Introduction

Toxocariasis is a zoonotic disease caused by the larvae of Toxocara canis and Toxocara cati, the common roundworms of dogs and cats, respectively. Humans become accidental hosts through the ingestion of embryonated eggs from contaminated environments, leading to various clinical manifestations that depend on the affected organs. While toxocariasis is a significant health concern on its own, its interaction with other parasitic infections can complicate diagnosis, treatment, and overall disease burden. This article delves into the complexity of toxocariasis, its co-occurrence with other parasitic diseases, and its implications for public health.

Toxocariasis - overview 

Lifecycle of toxocara

The lifecycle of Toxocara begins when adult worms residing in the intestines of definitive hosts (dogs for T. canis and cats for T. cati) release eggs into the environment through faeces. These eggs become infectious after embryonation, which takes approximately two to three weeks under favourable conditions. Humans, especially children, can ingest these embryonated eggs through contaminated soil, unwashed hands, or improperly washed vegetables. Once ingested, the larvae hatch in the small intestine and migrate through various tissues, leading to different clinical manifestations depending on their location and the host's immune response.^1,2

Clinical manifestations

1. Visceral larva migrans (VLM): This syndrome occurs when larvae migrate through major organs such as the liver, lungs, and heart. Symptoms include fever, hepatomegaly, pulmonary manifestations such as cough and wheezing, and marked eosinophilia
2. Ocular larva migrans (OLM): When larvae localise in the eye, they can cause granulomatous inflammation, promoting visual disturbances, strabismus, or even blindness. OLM typically presents unilaterally and is more common in older children and young adults^3,4
3. Covert toxocariasis:  It shows nonspecific symptoms such as abdominal pain, headache, and behavioural disturbances, which engender challenges to diagnosis without specific testing

Diagnosis and detection

Toxocariasis diagnosis involves a collaboration of clinical evaluation, serological testing, and imaging studies. Enzyme-linked immunosorbent assay (ELISA) is commonly used for serodiagnosis targeting Toxocara excretory-secretory antigens. Imaging modalities like ultrasound or MRI can assist in identifying organ involvement, especially in cases of VLM or OLM.⁵

Treatment options

The management of toxocariasis includes antiparasitic therapy and supportive care. Albendazole and mebendazole are the drugs of choice, often administered for 5 to 14 days. Corticosteroids may be prescribed to manage severe inflammatory responses, particularly in cases that concern significant organ involvement or ocular disease.

Co-infection of toxocara with other parasitic infections

Parasitic co-infections

In resource-limited regions, co-infections with multiple parasites are common due to overlapping transmission routes, such as contaminated soil, water, and inadequate sanitation. These co-infections can lead to compounded health issues, complicating diagnosis and treatment strategies.

Interactions with other parasites

1. Toxocariasis and soil-transmitted helminths (STH)

Co-infection with Toxocara and other soil-transmitted helminths, such as Ascaris lumbricoides, Trichuris trichiura, and hookworms, is prevalent in tropical and subtropical regions. They can exacerbate malnutrition and anaemia due to the combined nutritional burdens imposed by the parasites. Additionally, the immune responses induced by these helminths can modulate each other's survival and pathogenicity within the host.^{6, 7}

2. Toxocariasis and protozoan infections

Interactions between Toxocara and protozoan parasites such as Giardia lamblia and Entamoeba histolytica can intensify gastrointestinal symptoms. The concurrent presence of these parasites increases gut inflammation, diarrhoea, and malabsorption, further compromising the nutritional status of patients.

3. Toxocariasis and malaria (Plasmodium species)

The coexistence of Toxocara and Plasmodium species, the pathogens of malaria, can influence disease outcomes. Immune modulation by Toxocara may affect the severity of malaria, potentially altering the host's susceptibility and response to antimalarial treatments. Overlapping clinical features, such as fever and hepatosplenomegaly, can also complicate the differential diagnosis.⁸

4. Toxocariasis and schistosomiasis

Co-infection with Schistosoma species enhances liver pathology due to the additive effects of both parasites. The immune responses triggered by Toxocara may exacerbate schistosomiasis-related morbidity, including increased fibrosis and portal hypertension.

5. Toxocariasis and cysticercosis (Taenia solium larval infection)

When Toxocara co-infects with Taenia solium larvae, particularly in the central nervous system, there is a high risk of neurological complications. The coordination of these parasites leads to severe inflammatory responses, increasing the likelihood of seizures and other neurological deficits.

Immunological aspects of co-infection

Host immune response to Toxocara

The human immune response to Toxocara infection is characterised by a Th2-type response, triggering eosinophil activation and cytokine production such as interleukin (IL)-4, IL-5, and IL-13. This response aims to contain and eliminate the migrating larvae, but it can also result in tissue damage and allergic manifestations.⁹

Impact on immune response in co-infections

In co-infection scenarios, the immune response to one parasite can influence the host's susceptibility and reaction to another. For instance, the Th2-skewed response induced by Toxocara may suppress Th1-mediated immunity, potentially increasing vulnerability to intracellular pathogens. Conversely, some parasites may modulate the immune system to create a more favourable environment for survival, complicating the host's ability to control multiple infections simultaneously.

Public health concerns and control measures

Challenges in the diagnosis and treatment of co-infections

One of the significant obstacles in managing toxocariasis co-infections is the struggle to diagnose multiple parasites simultaneously. Serological tests such as ELISA and Western blot are specific for Toxocara, but may not detect other co-infecting parasites, requiring additional diagnostic procedures. Moreover, symptoms such as fever, hepatomegaly, eosinophilia, and gastrointestinal disturbances overlap with several other parasitic infections, leading to frequent misdiagnosis or underdiagnosis.

Treatment presents another challenge, as antiparasitic medications like albendazole or mebendazole, which are commonly used for toxocariasis, may not be effective against other co-infecting parasites such as Plasmodium, Schistosoma, or Giardia. Combination therapy is often required, but drug interactions and potential side effects must be carefully managed. For example, corticosteroids are sometimes used to control severe inflammation in toxocariasis, but their immunosuppressive effects could worsen other infections.¹⁰

Preventive strategies

Hygiene and sanitation

Since Toxocara eggs are transmitted through contaminated soil, water, and food, proper sanitation is the first line of defence. Measures such as:

• Regular deworming of pet dogs and cats to prevent shedding of Toxocara eggs
• Handwashing and hygiene education, particularly for children who play in soil
• Proper disposal of pet faeces in urban and rural settings
• Washing fruits and vegetables before consumption

Deworming programs

Mass drug administration (MDA) programs succeed in reducing disease prevalence in endemic areas through targeting multiple parasitic infections.  For example, the WHO recommends deworming children at regular intervals in regions where soil-transmitted helminths, including Toxocara, are common.¹¹

Health education and awareness campaigns

Public awareness is crucial, especially in high-risk populations, such as children, farmers, and pet owners. Community outreach programs, school-based education, and veterinary involvement can help reduce exposure risks and improve compliance with preventive measures.¹²

Vaccination research

Although no vaccine currently exists for toxocariasis, research into anti-parasitic vaccines targeting Toxocara larvae and other helminths is ongoing. Advances in molecular parasitology could pave the way for multivalent vaccines that protect against multiple parasites simultaneously.

Future research directions

Need for more studies on co-infections

The interactions between Toxocara and other parasites remain underexplored, particularly in terms of how immune responses influence co-infection dynamics. Future studies should focus on:

1. The role of the gut microbiome in shaping parasitic co-infections.
2. Long-term effects of chronic co-infections on child development and cognitive function.
3. Geographical and epidemiological trends, identifying high-risk populations.

Development of multi-targeted treatments

Given the burden of multiple parasitic infections in low-resource settings, research should focus on:

• Broad-spectrum anti-parasitic medications are effective against helminths and protozoa.
• Combination therapies that minimise drug resistance and enhance efficacy.
• Host-directed therapies that modulate the immune response rather than directly targeting parasites.

Summary

Toxocariasis is a significant zoonotic parasitic infection with widespread global implications. Beyond its impact, it frequently coexists with other parasitic infections, leading to compound health risks, diagnostic challenges, and treatment complexities. Co-infections with soil-transmitted helminths, protozoa, malaria, and schistosomiasis highlight the need for integrated disease management approaches.

Public health interventions, including sanitation improvements, deworming programs, and health education, are crucial to prevent transmission. Meanwhile, research into immune interactions, multi-target treatments, and potential vaccines will be critical for reducing the burden of parasitic co-infections worldwide.

By addressing toxocariasis and its interactions with other parasitic diseases holistically, we can develop better disease management strategies and improve health outcomes in affected populations.