Toxocara canis

Geographic Range

Toxcocara canis has a worldwide distribution. It is prevalent in all locations that have domestic dogs, puppies, and other canids. Toxocara canis is also found in places that have other various mammals such as mice, pigs, birds, and foxes, but these hosts are only paratenic hosts. Hosts are terrestrial mammals and therefore T. canis is mainly found in terrestrial terrain. (Xi and Jin, 1998)

Habitat

The eggs of T. canis are excreted in the feces of an infected canid host. The embryonated eggs can live in the feces for up to three weeks. The feces are often deposited in soil or sandy areas. A host must ingest the eggs for the life cycle to continue. If ingested, the new habitat becomes the internal organs of the host. The gut is the first area T. canis larvae reside. If the host has not been previously infected, hatched juveniles go throught the circulation to the lungs, then back to the gut. If in a canid host, they take up residence in the intestine and develop into adults. If hosts have been previously "immunized" junveniles go to the body tissues and become dormant as if they were in a paratenic host. Often the infectious larvae stay in the mammary glands until a pregnancy where they are passed on to a nursing pup. If in a human or other non-canid host the larvae will wonder throughout the organs. These wandering larvae are called visceral larva migrans. They may travel to the eyes, lungs, brain, heart, muscles, liver, and other organs. Here they do not develop further but can cause severe local reactions. (Helwigh, et al., 1999; Xi and Jin, 1998)

Physical Description

Toxocara canis is smaller than most of the other species in the family Ascarididae. It has a complete gut in the form of a simple tube. It is a "round worm" implying the shape of the outer layer to be round (if seen in a cross section ). Depending on the host the worm gets into T. canis will have different number of larval stages. Most worms have three larval stages before becoming infective.

Toxocara canis is dioecious having morphology distinctly different for the male and female. Males, 4-6 cm long, are smaller than females. The male's posterior end is curved ventrally and the tail is bluntly pointed. The male has a single tubular testis. He also has simple spicules, which allows for direct sperm transfer. The female worms are generally around 6.5 cm but can be as long as 15 cm long. In the female the vulva is about one-third the body length from the anterior end. The ovaries are very large and extensive. The uteri contain up to 27 million eggs at a time.

Both males and females have three prominent lips. Each lip has a dentigerous ridge. The lateral hypodermal cords are visible with the naked eye. No gubernacullum is present. In both sexes there are prominent cervical alae. The eggs are brownish and almost spherical. The eggs measure 75-90 micrometers. The eggs are embryonated when laid and have surficial pits. These eggs are very resistant to various weather and chemical conditions. (Brunaska, et al., 1995; Roberts and Janvory, Jr, 2000)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    4 to 15 cm
    1.57 to 5.91 in

Development

Toxocara canis is a canid parasite. Humans acquire the parasite as accidental hosts. In the tissues of all dogs, in many birds, and other mammals, the larval form of T. canis is found. The dog or canid host is the definitive host and only there will T. canis develop further than the larval stage. The name for the disease when in T. canis is in a host is Toxocariasis. Many animals such as mice, rabbits, and monkeys can serve as paratenic hosts.

Regardless of the path T. canis larvae take to get to the canid intestine once there the third stage larvae molt into adults. The adult worm remains in the intestine and produces an enormous number of eggs each day. Not until the fifth day post-infection do the eggs begin to appear in the canid feces.

Toxocara canis has a complex life cycle. Similar to other nematodes, T. canis is not infectious immediately when it leaves the definitive host. It needs to grow and develop into the stage that is infectious, ensheathed L3. Only this stage can infect other definitive hosts. There is strong evidence of two molts taking place inside the developing eggs, before the eggs even hatch. The molting process involves a separation of the cuticle from the epidermis. This causes a formation of the new cuticle, which is arising from the outermost surface of the epidermis. It also includes the shedding of the old cuticle. (Brunaska, et al., 1995; Xi and Jin, 1998)

Reproduction

Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella. The adult worm remains in the intestine and produces an enormous number of eggs each day. Not until the fifth day post-infection do the eggs begin to appear in the canid feces. There is strong evidence of two moults taking place inside the developing eggs. (Brunaska, et al., 1995; Roberts and Janvory, Jr, 2000; Xi and Jin, 1998)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • provisioning
      • female

Behavior

There are four ways of infection or transmission from one host to the next. The life cycle of T. canis is different in puppies than in dogs over six months of age. Toxocara canis can be transmitted to nursing pups by transmammary transmission from larvae that were in their mothers milk. This is the least common form of transmission. The ingested milk containing the infected stage three larvae goes directly to the newborns' small intestine. Here the larvae develop directly into adults.

Prenatal transmission is the second form of obtaining T. canis. Infected pups are born with larvae already in their bodies. Stage two larvae (not yet infective) migrate from tissue in the pregnant mother. The larvae travel across the placenta and through the umbilical cord to the fetal liver. The larvae remain in the liver until birth. Once the pup is born the larvae resume their migration to the lungs of the newborn.

Another form of transmission is direct transmission. This is the only transmission to humans of T. canis. In this form of transmission the new host directly ingests the eggs from the feces of another infected canid. Toxocara canis larvae migrate from the gut(where they were ingested) to the small intestine of newborns. Toxocara canis also migrates to the lungs from the intestines. They move up the bronchial tree and the trachea to the pharynx, here they are swallowed by the newborn to finally reach the intestine. In the intestine the larvae mature. Only a small number of the larvae infecting the host actually undergo migration to the trachea. The majority of larvae continue to migrate through the lungs and the pulmonary veins of the host. The larvae migrate to the heart and there they are distributed to the somatic tissues via the peripheral circulation. The conditions for prenatal and transmammary transmission to the pups is set up by these somatic migration conditions.

The last form of transmission is paratenic host transmission. Many hosts as described as above contain the non-infective larvae of T. canis. When one of these animals is eaten by another host transmission takes place. A good example is when a dog eats an infected mouse. In the case of ingesting an infected paratenic host no further migration takes place in the dog since the requirement for the lifecycle migration is already satisfied in the paratenic (i.e. mouse) host. (Helwigh, et al., 1999; Roberts and Janvory, Jr, 2000)

Communication and Perception

Nematodes within the Secernentea have phasmids, which are unicellular glands. Phasmids likely function as chemoreceptors. Females may produce pheromones to attract males.

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Roberts and Janvory, Jr, 2000)

Food Habits

The location of T. canis in hosts is in the small intestine. There they feed on intestinal contents. The adults have a specialized anaerobic metabolism. This specialized metabolism gives the adult worms an extra ATP. Adult T. canis worms are very host specific.

Pharyngeal glands and intestinal epithelium produce digestive enzymes to feed on the hosts’ body fluids. Extracellular digestion begins within the lumen and is finished intracellularly. (Barnes, 1987; Roberts and Janvory, Jr, 2000)

  • Animal Foods
  • body fluids

Predation

These parasites are usually not preyed on directly, but are ingested from host to host.

Ecosystem Roles

Toxocara canis is a canid parasite. Humans acquire the parasite as accidental hosts. In the tissues of all dogs, in many birds, and other mammals, the larval form of T. canis is found. The dog or canid host is the definitive host and only there will T. canis develop further than the larval stage. The name for the disease when in T. canis is in a host is Toxocariasis. Many animals such as mice, rabbits, and monkeys can serve as paratenic hosts. (Brunaska, et al., 1995; Xi and Jin, 1998)

Species Used as Host

Economic Importance for Humans: Negative

Toxocara canis is widespread, causing disease in many mammals including humans. Many humans are infected with T. canis larvae. The larvae can cause serious damage to the human paratenic host. It can be found in wealthy and well-developed countries just as much as poor and under-developed places. In the United States about 98% of puppies and 20% of adult dogs are infected with T. canis. The means the risk of exposure to humans in the United States is very high. Most cases go unreported or are unrecognized. All small mammals can be paratenic hosts especially small children. The disease is most common in children between the ages one and three. Ingesting embryonated eggs from the feces of dogs and other canids spreads the diease. Often pet owners take their dog for a walk in the park. During the walk the dog may deposit egg-bearing feces in the park soil or sand. The next day an unsuspecting parent brings their small child to play in the park. The young child is at an age where everything is picked up and tasted, including the contaminated soil. The eggs of T. canis are most commonly ingested this way. The eggs once excreted from the definitive host can survive for 10-20 days in the external environment. This means even long after the dog has been in the park humans can still be infected. In Britain, one study showed that the climate conditions there allow for some T. canis eggs to survive in the soil for up to three years!

Many of the wandering larvae end up in the brain causing serious reactions, which can lead to death of the host. The most common place in the body of infection is the liver, but it can be found in every organ. The amount of damage is related to the number of juveniles in the body of the host. One of the more serious results of visceral larva migrans is blindness. The worms that infect the eye are called ocular larval migrans. Blindness occurs from the infection when a larva becomes trapped in the blood vessels at the back of the eye. (Jeanfaivre, et al., 1996; Takayanagi, et al., 1999; Xi and Jin, 1998)

Conservation Status

Other Comments

If a worm gets into an improper host such as humans the juveniles migrate through the body. The juveniles begin a typical tissue migration. They do not undergo development nor do they complete the normal migration, instead they will randomly wander through the body. Visceral larva migrans (VLM) is the resulting disease.

Toxocara canis infection is largely preventable. Worming pets often, with worming agents (such as antihelmintics: fenbendazole, piperazine, and Dichlorvos) from a veterinarian will reduce the possibility of human infection. This drugs also help in human treatment. Also careful and prompt disposal of dog feces will help. Humans should also wash their hands and the hands of children after handling dogs or dog feces, and especially before handling food. Lastly, parents and childcare givers need to watch out for toddlers eating soil and try to prevent it. (Jeanfaivre, et al., 1996)

Contributors

Renee Sherman Mulcrone (editor).

Megan Harris-Linton (author), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scrub forest

scrub forests develop in areas that experience dry seasons.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5? N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

References

Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

Brunaska, M., P. Dubinsky, K. Teiterova. 1995. Toxocara canis: Ultrasturctual Aspects of Larval Moulting in teh Maturing Eggs. International Journal of Parasitology, 25(5): 683-690.

Helwigh, A., P. Lind, P. Nansen. 1999. Visceral larva migrans: migratory pattern of Toxocara canis in pigs. International Journal of Parasitology, 29(4): 559-565.

Jeanfaivre, T., B. Cimon, N. Tolstuchow, L. de Gentile, D. Chabasse. 1996. Pleural effusion and toxocariasis. Thorax, 51(1): 106-107.

Roberts, L., J. Janvory, Jr. 2000. Gerald. D. Schmidt & Larry S. Roberts' Foundations of Parasitology, Sixth Edition. United States: McGraw-Hill Companies, Inc..

Takayanagi, T., N. Akao, A. Suzuki, M. Tomoda, S. Tsukidate. 1999. New animal model for human ocular toxocariasis: ophthalmoscopic observation. Birtish Journal of Ophthalomogy, 83: 967-972.

Xi, W., L. Jin. 1998. A novel method for the recovery of Toxocara canis in mice. Journal of Helminthology, 72: 183-184.