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Everything about roundworm


Roundworms are commonly referred to as ascarids and are the most common parasites found in dogs.1 There are basically 4 species of roundworms that can affect dogs and/or cats: Toxocara canis, Toxocara leonine, Toxocara cati, and Baylisascaris procyonis (raccoon roundworm).1 Roundworms live an average of 4 months and have a life cycle ranging from as early as 14 days to as long as 80 days depending on the species.

The geographic distribution of Toxocara spp. is widespread in the pet population around the United States. It is estimated that more than 25% of cats have tested positive for Toxocara canis and more than 30% of dogs younger than 6 months of age are shedding Toxocara canis eggs.1 Some studies have shown that virtually all pups are born infected with T. canis.1 The raccoon roundworm Baylisascaris procyonis is occasionally found in dogs as well as other wildlife. 

Life Cycle:

Dogs are infected by ingesting an egg containing a larvae from a contaminated environment, ingestion of other animals whose tissues are infected with larvae, or by transplacental transmission of larvae from the mother to the pups prior to birth.1 The transplacental transmission occurs in T.canis infections and not in T. cati or T. leonine infections. Eggs will be passed in the stool of an infected dog and will contain an infective larvae within the next couple of weeks in the environment. The infective larvae will then be ingested by the dog either directly as an egg from the contaminated environment or secondarily by ingesting an animal already containing infective larvae. When the infective egg is ingested directly, the egg will travel to the first part of the intestine and hatch, releasing the larvae to migrate through the liver and lungs. The larvae will be transported up the trachea by coughing and will be swallowed to develop in the small intestine. The larvae will mature into the adult stage once in the small Larvae can be found in the dog’s intestine approximately 8 days after infection. The roundworm T. leonina are unique to the other species as they do not migrate outside the intestinal tract. The raccoon roundworm B. procyonis is similar to the T. canis and T. cati; however, they tend to invade the central nervous system. B. procyonis can be harbored in the intestinal tract of the raccoon or canine. Eggs that do not have time to become embryonated may be passed in the stool of an infected dog incapable of maturing into the infective larvae stage. An unborn puppy will be infected with T. canis by the larvae passing transplacentally prior to birth where the larvae wait in the liver and lung tissues until the puppy is born. Once the puppy is born, the larvae proceed with the migration through the tissues and eventually settle in the intestine after being coughed up and swallowed.

The time from when a dog is infected with a parasite to when the parasite is detected is known as the prepatent period. The prepatent period can vary depending on the species of the roundworm.1 Toxocara canis, commonly found in both dogs and cats, have a prepatent period of 14 to 28 days. The prepatent period of Toxocara cati is typically around 56 days and Toxocara leonina ranges from 56 to 80 days. Baylisascaris procyonis has a prepatent period of 49 to 70 days in the raccoon and 28 to 35 days in other animals such as dogs and other wildlife.

Disease State:

Disease caused by the roundworm T.canis is most severe in young pups and less commonly found in adult dogs. Repeat infections can occur if they ingest infective larvae. Puppies born with roundworm infection may have failure to thrive, poor hair coat, pot-bellied appearance, and ill thrift. Severe infections can result in acute death of puppies a few days old. Puppies with heavy fetal infections may vomit large amounts of worms around 4 to 6 months of age which can be distressing to the pet owner. The roundworm T. cati can cause pot-bellied appearance in kittens and ill thrift. Cats are susceptible to infection throughout their life with vomiting as the most common sign seen. The roundworm species T. leonina is not associated with clinical disease in pets. The raccoon roundworm B. procyonis can cause mild gastrointestinal symptoms such as enteritis (inflammation of the small intestine) or diarrhea similar to other roundworm infections in adult dogs. The most severe pathology caused by B. procyonis is neurologic signs due to migration of the worm into the central nervous system.  Dramatic intestinal obstruction can occur with high worm burdens of roundworms.  Some roundworm infections may be asymptomatic and detected incidentally on a stool test.


Fecal flotation with centrifugation is the most common test used to identify ova or eggs of adult roundworms present in the intestine. Visibly identifying the worms either in feces or vomitus is common especially in puppies. The Companion Animal Parasite Council (CAPC) recommends puppies and kittens be tested more frequently for roundworm infections than adult dogs and cats.1 Reinfection is possible especially if an area where the pet visits is highly contaminated. Roundworms are detected simply by fecal flotation with centrifugation due to the eggs being less dense and float easily. Additionally, roundworms produce eggs in abundance. A single adult female T. canis can produce up to 85,000 -200,000 eggs in a day. Fecal ELISA tests diagnose infection by detecting antigen or a particular protein produced by immature male and female worms of all roundworm species allowing for earlier detection of disease and single sex infections.2


There are a number of products approved for the treatment of Toxocara spp infections in dogs and cats. Some products are specifically formulated for only intestinal parasites while others are formulated in combination with heartworm prevention. It is highly recommended that puppies and kittens be dewormed every 2 weeks starting at 2 weeks of age to rid any existing infection and environmental contamination. It is recommended that the mother and her litter be dewormed at same time. There are various deworming schedules followed among veterinarians across the United States. Most of the products used to treat T. canis will treat B. procyonis although none are labeled for the parasite.

Protecting the environment by promptly removing feces, preventing scavenging tendencies by leash walking or keeping in a fenced yard, and administering monthly deworming are all ways in which to control and prevent roundworm infections. Cleaning the environment of stool prevent eggs from becoming infective is crucial because Roundworm eggs can be found in large numbers, are very hardy and may live for years in the environment. Extreme measures such as paving an area, complete removal of topsoil, controlled burns of the contaminated areas, or treating with steam have shown to effectively remove or destroy roundworm eggs in the environment.

Public Health:

Roundworms can pose as serious public health risks. Toxocara spp and B. procyonis are capable of being zoonotic or infecting humans. Children are the most commonly infected due to ingestion of infective eggs from a contaminated environment a sand box, beach, playground, park, etc. Syndromes identified include visceral larval migrans, neural larva migrans, ocular larva migrans, and the development of chronic abdominal pain. Visceral larval migrans is used to describe a roundworm infection found in the organs of a person. Neural larval migrans describes a condition where the larvae migrate into the neural tissues of a person. Lastly, ocular larva migrans occurs when the larvae infects the eye of a person.

The CAPC website and the website Pets and Parasites by CAPC are reliable resources for additional information regarding roundworms and your pet.


1. CAPC guideline: ascarid. Companion Animal Parasite Council.   

   (Accessed 24 August 2018)

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