Toxocariasis is spread in the faeces (stools) of infected dogs. To prevent the spread of the infection, dog-owners should always dispose of dog faeces in a responsible way. 

Toxocariasis is an uncommon infection caused by parasites that are known as toxocara canis. Toxocara canis are more commonly known as roundworm.

Toxocariasis is a zoonotic condition. Zoonotic conditions are conditions that are spread from animals to humans.

The toxocara canis parasites live in the digestive system of dogs and foxes. Parasite eggs can be released in the faeces of infected animals and contaminate soil. If someone accidently ingests small particles of contaminated soil, they may develop toxocariasis.

Types of toxocariasis

The symptoms of toxocariasis can vary depending on whereabouts in the body the infection occurs. There are three main types of toxocariasis:

  • covert toxocariasis,
  • visceral larva migrans, and
  • ocular larva migrans.

Covert toxocariasis

Covert toxocariasis is the most common and mildest form of toxocariasis. Symptoms of covert toxocariasis include abdominal pain, cough, and headache.

Visceral larva migrans

Visceral larva migrans develops when large numbers of parasites spread through different organs of the body, such as the lungs, liver, and heart.

The main symptoms of visceral larva migrans are fever, abdominal pain, and shortness of breath.

Ocular larva migrans

Ocular larva migrans is the least common, but potentially most serious, type of toxocariasis. The condition can develop if the toxocara canis parasites move into the eyes.

The main symptoms of ocular larva migrans are disturbed vision and irritation of the eyes. Left untreated, ocular larva migrans can result in permanent vision loss, although only one eye is usually affected.

How common is toxocariasis?

Toxocariasis is a rare condition, with an average of 10 cases occurring each year in England.

Toxocariasis usually affects children who are between 1-4 years of age. However, cases of toxocariasis have been reported in people of all ages.

Young children are particularly at risk of getting toxicariasis because their play habits make them more likely to come into contact with contaminated soil. Many young children also have a habit of eating soil.


With treatment, the outlook for toxocariasis is very good. Treatment involves taking medication designed to kill the parasites. Most people will quickly make a full recovery and will not experience any long-term complications.

Due to advances in treatment, the potential risk of blindness is now a very rare complication of toxocariasis.

The life cycle of toxocara canis

To better understand the causes of toxocariasis, it is useful to learn a little more about the life cycle of the toxocara canis parasites.

Adult parasites live in the small intestines of dogs and puppies (as well as a number of other mammals, such as foxes). They range from 4-12cm in length (1.5-5 inches).

The adult parasites are capable of producing a large number of eggs. The eggs are passed out in the infected animal’s stools (faeces).

The eggs have a tough outer shell and are able to survive for up to five years once they have been passed into the outside world.

Most cases of toxocariasis develop when someone touches contaminated soil and then transfers the eggs into their mouth.

Once the eggs are inside the human body, they move into the intestine before hatching and releasing parasite larvae (insects in their earliest stage of development). The larvae are capable of travelling to all parts of the body.

Most of the symptoms of toxocariasis are caused by people having an allergic reaction to the larvae. In many cases, people are infected by the parasites but do not experience any symptoms.


A blood test can be used to diagnose toxocariasis. If you have the parasites in your body, your immune system will produce a specific type of antibody that can be detected in your blood.

Covert toxocariasis

The symptoms of covert toxocariasis include:

  • cough,
  • difficulty sleeping,
  • abdominal pain,
  • headaches, and
  • swollen lymph nodes (glands).

Children with covert toxocariasis may also exhibit a change in their usual behaviour, such as appearing unusually ‘cranky’ and irritable.

Visceral larva migrans

Depending on what parts (or organs) of the body are infected with parasites, visceral larva migrans can cause a wide range of possible symptoms. Possible symptoms may include:

  • fatigue,
  • loss of appetite,
  • weight loss,
  • high temperature (fever) of 38C (100.4F) or above,
  • coughing,
  • breathing difficulties,
  • abdominal pain,
  • headaches,
  • skin rashes,
  • swollen lymph nodes and, less commonly,
  • seizures (fits).

Ocular larva migrans

Symptoms of ocular larva migrans include:

  • a disturbance (or reduction) in vision, such as blurred, or cloudy, vision that usually only affects one eye,
  • a very red and painful affected eye, and
  • light sensitivity (photophobia). 

All three types of toxocariasis can be treated with a medication called mebendazole. Mebendazole stops the parasites from being able to use glucose as food. Without a food source, the parasites will die.

Most people are required to take a four week course of mebendazole. Mebendazole does not usually cause side effects although some people may experience headaches at the start of their treatment.

Visceral larva migrans

In cases of visceral larva migrans that are causing particularly severe breathing difficulties, admission to hospital may be required so that your breathing can be supported while the infection is being treated.

Ocular larva migrans

In cases of ocular larva migrans, additional treatment may be required in order to prevent damage to the eye. For example, steroid medications can be used to help reduce eye inflammation and irritation.

In a number of cases, laser treatment is required to kill any parasites that are present in the eye.

A type of treatment that is known as laser photocoagulation can be used to kill the parasites. Treatment is usually available on an out-patient basis, which means that you will not have to stay in hospital overnight.

Drops are put into your eyes to numb the surface. A special contact lens is placed on your eye to hold your lids open and to focus the laser beam on to your retina. A laser is then used to burn away any parasites in your eyes.

Laser photocoagulation is not usually painful, although you may feel an occasional sharp pricking sensation when certain areas of your retina are being treated.

Following treatment, your vision may be blurred. However, it should return to normal after a few hours.

Some people who have had laser photocoagulation experience some loss of peripheral vision (side vision) and night vision. One study found that over 50% of people who were treated noticed some difficulty with their night vision, and 25% noticed some loss of their peripheral vision.