OVERVIEW

Left image courtesy of PetEcology and right image courtesy of Dan Heller Photography

Toxocariasis is a zoonotic (animal-to-human) disease caused by infection of the parasitic nematodes (roundworms), Toxocara canis and Toxocara cati. These parasites flourish in the lumen of the small intestine of dogs and cats and the eggs are excreted into the environment.

After 2-4 weeks in the soil, the non-infective eggs molt to become infective eggs. There is no vector for the parasite; it is transmitted to humans by ingestion of the infective eggs. Once inside the human body, the eggs hatch and the larvae migrate through the blood and capillaries, taking up residence in any location of the body (the most common locations include the eye, brain, liver, and lung).

Most infestations cause only mild symptoms and are not serious. However if the parasite infects the eye, local inflammation may cause a variety of eye problems, from partial loss of vision to complete blindness (this is called Ocular Larva Migrans.) If a large number of parasites infect the other tissues, or if the parasite enters the Central Nervous System, other damage may also occur; the precise effects are mostly determined by the site of infection. This is called Visceral Larva Migrans.

PARASITE CHARACTERISTICS

Vector
None

Hosts
cat, dog, mouse, fox, raccoon, and other mammals

Transmission
Toxocariasis is usually transmitted through ingestion of an infective egg in the soil, however recent reports suggest that it may also be transmitted through ingestion of raw meat from an infected chicken, rabbit, or lamb.

Synonyms
The common name of the parasite is simply the "common roundworm of dogs and cats," most likely due to its prevalence in these animals. The diseases, Ocular Larva Migrans and Visceral Larva Migrans, are both specific forms of the overarching disease, Toxocariasis, but they are often used as synonyms for Toxocariasis. It is also sometimes called Toxocaral Larva Migrans or Toxocarose.

Morphology
Nematodes are distinguished by their cylindrical shape and impressive length (for example, adult Toxocara worms measure 10 cm for males and 18 cm for females.)

These pictures show the morphology of the Toxocara parasite. The left image depicts the head of the parasite (picture courtesy of Toxocara canis Homepage, University of Pennsylvania.) The middle images depict the stomodeum (mouth and lip region) of the parasite (top picture courtesy of the Dreddyclinic.com and the bottom picture is from Kelcliffe Kennels.) The right images show the worm in entirety, giving indications of its impressive size (top picture courtesy of Barrier Animal Care Clinic and bottom picture courtesy of Ohio State University).

The eggs, which are roughly circular in shape, are light brown, with a thick, pitted protein coat. The eggs of T. canis measure approximately 85µm by 75µm while those of T. cati measure approximately 65µm by 70µm.

These images show the Toxocara eggs. (The left picture is courtesy of Toxocara canis Homepage, University of Pennsylvania and the right picture is courtesy of the GIDEON database)

Taxonomy

Phylum: Nemathelminthes
Class: Nematoda
Order: Ascaridida
Family: Ascaridida
Genus: Toxocara
Species: canis or cati

back to top

HISTORY OF DISCOVERY


Toxocariasis has attracted the attention of healthcare workers relatively recently. The first case of infection with T. cati was reported in 1824, however since this parasite occurs much less frequently in humans (there have only been 24 reported cases of T. cati infection since its discovery), attention has mainly been focused on T. canis. T. canis was identified in dogs in the 18th century but was not studied until 1908, when G.H.F. Nutall and C. Strickland examined dogs in Cambridge, England, finding 17 out of 24 infested with the parasite. The first human cases of T. canis infection did not occur until 1950 (observed in the liver by Mercer, et al, and in a retinal granuloma by H.C. Wilder.) In 1952, similar cases were reported and the connection between Toxocariasis and dogs was established.

G.H.F. Nutall (above) worked with C. Strickland in 1908 to conduct the first study of T. canis prevalence and distribution. Image courtesy of the University of Cambridge.

One of the biggest breakthroughs in understanding Toxocariasis was made by J.F.A. Sprent in 1958 when he worked out the T. canis life cycle and outlined the mechanism of transmission. Future studies elucidated many other important features of Toxocariasis, although attention was mainly focused on the more easily diagnosable Ocular Larva Migrans.

Over time it was discovered that Visceral Larva Migrans occurs much more frequently. Studies are still conducted today, with the hope of further understanding the intricacies of Toxocariasis and potential new treatments to reduce morbidity.

back to top

EPIDEMIOLOGY

Toxocariasis is found worldwide, although the majority of cases occur where dogs and cats are kept in close proximity to humans (usually household pets). Most cases are reported from the Southeastern United States, Mexico, Hawaii, East and Western Europe, Australia, the Philippines, and South Africa.

 


This map shows the distribution of Toxocariasis. Red dot indicates that Toxocara has been reported in that country. Map courtesy of the GIDEON database

Within these countries, pet owners (who live in close proximity to infected animals) and children (who are more likely to play in or eat contaminated dirt) are most susceptible to Toxocariasis.

Although both dogs and cats can transmit Toxocariasis, T. canis is much more common due to the indiscriminate defecatory habits of dogs. A survey of animal shelters in the United States found that 36% of dogs nationwide, and 52% of dogs in the Southeastern United States, carried the infective T. canis parasite. A different study found that one third of all soil specimens collected from the St. Joseph-Benton Harbor area of Michigan contained T. canis eggs, including 11% of back yards and gardens.

Similar studies indicated that approximately 16.4% of cats in the United States carried T. cati parasite.

The prevalence of Toxocara in humans remains relatively low. Although there are approximately 10,000 new infections each year, only a few dozen people each year develop severe conditions. Seroprevalence tests have indicated that approximately 5% of children, and 50% of children who have regular contact with puppies and soil, or who have chronic respiratory problems, carry Toxocara antibodies.

Left image courtesy of Animal Actors International and right image courtesy of Rescue Cats, Inc.

back to top

LIFE CYCLE

ZOONOTIC TRANSMISSION (Life Cycle in Dogs)

Adult worms are found in the lumen of the small intestine of dogs and cats. Although animals of all ages may transmit Toxocara parasites, puppies less than 6 months of age cause the most human infections. Older dogs have a more developed immune system which halts larvae in the L3 stage; thus the worms rarely reach adulthood and very few eggs are excreted into the environment. However these larvae will again begin to grow and develop on day 45 of pregnancy, such that the bitch may transmit T. canis to her offspring (through the placenta or from her milk) or she can contaminate the external environment with excreted T. canis parasites. Puppies may additionally contract the parasite after birth through ingestion of an infective egg. Incubation time in the puppy is 30-34 days following ingestion of an egg and 21-40 days following transmission from the bitch.

Image courtesy of Toxocara canis Homepage, University of Pennsylvania

T. canis larvae pass through the intestinal wall of puppies and into the vascular system. They travel to the lungs, where they develop to become L3 larvae. These larvae are coughed into the trachea and swallowed. The parasites return to the lumen of the small intestine where they develop to adulthood. The adult worms lay up to 100,000 eggs each day, which are then excreted into the environment along with the dog's feces. These eggs must remain in the environment for 2-4 weeks, depending on ambient temperature and humidity, in order to become infective; once they become infective, they may survive in the environment for years. The infective eggs may be ingested by other dogs (thus perpetuating the zoonotic cycle) or by a human.

T. cati will mature in cats of all ages.

DIRECTLY TRANSMITTED ZOONOSIS (life cycle in dogs and humans)

Toxocara parasites cannot mature in humans. If an infected egg is ingested by a human (usually a young child), the eggs hatch but the larvae do not develop past the L2 stage. The larvae travel through the blood and lymph to any tissue in the body. Incubation period in humans is quite variable and lasts between one week and two years. Local inflammation may occur at whichever organ T. canis invades, potentially causing Visceral Larva Migrans; this disease has numerous nonspecific symptoms which vary depending on the site of infection. Ocular Larva Migrans occurs when one or more larvae migrate to the eye (either through the central retinal artery or the short posterior ciliary artery). This infection causes the formation of a granuloma may destroy other tissues critical for normal eye functioning. The nematodes eventually die within the human host, although any incurred tissue damage (especially in the eye) may be irreparable. As accidental hosts, humans do not transmit Toxocariasis.

These images show the life cycle of T. canis and T. cati in the animal hosts and in humans. Left diagram is courtesy of the GIDEON database (originally courtesy of the Center for Disease control. Right diagram is courtesy of TodoPerros.

OVERVIEW

1.) Ingestion of infected egg or infestation of T. canis through maternal transmission
2.) Adult worms in small intestine lay eggs
3.) Eggs are excreted into the environment along with the feces
4.) Eggs mature in soil
5.) Dog or human eats infective egg
6.) If eaten by a dog, eggs hatch in small intestine
7.) Larvae pass through the intestinal wall and into the vascular system
8.) Larvae travel to lungs
9.) L2 larvae molt to become L3 larvae
10.) Larvae coughed into trachea and swallowed
11.) Larvae mature to adulthood in the small intestine and begin laying eggs

6.) If eaten by a human, eggs hatch in small intestine
7.) Larvae go to liver or to any tissue (via blood/lymph)
8.) Local inflammation in target organ causing related symptoms
9.) Worms die

 

 

back to top

CLINICAL SYMPTOMS

Most Toxocariasis infections are not serious and do not cause overt symptoms. Visceral Larva Migrans is caused by chronic or heavy infections and may cause:

Common
Less Common
gastrointestinal and intraabdominal dysfunction
headache
weakness
lethargy
fever
coughing
asthma
pneumonia
eosinophilia

myalgia (muscle pain)
seizures
urticaria (hives)
hepatatomegaly (enlarged liver)

 

 

These symptoms usually disappear without causing serious damage, although chronic infection may cause persistent health problems or the development of epilepsy in young children.

Ocular Larva Migrans generally presents with conditions due to the inflammation of ocular tissues; infection usually occurs unilaterally but occurs bilaterally in approximately 3% of patients. The most common symptoms include:

Common
Less Common

white, elevated granuloma on either the retina or the optic disk

 

 

uveitis (inflammation of the uvea)
vitritis
neuroretinitis (inflammation of the optic nerve and retina)
papillitis (inflammation of the papilla)
chronic endophthalmitis (inflammation of the interior eyeball)

 

These conditions are harder to treat, since the side effects of antiparasitic drugs could be devastating to the eye, and may cause strabismus, iridocyclitis, glaucoma, papillitis, or visual loss.

These photos show eyes damaged by Ocular Larva Migrans. Left image is courtesy of Handbook of Ocular Disease Management and the right image is courtesy of imagesMD

back to top

DIAGNOSIS

Because many of the symptoms of Toxocariasis are nonspecific and mild, diagnosis of the disease can be quite difficult. Chronic eosinophilia, hepatomegaly, chronic pulmonary disease, or a history of exposure to puppies or contact with feces-contaminated soil are common indicators of infection. Serologic tests may also be used to detect L2 larval antigens.

Ocular Larva Migrans can additionally be recognized by the presence of a granuloma or inflammation of the eye tissues, although care should be taken not to mistake Ocular Larva Migrans for retinoblastoma, which presents with similar symptoms.

back to top

TREATMENT

Since most cases of Toxocariasis are not serious, drugs are not usually given unless the symptoms are severe. Most of these cases will clear up without any medication or treatment.

Visceral Larva Migrans is primarily treated through antihelmintic drugs, usually Albendazole (400 mg BID for 5 days) or Mebendazole (100-200 mg BID for 5 days.) Other drugs such as Febantel, Fenbendazole, Ivermectin, Milbemycin oxime, Piperazine, and Pyrantel Pamoate may also be used

Risks of this medication go up for the following people: Side effects of this medication may include:
anyone with allergies
pregnant or lactating women
children less than 30 pounds
older adults
anyone simultaneously taking other medications
confusion
severe diarrhea
hallucinations
irritability
loss of appetite
severe nausea and vomiting
numbness or tingling in the hands or feet
skin rash
itching
muscle and joint aches
blurred or yellow vision
chills
seizures
dark urine
fever
back pain
pain while urinating
yellow eyes and skin
extreme lethargy
redness, blistering, peeling, or loosening of the skin

 

Left image courtesy of Vetonline. Middle image courtesy of ChemicalLand21.com. Right image courtesy of KMU.

 

There is no successful treatment of Ocular Larva Migrans since treatment with antihelmintic drugs may cause a high degree of inflammation in the eye. Oral steroids are sometimes used to decrease the effects of inflammation. Laser photocoagulation (the use of a laser to solidify the affected tissue into a single mass) and cryoretinopexy may also be used to treat severe cases.

back to top

PREVENTION

Prevention of Toxocariasis is relatively easy to do.

To prevent contamination of the environment:

To prevent human infection with T. canis or T. cati:

back to top

RELATED LINKS

Center for Disease Control, Division of Parasitic Diseases. http://www.cdc.gov/ncidod/dpd/default.htm

GIDEON Database. http://www.gideononline.net/web/epidemiology/index.php

Handbook of Ocular Disease Management. http://www.revoptom.com/handbook/oct02_sec5_4.htm

Human Biology 103 Website, Stanford University. http://www.stanford.edu/class/humbio103

Nemours Foundation: Kids Health for Parents. http://kidshealth.org/parent/infections/parasitic/toxocariasis.html

top

REFERENCES

Animal Actors International. "Puppies". http://www.animal-actors.com/animal_actors_international.htm. Accessed May 23, 2005.

Barrier Animal Care Clinic. "Dog Worms...". http://www.barrieranimalcareclinic.co.uk/Dworms.htm. Last modified April 9, 2005. Accessed May 22, 2005.

Center for Disease Control, Division of Parasitic diseases. "Guidelines for Veterinarians: Prevention of Zoonotic Transmission of Ascarids and Hookworms of Dogs and Cats." http://www.cdc.gov/ncidod/dpd/parasites/ascaris/prevention.htm. Last updated September 27, 2004. Accessed May 15, 2005.

Center for Disease Control, Division of Parasitic diseases. "Fact Sheet: Toxocariasis." http://www.cdc.gov/ncidod/dpd/parasites/toxocara/factsht_toxocara.htm. Last updated March 20, 2002. Accessed May 15, 2005.

Center for Disease Control, Division of Parasitic diseases. "What Every Pet Owner Should Know About Roundworms and Hookworms." http://www.cdc.gov/healthypets/Merial_CDCBroch_rsgWEB.pdf. Accessed May 15, 2005.

ChemicalLand21.com. "Albendazole". http://www.chemicalland21.com/arokorhi/lifescience/phar/ALBENDAZOLE.htm. Accessed May 23, 2005.

DreddyClinic.com. "Special Report on Parasites." http://www.dreddyclinic.com/integrated_med/parasites.htm. Last updated March 23, 2005. Accessed May 22, 2005.

GIDEON. "Toxocariasis". http://www.gideononline.net/web/epidemiology/index.php. Copyright 1994-2005. Accessed May 15, 2005.

Handbook of Ocular Disease Management. "Toxocariasis: Ocular Larva Migrans". http://www.revoptom.com/handbook/oct02_sec5_4.htm Accessed May 22, 2005.

Heller, Dan. "My New Puppy." http://www.danheller.com/loophead.html. Accessed May 26, 2005.

ImagesMD: The Online Encyclopedia of Medical Images. "Ocular Toxocariasis." http://images.md/users/image_show.asp. Copyright 2005. Accessed May 26, 2005.

Kelcliffe Kennels. "Worms, Dogs, and People." http://www.effect.net.au/thomas/CAREWORM.HTM. Accessed May 22, 2005.

KMU. "Mebendazole". http://www.kmhk.kmu.edu.tw/medhome/Intra_med/med_o/search/ENdata.asp?EN_data=M. Accessed May 22, 2005.

Markell, John, and Krotoski. "Markell and Voge's Medical Parasitology, Eighth Edition." Copyright 1958 by Saunders, renewed 1986 by Markell. Philadelphia: Saunders Company.

MedlinePlus, a service of the U.S. National Library of Medicine and the National Institutes of Health. "Thiabendazole". http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202558.html. Last updated July 26, 2001. Accessed May 15, 2005.

Nemours Foundation: Kids Health for Parents. "Infections: Toxocariasis". http://kidshealth.org/parent/infections/parasitic/toxocariasis.html. Copyright 1995-2005. Accessed May 15, 2005.

Nolan, M.D., University of Pennsylvania."Toxocara canis Homepage". http://cal.vet.upenn.edu/dxendopar/parasitepages/ascarids/t_canis.html. Copyright 2004. Accessed May 15, 2005.

Ohio State University. "Graphic Images of Parasites: Toxocara canis". http://www.biosci.ohio-state.edu/~parasite/toxocara.html. Accessed May 23, 2005.

PetEcology: Animal Health Management. "Girl With Kitten." http://www.petecology.com/links.htm. Copyright 2004. Accessed May 26, 2005

Rescue Cats, inc. "Three Little Kittens". http://www.rescuecats.org/. Last updated April, 2005. Accessed May 23, 2005.

TodoPerros. "La Toxocariasis Canina y Humana". http://www.todoperros.com/numero20.php#raza. Copyright 2001-2002. Accessed May 22, 2005.

University of Cambridge, Department of Pathology. "Kanthack and Nutall Library." http://www.path.cam.ac.uk/resources/library/. Copyright 2005. Accessed May 23, 2005.

University of Pennsylvania. "Toxocara canis Homepage." http://cal.vet.upenn.edu/dxendopar/parasitepages/ascarids/t_canis.html. Copyright 2004. Accessed May 23, 2005.

Vetonline. "Albendazole". http://www.vetonline.cu.edu.eg/specialpharma1d.htm. Copyright 2001. Accessed May 23, 2005.

Top

 

Website created by Katie Rice for Humbio 103: Parasites and Pestilence, Stanford University. Course taught by Dr. Scott Smith