Introduction

Toxocariasis is a serious human disease caused by the larvae of parasitic roundworms. The disease is classified as a zoonotic disease since it is transmitted from animals to humans. Covert toxocariasis is an important disease since it may remain clinically hidden but has the potential to cause critical illness and cognitive impairment in children.³

Toxocariasis can be categorised into different clinical forms, depending on which part of the body the roundworm larvae travel.¹ See Table 1 below for more information. The most common form of the disease is characterised by mild, non-specific symptoms and is called covert toxocariasis in children, or common toxocariasis in adults.² Due to the vague symptoms of covert toxocariasis, this disease often remains undetected in children.³ This article will discuss the causes, health implications and treatment of this often-overlooked disease. 

Table 1: The four different forms of clinical toxocariasis based on the location of larvae in the body.^1,2 

What causes covert toxocariasis? 

Covert toxocariasis is an illness caused by infection with the larvae of dog roundworms (Toxocara canis), or less frequently cat roundworms (Toxocara cati). The larvae migrate through the body, where they cause damage and inflammation. When the disease has vague, non-specific symptoms in children, it is called covert toxocariasis. 

How is toxocariasis transmitted?

Dogs and cats are the final hosts of the Toxocara worms. In some developed countries (such as the UK), foxes also play an important role in spreading Toxocara eggs.¹ The adult worms live in the intestines of these animals and shed their eggs into the environment with faeces. The eggs are not immediately infectious and need two to four weeks to mature in the soil. People can become infected by ingesting the mature eggs in contaminated soil or via larvae in undercooked meat of infected animals such as rabbit, chicken, ducks, pork or lamb. The larvae within the eggs hatch in the intestines of the person and then migrate through the body. Since humans are the dead-end hosts, the larvae die in the tissues and cause inflammation. 

Who is at risk of covert toxocariasis?

Covert toxocariasis affects millions of children worldwide, although the disease is more common in impoverished communities in tropical and subtropical countries.^1,3

The soil in public parks and playgrounds often has high levels of contamination by Toxocara eggs, especially if dogs, cats and foxes have access to these spaces. Young children are at a particularly high risk of infection since they typically visit these areas and often ingest soil during play. Children also generally have poorer hygiene, further increasing their risk of infection. Additionally, children living in households with dogs and cats have a higher chance of becoming infected. 

Is covert toxocariasis serious?

Most children who swallow Toxocara eggs do not become ill. The larvae can migrate to all parts of the body, such as the liver, lungs, brain or eyes, where they usually die without causing further illness. In most cases, the infection resolves within 3 months.⁴ However, sometimes the larvae can cause serious illness. Covert toxocariasis has also been linked to cognitive impairment³ and behavioural disorders in school-aged children.³ 

What are the symptoms of covert toxocariasis? 

Possible symptoms of covert toxocariasis in children include the following:^1,3

• Fever
• Headache
• Loss of appetite
• Nausea or vomiting
• Tummy pain
• Tiredness and changes in sleep patterns
• Changes in behaviour 
• Cognitive impairment 
• Asthma-like symptoms such as coughing and wheezing
• Pneumonia
• Itching
• Rash
• Pain in the arms or legs
• Enlargement of the lymph nodes 

How is covert toxocariasis diagnosed?

Covert toxocariasis can be difficult to diagnose since the symptoms are vague and often similar to the clinical signs of other diseases. The most accurate diagnosis is often based on a combination of: 

• Clinical signs of the disease
• History of exposure to potential sources of Toxocara eggs in the soil or to the larvae in undercooked infected meat
• Blood tests of infected children usually show an increase in a specific group of white blood cells called eosinophils.⁴ However, this is not always reliable³
• Detecting antibodies to Toxocara in the blood through laboratory tests such as the ELISA (enzyme-linked immunosorbent assay). The problem with this test, however, is that the presence of antibodies does not differentiate between current and previous infections. Disease detection and treatment are therefore not always straightforward

How is covert toxocariasis treated?

Not all cases of mild covert toxocariasis are treated since the infection can resolve on its own. Symptomatic cases are usually treated with anthelmintic drugs. Anthelmintics are a group of drugs that are used to expel parasites from the body. Albendazole is currently the drug of choice since it is widely available and has low side effects during short treatment courses.⁵ Another anthelmintic drug commonly used is mebendazole.  

Although anthelmintic drugs destroy the parasite, the dying larvae may result in severe inflammatory and allergic reactions, which can cause further complications. In these cases, steroids are often prescribed. 

How can covert toxocariasis be prevented?

Toxocariasis can be prevented through a combination of proper veterinary care for your pets and improved personal hygiene practices. The key prevention strategies to reduce the risk of infection⁶ are:  

• Take your dogs and cats for regular veterinary visits. Puppies and kittens should be dewormed regularly from the age of 2 weeks. Your veterinarian will recommend a testing and deworming schedule for your pets
• Regularly clean and disinfect your pet’s living area
• Pet faeces should be buried or bagged and disposed of in the trash
• Don’t allow your pet to foul in sandpits or playgrounds. Keep sandboxes covered when not in use
• Teach children the importance of good hygiene, such as proper handwashing with soap and water
• Teach children that eating soil is dangerous
• Ensure meat is hygienically prepared and thoroughly cooked
• Wash all fruits and vegetables before eating
• Public health measures should include controlling stray dog and cat populations, and including stray pets in community deworming programs

Summary 

Covert toxocariasis is an important disease in children which is often overlooked. The symptoms can be vague and therefore difficult to diagnose. Early recognition and management is important to prevent serious complications and cognitive impairment in younger children. 

Frequently asked questions (FAQs)

How does covert toxocariasis differ from the other forms of toxocariasis? 

Toxocariasis can be categorised into different clinical forms, depending on which part of the body the roundworm larvae travel:¹

• Visceral toxocariasis (also known as visceral larva migrans or VLM) occurs when Toxocara larvae migrate through organs such as the liver, kidneys, heart and lungs
• Ocular toxocariasis (also known as ocular larva migrans or OLM) occurs when Toxocara larvae migrate to the eye, often leading to permanent vision loss
• Neurotoxocariasis occurs when Toxocara larvae migrate to the brain and spinal cord, often causing neurological symptoms such as epilepsy
• Covert toxocariasis is the most common form of the disease in children and occurs when the larvae don’t migrate to a specific site. Covert toxocariasis is milder and characterised by non-specific symptoms

Is covert toxocariasis a common disease? 

Yes. Toxocariasis affects millions of children all over the world. Covert toxocariasis is the most common clinical presentation of the disease.

Does covert toxocariasis only occur in poorer communities? 

No. Toxocariasis can affect children from all walks of life.

Can covert toxocariasis occur without direct contact with dogs and cats? 

Yes. Toxocara eggs are shed in the faeces of infected dogs and cats, but the eggs are not immediately infectious. They need two to four weeks to mature in the soil.

Can I see toxocara eggs in dog or cat faeces? 

No. The eggs are too small to be seen with the naked eye.

Can toxocara eggs be detected in human faeces? 

No. Since humans are not the usual hosts, the larvae cannot mature to complete their life cycle.

Can toxocariasis spread from person to person? 

No. Toxocariasis mainly spreads from dogs and cats (and sometimes foxes) who shed the Toxocara eggs in their faeces. You can also get infected with Toxocara larvae by eating undercooked infected meat, such as chicken, duck, pork, lamb, etc.

What should I do if I think my child has toxocariasis? 

Speak to your healthcare provider as soon as possible. Your doctor will evaluate the specific risk factors in your case and may take a blood sample for further testing.

Is covert toxocariasis always treated in children? 

No. Since toxocariasis is often a mild self-limiting disease, many healthcare practitioners will choose to observe the disease and wait for it to resolve by itself. However, due to risks of complications, it is generally recommended to treat all infected children.⁷