2. Importance
• Members of the genus Toxocara are zoonotic intestinal nematodes
(roundworms) that mature in various mammals, including some domesticated
species.
• Parasitized animals can shed large numbers of eggs in the feces, infecting people
(particularly children) who ingest these eggs in contaminated soil, or on hands
or objects.
• Although Toxocara eggs do not complete their maturation in humans, the
developing larvae can migrate through the body for a time. In some cases, they
cause symptoms ranging from mild, vague discomfort to ocular disturbances,
blindness and neurological syndromes.
3. Etiology
• Toxocariasis is caused by members of the genus Toxocara, nematodes in the family
Toxocaridae, superfamily Ascaridoidea.
• Recognized species include Toxocara canis, T. cati, T. malaysiensis and T.
vitulorum, which have domesticated animals as their definitive hosts, which mature
and shed eggs in wild animals.
• Adult Toxocara can cause intestinal illnesses in their definitive hosts, while larvae
migrating through the tissues can affect both definitive and paratenic hosts.
• In paratenic hosts, toxocariasis is often called larva migrans.
• Visceral larva migrans is a general term used to indicate the presence of larvae in
various internal organs, while larvae in the eye are termed ocular larva migrans.
• Larvae in the brain are sometimes called cerebral larva migrans.
6. Transmission
• Transmission of Toxocara to humans is usually through ingestion of infective eggs.
• T. canis can lay around 200,000 eggs per day, These eggs are passed in cat or dog feces, but the
defecation habits of dogs cause T. canis transmission to be more common than that of T. cati.
• Both Toxocara canis and Toxocara cati eggs require a several week incubation period in moist,
humid, weather, outside a host before becoming infective, so fresh eggs cannot cause
toxocariasis.
• Many objects & surfaces can become contaminated with infectious eggs. Flies that feed on
feces can spread Toxocara eggs to surfaces or foods. Young children who put contaminated
objects in their mouths or eat dirt are at risk of developing symptoms. Humans can also
contaminate foods by not washing their hands before eating.
• Humans are not the only accidental hosts of Toxocara. Eating undercooked rabbit, chicken,
or sheep can lead to infection;
7. Incubation period
• The incubation period for Toxocara canis and cati eggs depends on temperature and
humidity.
• T. canis females, specifically, are capable of producing up to 200,000 eggs a day that
require 2-6 weeks minimum up to a couple months before full development into the
infectious stage.
• Under ideal summer conditions, eggs can mature to the infective stage after two
weeks outside of a host. Provided sufficient oxygen and moisture availability,
Toxocara eggs can remain infectious for years, as their resistant outer shell enables
the protection from most environmental threats.
• However, the second stage of larvae development poses strict vulnerabilities to
certain environmental elements. High temperatures and low moisture levels will
quickly degrade the larvae during this stage of growth.
8. Clinical Signs
• Toxocara larvae can cause symptoms related to the internal organs ,eye or brain or mild
syndromes known as covert or common toxocariasis.
• Small numbers of larvae are usually asymptomatic. Most cases of visceral larva migrans are
subclinical, and are recognized mainly by persistent eosinophilia. Typical signs in more
severely affected patients can include malaise, fever, hepatomegaly and upper abdominal
discomfort.
• Some patients may also have nausea, vomiting, or respiratory signs such as wheezing,
coughing and dyspnea. Pruritic rashes, chronic urticaria, lymphadenopathy, arthralgia
and myalgia have also been described, and there are rare reports of cardiac involvement.
• Invasion of the CNS seems to be uncommon, but can result in various syndromes including
meningoencephalitis, myelitis, cerebral vasculitis, or signs of a space-occupying lesion.
• Deaths are rare but have been seen in cases of CNS disease, severe pneumonia or cardiac
involvement.
10. Diagnostic Tests
• Finding Toxocara larvae within a patient is the only definitive diagnosis for
toxocariasis; however, biopsies to look for second stage larvae in humans are
generally not very effective.
• PCR, ELISA, and serological testing are more commonly used to
diagnose Toxocara infection. Serological tests are dependent on the number of
larvae within the patient, and are unfortunately not very specific.
12. • The optimal treatment of larva migrans is incompletely understood, and
recommendations for various syndromes can differ between authors.
Anthelmintic drugs can be used to destroy at least some of the larvae, but the
dying larvae may cause severe hypersensitivity reactions, and antiinflammatory
drugs (e.g., corticosteroids) are often given concurrently.
• Treatment of ocular disease may include surgery and/or anti-inflammatory
drugs to decrease further damage to the eye, with or without anthelmintics.
• Other therapies such as laser photocoagulation have also been used.
• However, is recommend treating all cases to reduce the risk of serious
complications such as ocular or cerebral disease.
13. Control
• Environmental contamination can be reduced by preventing
and/or treating Toxocara infections in animals, and by
removing feces before the eggs can become embryonated,
especially in areas where children play.
• Dog owners are encouraged to collect their animal's feces
in public areas.
• Patent infections are especially prevalent among dogs and
cats less than 6 months of age, with 3-week-old to 3-
month-old puppies shedding particularly large numbers of
T. canis eggs.
• Good hygiene can help prevent infections or reduce the
severity of illness by decreasing the dose of parasite eggs.
• Hands and raw fruits or vegetables should be washed or
peeled before eating.
• Children should be taught not to eat soil, and to wash
their hands after playing with pets or outdoor activities.
• Tissues from mammals, birds and other potential paratenic
hosts should be cooked before eating. Liver seems to be a
particularly common source of larvae.
14. Morbidity and Mortality
• Infections with Toxocara larvae are thought
to be widespread and common in human
populations. In serological studies,
exposure rates may be as low as 1-20% in
some developed countries with temperate
climates, and as high as 30-85% in some
tropical regions.
• Symptomatic visceral larva migrans is
particularly common between the ages of 1
to 7 years.
• The most severe cases are often reported in
toddlers 1- 3 years of age.
• Ocular migrans tends to be more common
in older children and young adolescents.
• Fatalities are rare but have occurred in
cases with severe pneumonia, cardiac
involvement or neurological disease.
• One report found that 1% of patients with
uveitis had ocular larva migrans, and a
survey of Irish school reported 6.6 cases of
ocular larva migrans per 100,000
population.
• Damage to the eye can be permanent.
15. Internet Resources
1. Centers for Disease Control and Prevention (CDC)
http://www.cdc.gov/parasites/toxocariasis/
2. CDC Guidelines for Veterinarians: Prevention of
Zoonotic Transmission of Ascarids and Hookworms
of Dogs and Cats
http://www.cdc.gov/parasites/zoonotichookworm/
resou rces/prevention.pdf
3. Companion Animal Parasite Council Current
Advice on Parasite Control: Intestinal Parasites
https://www.capcvet.org/capcrecommendations/as
carid-roundworm
4. The Merck Manual
https://www.merckmanuals.com/professional
5. The Merck Veterinary Manual
http://www.merckvetmanual.com/mvm/index.html
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Parasitoses. 3rd ed. Washington DC: PAHO; 2003.
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15.
