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ToxocariasisSynonyms: toxocarosis, visceral larva migrans, ocular larva migrans, covert toxocariasis, Toxocara canis, Toxocara cati

Human toxocariasis is caused by migration of the larvae of Toxocara canis (dog roundworm) and less frequently of T. cati (catroundworm) through the human tissues. Toxocara are nematode parasites of animals. Despite cases of illness due to T. cati (which canbe difficult to distinguish from T. canis as they share many common antigens), the importance of this zoonosis has yet to be fullyestablished. [1]

Humans become infected by ingestion of eggs in soil contaminated by dog faeces. Larvae hatch out in the small intestine and migratevia the liver and lungs to other tissues, though they never mature in humans. In most cases the larvae are probably eliminated, but asurrounding granuloma may form in some patients. Rarely, T. cati can mature in humans and be transmitted by vomit or faeces.However, patients with adult T. cati don't have antecedent symptoms, eosinophilia or antibodies, suggesting that they are acquired byingestion of adult worms or advanced larval stages from cat vomit or faeces.

Life cycle [2]

The definitive hosts of T. canis are dogs, with humans acquiring the infection as accidental hosts. Unembryonated eggs areshed in the faeces of the definitive host. Following ingestion by dogs, the infective eggs hatch and larvae penetrate the gutwall.In younger dogs, the larvae migrate through the lungs, bronchial tree, and oesophagus. Adult worms develop and depositeggs in the small intestine. In older dogs, infections may occur but larval encystment in tissues is more common.Encysted stages are reactivated in female dogs during late pregnancy with transplacental and transmammary infection of thepuppies, in whose small intestine adult worms become established. Puppies are a major source of environmental eggcontamination.T. canis can also be transmitted by ingestion of other hosts: eggs ingested by small mammals (eg, rabbits) hatch and larvaepenetrate the gut wall and migrate into various tissues where they encyst. The life cycle is completed when dogs eat thesehosts and the larvae develop into egg-laying adult worms in the small intestine.Humans are accidental hosts who become infected by ingesting infective eggs in contaminated soil or infected animals suchas rabbits. After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a widevariety of tissues (liver, heart, lungs, brain, muscle, eyes). The larvae do not undergo any further development in these sitesbut they can cause severe local reactions that are the basis of toxocariasis.

EpidemiologyDistribution is worldwide. [3]

Exposure is common, especially among children from socio-economically disadvantaged populations both in the tropics andsub-tropics and in industrialised nations. [4]

Seroprevalence studies (particularly tropical and relatively undeveloped regions) show that the vast majority of the populationhas been exposed at some time. [5]

Many studies show contamination of children's play areas/sandpits with eggs of T. canis, in about 15-25% of thosesurveyed. [6]

Prevalence rates for visceral larva migrans are difficult to estimate, as the disease is largely asymptomatic in many, andpresents in a variety of ways. [7]

Risk factorsContact with soil contaminated with dog faeces; children are at greatest risk as they may ingest soil with contaminated eggsin playing areas.Cohabitation with dogs and cats.Eating without hand washing.Socio-economic deprivation.Rural dwelling.Travel to areas of high prevalence.

Presentation [8]

Studies suggest symptomatic illness may be due to induced autoimmunity associated with the infestation. Affected areasinclude liver, lungs, skin, joints, eyes, heart and brain.Many human infections are asymptomatic, with only eosinophilia and positive serology. The two main clinical presentations oftoxocariasis are visceral larva migrans and ocular larva migrans. [9]

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Visceral larva migrans:Occurs mostly in preschool children.The larvae invade multiple tissues and clinical features include fever, anorexia, abdominal pain, weight loss, cough,wheezing, pleural effusion, hepatosplenomegaly, urticaria, subacute meningitis/encephalitis, seizures,monoarthritis, cutaneous vasculitis, myocarditis and hypereosinophilia.Death (from severe cardiac, pulmonary or neurological involvement) may occur but is rare.

Ocular larva migrans: [3]

Often occurs in older children or young adults, with only rarely associated eosinophilia or visceral manifestations.Clinical features include uniocular decreased visual acuity or blindness, strabismus, seeing floaters or 'bubbles',peripheral retinal lesions, retinal detachment, peripheral retinal exudates, optic neuritis, vitreoretinal bands, uveitis,vitreous abscess.

Differential diagnosisVisceral form: other causes of acute hepatitis, adverse drug reactions, pulmonary eosinophilia, other helminth infestations,allergic bronchopulmonary aspergillosis, angio-oedema, hypereosinophilic syndrome, eosinophilic pneumonia, systemiclupus erythematosus.Ocular form: this should be investigated by ophthalmologists to exclude other possible causes. Some cases of ocular larvamigrans have been misdiagnosed as retinoblastoma, resulting in surgical enucleation.

Investigations [10]

FBC: leukocytosis with marked eosinophilia (20-80% of total white cell count). Eosinophilia is less common in ocular andoccult forms.Serology may be positive (in the ocular form, test vitreous fluids for antibodies). The currently recommended serology test fortoxocariasis is enzyme immunoassay. [11]

Ultrasound of the liver may show multiple hypoechoic areas.CXR may show nodular infiltrates if there is lung involvement.CT/MRI scan of the brain may show meningeal/cerebral involvement.Tissue biopsy may be necessary.

ManagementVisceral [4, 12]

Most patients recover without treatment.Anthelmintics. Albendazole or mebendazole are usually used in advanced or highly symptomatic cases or where there isorgan damage.Corticosteroids play a role in suppressing intense allergic manifestations of the disease.

Ocular [3]

Surgical intervention may be required.Corticosteroids have an important role.

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Prognosis and complicationsIn the ocular form, outcome is variable but uniocular visual loss is not uncommon.In the visceral form, outcome is usually good but marked organ damage and even death can occur in extreme cases.Epilepsy may be associated with toxocariasis. [13]

PreventionRemoval of pet faeces ('poop scoops', bins, and enforcement fines).Personal hygiene.Wearing gloves when gardening.Vegetables to be eaten raw should be washed.Regular worming of pets, especially puppies.

Further reading & references1. Fisher M; Toxocara cati: an underestimated zoonotic agent. Trends Parasitol. 2003 Apr;19(4):167-70.2. Toxocariasis; DPDx - Centers for Disease Control and Prevention3. Ahn SJ, Ryoo NK, Woo SJ; Ocular toxocariasis: clinical features, diagnosis, treatment, and prevention. Asia Pac Allergy. 2014 Jul;4(3):134-41. doi:

10.5415/apallergy.2014.4.3.134. Epub 2014 Jul 29.4. Macpherson CN; The epidemiology and public health importance of toxocariasis: a zoonosis of global importance. Int J Parasitol. 2013 Nov;43(12-

13):999-1008. doi: 10.1016/j.ijpara.2013.07.004. Epub 2013 Aug 14.5. Obwaller A, Duchene M, Walochnik J, et al; Association of autoantibodies against small nuclear ribonucleoproteins (snRNPs) with symptomatic

Toxocara canis infestation. Parasite Immunol. 2004 Aug-Sep;26(8-9):327-33.6. Chorazy ML, Richardson DJ; A survey of environmental contamination with ascarid ova, Wallingford, Connecticut. Vector Borne Zoonotic Dis. 2005

Spring;5(1):33-9.7. Good B, Holland CV, Taylor MR, et al; Ocular toxocariasis in schoolchildren. Clin Infect Dis. 2004 Jul 15;39(2):173-8. Epub 2004 Jun 22.8. Lee RM, Moore LB, Bottazzi ME, et al; Toxocariasis in north america: a systematic review. PLoS Negl Trop Dis. 2014 Aug 28;8(8):e3116. doi:

10.1371/journal.pntd.0003116. eCollection 2014 Aug.9. Despommier D; Toxocariasis: clinical aspects, epidemiology, medical ecology, and molecular aspects. Clin Microbiol Rev. 2003 Apr;16(2):265-72.

10. Nicoletti A; Toxocariasis. Handb Clin Neurol. 2013;114:217-28. doi: 10.1016/B978-0-444-53490-3.00016-9.11. Carvalho EA, Rocha RL; Toxocariasis: visceral larva migrans in children. J Pediatr (Rio J). 2011 Mar-Apr;87(2):100-10. doi: doi:10.2223/JPED.2074.12. British National Formulary13. Quattrocchi G, Nicoletti A, Marin B, et al; Toxocariasis and epilepsy: systematic review and meta-analysis. PLoS Negl Trop Dis. 2012;6(8):e1775. doi:

10.1371/journal.pntd.0001775. Epub 2012 Aug 14.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. PatientPlatform Limited has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctoror other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

Author:Dr Colin Tidy

Peer Reviewer:Dr Jacqueline Payne

Document ID:2880 (v23)

Last Checked:15/10/2014

Next Review:14/10/2019

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