Toxocariasis: a verminous zoonosis

Toxocariasis is a verminous zoonosis caused by parasitic nematodes, mainly Toxocara canis in dogs and exceptionally Toxocara cati in cats. This disease can affect various animals and, accidentally, humans. Toxocariasis is widespread throughout the world and poses a public health problem, especially for children.

What infectious agent is responsible?

Toxocariasis is a disease caused by parasitic nematodes of the Ascarididae family, mainly Toxocara canis in dogs and Toxocara cati in cats. These parasites, often called roundworms, are white to pink nematodes.

Toxocara eggs are excreted in the faeces of infected animals, contaminating the environment. These eggs require between one and three weeks in favourable conditions to become embryonated and infectious. Once ingested by a definitive host, such as a dog or cat, the eggs hatch in the intestine, releasing larvae. These larvae then penetrate the intestinal wall and migrate through various host tissues, including the liver, lungs and other organs, before returning to the small intestine to reach the adult stage.

Infection in humans generally occurs through ingestion of embryonated eggs present in contaminated soil or on poorly washed vegetables. The larvae hatch in the human intestine and migrate through various organs, but generally fail to develop into adult worms. This migration of larvae can cause mechanical damage and inflammatory reactions in the affected tissues.

Toxocara larvae can remain alive for several months in human tissues, causing various symptoms depending on the organs affected. Toxocariasis is therefore a cosmopolitan zoonosis, mainly affecting young children because of high-risk behaviour such as geophagy. Prevalence is high in both industrialised and developing countries.

Adult Toxocara canis worms can measure up to 20 cm in length. The life cycle of these parasites is particularly complex and includes phases of migration and encystment in various tissues, which explains the diversity of symptoms observed in infected hosts.

How does this disease manifest itself in animals?

Toxocariasis mainly affects dogs and, more rarely, cats. Young animals, particularly puppies, are the most likely to be infested with Toxocara canis. Adults can also be infected, but symptoms are often less severe.

Puppies infested with Toxocara canis show more marked clinical signs than kittens. Symptoms include slowed growth: infested puppies often have an abnormal growth curve and may remain small for their age. Gastrointestinal problems such as vomiting, diarrhoea alternating with constipation, and abdominal distension are common. Dull coats and asthenia (loss of vitality) are common in infested puppies. Appetite abnormalities, including loss of appetite or increased appetite to compensate for the nutritional deficit caused by parasites, also occur.

Transmission of toxocariasis in animals occurs mainly via the digestive tract, via ingestion of embryonated eggs present in the excrement of puppies and kittens. The eggs become infectious after one to three weeks in a favourable environment. Transmission can also occur through ingestion of animal organs infested with encysted larvae. Puppies are often contaminated from birth via the transplacental route or through breast-feeding.

Young animals that have not been wormed are particularly at risk, and infested puppies can become a major source of environmental contamination.

Toxocara-infected animals can present a variety of symptoms, from asymptomatic infestation to severe clinical signs, particularly in cases of high parasite load. In some severe cases, massive infestation can lead to intestinal obstruction due to the formation of worm balls, requiring urgent veterinary intervention.

How is it transmitted?

Toxocariasis is transmitted mainly via the digestive tract. Humans, particularly children, become infected by ingesting embryonated eggs present in the soil or on soiled plants.

Methods of contamination include ingestion of embryonated eggs present in contaminated soil. These eggs can be ingested accidentally when children put soiled hands in their mouths after playing in contaminated sandboxes or gardens. Vegetables, particularly salads, grown in contaminated soil and insufficiently washed, can also be a source of contamination.

Occupational contamination is rare but possible. People working in contact with environments contaminated by dog excrement, such as urban cleaners, gardeners, breeders and vets, are also at risk.

Children, especially those aged 2 to 7, are the most vulnerable due to geophagy (pica) and playing in soiled environments. Young children tend to explore their environment by putting objects and hands in their mouths, which increases the risk of ingesting Toxocara eggs.

Toxocara eggs are extremely hardy and can survive in the soil for months or even years, increasing the risk of ongoing contamination. Once ingested, the eggs hatch in the host’s intestine, releasing larvae that migrate through various organs. In humans, these larvae generally fail to develop into adult worms, but they can cause significant damage by migrating through tissues.

Transmission of toxocariasis is therefore mainly fecal-oral, with infected animals playing a key role in contaminating the environment. Puppies, in particular, are a major source of Toxocara eggs in the environment.

What are the symptoms of this infection in humans?

Human toxocariasis is often asymptomatic, but it can lead to various clinical manifestations depending on the migration of the larvae and the organs affected. Symptoms vary according to the parasite load and the host’s immune response.

Common symptoms

Allergic symptoms are common and can resemble asthma, with episodes of coughing, wheezing and hives. These symptoms are due to the immune response against the migrating larvae.

Ocular damage, known as ocular toxocariasis, can cause reduced vision and strabismus, as a result of a larva penetrating the eye. A single larva can be enough to trigger a local inflammatory reaction, causing uveitis or chorioretinitis. Eye damage can lead to partial or total loss of vision if not treated quickly.

Visceral forms of toxocariasis, known as visceral larva migrans, mainly affect children aged 2 to 7. Visceral larva migrans can reach the liver and lungs, causing symptoms such as fever, cough, hepatomegaly (enlargement of the liver) and breathing difficulties. These symptoms are the result of the larvae migrating through the tissues and the resulting immune response.

Severe forms

Neurological toxocariasis occurs when the larvae reach the central or peripheral nervous system. Symptoms can include fever, headaches, epilepsy, meningoencephalitis and other serious neurological manifestations. This form of the disease is rare but can have devastating consequences.

Visceral larva migrans includes symptoms such as fever, anorexia, hepatosplenomegaly (enlargement of the liver and spleen), skin rashes, pneumonia and asthmatic symptoms. These symptoms are more frequent in children with a history of geophagia.

Toxocariasis is often benign and asymptomatic, but it can lead to serious complications depending on the parasite load and theimmune status of the host. Serious forms of the disease, although rare, require urgent and specialised medical treatment. Raising awareness of the symptoms and implementing preventive measures are essential to reduce the incidence and severity of this zoonosis.

How is the disease diagnosed?

Diagnosis of toxocariasis in humans is complex due to the polymorphism of symptoms and the absence of specific clinical signs. Clinical manifestations vary according to the organs affected, making diagnosis based on symptoms alone difficult.

Blood tests are often the first step in diagnosing toxocariasis. Hypereosinophilia, hyperleukocytosis and high levels of immunoglobulin E (IgE) may indicate parasitic infection, although these findings are not specific to toxocariasis. These blood abnormalities can also be seen in other parasitic or allergic infections.

Serology is essential to confirm the diagnosis of toxocariasis. Detection of anti-Toxocara antibodies by enzyme-linked immunosorbent assay (ELISA) and confirmation by western blot identify an active infection. However, the presence of antibodies can also indicate past exposure and a resolved infection, which can complicate interpretation of the results.

Medical imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), can reveal lesions in affected organs, such as nodules in the liver or lung infiltrates. These imaging techniques are particularly useful for detecting the visceral complications of toxocariasis.

Biopsies may sometimes be necessary to confirm the presence of larvae in tissues, although this procedure is invasive and rarely performed. Direct microscopic visualisation of larvae in biopsy samples or in body fluids such as cerebrospinal fluid can provide definitive proof of infection. However, the probability of obtaining tissue containing a Toxocara larva is low, depending on the larval load and the stage of infection.

What is the appropriate treatment?

Treatment for toxocariasis varies according to the clinical form and severity of the symptoms. Doctors often prescribe anthelmintics to eliminate the parasites. They commonly use anthelmintics such asalbendazole (400 mg twice a day for 5 days) and mebendazole (100 to 200 mg twice a day for 5 days) to treat toxocariasis. These drugs effectively kill Toxocara larvae and reduce the parasite load.

To reduceinflammation, especially in serious cases or when symptoms are severe, doctors administer corticosteroids such as prednisone (20 to 40 mg a day). Corticosteroids can help control immune reactions and relieve the allergic and inflammatory symptoms associated with larval migration.

Antihistamines can be used to relieve symptoms of pruritus and skin rashes. These drugs help to reduce allergic reactions and improve the comfort of patients suffering from toxocariasis.

For ocular toxocariasis, ophthalmological expertise is essential. Local and oral corticosteroids are needed to reduce inflammation in the eye. Specialists consider laser photocoagulation and cryosurgery to treat larvae in the retina and prevent permanent damage to vision.

Prophylaxis against recontamination is often the best treatment. Regular worming of pets and the adoption of rigorous hygiene practices can reduce the risk of infection. Individual and collective preventive measures play a crucial role in the management of toxocariasis.

What preventive measures are available?

Toxocariasis prevention is based on a number of measures designed to limit environmental contamination by Toxocara eggs and reduce the risk of infection in humans and animals. For animals, regular deworming is essential. Puppies should be wormed from two weeks of age, then every fortnight until they are eight weeks old, and then monthly until they are six months old. Adult dogs should be wormed four times a year. Pregnant and lactating mothers should also be treated. It is crucial to systematically collect the excrement after each worming session to eliminate the eggs before they become infectious.

For humans,general hygiene is the first line of defence. It is important to restrict the roaming of dogs in public areas and to systematically collect their excrement. Daily cleaning of premises where animals are kept is essential to reduce contamination. Dog owners must be made aware of these practices to protect their health and that of those around them.

Exposed workers, such as urban cleaners, gardeners, breeders and vets, must be trained in the risks and hygiene measures. Hygiene facilities, such as drinking water, soap and single-use wiping equipment, must be available. Separate lockers for street clothes and work clothes are necessary to avoid cross-contamination. Wearing gloves when collecting excrement and cleaning premises is also essential.

Compliance with personal hygiene rules is crucial. Washing your hands systematically with soap and drinking water after any contact with animals, waste or animal faeces, and before meals or breaks, is essential.

Some epidemiological data…

Toxocariasis is found throughout the world. Epidemiological studies show that prevalence rates vary from region to region. In urban areas of Western countries, prevalence varies from 2% to 5% in healthy adults, while in rural areas it can be as high as 14.2% to 37%.

In tropical countries, prevalence rates reach even higher levels. For example, 63.2% in Bali, 86% in Saint Lucia and 92.8% in La Réunion. These figures reflect the high prevalence of toxocariasis in regions wherehygiene conditions are less strict and pets are often left undewormed.

Sero-epidemiological surveys show that human toxocariasis is one of the most common helminthiases in the world. In G8 countries, seroprevalence varies from one area to another: it exceeds 35-42% in rural areas, reaches 15-20% in semi-rural areas, and remains between 2-5% in urban areas.

In developing countries, seroprevalence rates are even higher. For example, 30% in Nigeria, 36% in Brazil, 44.6% in Swaziland, 58% in Malaysia, 63.2% in Indonesia, 81% in Nepal, 86.8% in the Marshall Islands and 93% in Réunion. These data show widespread exposure to infection by Toxocara canis and Toxocara cati.

The different methods used to detect infection, such as Western blot or ELISA, may bias the analysis of seroprevalence between different countries and studies. In addition, antibody titration thresholds and the difficulty of linking infection to symptomatic disease can also influence results.