Roundworms in Dogs: Symptoms, Treatment & Deworming

Roundworms are intestinal parasites that live in the small intestine of dogs. The most common canine roundworm is Toxocara canis, while Toxascaris leonina is seen less often. Adult worms are long, pale, and tube-shaped, often described as looking like spaghetti.

Roundworms are especially common in puppies. Larvae can reactivate in a pregnant dog and pass to puppies through the placenta, and some can also pass through milk after birth. That is why many puppies are treated on a routine deworming schedule even if they seem healthy and even if an early fecal test is negative.

After a dog swallows infective eggs or eats infected prey, larvae hatch and migrate through the body. In young puppies, they commonly move through the liver and lungs before returning to the intestine to mature into adults. Adult dogs are more likely to wall off larvae in body tissues, where they can stay dormant for months or years.

Adult female roundworms produce large numbers of eggs that pass in stool. Those eggs become infective in the environment after about 2 to 4 weeks and can persist in soil for a long time. That combination makes roundworms common, frustrating, and very preventable with steady parasite control.

The lifecycle of Toxocara canis is more complex than most intestinal parasites, which is part of what makes the species so successful and so difficult to eliminate from a breeding population.

Egg Morphology and Environmental Stage

Adult female T. canis in the small intestine are prolific egg producers, shedding an estimated 85,000 to 200,000 eggs per day. The eggs passed in feces are subspherical, thick-shelled, and measure approximately 75 to 90 micrometers in diameter. They have a characteristic dark-brown, pitted outer shell that is highly resistant to environmental degradation. Freshly passed eggs are not immediately infective. Under favorable conditions of warmth and moisture, the eggs embryonate and reach the infective second-stage larva (L2) within approximately 2 to 4 weeks, though this can take longer in cold climates. Once embryonated, eggs can remain viable in soil for years, making environmental decontamination extremely difficult.

Routes of Infection

T. canis reaches new hosts through four distinct transmission routes:

1. Direct ingestion of embryonated eggs. Dogs of any age can become infected by swallowing infective eggs from contaminated soil, grass, or fomites. In puppies under about 5 weeks of age, ingested larvae undergo hepato-tracheal migration: they penetrate the intestinal wall, travel via the portal circulation to the liver, then migrate to the lungs via the pulmonary vasculature. In the lungs, larvae break out of the capillaries into the alveoli, are coughed up and swallowed, and return to the small intestine where they mature into adults. This full somatic migration takes approximately 4 to 5 weeks from ingestion to the appearance of patent (egg-shedding) adults.

2. Transplacental (prenatal) transmission. This is the most clinically important route. In older dogs and previously infected bitches, ingested larvae do not complete the hepato-tracheal migration. Instead, they become arrested (hypobiotic) as second-stage larvae encysted in somatic tissues, particularly skeletal muscle, kidneys, and liver. During late pregnancy (around day 42 of gestation), hormonal changes reactivate dormant larvae, which cross the placenta and infect puppies in utero. Puppies can therefore be born with developing roundworm infections, and patent infections may be present by 2 to 3 weeks of age.

3. Transmammary (lactogenic) transmission. Some reactivated larvae migrate to the mammary glands and are passed to nursing puppies through milk during the first 3 weeks of lactation. This route supplements transplacental transmission and means that even puppies born without prenatal infection can acquire worms while nursing.

4. Paratenic (transport) host ingestion. A range of vertebrate species, including rodents, rabbits, birds, and earthworms, can serve as paratenic hosts. When these animals ingest embryonated T. canis eggs, the larvae hatch and migrate into tissues but do not develop further. They remain as encysted L2 larvae in the paratenic host's tissues indefinitely. When a dog eats an infected prey animal, the larvae are released during digestion and can develop to adulthood in the dog's intestine without undergoing hepato-tracheal migration. This route is particularly important for hunting dogs, terriers, and dogs with high prey drive.

Larval Migration in Older Dogs

In dogs older than about 5 to 6 weeks, the immune system increasingly diverts migrating larvae into somatic tissues rather than allowing them to complete the tracheal migration route. These larvae encyst in muscle, liver, kidneys, and other organs as hypobiotic L2 larvae. They can persist in this arrested state for the lifetime of the dog and cannot be killed by standard anthelmintic treatments. In intact female dogs, these dormant larvae represent the reservoir for transplacental infection of future litters, which is why routine deworming of puppies is practiced regardless of the dam's apparent health status.

Toxocara canis is considered the most common intestinal helminth of dogs worldwide and is found on every inhabited continent. Its success as a parasite is driven by the combination of transplacental transmission, massive egg output, extreme environmental egg persistence, and multiple transmission routes.

Puppy Infection Rates

Because of transplacental transmission, virtually all puppies born to previously infected dams harbor T. canis larvae at birth. Studies consistently show that the majority of puppies shed roundworm eggs in their feces by 3 weeks of age if not dewormed. This near-universal prenatal infection rate is why veterinary guidelines recommend beginning deworming at 2 weeks of age, well before any diagnostic test could be performed.

Prevalence in Shelter and Owned Dogs

Prevalence figures vary by population, geography, and diagnostic method. In shelter and rescue dogs, fecal surveys in the United States have documented Toxocara prevalence rates of 30% to over 50%, reflecting the higher parasite burden typical of dogs from crowded, uncontrolled environments. Among owned dogs receiving veterinary care, prevalence is considerably lower but still significant, generally reported between 2% and 15% depending on the region, the dog's age, and whether the animal receives regular preventive care.

The Companion Animal Parasite Council (CAPC) maintains continuously updated parasite prevalence maps based on millions of diagnostic test results from veterinary practices across North America. These maps consistently show higher Toxocara prevalence in the southeastern United States and in areas with warmer, more humid climates that favor egg embryonation and survival.

Environmental Contamination

The environmental persistence of T. canis eggs is a major public health and epidemiologic concern. Embryonated eggs can survive in soil for years under a wide range of conditions, and they are resistant to most common disinfectants. Soil surveys from public parks, playgrounds, and residential yards in multiple countries have found that 10% to 30% or more of soil samples contain Toxocara eggs. Contamination is typically highest in areas frequented by uncontrolled dogs, in warm and humid climates, and in urban areas with high dog density.

Geographic and Seasonal Variation

While T. canis is found globally, prevalence and environmental contamination are highest in tropical and subtropical regions where year-round warm temperatures support continuous egg embryonation. In temperate climates, egg development slows or stops during winter but resumes in spring, creating a seasonal pattern of transmission risk. However, because transplacental transmission does not depend on environmental conditions, puppies can be born infected at any time of year regardless of climate.

Risk Factors

Factors associated with higher roundworm prevalence include young age (puppies under 6 months), lack of regular deworming or preventive medication, crowded housing (shelters, breeding kennels, puppy mills), outdoor access with soil contact, hunting or scavenging behavior, and geographic location in warmer or more humid regions. There is no meaningful breed or sex predisposition beyond lifestyle and management factors.

Dogs can pick up roundworms in several different ways. In puppies, the most important route is from their mother. Dormant larvae in the mother's tissues can reactivate during pregnancy and cross the placenta, and some can also pass through milk during nursing.

Dogs of any age can also become infected by swallowing infective eggs from contaminated soil, grass, paws, toys, or other objects. Eggs are passed in stool, then become infective in the environment after roughly 2 to 4 weeks. Because the eggs are hardy, contaminated yards, runs, and outdoor spaces can stay risky for a long time.

Another route is eating infected prey such as rodents or birds. These animals can carry larval stages in their tissues, and dogs that hunt, scavenge, or eat carcasses have a higher exposure risk.

Risk tends to be highest in puppies, dogs from crowded environments, dogs with frequent outdoor exposure, and dogs not on regular parasite prevention. There is no meaningful breed predisposition. Any dog can get roundworms if the exposure is right.

The most common test is a fecal flotation, where your vet checks a stool sample under the microscope for roundworm eggs. This is still the standard first-line test in general practice. If your dog passes a visible worm, bringing a photo or sealed sample can also be very helpful.

Some clinics also use fecal antigen testing, which can improve detection in certain cases, including low egg shedding. This can be useful when suspicion is high but a routine flotation is negative.

A negative fecal test does not always rule roundworms out. Puppies may be infected before worms are mature enough to shed eggs, and adult dogs may carry dormant larvae in tissues with little or no egg shedding. That is one reason routine puppy deworming schedules are used even when early testing is unrevealing.

For most dogs, diagnosis is straightforward and does not require advanced imaging or blood work. Those tests are more likely if a puppy is very ill, severely underweight, dehydrated, or showing signs that raise concern for a blockage or another problem happening at the same time.

Toxocara canis is one of the most important zoonotic parasites worldwide, and human infection (toxocariasis) is far more common than most people realize. Humans are accidental hosts: they become infected by ingesting embryonated T. canis eggs from contaminated soil, unwashed produce, or fomite contact. Because humans are not the parasite's definitive host, larvae never mature into adult worms in the human body. Instead, they migrate through tissues, causing inflammatory damage that can range from asymptomatic to severe depending on the organs involved and the larval burden.

Forms of Human Toxocariasis

Visceral larva migrans (VLM) is the classic presentation. Larvae migrate through the liver, lungs, central nervous system, and other organs, provoking eosinophilic inflammation. Symptoms can include fever, cough, wheezing, hepatomegaly, and marked eosinophilia. VLM is most commonly diagnosed in children under 5 years old, who are at greatest risk because of hand-to-mouth behavior, pica (eating non-food items including soil), and frequent contact with contaminated ground surfaces.

Ocular larva migrans (OLM) occurs when a larva migrates to the eye, typically lodging in the retina or vitreous. Even a single larva can cause significant damage, including granulomatous retinitis, endophthalmitis, and retinal detachment. OLM can result in permanent unilateral vision loss and is sometimes initially misdiagnosed as retinoblastoma, leading to unnecessary enucleation. The CDC has estimated that approximately 70 people per year in the United States experience vision loss attributable to OLM. OLM tends to present in slightly older children (ages 5 to 10) compared with VLM.

Neural larva migrans (NLM) involves larval migration into the brain or spinal cord. Clinical presentations can include eosinophilic meningoencephalitis, seizures, cognitive or behavioral changes, and myelitis. NLM is less commonly diagnosed than VLM or OLM but may be underrecognized.

Covert or common toxocariasis describes a milder, more chronic form characterized by nonspecific symptoms such as abdominal pain, headache, cough, sleep disturbance, and behavioral changes, often accompanied by peripheral eosinophilia and elevated Toxocara serology. This is likely the most common clinical presentation in seropositive individuals and may go undiagnosed because symptoms are nonspecific.

Seroprevalence and Public Health Burden

National seroprevalence surveys in the United States have found Toxocara antibodies in approximately 14% of the general population, indicating past or current exposure. Seroprevalence is significantly higher in certain populations: rates exceed 20% in some southern states, and higher rates are consistently found in African American populations, people living in poverty, and residents of rural or urban areas with high stray dog density. Among children in some underserved urban communities, seroprevalence has been reported as high as 30% or more.

Globally, toxocariasis seroprevalence is even higher in many tropical and subtropical countries, with some surveys reporting rates above 50% in pediatric populations.

Risk Factors

The primary risk factors for human toxocariasis include age under 5 years, pica or geophagia (soil eating), contact with soil contaminated by dog (or cat) feces, ownership of puppies or dogs not on regular parasite prevention, living in areas with stray or free-roaming dog populations, and lack of access to veterinary care in the community. Children's playgrounds, sandboxes, and public parks are common sites of Toxocara egg contamination.

Prevention of Human Infection

Preventing human toxocariasis depends primarily on veterinary public health measures: routine deworming of puppies beginning at 2 weeks of age, year-round parasite prevention for adult dogs, prompt removal and disposal of dog feces from yards and public spaces, and community-level control of stray and free-roaming dog populations. On the human side, key measures include hand hygiene (especially after contact with soil, dogs, or outdoor play areas), covering sandboxes when not in use, washing produce, and discouraging pica in young children. Eggs are not immediately infective when passed in feces, so prompt daily stool cleanup significantly reduces environmental contamination risk.

Prevention starts with a realistic routine. Puppies are commonly dewormed beginning around 2 weeks of age, then repeated every 2 weeks until about 12 to 16 weeks old, followed by monthly parasite control through 6 months of age. Exact schedules vary by your vet, your puppy's risk, and the products being used.

For adult dogs, year-round parasite prevention is often the easiest long-term plan. Many monthly heartworm preventives also help control common intestinal parasites, including roundworms. Your vet can help match the product to your dog's age, lifestyle, and regional parasite risk.

Environmental control matters because eggs need time in the environment before they become infective. Picking up stool promptly, ideally daily, lowers risk for your dog and for people. Prevent scavenging and hunting when possible, and wash hands after handling stool, soil, or a dog that may have been in contaminated areas.

Roundworms are zoonotic, but people do not usually catch them directly from touching an infected dog. The main risk is accidentally swallowing infective eggs from contaminated soil or surfaces. In people, migrating larvae can cause visceral larva migrans or ocular larva migrans, which is why child-safe hygiene, covered sandboxes, and regular deworming are important public health steps as well as pet care steps.