Toxocariasis: Symptoms, Treatments, Medications and Prevention

Toxocariasis is a parasitic infection caused by the larvae of Toxocara canis and Toxocara cati, which are intestinal roundworms found in dogs and cats, respectively. The disease occurs when humans accidentally ingest the eggs of these parasites, leading to a condition known as visceral larva migrans or ocular larva migrans, depending on the organs affected. Though most commonly seen in children, toxocariasis can affect people of all ages and is considered a neglected tropical disease (NTD) by the World Health Organization (WHO). Toxocariasis can cause a range of symptoms from mild to severe, depending on the organ systems involved and the immune response.

What is Toxocariasis?

The Causative Agent: Toxocara Species

Toxocariasis is caused by two species of roundworms:

• Toxocara canis: Found in dogs and is the most common cause of human toxocariasis.
• Toxocara cati: Found in cats and is less commonly involved in human infections.

These parasites have a complex life cycle that involves definitive hosts (dogs and cats) and accidental human hosts. In animals, the adult worms live in the intestines and shed eggs in the feces. Once excreted into the environment, the eggs can survive in the soil for long periods. Humans can become infected when they accidentally ingest the infective eggs through contaminated hands, food, or water.

Life Cycle of Toxocara

The life cycle of Toxocara is complex, involving both definitive hosts (dogs and cats) and humans as accidental hosts.

1. Egg shedding: Adult female worms in the intestines of infected dogs and cats produce eggs, which are passed into the environment through the animal’s feces.
2. Egg maturation: The eggs become infectious after 2 to 4 weeks in the environment, typically in soil or sand contaminated with animal feces.
3. Human infection: Humans become infected when they accidentally ingest infectious Toxocara eggs. This can occur through contaminated soil, food, or surfaces.
4. Larval migration: Once ingested, the eggs hatch into larvae in the human intestines. Since humans are not the parasite’s natural host, the larvae cannot complete their life cycle. Instead, they migrate through various tissues and organs, causing damage as they go.
5. Larval persistence: In humans, the larvae can persist in tissues for months or years, leading to inflammatory responses and organ damage, known as larva migrans.

Modes of Transmission

Humans contract toxocariasis through accidental ingestion of Toxocara eggs. The primary modes of transmission include:

• Ingestion of contaminated soil or sand: Children playing in contaminated sandboxes or parks, where infected animals have defecated, are at high risk of ingesting the eggs.
• Contact with contaminated surfaces: Touching surfaces that have come into contact with infected feces and then touching the mouth can lead to infection.
• Consuming contaminated food or water: Eating fruits, vegetables, or other foods that have been contaminated with soil containing Toxocara eggs can result in infection.

Who is at Risk of Toxocariasis?

Geographic Risk Factors

Toxocariasis is a worldwide infection, but it is more prevalent in regions with poor sanitation, high populations of stray dogs and cats, and warm, humid climates. The parasite’s eggs can survive in the environment for extended periods under such conditions.

Occupational and Behavioral Risk Factors

Several groups are at higher risk of contracting toxocariasis due to increased exposure to contaminated environments:

1. Children

Children, particularly those aged 1 to 4 years, are the most at-risk group for toxocariasis. Young children are more likely to play in sandboxes or dirt, and they often put their hands in their mouths after playing, increasing the likelihood of ingesting Toxocara eggs. Children in urban settings with many stray dogs or cats are especially vulnerable.

2. Dog and Cat Owners

Individuals who own or frequently interact with dogs and cats, especially those who do not regularly deworm their pets, are at higher risk of exposure to Toxocara eggs. Pet feces that is not promptly cleaned up can contaminate the surrounding environment.

3. Gardeners and Landscapers

People who work with soil, such as gardeners or landscapers, may come into contact with contaminated soil, particularly in areas where stray animals are common.

4. Hunters and Farmers

Hunters and farmers who work in rural environments with high populations of feral or stray dogs and cats are at increased risk of exposure to contaminated soil or water.

5. Individuals Living in Poverty

Communities with poor sanitation and large populations of stray animals are at higher risk for toxocariasis. Open defecation by animals in these areas increases the likelihood of environmental contamination with Toxocara eggs.

6. Travelers

Travelers visiting countries with high rates of toxocariasis, especially those involving outdoor activities, may be at risk if they come into contact with contaminated soil or surfaces.

Symptoms of Toxocariasis

The symptoms of toxocariasis vary widely, depending on the number of larvae, the organ systems involved, and the immune response of the infected individual. The infection can range from asymptomatic to severe, with symptoms often classified into two main forms: visceral larva migrans (VLM) and ocular larva migrans (OLM).

1. Visceral Larva Migrans (VLM)

Visceral larva migrans is the more common form of toxocariasis and occurs when the larvae migrate through various internal organs, including the liver, lungs, and heart. VLM is primarily seen in young children and can cause a range of systemic symptoms.

Symptoms of VLM:

• Fever: Often one of the first signs, accompanied by general malaise.
• Cough and wheezing: As the larvae migrate through the lungs, individuals may develop respiratory symptoms, including a persistent cough, wheezing, or even pneumonitis.
• Abdominal pain: The migration of larvae through the intestines and liver can cause abdominal pain or tenderness.
• Hepatomegaly: Enlarged liver is common due to inflammation caused by the larvae. In some cases, the spleen may also become enlarged (splenomegaly).
• Fatigue: Due to the systemic nature of the infection, individuals may experience prolonged fatigue and weakness.
• Rashes: Some individuals may develop itchy skin rashes (urticaria) as part of an allergic reaction to the migrating larvae.
• Encephalitis or seizures: In rare, severe cases, larvae can migrate to the central nervous system, leading to seizures or encephalitis.

2. Ocular Larva Migrans (OLM)

Ocular larva migrans occurs when Toxocara larvae migrate to the eye, leading to inflammation and damage to ocular tissues. OLM is more common in older children and adults and may affect one or both eyes.

Symptoms of OLM:

• Blurred vision: One of the most common symptoms is blurry vision in one eye.
• Eye pain: The inflammation caused by the larvae can lead to discomfort or pain in the affected eye.
• Redness and tearing: Some individuals may develop red eyes and excessive tearing.
• Strabismus (cross-eye): In severe cases, OLM can cause strabismus, or misalignment of the eyes, particularly in children.
• Vision loss: If untreated, OLM can lead to permanent vision loss in the affected eye due to retinal damage or scarring.

3. Asymptomatic Toxocariasis

In many cases, toxocariasis remains asymptomatic, especially in individuals with a low parasite load or a robust immune response. These individuals may not show any obvious symptoms but can still have mild systemic inflammation caused by the presence of migrating larvae.

Diagnosis of Toxocariasis

Diagnosing toxocariasis can be challenging, as the symptoms are nonspecific and can resemble other conditions. A combination of clinical evaluation, laboratory tests, and imaging is typically required to make an accurate diagnosis.

1. Clinical Evaluation

A detailed medical history is essential for diagnosing toxocariasis, especially in individuals with risk factors such as recent exposure to pets, contaminated soil, or travel to endemic areas. The physician will evaluate symptoms such as fever, abdominal pain, cough, and blurred vision in cases of ocular involvement.

2. Serological Tests

Serological testing is the most common and reliable method for diagnosing toxocariasis. These tests detect antibodies to Toxocara larvae in the patient’s blood.

1. Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA is the primary test used to detect antibodies to Toxocara larvae. A positive result suggests active or recent infection. However, a positive serology does not always indicate disease, as asymptomatic individuals may also test positive, especially in endemic regions.

2. Western Blot

Western blot testing can be used to confirm ELISA results in cases where the diagnosis is uncertain. It is more specific than ELISA and helps rule out cross-reactivity with other parasitic infections.

3. Complete Blood Count (CBC)

A complete blood count may reveal eosinophilia (elevated levels of eosinophils, a type of white blood cell), which is commonly seen in parasitic infections. Eosinophilia is a hallmark of VLM and can be an important clue in diagnosing toxocariasis.

4. Imaging Studies

In cases where visceral organs or the eye are affected, imaging studies may be necessary to assess the extent of the damage caused by migrating larvae.

1. Ultrasound

Ultrasound imaging of the liver may reveal hepatic lesions or inflammation caused by migrating larvae in individuals with VLM.

2. Chest X-ray

A chest X-ray may show evidence of pneumonitis or pulmonary infiltrates, which can occur when larvae migrate through the lungs.

3. Ophthalmological Examination

An eye exam using fundoscopy or ophthalmoscopy may reveal retinal damage, granulomas, or other lesions associated with OLM. In some cases, imaging techniques such as optical coherence tomography (OCT) may be used to assess retinal damage.

5. Biopsy

In rare cases where a definitive diagnosis is needed, a biopsy of affected tissues (such as liver or lung) may be performed to directly visualize the Toxocara larvae. However, this invasive procedure is rarely necessary due to the availability of reliable serological testing.

Treatments for Toxocariasis

Antiparasitic Treatment

The mainstay of treatment for toxocariasis is antiparasitic medications aimed at killing the larvae. Treatment should be started as soon as the diagnosis is confirmed to prevent further tissue damage.

1. Albendazole

Albendazole is the drug of choice for treating toxocariasis. It works by inhibiting the parasites’ ability to absorb nutrients, ultimately killing the larvae.

• Dosage: Albendazole is typically given at a dose of 400 mg twice daily for 5 to 7 days.
• Side effects: Albendazole is generally well-tolerated, with mild side effects such as nausea, abdominal pain, and headache.

2. Mebendazole

Mebendazole is an alternative to albendazole and is also effective in treating toxocariasis.

• Dosage: The typical dosage is 100 mg twice daily for 5 to 7 days.
• Side effects: Similar to albendazole, common side effects include nausea, diarrhea, and abdominal discomfort.

Supportive and Symptomatic Treatment

In addition to antiparasitic therapy, supportive treatments may be necessary to manage inflammation and prevent long-term complications.

1. Corticosteroids

Corticosteroids are often prescribed in cases of severe toxocariasis, especially in individuals with significant inflammation or organ involvement. Corticosteroids help reduce the immune response to the migrating larvae, alleviating symptoms such as fever, abdominal pain, and lung inflammation.

• Dosage: Prednisone or another corticosteroid is typically prescribed in conjunction with antiparasitic medications to reduce inflammation.
• Side effects: Long-term use of corticosteroids can lead to side effects such as weight gain, increased blood pressure, and weakened bones.

2. Treatment for Ocular Toxocariasis

In cases of ocular larva migrans, additional treatments may be necessary to prevent vision loss.

• Laser therapy: Laser photocoagulation may be used to destroy larvae in the retina and prevent further damage.
• Surgery: In cases of severe retinal damage or detachment, vitrectomy or other surgical interventions may be necessary to restore vision.

Most Common Medications for Toxocariasis

The most common medications prescribed for toxocariasis include:

1. Albendazole

Albendazole is the first-line treatment for toxocariasis due to its high efficacy in killing Toxocara larvae. It is effective in treating both VLM and OLM.

2. Mebendazole

Mebendazole is an alternative antiparasitic drug that is used when albendazole is not available or when patients cannot tolerate it.

3. Corticosteroids

Corticosteroids such as prednisone are often used in conjunction with antiparasitic medications to reduce inflammation, especially in cases of severe VLM or OLM.

Where is Toxocariasis Most Prevalent?

Toxocariasis is found worldwide, but it is more common in regions where dogs and cats are widespread, and sanitation is poor. The following areas have the highest rates of infection:

1. North America

Toxocariasis is endemic in parts of the United States and Canada, especially in urban areas with large populations of stray animals. Studies have shown that children living in low-income urban environments, particularly in cities with high numbers of stray dogs and cats, are at increased risk.

2. Latin America

Countries in Latin America, including Brazil, Mexico, and Peru, report high rates of toxocariasis due to the prevalence of stray dogs and cats, as well as poor sanitation in rural and urban areas.

3. Africa

Toxocariasis is common in parts of sub-Saharan Africa, particularly in regions where stray animals are abundant, and access to veterinary care is limited.

4. Southeast Asia

Countries such as Thailand, Vietnam, and Indonesia have high rates of toxocariasis, particularly in areas where stray animals are common and sanitation is inadequate.

5. Eastern Europe

Toxocariasis is also found in Eastern Europe, particularly in rural communities where there is limited access to clean water and sanitation.

Prevention of Toxocariasis

Preventing toxocariasis requires a combination of public health interventions, improved sanitation, and personal protective measures. Key prevention strategies include:

1. Deworming Pets

Regular deworming of dogs and cats is one of the most effective ways to prevent toxocariasis. Veterinary care should include routine deworming of pets, especially puppies and kittens, as they are most likely to carry Toxocara parasites.

• Deworming schedule: Puppies and kittens should be dewormed every 2 to 3 weeks until they are 3 months old, and then once every 3 to 6 months thereafter.

2. Proper Disposal of Pet Feces

Pet owners should ensure that pet feces is promptly and properly disposed of to prevent contamination of soil and the environment. Cleaning up after pets in public areas and ensuring that animals do not defecate in sandboxes or play areas can reduce the risk of Toxocara eggs contaminating the environment.

3. Hand Hygiene

Good hand hygiene is essential in preventing the accidental ingestion of Toxocara eggs, particularly for children who play in sandboxes or areas frequented by animals. Key measures include:

• Washing hands with soap and water after playing outdoors or handling animals.
• Supervising children to prevent them from putting dirty hands or objects in their mouths.

4. Education and Public Awareness

Raising awareness about the risks of toxocariasis and the importance of deworming pets, proper hygiene, and sanitation can help reduce the spread of the disease. Public health campaigns should focus on:

• Educating communities about the importance of regular pet deworming and proper disposal of pet feces.
• Informing parents about the risks of allowing children to play in areas contaminated with animal feces.
• Promoting hand hygiene, especially in areas with high populations of stray animals.

5. Safe Play Areas

Ensuring that children have access to clean and safe play areas can reduce the risk of exposure to contaminated soil. Sandboxes should be covered when not in use to prevent animals from defecating in them, and play areas should be regularly cleaned and maintained.

6. Controlling Stray Animal Populations

Efforts to control the population of stray dogs and cats through spaying, neutering, and adoption programs can reduce the number of animals shedding Toxocara eggs into the environment. Reducing the stray animal population in urban and rural areas can significantly lower the prevalence of toxocariasis.