Toxocariasis

Toxocariasis is caused by the nematodes Toxocara canis and T. cati. These species frequently infect dogs and cats and are most commonly transmitted to humans via accidental ingestion of eggs through the fecal–oral route. Toxocara are not able to complete their life cycle in humans, but they do migrate to organs (including the liver, lungs, heart, brain, and eyes), where they cause inflammation and tissue damage. Depending on the affected site, patients may develop visceral larva migrans or ocular larva migrans. Visceral larva migrans can present with flu-like symptoms, as well as hepatic, pulmonary, neurologic, and cardiac manifestations. Ocular larva migrans presents with unilateral vision impairment, and a white, elevated granuloma may be seen on fundus examination. The diagnosis is guided by clinical suspicion and may be supported with serology. Management includes anthelmintic therapy and steroids for severe disease.

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General Characteristics and Epidemiology

Basic features of Toxocara

  • Nematode (roundworm)
  • A dioecious helminth (males and females have different morphologies):
    • Females are longer than males.
    • Males have a curved tail with paired spicules.
  • Size: 4–12 cm long
  • Have complete digestive systems
  • Move in contractile waves
  • Eggs:
    • Brownish
    • Spherical 
    • Granulated or pitted surface
    • Very resistant to adverse environmental conditions
Toxocara (roundworm)

Spherical Toxocara egg with a brown, granulated appearance

Image: “Toxocara (roundworm)” by SuSanA Secretariat. License: CC BY 2.0

Clinically relevant species

Toxocariasis is caused by the following species:

  • Toxocara canis (dog roundworm)
  • Toxocara cati (cat roundworm)

Epidemiology

  • United States:
    • Prevalence: 5%–15% of the population
    • Approximately 10,000 cases are diagnosed per year.
  • International:
    • 2%–5% positive rate in healthy adults from urban Western countries
    • 14%–37% in rural areas
    • More common in tropical and subtropical regions
  • Race:
    • No ethnic predisposition has been reported
    • More common in non-Hispanic Blacks
  • Age: common in young children and persons < 20 years of age

Pathogenesis

Reservoir

  • T. canis:
    • Dogs (most puppies are infected soon after birth)
    • Foxes
  • T. cati: cats

Transmission

Transmission occurs through:

  • Fecal–oral route
  • Ingestion of infected animals

Host risk factors

  • Young age
  • Living with dogs and cats
  • Poor sanitation and handwashing
  • Consumption of:
    • Undercooked meat
    • Unwashed fruits and vegetables
  • Contact with infested soil from a yard, sandbox, park, or playground
  • Living in:
    • Rural areas
    • Poverty
    • Hot, humid environment (favors the survival of eggs)

Life cycle and pathophysiology

In animals:

  • Adult Toxocara worms live in the small intestines of dogs and cats.
  • Eggs are excreted in feces into the environment → transmission to a new host
  • Eggs hatch in the intestine and release larvae → burrow through the bowel wall → enter the bloodstream 
  • Larvae migrate through the lungs and trachea → coughed up → swallowed into the GI tract 
  • Adult worms develop in the small intestine → lay eggs → excreted in the stool

In humans:

  • Infection occurs through fecal–oral transmission.
  • Eggs hatch in the intestine and release larvae → burrow through the bowel wall → enter the bloodstream
  • Invade organs throughout the body, most notably:
    • Muscles
    • Liver
    • Heart
    • Lung
    • Eyes
    • Brain
  • Immune response against larvae antigens → inflammation → tissue damage → clinical signs and symptoms
  • Toxocara cannot complete the life cycle in humans.

Clinical Presentation

Most Toxocara infections are asymptomatic and have a benign course. The 2 major forms of toxocariasis are visceral larva migrans and ocular larva migrans.

Visceral larva migrans

General:

  • Fever
  • Anorexia
  • Malaise
  • Lymphadenopathy
  • Pruritic urticaria-like lesions

Hepatic:

  • Abdominal pain
  • Hepatomegaly 
  • Liver nodularity

Pulmonary:

  • Dyspnea
  • Nonproductive cough
  • Wheezing
  • Chest tightness

Less common manifestations:

  • Muscle involvement
  • CNS involvement:
    • Eosinophilic meningitis
    • Meningoencephalitis
    • Cerebral vasculitis
  • Cardiac involvement:
    • Myocarditis
    • Pericarditis
    • Loeffler (eosinophilic) endocarditis
    • Cardiac tamponade

Ocular larva migrans

Unilateral symptoms:

  • Vision impairment
  • Floaters
  • Strabismus

Findings:

  • White, elevated granuloma (may be confused with retinoblastoma)
  • Uveitis
  • Retinitis
  • Endophthalmitis
  • Scleritis

Complications:

  • Retinal detachment
  • Blindness

Diagnosis

The diagnosis of toxocariasis requires a high index of suspicion.

Laboratory evaluation

Serologic testing:

  • ELISA:
    • Detects antibodies to Toxocara excretory/secretory antigens
    • A positive test does not indicate active infection.
    • May be negative in ocular larva migrans
  • Western blot:
    • More sensitive and specific
    • Expensive
    • Can be used as a confirmatory test

Supporting evaluation:

  • Leukocytosis
  • Eosinophilia
  • Transaminitis → hepatic involvement

Biopsy

  • Provides a definitive diagnosis but is rarely indicated
  • Findings: Toxocara larvae within eosinophilic granulomatous lesions

Imaging studies

Imaging is usually guided by the patient’s clinical presentation.

  • Chest X-ray:
    • Bilateral peribronchial infiltration
    • Parenchymal infiltrates
    • Pleural effusion
  • Ultrasonography: multiple hypoechoic areas in the liver
  • CT scan:
    • Lungs: multiple pulmonary nodules with surrounding ground-glass opacities
    • Hepatic: low-density lesions
    • Brain: hyperintense cortical or subcortical granulomas

Management and Prevention

Management of visceral larva migrans

Patients with mild symptoms do not require therapy, since the disease is self-limited. Patients with moderate to severe disease may be given:

  • Anthelmintic therapy:
    • Albendazole
    • Mebendazole
  • Prednisone for severe respiratory, myocardial, or CNS disease.

Management of ocular larva migrans

  • Antihelmintic therapy
  • Prednisone for sight-threatening inflammation

Prevention

  • Proper disposal of dog and cat feces
  • Deworming pets
  • Good hygiene practices
  • Cover sandboxes when not in use.

Comparison of Similar Helminths

Table: Comparison of similar helminths and their associated diseases
OrganismEnterobius vermicularisToxocara canisAscaris lumbricoidesStrongyloides stercoralisSchistosoma mansoni
CharacteristicsNematodeNematodeNematodeNematodeTrematode
ReservoirHumansDogsHumans
  • Humans
  • Dogs
  • Cats
Humans
TransmissionFecal–oralFecal–oralFecal–oralSkin contact with contaminated soilSkin contact with contaminated water
Clinical
  • Pruritus ani
  • Abdominal pain and vomiting are less common.
  • Visceral larva migrans
  • Ocular larva migrans
  • Cough
  • Wheezing
  • Hemoptysis
  • Abdominal cramping
  • Nausea
  • Malnutrition
  • Cough
  • Wheezing
  • Abdominal pain
  • Diarrhea
  • Rash
  • Swimmer’s itch
  • Katayama fever
  • Chronic infections lead to granuloma formation, which causes brain, lung, intestinal, and liver disease.
Diagnosis
  • Clinical
  • Cellophane tape test
  • Serology
  • Biopsy
Stool analysis
  • Stool analysis
  • Serology
  • Stool analysis
  • Serology
Management
  • Albendazole
  • Mebendazole
  • Pyrantel pamoate
  • Albendazole
  • Mebendazole
  • Albendazole
  • Mebendazole
  • Ivermectin
  • Albendazole
Praziquantel
PreventionGood hygiene
  • Good hygiene
  • Deworm dogs.
  • Proper disposal of dog feces
  • Good hygiene
  • Clean raw fruits and vegetables before consuming.
  • Wear shoes and protective clothing.
  • Improve sanitation.
  • Avoid swimming or bathing in contaminated water.
  • Drink boiled or bottled water.
  • Improve sanitation.

Differential Diagnosis

  • Allergic bronchopulmonary aspergillosis (ABPA): hypersensitivity reaction to Aspergillus: Patients may have symptoms of airway obstruction, such as dyspnea, wheezing, productive cough, and fever. Bronchiectasis and pulmonary fibrosis may occur if left untreated. Eosinophilia may be seen. The diagnosis is made with imaging, IgE levels, skin prick testing, and serology. Management includes steroids and antifungal therapy.
  • Retinoblastoma: the most common primary intraocular malignancy of childhood: Retinoblastoma typically presents as leukocoria (abnormal white reflection in the eye) in a child under the age of 2 years. The disease may affect 1 or both eyes (unlike toxocariasis). The diagnosis is made with fundus examination and imaging. Management may include enucleation, laser photocoagulation, radiation, and chemotherapy.
  • Toxoplasmosis: infectious disease caused by Toxoplasma gondii: The clinical presentation and complications of toxoplasmosis depend on the host’s immune status and can vary greatly, including an acute, flu-like syndrome, CNS toxoplasmosis, chorioretinitis, or pneumonia. The diagnostic workup depends on the presentation, but can include lumbar puncture, imaging, biopsy, serology, or PCR.  Management is with antimicrobial therapy.
  • Viral hepatitis: liver inflammation caused by infection with the hepatitis virus: Patients may present with a viral prodrome of fever, anorexia, and nausea. Right upper quadrant abdominal pain, jaundice, and transaminitis may also occur. The diagnosis is made with viral serologic testing and will differentiate viral hepatitis from visceral larva migrans. Management of acute hepatitis is supportive.

References

  1. Woodhall DM, Fiore AE. (2014). Toxocariasis: a review for pediatricians. J Pediatric Infect Dis Soc 3(2):154–159.
  2. Woodhall D, Jones JL, Cantey PT, Wilkins PP, Montgomery SP. (2014). Neglected parasitic infections: what every family physician needs to know. Am Fam Physician 89(10):803–811.
  3. Hotez PJ, Wilkins PP. (2009). Toxocariasis: America’s most common neglected infection of poverty and a helminthiasis of global importance? PLoS Negl Trop Dis 3(3).
  4. Despommier D. (2003). Toxocariasis: clinical aspects, epidemiology, medical ecology, and molecular aspects. Clin Microbiol Rev 16:265–272.
  5. Herrmann N, Glickman LT, Schantz PM, Weston EI, Domanski LMA. (1985). Seroprevalence of zoonotic toxocariasis in the United States: 1971-1973. J Epidemiol 122(5):890–896.
  6. Weller PF, Leder K. (2020). Toxocariasis: visceral and ocular larva migrans. In Baron EL (Ed.), UpToDate. Retrieved April 12, 2021, from https://www.uptodate.com/contents/toxocariasis-visceral-and-ocular-larva-migrans
  7. Huh S, Lee S. (2019). Toxocariasis. In Brusch JL (Ed.), Medscape. Retrieved April 12, 2021, from https://emedicine.medscape.com/article/229855-overview
  8. Pearson RD. (2020). Toxocariasis. MSD Manual Professional Version. Retrieved April 12, 2021, from https://www.msdmanuals.com/professional/infectious-diseases/nematodes-roundworms/toxocariasis
  9. Winders WT, Merkin-Smith L. (2020). Toxocara canis. StatPearls. Retrieved April 12, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK538524/

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