Roundworms in dogs: symptoms, transmission and treatment

Overview

Two roundworm species occur in dogs worldwide: Toxocara canis and Toxascaris leonina. Toxascaris leonina can infect both dogs and cats. Toxocara cati is the feline roundworm. Adult Toxocara canis are usually pale and around 10–15 cm long.

Adult Toxascaris leonina are usually 5–10 cm long. However, individuals can reach 12 cm. Adult females lay eggs in the small intestine, which pass out in faeces. A dog can shed eggs without visible symptoms.

Toxascaris leonina is less common in dogs than Toxocara canis. Canine and feline Toxocara species are among the causes of human toxocariasis. Toxascaris leonina infection is not considered transmissible to humans.

Causes

Toxocara canis can infect puppies through the placenta from around day 42 of pregnancy. Puppies can also acquire larvae through their mother's milk. Larvae remaining in maternal tissues can reactivate late in pregnancy. Dogs can become infected by swallowing infective eggs from soil, objects or elsewhere in the environment.

A rodent carrying larvae in its tissues can also be a source. Newly passed Toxocara eggs do not yet contain an infective larva. In warm, humid surroundings, a larva can develop inside the egg and become infective in around two weeks. The usual environmental maturation time for Toxocara canis eggs is around two to four weeks. Toxocara egg maturation can take one to four weeks. Under suitable conditions, mature eggs can survive in the environment for years.

In puppies under three months old, larvae hatching from swallowed Toxocara eggs can pass through the intestinal wall and liver to the lungs. They travel from the lungs in coughed-up material to the throat and are swallowed back into the small intestine. In older dogs, larvae more often become dormant in tissues. Toxascaris leonina develops in the intestinal wall and lumen without similar lung migration. It does not pass to puppies through the placenta or milk.

Toxascaris leonina eggs can become infective just one week after being passed. Infective eggs can remain in soil after faeces have broken down and are no longer visible. Larvae acquired before birth can be found in a puppy's intestine by one week old.

Risk factors

Toxocara canis most often causes symptomatic disease in young puppies. Resistance to the parasite usually increases with age. Outdoor risk increases with access to faecally contaminated areas or opportunities to eat rodents. In kennels, faeces can expose dogs sharing the environment.

A study from northern Europe published in 2024 examined faecal samples from 664 dogs over six months old. Toxocara canis eggs were present in 0.9% of samples. They were more commonly found in dogs under one year old than in older dogs. In an earlier northern European study of 541 dogs published in 2006, roundworm eggs were found in 17 dogs, or 3.1%.

Samples for the older study were collected between autumn 2001 and autumn 2002. In that study, roundworm and hookworm findings were more likely in dogs that had travelled outside the Nordic countries during the preceding year. The studies' age limits matter when comparing results, as the youngest puppies were excluded from the newer sample.

Typical symptoms

Roundworm infection in adult dogs often causes no symptoms. Slower-than-normal weight gain may be a puppy's first sign.

• A rounded, swollen-looking abdomen.
• Loose, mucoid faeces.
• Vomiting.
• Long, pale worms visible in vomit.
• A duller coat.
• Weight loss.
• Coughing in a puppy when larvae migrate through the lungs.

Adult worms can also be visible in faeces. A puppy's symptoms can start before worm eggs are detectable in faeces.

An infected puppy can be otherwise thin despite a rounded abdomen. Toxascaris leonina rarely causes apparent illness in dogs, even with adult worms in the intestine.

Complications and consequences

A heavy roundworm infection can impair a puppy's nutritional status and slow growth. Rarely, large numbers of worms can obstruct the intestine. Migrating larvae can inflame lung tissue. In severe infection, the puppy's intestine may become inflamed and produce excessive mucus.

People can acquire toxocariasis by swallowing infective Toxocara eggs. In humans, larvae can migrate to the liver, lungs or eye, for example. They do not develop into reproducing adult worms in the human intestine. Rarely, a larva reaching the eye can damage vision.

A child's risk includes swallowing mature eggs from soil in a play area. Most human Toxocara infections cause no apparent symptoms. Larvae migrating into a newborn puppy's lungs can cause severe illness during the first days of life.

Diagnosis at the vet

Faecal testing and egg identification

Roundworm infection can be diagnosed by finding parasite eggs in faeces. Flotation brings eggs to the liquid surface for microscopic identification. Centrifugation improves test sensitivity. Toxocara canis eggs have a pitted surface and measure around 80–90 × 75 µm. Toxascaris leonina eggs are oval with smooth shells.

Collecting adult dogs' samples on several consecutive days may be useful because eggs are not necessarily shed daily. A worm seen in vomit or faeces can be identified. If a dog eats other animals' faeces, eggs can pass into the sample without adult worms living in the intestine.

Toxascaris leonina eggs measure around 75–85 µm long and 60–75 µm wide. Toxocara canis egg size can also be reported as around 85–90 × 75 µm.

When does infection become detectable?

An early faecal sample can be negative if worms are not yet producing eggs. A puppy infected before birth may begin shedding Toxocara canis eggs from around 16 days old at the earliest. With placental infection, eggs may be found in faeces at two to three weeks old. With Toxocara canis infection acquired from swallowed eggs or prey, eggs may appear in faeces after four to five weeks. Egg shedding after infection from swallowed eggs can begin after four to eight weeks.

In a young dog, shedding may begin four to six weeks after swallowing eggs. In a dog around one year old, the corresponding interval is six to eight weeks. Toxascaris leonina eggs can be found around seven to ten weeks after infection.

Faecal antigen testing looks for substances produced by worms and does not require eggs in the sample. It may be useful with only immature worms or worms of one sex. Faecal PCR can detect worm DNA even when no eggs are visible microscopically.

Antigen testing can complement microscopy because the methods detect infection differently. An antigen finding alone does not identify the roundworm species.

Treatment

Choosing a wormer and assessing the response

Confirmed roundworm infection is treated with a wormer effective against roundworms. Different medicines may act differently against the developmental stages. If a product kills only adults, treatment may need repeating once younger stages have developed further. With a medicine effective only against adults, a faecal check can be performed 7–14 days after treatment ends.

Most dogs recover after appropriate treatment. If a post-treatment sample still contains eggs, the possibility of eating contaminated faeces is also investigated. Antigen testing may then help distinguish worms living in the intestine from eggs passing through.

Worm control for litters and adult dogs

A puppy needs an age-appropriate worming plan because infection can occur before birth. Veterinary-directed treatment of a pregnant bitch can reduce transmission to puppies. A nursing bitch's worming is planned alongside the litter's care, as she may also shed eggs. Adult dogs' worming needs are assessed from exposure and faecal testing.

When other intestinal worms are also suspected, product effectiveness is considered against every identified species.

Prevention

Picking up faeces soon after defaecation reduces environmental egg accumulation. Remove dog faeces regularly from the garden even without a confirmed infection. Ordinary disinfectants do not reliably destroy roundworm eggs in the environment. Preventing access to rodents reduces infection risk.

Cover children's sandpits when not in use. Wash hands with soap after outdoor activities and before eating. Wash vegetables eaten raw too.

Sunlight and drying can reduce egg survival in contaminated areas. Gardening gloves reduce hand contact with potentially contaminated soil. Cooking meat fed to dogs reduces infection risk from larvae in tissues.

Important for dog owners

Collect a faecal sample as fresh as possible. A tablespoonful is usually enough for a routine parasite examination. Store it in a sealed container at refrigerator temperature if directed by the vet or laboratory. If you find a visible worm, place it in a separate container for identification.

Wash your hands after collecting faeces and handling the sample. Keep children away from places where pets or wildlife defaecate.

Place the tightly sealed sample in an additional bag if sending it to a laboratory. Record whether the dog ate another animal's faeces before sampling.

Frequently asked questions

Can a symptom-free dog spread roundworms?

Yes: egg-producing roundworms can live in an adult dog's intestine without symptoms. Eggs passed into the environment can later mature and become infective. Prompt faeces removal reduces exposure for dogs and people.

Can a puppy become infected before birth?

Toxocara canis can pass through the placenta before birth. Dormant larvae in the mother's tissues can reactivate late in pregnancy. After birth, a puppy can also acquire larvae of the same parasite through milk during nursing.

Can a person catch roundworms by stroking a dog?

Infection occurs when a person swallows infective eggs, for example from dirty hands or soil-contaminated food. Contact with a dog alone does not mean eggs have reached the mouth. Wash hands with soap after outdoor activities and handling animals.

Does a negative faecal sample rule out infection?

No: immature worms do not yet produce eggs even when a dog has symptoms. The vet may use antigen testing to detect substances produced by worms before egg shedding begins. Sampling can be repeated if the result does not fit the puppy's symptoms.

Can a dog get roundworms again?

Yes: eggs can remain infective in the environment for years. A dog can be reinfected by swallowing environmental eggs or eating an infected rodent. Prompt, thorough faeces removal reduces the addition of new eggs to the dog's surroundings.