Roundworms

• Roundworms are the most common intestinal parasite in dogs worldwide. They look exactly as you'd imagine — long, spaghetti-like white worms, up to 7 inches long.

• How dogs get them:

  • Puppies most commonly get roundworms from their mother — larvae pass through the placenta before birth or through the mother's milk during nursing

  • Older dogs get them by swallowing eggs from contaminated soil, water, or feces, or by eating an infected rodent or bird

• Signs: Many adult dogs show no signs at all. Heavy infections — most common in puppies — cause a pot-bellied appearance, poor coat, failure to thrive, vomiting (sometimes with visible worms), and diarrhea. In severe puppy infections, roundworms can cause intestinal blockage and death.

• Zoonotic risk: High. Toxocara eggs in soil can infect humans — particularly children who play in contaminated dirt or sandboxes. In humans, larvae can migrate to the eyes (ocular larva migrans, potentially causing blindness) or other organs (visceral larva migrans). Handwashing after outdoor play and picking up pet waste promptly are critical.

• Treatment: Pyrantel pamoate, fenbendazole, or milbemycin — typically multiple doses. Most monthly heartworm preventives that contain pyrantel or milbemycin also control roundworms, providing ongoing prevention.

• Testing: Fecal exam. Note that because eggs are shed intermittently, a single negative fecal doesn't rule out infection.

Hookworms

• Hookworms are small — only about half an inch long — but they are one of the most dangerous parasites for puppies. They attach to the intestinal lining and actively feed on blood. A heavy hookworm infection in a young puppy can cause life-threatening anemia.

• How dogs get them:

  • Skin penetration — larvae burrow directly through a dog's paws or belly from contaminated soil

  • Ingestion of contaminated soil or feces

  • From mother to puppy through the placenta or milk

• Signs:

  • Pale gums (from blood loss)

  • Weakness, lethargy

  • Dark, tarry, or bloody diarrhea

  • Weight loss and poor growth in puppies

  • Puppies can become critically ill and die without treatment

• Zoonotic risk: Moderate. Hookworm larvae can penetrate human skin — typically the feet — causing a condition called cutaneous larva migrans: an intensely itchy, winding red rash that tracks the path of the migrating larva. Walking barefoot in areas where dogs defecate is the primary risk.

• Treatment: Pyrantel pamoate, fenbendazole, or moxidectin. Most monthly heartworm preventives with pyrantel or milbemycin also control hookworms.

Whipworms

• Whipworms live in the large intestine and cecum and are one of the harder parasites to clear due to the long lifespan of their eggs in the environment.

• How dogs get them: Swallowing whipworm eggs from contaminated soil. The eggs can survive in soil for years — re-infection is common in yards where an infected dog has defecated.

• Signs:

  • Chronic, watery diarrhea — often with blood or mucus

  • Weight loss

  • Lethargy

  • Many dogs show no signs with light infections

• Zoonotic risk: Low. Trichuris vulpis does not infect humans.

• Treatment: Fenbendazole (given for 3–5 days, often repeated in 3 weeks and again in 3 months to break the life cycle), milbemycin oxime, or febantel. Environmental decontamination and prompt waste removal are critical — eggs survive a long time in soil.

Tapeworms

• Tapeworms are flat, segmented worms that can grow several feet long, though individual segments (called proglottids) are what dog owners typically notice. They look like small grains of white rice — you'll find them near your dog's rear end, in their feces, or on their bedding.

• How dogs get them:

  • The most common tapeworm is transmitted by ingesting an infected flea during grooming. If your dog has tapeworms, they almost certainly have or recently had fleas.

  • Less common tapeworms (Taenia species) are transmitted by eating infected prey animals — rodents, rabbits.

• Signs: Tapeworms rarely cause serious illness in adult dogs. The main signs are:

  • Visible rice-like segments on the fur around the tail, in feces, or on bedding

  • Scooting (dragging bottom on the ground) from irritation

  • Occasional vomiting or weight loss in heavy infections

• Zoonotic risk: Low for common tapeworms. Humans would have to swallow an infected flea to get Dipylidium (which happens rarely, most often in small children). However, Echinococcus tapeworms — a less common species found primarily in dogs that eat raw prey — pose a serious public health concern and can cause life-threatening cysts in human organs.

• Treatment: Praziquantel — highly effective and typically works after a single dose. Flea control is essential alongside treatment, or re-infection will occur.

Giardia

• Giardia is not a worm — it's a microscopic, single-celled parasite (a protozoan) that infects the small intestine. It's one of the most common intestinal parasites in dogs in the United States.

• How dogs get it: Swallowing Giardia cysts from contaminated water (lakes, streams, puddles), soil, or feces. Dogs that drink from natural water sources, go to dog parks, or live in group settings (shelters, breeders) are at higher risk.

• Signs:

  • Soft, greasy, foul-smelling diarrhea — often sudden in onset

  • Intermittent diarrhea that seems to come and go

  • Mucus in stool

  • Weight loss with prolonged infection

  • Many infected dogs, especially healthy adults, show no signs

• Giardia can be particularly persistent and difficult to clear completely, especially in households with multiple pets.

• Zoonotic risk: Debated. The strains of Giardia that affect dogs are generally different from those that affect humans, and the CDC considers the risk of transmission from dog to human to be low — but not zero. Good hygiene (handwashing, prompt waste removal) is recommended.

• Treatment: Fenbendazole (5 days) and/or metronidazole. Sometimes both are used together for resistant cases. Environmental decontamination matters — bathe your dog to remove cysts from the coat, disinfect water bowls and bedding with dilute bleach, and pick up all feces immediately.

Coccidia

• Like Giardia, coccidia is a microscopic protozoan — not a worm. It's found in soil contaminated with infected feces and is especially common and dangerous in puppies and young dogs.

• How dogs get it: Swallowing oocysts (eggs) from contaminated soil or feces. Puppies are commonly infected by their mother or the environment of their birth.

• Signs: Most healthy adult dogs infected with coccidia show no signs. Puppies are the most vulnerable:

  • Watery, mucus-laden, or bloody diarrhea

  • Dehydration

  • Lethargy and loss of appetite

  • Failure to thrive

  • In severe cases in very young puppies, it can be fatal

• Stress — from weaning, travel, or a new home — often triggers a clinical outbreak in puppies that were previously carrying the parasite without symptoms. This is why many newly adopted puppies develop diarrhea in the first week in their new home.

• Zoonotic risk: None. The species of coccidia that infect dogs do not infect humans.

• Treatment: Sulfadimethoxine (Albon) — the only FDA-approved treatment — typically given for 5–20 days. Ponazuril is commonly used off-label with good results. Keep the environment clean and dry — oocysts are killed by most disinfectants and direct sunlight.

Parasite Prevention: The Full Plan

The Companion Animal Parasite Council (CAPC) — the leading authority on parasite control in the U.S. — recommends a year-round, comprehensive approach:

For puppies:

• Deworm starting at 2 weeks of age, repeating every 2 weeks until 8 weeks old

• Then monthly until 6 months of age

• Then quarterly (every 3 months) for the first year

• Fecal exams at least 4 times in the first year of life

For adult dogs:

• Fecal exam at least twice per year (more often for dogs with higher exposure risk)

• Year-round broad-spectrum parasite prevention with coverage against heartworm, intestinal parasites, fleas, and ticks

• Prompt removal and disposal of feces — every single day if possible

What "broad-spectrum prevention" looks like in practice:

Most monthly heartworm preventives also cover the most common intestinal parasites. Products containing milbemycin oxime (Interceptor Plus, Sentinel Spectrum) cover roundworms, hookworms, and whipworms in addition to heartworm. Products containing pyrantel (Heartgard Plus) cover roundworms and hookworms. Ask your vet which product provides the right coverage for your dog's specific risk profile.

Tapeworms and Giardia are generally not covered by monthly preventives and require targeted treatment when detected.

Good hygiene habits that make a real difference:

• Pick up feces immediately — don't let it sit in the yard

• Wash hands after handling feces, soil, or your dog's rear end

• Keep children from playing in areas where dogs defecate

• Don't let your dog drink from stagnant water, puddles, or natural water sources if avoidable

• Control fleas year-round to prevent tapeworm transmission

• At dog parks, watch what your dog sniffs, licks, and eats from the ground

When to Bring a Stool Sample to the Vet

Bring a fresh fecal sample to your dog's annual (or semi-annual) wellness visit even if your dog seems completely healthy. Many parasitic infections are silent. A fecal floatation test — which concentrates and examines the stool under a microscope — is the only reliable way to detect most intestinal parasites.

Call your vet for a fecal exam sooner if your dog has:

• Diarrhea lasting more than 48 hours

• Blood or mucus in the stool

• Visible worms or rice-like segments in feces or on the fur

• Scooting or excessive licking of the rear

• Unexplained weight loss

• A pot-bellied appearance (especially in puppies)

• Just joined your household from a shelter, breeder, or rescue