Toxocara Species - Toxocariasis

Toxocariasis is caused by the parasitic roundworms Toxocara canis and T. cati, which are often found in the intestines of dogs and cats, respectively. Although anyone can be infected with Toxocara, infection is more common in young children and in people who own or live with cats and dogs. Infection most often occurs when people come into contact with Toxocara eggs that have been shed in animal feces. Infection is also possible through ingestion of infected meat or offal. Toxocariasis is often asymptomatic. When present, symptoms may manifest ocularly or viscerally. Ocular toxocariasis (OT) occurs when larvae migrate to the eyes. Symptoms include vision loss, eye inflammation, and retinal damage. Visceral toxocariasis (VT) occurs when larvae migrate to organs such as the liver or central nervous system (CNS). Symptoms include fever, fatigue, coughing, and abdominal pain. Toxocariasis can be difficult to diagnose because its symptoms are often nonspecific. Diagnosis of VT and OT is based on clinical evaluation, patient history, and laboratory testing. The recommended laboratory test is serology to detect antibodies against Toxocara. 

Quick Answers for Clinicians

How is toxocariasis diagnosed?

Toxocariasis diagnosis should be based on a detailed clinical evaluation to identify potential signs and symptoms of infection. Additionally, patient history should be obtained to determine a potential source of exposure. Finally, laboratory testing should be used to find supportive evidence of infection. Serology is the preferred laboratory test for diagnosis. In patients with ocular toxocariasis (OT), serum antibody concentrations may be low or absent even in the presence of clinical disease. In these cases, antibodies may be detected in aqueous or vitreous fluid samples from the affected eye. 

How is toxocariasis spread?

Toxocara eggs are shed by infected animals (usually dogs and cats) in their feces. Over the next 2-4 weeks, larvae develop inside the eggs and become infectious. These may be ingested by humans or other animals such as rabbits or pigs, causing infection. In rare cases, ingestion may occur from consumption of undercooked or raw meat from an infected animal such as a rabbit or lamb. Once ingested, the eggs hatch and the larvae travel to different parts of the body (eg, liver, heart, or eyes) through the bloodstream. Generally, symptoms are caused by the migration of larvae. 

What should be considered in the differential diagnosis of toxocariasis?

Other helminths may cause infections with signs and symptoms that mimic visceral toxocariasis (VT) and ocular toxocariasis (OT). These include Baylisascaris procyonis (infection resembles both VT and OT), Strongyloides spp (infection mimics VT), and Paragonimus spp (infection resembles VT). 

Indications for Testing

Testing should be considered in patients with signs and symptoms of OT or VT and a patient history that includes potential exposure to Toxocara eggs. This history may include coming into contact with animal feces or ingesting undercooked meat or offal from an animal such as lamb or rabbit. 

Laboratory Testing

Serology to detect Toxocara antibodies is recommended to aid in the diagnosis of toxocariasis (OT and VT). This assay is generally very specific, but cross-reactivity with antibodies against Ascaris lumbricoides (a human roundworm) is possible. A positive Toxocara antibody test result should be interpreted in the context of a patient’s clinical evaluation and patient history. Seropositivity may occur in patients who have been infected with Toxocara in the past or who have an active, asymptomatic infection. Observation of a significant increase in antibody concentration between paired acute and convalescent samples may be useful to confirm an active infection.

Toxocara antibody concentrations are often low in cases of OT. In some cases, the antibodies may be detected in the aqueous or vitreous fluid samples from the affected eye.

ARUP Laboratory Tests

Aids in diagnosis of toxocariasis

If results are equivocal, repeat testing in 10-14 days

References

Medical Experts

Contributor

Jackson

Brian R. Jackson, MD, MS
Associate Professor of Pathology (Clinical); Adjunct Associate Professor, Biomedical Informatics, University of Utah
Medical Director, Support Services, IT, and Business Development, ARUP Laboratories