Toxocariasis is the parasitic disease caused by the larvae of two species of Toxocara roundworms: Toxocara canis from dogs and, less commonly, Toxocara cati from cats.

Most people infected with Toxocara do not have any symptoms. There are two major forms of toxocariasis, visceral toxocariasis (VT), also called visceral larva migrans (VLM), and ocular toxocariasis (OT), also called ocular larva migrans (OLM). The syndromes VLM and OLM can be caused by infection with the migrating larvae of other kinds of parasites which cause symptoms similar to those caused by migrating Toxocara larvae.

In a few people who are infected with high numbers of Toxocara larvae or have repeated infections, the larvae can travel through parts of the body such as the liver, lungs, or central nervous system and cause symptoms such as fever, coughing, enlarged liver or pneumonia. This form of toxocariasis is called visceral toxocariasis (VT). The larvae can also travel to the eye and cause ocular toxocariasis (OT). Ocular toxocariasis occurs when a microscopic Toxocaralarva enters the eye and causes inflammation and scarring on the retina. OT typically occurs only in one eye and can cause irreversible vision loss.

Infected dogs and cats shed Toxocara eggs in their feces into the environment. Once in the environment, it takes 2 to 4 weeks for Toxocara larvae to develop inside the eggs and become infectious. Toxocara eggs have a strong protective layer, which allows the eggs to survive in the environment for months or even years under the right conditions. Humans or other animals (e.g. rabbits, pigs, cattle, or chickens) can be infected by accidentally ingesting Toxocara eggs. For example, humans can become infected if they work with dirt and accidentally ingest dirt containing Toxocara eggs. Although rare, people can be infected by eating undercooked or raw meat from an infected animal such as a lamb or rabbit. Because dogs and cats are frequently found where people live, there may be large numbers of infected eggs in the environment. Once in the body, the Toxocara eggs hatch and the larvae can travel in the bloodstream to different parts of the body, including the liver, heart, lungs, brain, muscles, or eyes. Most infected people do not have any symptoms. However, in some people, the Toxocara larvae can cause damage to these tissues and organs. The symptoms of toxocariasis, the disease caused by these migrating larvae, include fever, coughing, inflammation of the liver, or eye problems.

A U.S. study in 1996 showed that 30% of dogs younger than 6 months deposit Toxocara eggs in their feces; other studies have shown that almost all puppies are born already infected with Toxocara canis. Research also suggests that 25% of all cats are infected with Toxocara cati. Infection rates are higher for dogs and cats that are left outside and allowed to eat other animals. In humans, it has been found that 5% of the U.S. population has been infected with Toxocara. Globally, toxocariasis is found in many countries, and prevalence rates can reach as high as 40% or more in parts of the world. There are several factors that have been associated with higher rates of infection with Toxocara. People are more likely to be infected with Toxocara if they own a dog. Children and adolescents under the age of 20 are more likely to test positive for Toxocara infection than adults. This may be because children are more likely to eat dirt and play in outdoor environments, such as sandboxes, where dog and cat feces can be found. This infection is more common in people living in poverty. Geographic location plays a role as well, because Toxocara is more prevalent in hot, humid regions where eggs are able to survive better in the soil.

Causal Agents

Toxocariasis in humans is caused by infection with larvae of Toxocara spp., which are common ascarid roundworms of mammals. Confirmed zoonotic species include the dog roundworm T. canis (presumed most common) and the cat roundworm T. cati (frequency not known). It is not known whether other closely-related Toxocara species can infect humans (e.g. T. malaysiensis of cats).

Life Cycle

Toxocara spp. can follow a direct (one host) or indirect (multiple host) life cycle. Unembryonated eggs are shed in the feces of the definitive host (canids: T. canis; felids: T. cati) (1). Eggs embryonate over a period of 1 to 4 weeks in the environment and become infective, containing third-stage (L3) larvae (2). Following ingestion by a definitive host (3), the infective eggs hatch and larvae penetrate the gut wall. In younger dogs (T. canis) and in cats (T. cati), the larvae migrate through the lungs, bronchial tree, and esophagus, where they are coughed up swallowed into the gastrointestinal tract; adult worms develop and oviposit in the small intestine (4). In older dogs, patent (egg-producing) infections can also occur, but larvae more commonly become arrested in tissues. Arrested larvae are reactivated in female dogs during late gestation and may infect pups by the transplacental (major) and transmammary (minor) routes (5) in whose small intestine adult worms become established (6). In cats, T. cati larvae can be transmitted via the transmammary route (5) to kittens if the dam is infected during gestation, but somatic larval arrest and reactivation does not appear to be important as in T. canis.

Toxocara spp. can also be transmitted indirectly through ingestion of paratenic hosts. Eggs ingested by suitable paratenic hosts hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst (7). The life cycle is completed when definitive hosts consume larvae within paratenic host tissue (8), and the larvae develop into adult worms in the small intestine.

Humans are accidental hosts who become infected by ingesting infective eggs (9) or undercooked meat/viscera of infected paratenic hosts (10). After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a variety of tissues (liver, heart, lungs, brain, muscle, eyes) (11). While the larvae do not undergo any further development in these sites, they can cause local reactions and mechanical damage that causes clinical toxocariasis.

Most people infected with Toxocara do not have any symptoms. There are two major forms of toxocariasis, visceral toxocariasis (VT), also called visceral larva migrans (VLM), and ocular toxocariasis (OT), also called ocular larva migrans (OLM). The syndromes VLM and OLM can be caused by infection with the migrating larvae of other kinds of parasites which cause symptoms similar to those caused by migrating Toxocara larvae.

In a few people who are infected with high numbers of Toxocara larvae or have repeated infections, the larvae can travel through parts of the body such as the liver, lungs, or central nervous system and cause symptoms such as fever, coughing, enlarged liver or pneumonia. This form of toxocariasis is called visceral toxocariasis (VT). The larvae can also travel to the eye and cause ocular toxocariasis (OT). Ocular toxocariasis occurs when a microscopic Toxocara larva enters the eye and causes inflammation and scarring on the retina. OT typically occurs only in one eye and can cause irreversible vision loss.

If you think you or your child may have toxocariasis, you should see your health care provider to discuss the possibility of infection and, if necessary, to be examined. Toxocariasis can be difficult to diagnose because the symptoms of toxocariasis are similar to the symptoms of other infections. A blood test is available that looks for evidence of infection with Toxocara larvae. In addition to the blood test, diagnosis of toxocariasis includes identifying the presence of typical clinical signs of VT or OT and a compatible exposure history.

Visceral toxocariasis can be treated with antiparasitic drugs such as albendazole or mebendazole. Treatment of ocular toxocariasis is more difficult and usually consists of measures to prevent progressive damage to the eye.

Controlling Toxocara infection in dogs and cats will reduce the number of infectious eggs in the environment and reduce the risk of infection for people. Have your veterinarian treat your dogs and cats, especially young animals, regularly for worms. This is especially important if your pets spend time outdoors and may become infected again.

There are several things that you can do around your home to make you and your pets safer, including the following:

• Clean your pet’s living area at least once a week; every day is better. Feces should be either buried or bagged and disposed of in the trash. Wash your hands after handling pet waste.
• Do not allow children to play in areas that are soiled with pet or other animal feces and cover sandboxes when not in use to make sure that animals do not get inside and contaminate them.
• Wash your hands with soap and warm water after playing with your pets or other animals, after outdoor activities, and before handling food or eating.
• Teach children the importance of washing hands to prevent infection.
• Teach children that it is dangerous to eat dirt or soil.

Toxocara eggs have a strong protective layer, which allows the eggs to survive in the environment for months or even years under the right conditions. Many common disinfectants are not effective against Toxocara eggs but extreme heat has been shown to kill the eggs. Prompt removal of animal feces can help prevent infection since the eggs require 2 to 4 weeks to become infective once they are passed out of the animal.

Although rare, people can also be infected by eating undercooked or raw meat from an infected animal such as a lamb or rabbit. Meat and offal should always be cooked thoroughly and to appropriate temperatures to prevent illness.

What is toxocariasis?

Toxocariasis is an infection transmitted from animals to humans (zoonosis) caused by the parasitic roundworms commonly found in the intestine of dogs (Toxocara canis) and cats (T. cati).

Who is at risk for toxocariasis?

Anyone can become infected with Toxocara. Young children and owners of dogs or cats have a higher chance of becoming infected.

Approximately 5% of the U.S. population has antibodies to Toxocara. This suggests that tens of millions of Americans may have been exposed to the Toxocara parasite.

How can I get toxocariasis?

Dogs and cats that are infected with Toxocara can shed Toxocara eggs in their feces. Adults and children can become infected by accidentally swallowing dirt that has been contaminated with dog or cat feces that contain infectious Toxocara eggs. Although it is rare, people can also become infected from eating undercooked meat containing Toxocara larvae.

What are the clinical manifestations of toxocariasis?

Many people who are infected with Toxocara do not have symptoms and do not ever get sick. Some people may get sick from the infection and may develop the following:

• Ocular toxocariasis: Ocular toxocariasis occurs when Toxocara larvae migrate to the eye. Symptoms and signs of ocular toxocariasis include vision loss, eye inflammation or damage to the retina. Typically, only one eye is affected.
• Visceral toxocariasis: Visceral toxocariasis occurs when Toxocara larvae migrate to various body organs, such as the liver or central nervous system. Symptoms of visceral toxocariasis include fever, fatigue, coughing, wheezing, or abdominal pain.

How serious is infection with Toxocara?

In most cases, Toxocara infections are not serious, and many people, especially adults infected by a small number of larvae (immature worms), may not notice any symptoms. The most severe cases are rare, but are more likely to occur in young children, who often play in dirt, or eat dirt (pica) contaminated by dog or cat feces.

How is toxocariasis spread?

The most common Toxocara parasite of concern to humans is T. canis, which puppies usually contract from their mother before birth or from her milk. The larvae mature rapidly in the puppy’s intestine; when the pup is 3 or 4 weeks old, the worms inside the puppy begin to produce large numbers of eggs that contaminate the environment through the puppy’s feces. After the eggs pass into the environment, it takes about 2 to 4 weeks for infective larvae to develop in the eggs. If a person ingests one of these infective eggs, then they can become infected with toxocariasis.

Toxocariasis is not spread by person-to-person contact like a cold or the flu.

What should I do if I think I have toxocariasis?

See your health care provider to discuss the possibility of infection and, if necessary, to be examined. Your provider may take a sample of your blood for testing.

What is the treatment for toxocariasis?

Visceral toxocariasis is treated with antiparasitic drugs. Treatment of ocular toxocariasis is more difficult and usually consists of measures to prevent progressive damage to the eye.

How do I prevent toxocariasis?

• Take your pets to the veterinarian to prevent infection with Toxocara. Your veterinarian can recommend a testing and treatment plan for deworming.
• Wash your hands with soap and water after playing with your pets or other animals, after outdoor activities, and before handling food or eating.
• Teach children the importance of washing hands to prevent infection.
• Do not allow children to play in areas that are soiled with pet or other animal feces.
• Clean your pet’s living area at least once a week. Feces should be either buried or bagged and disposed of in the trash. Wash your hands after handling pet waste.
• Teach children that it is dangerous to eat dirt or soil.