What is the cause of toxocariasis?

Toxocariasis: Causes and Transmission

Toxocariasis is caused by infection with larvae of the roundworm parasites Toxocara canis and Toxocara cati, which are intestinal parasites of dogs and cats, respectively. 1, 2

Primary Sources of Infection

• Humans become infected primarily through the oral route by accidental ingestion of embryonated Toxocara eggs present in contaminated soil, food, water, or from unwashed hands 3, 1
• The eggs are shed in dog and cat feces and become infectious within weeks after being deposited in the environment (e.g., sandboxes, parks, public beaches) 1, 2
• Children are at particularly high risk due to their play habits and tendency to put contaminated fingers in their mouths 2, 4

Specific Routes of Transmission

• Contact with soil contaminated with Toxocara eggs from cat or dog feces:

  • Touching mouth with hands after gardening
  • Cleaning a cat's litter box
  • Touching anything that has come into contact with cat or dog feces
  • Eating unwashed fruits or vegetables contaminated with soil 3

• Ingestion of larvae from tissues of paratenic hosts (animals that carry the parasite but are not the definitive host):

  • Consumption of undercooked meat from infected animals 2, 5

Risk Factors

• Young age (children under 5 years are at highest risk) 6, 2
• Living in areas with high stray dog or cat populations 2
• Poor hygiene practices 4
• Living in tropical or subtropical regions, though infection occurs worldwide 2
• Socioeconomic disadvantage is associated with higher seroprevalence 2, 5

Pathophysiology

• After ingestion, Toxocara eggs hatch in the intestine and release larvae that penetrate the intestinal wall 7
• Unlike in dogs and cats (definitive hosts), the larvae cannot complete their life cycle in humans 1
• Instead, larvae migrate through various tissues causing inflammation and damage, resulting in different clinical syndromes:
  • Visceral larva migrans (VLM): larvae migrate to organs like the liver, lungs, and brain 8, 2
  • Ocular larva migrans (OLM): larvae migrate to the eye 9, 4
  • Neurotoxocariasis: larvae invade the central nervous system 3, 8
  • Covert toxocariasis: milder, non-specific symptoms 2, 5

Clinical Manifestations

• Visceral larva migrans presents with fever, eosinophilia, wheeze, cough, abdominal pain, and hepatosplenomegaly 8
• Ocular toxocariasis presents with visual changes, retinal granuloma, or uveitis, with eosinophilia being less common in isolated ocular disease 3, 9
• Neurotoxocariasis can present with myelitis, encephalitis, or meningitis 3, 8
• Many infections remain asymptomatic despite serological evidence of exposure 2, 5

Prevention Strategies

• Regular deworming of pets, especially puppies and kittens 2
• Proper disposal of pet feces 4
• Good personal hygiene, particularly handwashing after outdoor activities 2, 4
• Covering sandboxes when not in use to prevent contamination by animals 1
• Washing fruits and vegetables thoroughly before consumption 3
• Avoiding consumption of undercooked meats 2

Common Pitfalls in Diagnosis

• Ocular toxocariasis may occur without systemic eosinophilia, making diagnosis challenging 9, 8
• Many infections remain asymptomatic or cause non-specific symptoms that may not prompt testing 2, 5
• Diagnosis often requires specialized serological, molecular, or imaging tests that may not be readily available 2