Intestinal parasites are among the most common health problems in dogs and cats worldwide. The Centers for Disease Control and Prevention (CDC) estimates that 34% of dogs in the United States have at least one intestinal parasite at any given time. Cats and dogs acquire parasitic infections through contaminated soil, prey animals, flea ingestion, or maternal transmission — and many infections cause no obvious symptoms while still affecting the animal's health and, in some cases, human health in the household.
This guide covers the major intestinal and common external parasites affecting dogs and cats, how infection occurs, diagnostic confirmation, treatment protocols, and year-round prevention programmes.
Why Parasite Control Matters Beyond the Individual Animal
Several common pet parasites are zoonotic — transmissible to humans. The CDC and AVMA identify the following as significant public health concerns in pet-owning households:
Toxocara canis/cati (roundworms): Estimated 14% of Americans are seropositive for Toxocara exposure. In humans, larval roundworms can migrate through tissues causing visceral larva migrans (liver, lung) or ocular larva migrans (eye), the latter causing vision loss or blindness. Children are at highest risk from soil contact in areas contaminated with infected pet faeces.
Ancylostoma/Uncinaria (hookworms): Hookworm larvae penetrate human skin through soil contact, causing cutaneous larva migrans — a painful, itchy, migrating track visible under the skin. Most common in warm climates and sandy environments.
Dipylidium caninum (tapeworm): Transmitted to humans (primarily children) through accidental ingestion of infected fleas. A reason flea control and tapeworm control are linked.
Toxoplasma gondii (cats only): The most clinically significant zoonosis associated with cats. Transmitted through handling infected cat faeces. Serious risk for immunocompromised individuals and pregnant women (risk of foetal damage). Toxoplasmosis from indoor cats that do not hunt is low risk; outdoor or hunting cats pose higher risk.
The Major Worm Types
Roundworms (Toxocara canis, Toxocara cati, Toxascaris leonina)
What they are: Large, white, spaghetti-like worms up to 10-18 cm long. The most common intestinal parasite in dogs and cats globally.
Transmission:
- Ingestion of infective eggs from contaminated soil
- Transplacental (puppies infected before birth through the placenta)
- Transmammary (kittens and puppies infected through mother's milk)
- Ingestion of paratenic hosts (rodents, earthworms, birds that carry larval roundworms)
Signs of infection: In puppies and kittens: pot-bellied appearance, weight loss, dull coat, vomiting, diarrhoea. In adults, often asymptomatic. Heavy infections can cause respiratory signs in puppies (larval migration through the lungs — "tracheal migration").
Diagnosis: Faecal flotation (examining stool under microscopy for eggs). Eggs are not shed continuously — multiple faecal samples improve detection rate.
Treatment: Pyrantel pamoate, fenbendazole, or milbemycin oxime. Repeat treatment is necessary to address migrating larvae (which are not killed by a single dose) — typically 2 weeks apart. Puppies and kittens are routinely treated preventatively at 2, 4, 6, and 8 weeks of age, then monthly until 6 months.
Hookworms (Ancylostoma caninum, A. braziliense, Uncinaria stenocephala)
What they are: Small, thin worms that attach to the small intestinal lining and feed on blood. Heavy infections cause significant blood loss.
Transmission:
- Larval penetration through the skin (the most common route in warm, humid environments)
- Ingestion of larvae from contaminated soil
- Transmammary transmission to puppies
Signs of infection: Pale gums, weakness, and diarrhoea (often with blood) from blood loss. Puppies with heavy infections can develop life-threatening anaemia rapidly. In adults, often asymptomatic unless heavy infection.
Diagnosis: Faecal flotation.
Treatment: Fenbendazole, pyrantel pamoate, or milbemycin. Puppies at risk for maternal transmission are treated on the same schedule as for roundworms.
Tapeworms (Dipylidium caninum, Taenia species, Echinococcus)
What they are: Flat, segmented worms that can grow to 50 cm or more in the intestine. Shed proglottid segments (egg packets) that appear as small white, rice-grain-like pieces around the anus or in fresh faeces.
Transmission:
- Dipylidium caninum: Ingestion of infected fleas (the most common tapeworm in pet dogs and cats)
- Taenia species: Ingestion of infected intermediate hosts — rodents, rabbits, livestock
- Echinococcus species: From raw meat or infected prey; significant zoonotic potential for cystic echinococcosis in humans
Signs of infection: Often none beyond visible proglottid segments. Scooting may occur due to anal irritation from proglottids. Heavy infections cause weight loss.
Diagnosis: Visual identification of proglottids in faeces or around the anus; faecal flotation detects Taenia eggs but often misses Dipylidium (eggs are inside proglottids). Clinical presentation drives diagnosis when faecal tests are negative.
Treatment: Praziquantel — effective as a single dose. Does not prevent re-infection. Dipylidium re-infection will recur if flea infestation is not controlled simultaneously.
Whipworms (Trichuris vulpis — dogs only)
What they are: Whip-shaped worms inhabiting the large intestine. Less common than roundworms or hookworms but persistent in contaminated soil (eggs survive for years).
Signs of infection: Watery diarrhoea, often containing blood or mucus, weight loss, dehydration in heavy infections.
Diagnosis: Faecal flotation, but eggs are shed intermittently and in low numbers — multiple samples may be needed. Faecal antigen tests are more sensitive.
Treatment: Fenbendazole (3-day course, repeated monthly for 3 months to address reinfection cycle). Milbemycin and febantel are also effective. Whipworm eggs in the environment persist for years; environmental decontamination is challenging.
Lungworm (Angiostrongylus vasorum — dogs; Aelurostrongylus abstrusus — cats)
What they are: Worms that inhabit the pulmonary arteries (dogs) or lung tissue (cats) rather than the intestinal tract. Increasingly recognised in Europe and parts of North America.
Transmission: Ingestion of infected slugs or snails (dogs); ingestion of paratenic hosts (cats).
Signs in dogs: Exercise intolerance, coughing, breathlessness, bleeding disorders (A. vasorum interferes with clotting), sudden death in heavy infections.
Diagnosis: The Baermann technique on faeces, ELISA antigen test (Angio Detect test for dogs), or bronchoalveolar lavage. Standard faecal flotation misses lungworm.
Treatment: Monthly spot-on products containing moxidectin/imidacloprid (Advocate) prevent lungworm in dogs. Fenbendazole treats established infections.
Heartworm (Dirofilaria immitis)
What it is: A large worm inhabiting the pulmonary arteries and right side of the heart. Transmitted by mosquitoes — not orally acquired.
Geographic distribution: Endemic in the southeastern USA, Gulf Coast, Atlantic seaboard, parts of California, and southern Europe. Expanding northward.
Signs of infection: Early infection asymptomatic. Advanced infection: exercise intolerance, coughing, weight loss, right heart failure. "Caval syndrome" — a severe, rapidly fatal form — occurs with massive worm burden.
Diagnosis: Annual antigen blood test (mandatory in endemic regions). Microfilaria detection.
Treatment of established infection: Complex, expensive, and risky — an extensive protocol involving doxycycline (eliminating Wolbachia bacteria in worms), melarsomine injections, and strict exercise restriction for months. Treatment in cats is limited and high-risk.
Prevention (far preferable to treatment): Monthly oral or topical preventatives (ivermectin, selamectin, moxidectin, milbemycin) in endemic regions. Test annually before renewing prescription. Preventatives do not treat established infection.
External Parasites
Fleas
Why covered here: Fleas are not intestinal parasites but are an integral part of tapeworm transmission (Dipylidium) and cause skin disease (flea allergy dermatitis), severe anaemia in heavy infestations, and distress.
Life cycle: The flea life cycle has four stages: egg, larva, pupa, and adult. Only 5% of the flea population is on the animal at any time — 95% is in the environment (eggs and larvae in carpets, bedding, furniture). Treatment must address both the animal and the environment.
Treatment: Modern spot-on and oral flea preventatives (Frontline Plus, Bravecto, Nexgard, Seresto collar, Revolution/Advocate) are highly effective. Resistance to pyrethrins (the active ingredient in older products) is widespread. Environmental treatment with an insect growth regulator (IGR) such as methoprene or pyriproxyfen prevents egg and larval development in the home.
Natural products: Diatomaceous earth, essential oils, and other "natural" flea treatments have negligible efficacy compared to veterinary-grade products and are not recommended as primary flea control.
Ear Mites (Otodectes cynotis)
Ear mites are more common in cats than dogs and cause intense itching and dark, coffee-ground-like discharge in the ear canal. Highly contagious between cats.
Treatment: Topical treatments applied directly to the ear canal (selamectin, moxidectin products) or systemic preventatives that also cover ear mites.
Sarcoptic Mange (Sarcoptes scabiei)
An intensely pruritic (itchy) mite infestation causing hair loss, crusting, and secondary skin infection. Zoonotic — can cause temporary skin irritation in humans. Treated with ivermectin, selamectin, or milbemycin.
Standard Deworming Protocols
Puppies and Kittens
| Age | Treatment |
|---|---|
| 2 weeks | Begin deworming (puppies: pyrantel pamoate) |
| 4 weeks | Repeat |
| 6 weeks | Repeat |
| 8 weeks | Repeat; begin monthly prevention until 6 months |
| 3-6 months | Monthly broad-spectrum prevention |
Adult Pets
| Risk Category | Recommendation |
|---|---|
| Indoor only (cats); limited outdoor exposure (dogs) | Annual faecal examination; biannual prevention or as recommended |
| Outdoor access, hunting | Year-round monthly broad-spectrum prevention; biannual faecal examination |
| Hunting dogs; dogs with prey access | Year-round prevention including tapeworm coverage; quarterly faecal examination |
| Heartworm-endemic region | Year-round monthly heartworm prevention; annual antigen testing |
The Annual Faecal Examination
The Companion Animal Parasite Council (CAPC) recommends at least one faecal examination per year for adult pets, two per year for higher-risk animals. The faecal flotation exam detects eggs of roundworms, hookworms, whipworms, and some tapeworms. It does not detect all parasites — heartworm requires a blood test; lungworm requires specialised techniques.
"Routine faecal testing is the only way to detect many intestinal parasitic infections in asymptomatic animals. Most infected pets show no visible signs." — Companion Animal Parasite Council (CAPC)
Product Types and Active Ingredients
| Product Type | What It Covers | Examples |
|---|---|---|
| Oral dewormers (pyrantel) | Roundworms, hookworms | Nemex, Strongid |
| Fenbendazole (3-day course) | Roundworms, hookworms, whipworms, Giardia, some tapeworms | Panacur |
| Praziquantel | Tapeworms | Droncit; in combination products |
| Milbemycin oxime | Roundworms, hookworms, whipworms, heartworm prevention | Interceptor |
| Selamectin (spot-on) | Roundworms, hookworms, fleas, ear mites, heartworm prevention | Revolution |
| Moxidectin/imidacloprid (spot-on) | Roundworms, hookworms, lungworm, fleas, heartworm prevention | Advocate |
| Ivermectin | Roundworms, hookworms, heartworm prevention; mange | Heartgard; not for Collie-type breeds |
Ivermectin warning: Dogs carrying the MDR1/ABCB1 gene mutation (present in Collies, Australian Shepherds, Shelties, Border Collies, and related breeds) are severely sensitive to ivermectin and related drugs (milbemycin, moxidectin) at certain doses. Always inform your vet of your dog's breed. Genetic testing for MDR1 status is available.
References
Companion Animal Parasite Council (CAPC). (2023). CAPC Guidelines: Roundworms, Hookworms, Whipworms, Tapeworms. https://capcvet.org
Centers for Disease Control and Prevention (CDC). (2023). Toxocariasis: Epidemiology. https://www.cdc.gov/parasites/toxocariasis/
American Heartworm Society. (2022). Current Canine Guidelines for the Diagnosis, Prevention, and Management of Heartworm Infection in Dogs. https://www.heartwormsociety.org
Traversa, D., et al. (2010). "Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated." Parasites & Vectors, 3, 62. https://doi.org/10.1186/1756-3305-3-62
Blagburn, B.L., & Dryden, M.W. (2009). "Biology, treatment, and control of flea and tick infestations." Veterinary Clinics of North America: Small Animal Practice, 39(6), 1173–1200.
Related reading:
- How to Spot Fleas and Ticks on Dogs
- Common Dog Illnesses Explained
- Dog Vaccination Schedule Explained
- Signs of a Healthy Dog
- Signs of a Healthy Cat
Frequently Asked Questions
How do I know if my dog or cat has worms?
Many intestinal parasite infections cause no visible symptoms in adult pets. Signs that can indicate a heavy infection include: visible worm segments (rice-grain-like white pieces around the anus or in faeces — tapeworms), pot-bellied appearance in puppies or kittens, weight loss, diarrhoea, pale gums from blood loss (hookworms), and scooting. Annual faecal testing is the only reliable way to detect asymptomatic infections.
How often should I deworm my dog or cat?
Puppies and kittens should be dewormed at 2, 4, 6, and 8 weeks, then monthly until 6 months of age. Adult pets should receive year-round broad-spectrum prevention (especially in heartworm-endemic regions or for outdoor pets that hunt) with at least one annual faecal examination. Higher-risk animals (hunting dogs, outdoor cats with prey access) benefit from quarterly faecal testing and more comprehensive prevention.
Can humans get worms from dogs or cats?
Yes. Several pet parasites are zoonotic. Roundworm (Toxocara) larvae can migrate into human tissue, potentially causing vision loss — children are at highest risk. Hookworm larvae penetrate human skin from contaminated soil, causing itchy migrating tracks. Tapeworms (Dipylidium) can infect humans who accidentally swallow infected fleas. Cats can shed Toxoplasma gondii, a risk for immunocompromised people and pregnant women.
What is the best dewormer for dogs?
The best dewormer depends on which parasites are present or being prevented against. Fenbendazole (Panacur) treats roundworms, hookworms, whipworms, and Giardia. Praziquantel treats tapeworms. Milbemycin and selamectin-based spot-ons prevent heartworm, roundworms, and hookworms simultaneously. Your veterinarian can recommend the most appropriate product based on your dog's risk profile and region.
Do indoor cats need deworming?
Indoor-only cats have significantly lower parasite risk than outdoor cats, but can still be exposed through: tracking in infective eggs on shoes or clothing, accidental ingestion of insects or rodents that occasionally enter the home, and fleas (which can enter on human clothing and transmit tapeworms). Indoor cats should have at least annual faecal examination and flea prevention, with deworming as indicated.
What is the difference between heartworm prevention and intestinal worm treatment?
Heartworm is transmitted by mosquitoes (not orally acquired) and lives in the pulmonary arteries and heart, not the intestines. It requires monthly preventative medication (ivermectin, milbemycin, or moxidectin) and annual blood testing. Treating established heartworm infection is complex, risky, and expensive — prevention is essential in endemic areas. Intestinal dewormers do not prevent heartworm, and heartworm preventatives do not treat all intestinal parasites.