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 Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver, lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, heart, brain, and eyes), where they cause inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. 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 Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs and trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea → coughed up → swallowed into the GI tract 
  • Adult worms develop in the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. 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 Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. 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 Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever
  • Anorexia
  • Malaise
  • Lymphadenopathy Lymphadenopathy Lymphadenopathy is lymph node enlargement (> 1 cm) and is benign and self-limited in most patients. Etiologies include malignancy, infection, and autoimmune disorders, as well as iatrogenic causes such as the use of certain medications. Generalized lymphadenopathy often indicates underlying systemic disease. Lymphadenopathy
  • Pruritic urticaria Urticaria Urticaria is raised, well-circumscribed areas (wheals) of edema (swelling) and erythema (redness) involving the dermis and epidermis with associated pruritus (itch). Urticaria is not a single disease but rather is a reaction pattern representing cutaneous mast cell degranulation. Urticaria (Hives)-like lesions

Hepatic:

  • Abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain
  • Hepatomegaly 
  • Liver nodularity

Pulmonary:

  • Dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea
  • Nonproductive cough
  • Wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing
  • Chest tightness

Less common manifestations:

  • Muscle involvement
  • CNS involvement:
    • Eosinophilic meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis
    • Meningoencephalitis
    • Cerebral vasculitis
  • Cardiac involvement:
    • Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis
    • Pericarditis Pericarditis Pericarditis is an inflammation of the pericardium, often with fluid accumulation. It can be caused by infection (often viral), myocardial infarction, drugs, malignancies, metabolic disorders, autoimmune disorders, or trauma. Acute, subacute, and chronic forms exist. Pericarditis
    • Loeffler (eosinophilic) endocarditis Endocarditis Endocarditis is an inflammatory disease involving the inner lining (endometrium) of the heart, most commonly affecting the cardiac valves. Both infectious and noninfectious etiologies lead to vegetations on the valve leaflets. Patients may present with nonspecific symptoms such as fever and fatigue. Endocarditis
    • Cardiac tamponade Cardiac tamponade Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade

Ocular larva migrans

Unilateral symptoms:

  • Vision impairment
  • Floaters
  • Strabismus Strabismus Strabismus is the misalignment of the eyes while fixating the gaze on an object. Strabismus can be idiopathic, but it may also be caused by cerebral palsy, uncorrected refractive errors, and extraocular muscle or cranial nerve dysfunction. Strabismus

Findings:

  • White, elevated granuloma (may be confused with retinoblastoma Retinoblastoma Retinoblastoma is a rare tumor but the most common primary intraocular malignancy of childhood. It is believed that the condition arises from a neuronal progenitor cell. Retinoblastoma can be heritable or nonheritable. Retinoblastoma)
  • Uveitis Uveitis Uveitis is the inflammation of the uvea, the pigmented middle layer of the eye, which comprises the iris, ciliary body, and choroid. The condition is categorized based on the site of disease; anterior uveitis is the most common. Diseases of the Uvea
  • Retinitis
  • Endophthalmitis Endophthalmitis Endophthalmitis is an inflammatory process of the inner layers of the eye, which may be either infectious or sterile. Infectious endophthalmitis can lead to irreversible vision loss if not treated quickly. Based on the entry mode of the infectious source, endophthalmitis is divided into endogenous and exogenous types. Endophthalmitis
  • Scleritis

Complications:

  • Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment
  • Blindness

Diagnosis

The diagnosis of toxocariasis requires a high index of suspicion.

Laboratory evaluation

Serologic testing:

  • ELISA:
    • Detects antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins 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 Pleural Effusion Pleural effusion refers to the accumulation of fluid between the layers of the parietal and visceral pleura. Common causes of this condition include infection, malignancy, autoimmune disorders, or volume overload. Clinical manifestations include chest pain, cough, and dyspnea. Pleural Effusion
  • Ultrasonography: multiple hypoechoic areas in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. 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 Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. 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
Organism Enterobius Enterobius Enterobiasis is a helminth infection caused by Enterobius vermicularis, also known as a pinworm. This infection is typically seen in children and is transmitted through the fecal-oral route. The primary clinical feature is anal pruritus, but patients are often asymptomatic. Enterobius/Enterobiasis vermicularis Toxocara canis Ascaris Ascaris Ascaris is a genus of parasitic nematodes. The infection, ascariasis, is most often caused by A. lumbricoides. Transmission occurs primarily via ingestion of water or food contaminated with Ascaris eggs. Most patients with ascariasis are asymptomatic. Ascaris/Ascariasis lumbricoides Strongyloides stercoralis Schistosoma Schistosoma Schistosomiasis is an infection caused by Schistosoma, a trematode. Schistosomiasis occurs in developing countries with poor sanitation. Freshwater snails are the intermediate host and are transmitted to humans through skin contact with contaminated fresh water. The clinical presentation occurs as a result of the host's immune response to antigens from the eggs. Schistosoma/Schistosomiasis mansoni
Characteristics Nematode Nematode Nematode Nematode Trematode
Reservoir Humans Dogs Humans
  • Humans
  • Dogs
  • Cats
Humans
Transmission Fecal–oral Fecal–oral Fecal–oral Skin contact with contaminated soil Skin contact with contaminated water
Clinical
  • Pruritus ani
  • Abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain and vomiting are less common.
  • Visceral larva migrans
  • Ocular larva migrans
  • Cough
  • Wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing
  • Hemoptysis Hemoptysis Hemoptysis is defined as the expectoration of blood originating in the lower respiratory tract. Hemoptysis is a consequence of another disease process and can be classified as either life threatening or non-life threatening. Hemoptysis can result in significant morbidity and mortality due to both drowning (reduced gas exchange as the lungs fill with blood) and hemorrhagic shock. Hemoptysis
  • Abdominal cramping
  • Nausea
  • Malnutrition Malnutrition Malnutrition is a clinical state caused by an imbalance or deficiency of calories and/or micronutrients and macronutrients. The 2 main manifestations of acute severe malnutrition are marasmus (total caloric insufficiency) and kwashiorkor (protein malnutrition with characteristic edema). Malnutrition in children in resource-limited countries
  • Cough
  • Wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing
  • Abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain
  • Diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea
  • Rash
  • Swimmer’s itch
  • Katayama fever
  • Chronic infections lead to granuloma formation, which causes brain, lung, intestinal, and liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. 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
Prevention Good 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 Aspergillosis Aspergillosis is an opportunistic fungal infection caused by Aspergillus species, which are common spore-forming molds found in our environment. As Aspergillus species are opportunistic, they cause disease primarily in patients who are immunocompromised. The organs that are most commonly involved are the lungs and sinuses. Aspergillus/Aspergillosis (ABPA): hypersensitivity reaction to Aspergillus: Patients may have symptoms of airway obstruction Airway obstruction Airway obstruction is a partial or complete blockage of the airways that impedes airflow. An airway obstruction can be classified as upper, central, or lower depending on location. Lower airway obstruction (LAO) is usually a manifestation of chronic disease, such as asthma or chronic obstructive pulmonary disease (COPD). Airway Obstruction, such as dyspnea, wheezing, productive cough, and fever. Bronchiectasis Bronchiectasis Bronchiectasis is a chronic disease of the airways that results from permanent bronchial distortion. This results from a continuous cycle of inflammation, bronchial damage and dilation, impaired clearance of secretions, and recurrent infections. Bronchiectasis and pulmonary fibrosis Pulmonary Fibrosis Idiopathic pulmonary fibrosis is a specific entity of the major idiopathic interstitial pneumonia classification of interstitial lung diseases. As implied by the name, the exact causes are poorly understood. Patients often present in the moderate to advanced stage with progressive dyspnea and nonproductive cough. Pulmonary Fibrosis may occur if left untreated. Eosinophilia may be seen. The diagnosis is made with imaging, IgE levels, skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the 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 Toxoplasmosis Toxoplasmosis is an infectious disease caused by Toxoplasma gondii, an obligate intracellular protozoan parasite. Felines are the definitive host, but transmission to humans can occur through contact with cat feces or the consumption of contaminated foods. The clinical presentation and complications depend on the host's immune status. Toxoplasma/Toxoplasmosis: infectious disease caused by Toxoplasma Toxoplasma Toxoplasmosis is an infectious disease caused by Toxoplasma gondii, an obligate intracellular protozoan parasite. Felines are the definitive host, but transmission to humans can occur through contact with cat feces or the consumption of contaminated foods. The clinical presentation and complications depend on the host's immune status. Toxoplasma/Toxoplasmosis 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 Chorioretinitis Chorioretinitis is the inflammation of the posterior segment of the eye, including the choroid and the retina. The condition is usually caused by infections, the most common of which is toxoplasmosis. Some of these infections can affect the fetus in utero and present as congenital abnormalities. Chorioretinitis, or pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. 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 Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation caused by infection with the hepatitis virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology: Overview: Patients may present with a viral prodrome of fever, anorexia, and nausea. Right upper quadrant abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, jaundice Jaundice Jaundice is the abnormal yellowing of the skin and/or sclera caused by the accumulation of bilirubin. Hyperbilirubinemia is caused by either an increase in bilirubin production or a decrease in the hepatic uptake, conjugation, or excretion of bilirubin. 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

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