Toxocara

Author: MBLOGSTU

Introduction

The nematode worms Toxocara canis and Toxocara cati were first described by Werner and Schrank (1788), respectively.

Three clinical syndromes have now been associated with human infection: visceral larva migrans, ocular larva migrans, and covert toxocariasis.

Because Toxocara does not mature in humans, exposure can only be assessed indirectly via serology. Seroprevalence is typically higher in children.

Morphology

Stage	T. canis	T. cati
Adult (female)	6–18 cm	4–12 cm
Adult (male)	4–10 cm	3–6 cm
Egg size	75–90 µm	65–75 µm
Larva size	400 µm × 18–21 µm	400 µm × 15–17 µm

Eggs are nearly spherical, sometimes oval, with a thick, rough pitted shell and dark brown to black granular contents. Morphologically indistinguishable under light microscopy.

Pathogenesis

Pathology depends on infection intensity and location. Migrating larvae particularly affect the eye and brain. Inflammation and larval toxins cause tissue damage.

Clinical Manifestations

Visceral Larva Migrans (VLM)

Predominantly in children. Diagnosis: eosinophilia >30%, history of eating dirt. Symptoms: geophagia, anaemia, cough, wheeze, pulmonary infiltrates, hepatosplenomegaly, fever, anorexia.

Ocular Larva Migrans (OLM)

Larva migrates to the eye, potentially causing blindness. Can affect retina and other intraocular structures. Usually unilateral in children, rarely bilateral or in adults.

Covert Toxocariasis

Atypical presentation (Taylor et al. 1988). Common features: abdominal pain, hepatomegaly, anorexia, nausea, vomiting, sleep/behavior disturbances, pneumonia, cough, wheeze, pharyngitis, cervical adenitis, limb pain.

Laboratory Diagnosis

Larvae remain in tissues and do not appear in feces. Diagnosis relies on detecting Toxocara-specific serum antibodies.

Antigens vary in specificity. Biopsy is rarely productive. Larvae have been found post-mortem.

Serological tests include:

• Skin tests
• Complement fixation
• Bentonite flocculation
• Larval precipitation
• Gel diffusion
• Capillary tube precipitation
• Indirect haemagglutination
• Immunofluorescence (direct or indirect)
• ELISA
• Radioimmunoassay

PCR-based methods can identify ascarid larvae in cats, dogs, and foxes and differentiate species (T. canis, T. cati, Toxascaris leonina).