Endoparasites: Nematodes in Dogs and Cats (Study Guide)

Nematodes are the roundworms of veterinary parasitology: unsegmented worms that, in dogs and cats, live anywhere from the small intestine to the pulmonary arteries, the trachea, the kidney and the surface of the eye. Most clinic cases come down to a handful of species, and for each one you need the same four answers: where it lives, how the animal picks it up, whether people can catch it, and which test finds it.

This guide puts those answers in one table you can screenshot before an exam or a fecal run, then adds the life-cycle twists behind them. Flukes and tapeworms have their own guide: Endoparasites: Trematodes and Cestodes.

Nematodes at a glance

"Prey" here means a paratenic host: an animal such as a rodent that carries infective larvae without the worm developing in it. "Not known" means the sources below describe no human infection, which is not the same as proof that none can happen.

What makes a nematode a nematode

Nematodes are round in cross-section and covered by a tough cuticle that they shed in four molts as they grow, from first-stage larva (L1) to adult. The sexes are separate. The gut runs the length of the body from the mouth at the front to the anus near the tail, and there are no circulatory or respiratory organs.

Two terms come up in every section below:

• Prepatent period: the time from infection until the parasite's eggs or larvae first appear in the feces, blood or urine. A test can be negative during this window even when the animal is infected.
• Paratenic host: a "transport" animal that carries larvae without the worm developing; the dog or cat gets infected by eating it.

Size varies a lot. Fox lungworm males are only a few millimeters long, while an adult female heartworm in a dog reaches 25 to 30 cm, and Dioctophyme renale earns the name "giant kidney worm".

Intestinal nematodes

Roundworms (ascarids)

Toxocara canis is the one to know best. Pups pick it up before birth: larvae resting in the bitch's tissues wake up late in pregnancy and cross the placenta. Some pups also get larvae through the milk. Older dogs swallow eggs from contaminated soil or eat infected prey. CAPC reports that more than 30% of US dogs younger than 6 months shed T. canis eggs, and notes that some studies have found virtually all pups are born infected.

• Life cycle twist: in young pups the larvae migrate through the liver and lungs, get coughed up and swallowed, and mature in the intestine. In older dogs most larvae stop in body tissues instead, which is why the adult female becomes the source for the next litter.
• Prepatent period: 2 to 4 weeks depending on the route; pups infected before birth do not shed eggs before 2.5 to 3 weeks of age.
• Signs: often none. Heavy burdens in young pups cause poor growth, a pot belly and diarrhea, sometimes coughing from migrating larvae. Intestinal blockage and intussusception have been reported but are rare.
• People: most ascarid eggs need 2 to 4 weeks in the environment before they can infect anyone. Children who swallow contaminated soil can develop visceral, ocular or neural larva migrans.

Toxocara cati is the common roundworm of cats. Cats get it from eggs or by eating infected prey. Unlike T. canis, it does not cross the placenta, and kittens get it through milk only when the queen is infected while she is nursing. It is zoonotic like T. canis.

Toxascaris leonina infects dogs and cats worldwide. It does not cross the placenta or pass in milk, and is not known to infect people. Its eggs have a smooth shell, which tells them apart from the rough, pitted Toxocara egg. Prepatent period is about 8 to 10 weeks.

Baylisascaris procyonis is the raccoon roundworm, occasionally found in dogs. It rarely makes dogs sick, but in people, mostly young children, migrating larvae can damage the eyes, organs and brain. That is why CAPC asks clinics to diagnose and treat it properly in dogs. Fecal antigen tests for Toxocara also pick up Baylisascaris.

More on clinical roundworm cases in dogs: How to Get Rid of Roundworms in Dogs.

Hookworms

Hookworms attach to the small intestine with a large mouth cavity and feed on blood. Ancylostoma caninum is the heavy bleeder. It starts sucking blood before it lays eggs, so a nursing pup can become severely anemic, or die, before the fecal test turns positive.

Dogs pick up A. caninum four ways: larvae swallowed from the environment, larvae burrowing through the skin, larvae in the milk, and larvae in prey such as rodents. Larvae resting in an adult dog's tissues can reactivate and pass to pups through the milk. The prepatent period is usually 2 to 3 weeks, but pups infected through milk can shed eggs 10 to 12 days after birth.

Geography matters too. A 1996 US fecal survey cited by CAPC found Ancylostoma eggs in 19% of samples, from under 5% in the West to 36% in the Southeast.

Ancylostoma tubaeforme is the cat hookworm (prepatent period 18 to 28 days), and it also feeds on blood before eggs appear. Kittens do not get it through milk, only from the environment after birth. Ancylostoma braziliense infects dogs and cats in warm coastal areas of the US. Uncinaria stenocephala, the northern hookworm, prefers colder climates and is most common in the northern US and Canada.

In people, CAPC calls animal hookworms the most common cause of cutaneous larva migrans, the itchy, winding red tracks that appear where larvae entered bare skin on contaminated soil or sand. A. braziliense migrates the furthest in human skin.

Whipworms

Trichuris vulpis lives in the cecum and colon of dogs, with its thin front end threaded into the lining. Dogs swallow embryonated eggs from the ground, and those eggs can stay infective for many years. Heavy infections cause watery diarrhea that can contain blood.

Whipworms are the classic "negative fecal, sick dog" parasite:

• The prepatent period is long, so signs can start before eggs appear.
• Eggs are shed on and off.
• The eggs are dense and need centrifugal flotation to float reliably. A fecal antigen test can confirm infection in egg-negative suspects.

CAPC does not consider T. vulpis zoonotic, and whipworms are rare in cats in most of the US; barrel-shaped eggs with end plugs in a cat's sample are more likely a capillarid. More: Whipworms in Dogs.

Threadworm (Strongyloides)

Strongyloides stercoralis is the odd one out: dogs do not swallow eggs. Infective larvae penetrate the skin or the lining of the mouth, and pups can also get larvae through the milk. In rare cases the worm reinfects the same host (autoinfection), and disseminated infections are the most severe.

Young animals get diarrhea and absorb nutrients poorly. Larvae reach the feces 5 to 21 days after infection; look for them with a Baermann test on fresh feces, and run several samples, because larvae are shed inconsistently.

Dogs carry two lineages of S. stercoralis, and one looks like human isolates, so infected dogs can contaminate soil that people touch. Avoid skin contact with soil where dogs defecate.

Stomach and esophageal worms

Physaloptera spp. live in the stomach of dogs and cats, which get them by eating crickets, cockroaches, beetles or prey that ate those insects. Think of it in any dog or cat with chronic vomiting and a taste for bugs. Most infected animals carry only one to three worms, which may turn up in the vomit or at endoscopy; finding no worms in the vomit does not rule it out.

Spirocerca lupi forms nodules in the esophageal wall of dogs that eat dung beetles or paratenic hosts such as chickens, reptiles, rabbits and rodents. Many dogs show no signs; others vomit, lose weight, cough or struggle to swallow. The Merck Veterinary Manual notes that some nodules turn into sarcomas. Endoscopy is the gold standard because eggs are shed irregularly.

Heartworm

Dirofilaria immitis reaches the dog through a mosquito bite. Larvae mature over months and settle in the pulmonary arteries and heart; microfilariae appear in the blood 6 to 7 months after infection.

In dogs it causes cough and exercise intolerance and, in severe cases, right-sided heart failure or caval syndrome. CAPC recommends annual testing of every dog with both an antigen test and a microfilaria test (such as the modified Knott's test), including dogs on prevention. The pairing matters because the antigen test detects a protein made mostly by adult female worms: it picks up mature infections with at least one female, and can miss very low burdens and male-only infections.

Cats are atypical hosts. Most immature worms die before adulthood, and the inflammation they cause in the lungs and airways is what makes cats sick. Cats usually carry one to three worms, single-sex infections are common, and fewer than 20% of cats with adult worms have microfilariae in the blood. That is why antigen tests can miss cats, and why CAPC advises combining antibody and antigen tests for high-risk or suspect cats rather than relying on one test.

People occasionally get infected too. CAPC counts more than 100 human cases of pulmonary dirofilariasis reported in the US in fifty years, usually a single lung nodule that can be mistaken for a tumor.

More: Heartworm in Dogs: What You Should Know.

Lungworms

Lungworms split into two groups, and the split decides how they spread. Snail-borne species need the animal to eat a snail, slug or a small animal that ate one. Direct species pass straight from dog to dog, which is why they can sweep through a kennel.

Angiostrongylus vasorum lives in the pulmonary arteries, hence the nickname French heartworm. Beyond cough and exercise intolerance it can cause bleeding problems, and larvae occasionally wander to the brain and other organs. It occurs in continental Europe, Africa and South America, and has spread to England, Ireland and Atlantic Canada.

Aelurostrongylus abstrusus causes cough, wheezing and labored breathing that can look like feline asthma, though many cats show nothing. Keeping cats from hunting is the main prevention.

Eucoleus aerophilus (formerly Capillaria aerophila) threads itself into the lining of the trachea and bronchi. Its eggs have end plugs like whipworm eggs but are smaller, with a ridged shell instead of a smooth one, so the two are easy to confuse on a float.

Filaroides hirthi usually stays quiet but can cause fatal hyperinfection in severely stressed or immunocompromised dogs. Oslerus osleri forms wart-like nodules where the trachea divides; the classic sign is a hard, dry cough set off by exercise or cold air. The larvae of the two species look almost identical.

CAPC lists no human health hazard for Angiostrongylus, Aelurostrongylus, Crenosoma or F. hirthi, and only very rare human reports of E. aerophilus.

Eyeworms and kidney worms

Thelazia callipaeda lives in the conjunctival sac. A small fly, Phortica variegata, picks up larvae while feeding on tears and passes them to the next animal. Affected animals squint, tear and build up discharge; some show no signs. Diagnosis is simply seeing the worms in the eye, which the vet removes.

It is zoonotic, and in the US it is new. A 2024 report in the CDC journal Emerging Infectious Diseases describes its emergence: the first locally acquired case was a New York dog in 2020, and 11 dogs and 2 cats had since been confirmed in New York, New Jersey, Connecticut and Nevada.

Dioctophyme renale, the giant kidney worm, develops in the kidney (usually only the right one) and destroys it from the inside. Dogs get it by eating infected aquatic worms or the fish and frogs that carry larvae. Because the other kidney compensates, many cases are found by accident on imaging or surgery; eggs can be seen in urine sediment. People can be infected by eating raw or undercooked fish or frogs.

Which test finds which worm

Zoonotic nematodes: what to tell clients

Most people who catch a dog or cat nematode get it from the environment, not from petting the animal. Roundworm eggs need weeks in soil before they become infective, and hookworm larvae need soil or sand to develop. The CDC's advice for families is practical:

• Have your veterinarian test and treat your pets for worms, especially puppies, kittens and pets that spend time outdoors.
• Pick up pet waste every time.
• Wash hands after handling pets, after outdoor activities and before eating.
• Teach children not to eat dirt, and keep them away from areas where raccoons leave droppings.
• Wear shoes on soil or sand where dogs and cats may have defecated.

Treatment is a vet decision. Several drug classes act on nematodes, including benzimidazoles, macrocyclic lactones and pyrantel, and many heartworm preventives also control some intestinal worms. Which product, which schedule and whether a retest is needed depend on the parasite, the animal and the region, so your veterinarian chooses and doses it. Not every product covers every worm on this page.

For the clinic wall or a study corner, our Most Common Internal Parasites in Dogs and Cats poster sums up roundworms, hookworms and whipworms alongside coccidia, giardia and tapeworms.

Sources

• CAPC: Ascarid
• CAPC: Hookworms
• CAPC: Trichuris vulpis
• CAPC: Strongyloidiasis
• CAPC: Baylisascaris procyonis
• CAPC: Physaloptera spp.
• CAPC: Heartworm
• CAPC: Angiostrongylus vasorum
• CAPC: Aelurostrongylus abstrusus
• CAPC: Crenosoma vulpis
• CAPC: Eucoleus aerophilus
• CAPC: Filaroides hirthi
• CAPC: Filaroides osleri
• CAPC: Urinary tract nematodes (Dioctophyme renale)
• Merck Veterinary Manual: Spirocerca lupi in Small Animals
• Merck Veterinary Manual: Strongyloides sp in Small Animals
• WCVM Learn About Parasites: Toxocara canis
• WCVM Learn About Parasites: Toxascaris leonina
• dvm360: Controlling canine and feline gastrointestinal helminths (Little)
• CDC: About Toxocariasis
• CDC: About Baylisascaris
• Emerging Infectious Diseases: Emergence of Thelaziosis Caused by Thelazia callipaeda in Dogs and Cats, United States (2024)
• Northern Arizona University: Nematode biology lecture notes

This article is for general information and is not a substitute for advice from your veterinarian.