a heavy flea burden, or a suspected product reaction needs prompt veterinary care.
Frequently Asked Questions
Can a dog have parasites without showing symptoms?
Yes. Heartworm disease and several intestinal parasites can remain clinically silent, especially early in infection. That is why prevention, routine fecal examinations, and annual heartworm testing should not depend only on whether a dog appears ill.
Is ringworm actually a parasite?
No. Ringworm is a fungal infection, not a worm or parasite. It can cause circular hair loss, scaling, or crusting and may resemble other skin diseases, so veterinary examination is needed for confirmation.
Which dog parasites can infect people?
Some dog-associated organisms can affect people. Roundworms, hookworms, sarcoptic mites, and flea-associated tapeworms are the main concerns described here. Prompt feces removal, handwashing, avoiding contaminated soil, and appropriate veterinary treatment reduce risk. Giardia transmission from dogs to people is considered unlikely because the types commonly infecting people are usually different.
How often should dogs be tested for parasites?
There is no single schedule for every dog. Testing frequency depends on age, lifestyle, geography, prevention, and symptoms. Veterinary life-stage guidance commonly places fecal examinations between once and four times yearly, while heartworm testing is generally performed annually.
The Bottom Line
The safest approach is prevention plus testing, not waiting for visible worms or severe symptoms. Because different parasites require different tests and treatments, consult a veterinarian before starting, restarting, combining, or changing parasite products.
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