Introduction

Toxocariasis is a zoonotic disease, that is, an infectious disease that jumps from a non-human animal to humans.¹ This infection happens when people accidentally ingest the larvae of the roundworms Toxocara canis (dog roundworm) and Toxocara cati (cat roundworm).² These two animals pass the worm eggs in their faeces, and if faeces, soil or sand contaminated with these eggs, gets into humans’ mouths, they can get infected.³ With that being said, these parasites primarily infect dogs and cats, but humans can act as accidental hosts.

While most infections are asymptomatic, significant manifestations can occur, particularly, in children, highlighting the importance of understanding the aetiology of this infection.³

Aetiology and life cycle of Toxocara

The parasite Toxocara can have a life cycle with one host (direct life cycle) or with multiple hosts (indirect life cycle). The parasite’s embryonated eggs are expelled in the faeces of the host, which is the dog in Toxocara canis, and the cat in Toxocara cati. In a time period between 1 to 4 weeks, the eggs get embryonated in the environment, and in this stage, they become infective and in the form of a larva.⁴

These larvae can be ingested by dogs (Toxocara canis), or cats (Toxocara cati), and they travel through the lungs and respiratory system all the way to the gastrointestinal system. Here, the adult worms install themselves in the small intestine of the dog or cat. The female dogs can transmit the parasite during late gestation and may infect their pups, in whose small intestine adult worms become installed. In cats, the larvae can also be transmitted to kittens.⁴

With that being said, the life cycle of Toxocara canis involves several stages, including the egg, larva, and adult worm. The adult female worms live in the small intestine of the dog, where they produce eggs that are shed in the dog’s faeces.⁵ Afterwards, these eggs are embryonated in the environment, under suitable conditions of temperature and moisture, becoming infectious within a few weeks.⁶

The life cycle of Toxocara cati is similar to that in dogs. Adult female worms install themselves in the cat’s small intestine, producing eggs that are also released in the cat’s faeces. Similarly, these eggs are embryonated in the environment, becoming infective.

Humans can become accidental hosts that can be infected by ingesting infective eggs. This can happen through several routes:

• Geophagia: This is known as soil ingestion, and it is more common among children⁷ Contaminated soil in playgrounds, parks and gardens can have high concentrations of Toxocara eggs
• Hand-to-mouth contact: children might play in contaminated areas, and they frequently put their hands in their mouths, facilitating the ingestion of the eggs⁸
• Ingestion of contaminated food: unwashed fruits and vegetables that grow in contaminated soils and are consumed directly from the ground can transmit the infection⁹

The primary infection mechanism of Toxocara canis and Toxocara cati in humans is the accidental ingestion of embryonated eggs from contaminated sources. After human ingestion, the eggs and larvae enter the intestine and can be carried by the blood circulation to several organs, like the liver, heart, lungs, brain, muscles and eyes, leading to different clinical presentations, as will be discussed later.^4,10

Even though the majority of human infections are asymptomatic, the migrating larvae can lead to significant tissue damage, causing a range of clinical presentations.

Clinical presentations of toxocariasis

The principal clinical presentation of Toxocariasis are visceral larva migrans (VLM) and ocular larva migrans (OLM).⁴

The VLM occurs more commonly in preschool children, and in this case, the larvae invade multiple organs, such as the liver, lungs, muscles and the heart, occasionally. This can lead to several nonspecific symptoms like:^4,11

• Fever
• Muscle aches
• Weight loss
• Cough 
• Rashes

In OML, the larvae lead to several ophthalmologic lesions, such as inflammations, retinal detachment, and potential vision loss.^4,12 OML typically affects just one eye and occurs more frequently in older children or young adults, who uncommonly have visceral manifestations.⁴

In rare cases, larvae can migrate to the central nervous system, causing seizures, headaches and neurological deficits.^4,13

Pathogenesis and epidemiology

Toxocariasis is mostly caused by the host's immunological reaction to migratory larvae. The larvae cause an inflammatory reaction, as evidenced by the entry of eosinophils, neutrophils, and lymphocytes (some types of white blood cells) into the affected tissues. This inflammatory response might produce severe tissue damage, resulting in the observed clinical symptoms.¹⁴

Regarding the epidemiology, this infection is globally distributed, but it has a higher number of cases in areas with poor sanitation and high rates of pet ownership. As discussed before, children are particularly susceptible to Toxocariasis due to their frequent contact with soil.¹⁵

Prevention and control

To prevent toxocariasis, it is required a multifaceted approach:¹¹

• Pet hygiene: Regular deworming of dogs and cats to reduce egg shedding, as well as proper disposal of pet faeces to avoid environmental contamination
• Hygiene practices: Promoting good hand hygiene, particularly before eating, can greatly lower the risk of infection
• Education: Public health programs focused on informing the public about the dangers of Toxocariasis and preventive actions are critical
• Environmental sanitation: Improving sanitation techniques, such as regular playground and park cleaning, will help prevent contamination from Toxocara canis and Toxocara cati eggs

FAQs

What is Toxocariasis?

Toxocariasis is a zoonotic disease caused by the accidental ingestion of larvae from the roundworms Toxocara canis (dog roundworm) and Toxocara cati (cat roundworm).

How do humans get infected with Toxocariasis?

Humans become infected by accidentally ingesting embryonated eggs from contaminated sources, such as:

• Geophagia: Ingesting soil contaminated with eggs
• Hand-to-mouth contact: Transferring eggs from contaminated surfaces to the mouth
• Ingestion of contaminated food: Consuming unwashed produce or undercooked meat from infected animals

What are the symptoms of Toxocariasis?

Most infections are asymptomatic.

• Visceral larva migrans (VLM): Fever, cough, enlarged liver (hepatomegaly), eosinophilia, pulmonary infiltrates
• Ocular larva migrans (OLM): Inflammation, retinal detachment, vision loss
• Neurological larva migrans (NLM): Seizures, headaches, neurological problems (rare)

Who is at higher risk of Toxocariasis?

Children are at higher risk due to their frequent contact with soil and their tendency to engage in eating soil (geophagia).

How is Toxocariasis diagnosed?

Diagnosis typically involves:

• Blood tests: To detect antibodies against Toxocara
• Imaging tests: Such as ultrasound or CT scan, to assess organ involvement

How is Toxocariasis treated?

Treatment usually involves medications like albendazole or thiabendazole to kill the larvae.

Can Toxocariasis be prevented?

Yes, prevention strategies include:

• Pet hygiene: Regular deworming of pets and proper disposal of faeces
• Hand hygiene: Frequent handwashing, especially before eating
• Public education: Raising awareness about the risks and preventive measures
• Environmental sanitation: Maintaining clean play areas and improving sanitation practices

What are the long-term effects of Toxocariasis?

In most cases, there are no long-term effects. However, severe cases of OLM can lead to permanent vision loss.

Is Toxocariasis contagious between humans?

No, Toxocariasis is not contagious between humans.

Can pregnant women pass Toxocariasis to their unborn child?

While possible, it is rare for pregnant women to transmit it to their unborn child.

What animals can carry Toxocara?

Primarily dogs and cats, but other animals like foxes and raccoons can also be infected.

Summary

Toxocariasis is a zoonotic disease caused by the accidental ingestion of larvae from the roundworms Toxocara canis (dog roundworm) and Toxocara cati (cat roundworm). These parasites infect dogs and cats, respectively, and their eggs are shed in faeces.

The aetiology of toxocariasis centres around the transmission of these eggs to humans. Key routes of infection include ingestion of soil contaminated with eggs, particularly common in children; transfer of eggs from contaminated surfaces to the mouth; and consumption of unwashed produce or undercooked meat from infected animals. Once ingested, the eggs hatch in the human intestine, and the larvae migrate through various organs, including the liver, lungs, eyes, and brain. This larval migration triggers an immune response, leading to tissue damage and the development of clinical manifestations, such as visceral larva migrans (VLM) and ocular larva migrans (OLM).

Prevention strategies focus on regular deworming of pets and proper disposal of faeces; frequent handwashing, especially before eating; raising awareness about the risks of toxocariasis and preventive measures; maintaining clean play areas and improving sanitation practices.