Toxocara canis
Encyclopedia
Toxocara canis is worldwide distributed helminth parasite of dogs and other canids. T. canis are gonochorists
Gonochorism
In biology, gonochorism or unisexualism describes sexually reproducing species in which individuals have just one of at least two distinct sexes. The term is most often used with animals . The sex of an individual may change during its lifetime, this can for example be found in parrotfish...

, adult worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

s measure from 9 to 18 cm, are yellow-white in color, and occur in the intestine of the definitive host. In adult dogs, the infection is usually asymptomatic. By the contrast, massive invasion of T. canis can be fatal in puppies. As paratenic
Paratenic
In parasitology, the term paratenic describes a host which is not needed for the development of the parasite, but nonetheless serves to maintain the life cycle of the parasite. Alaria americana may serve as an example: the so-called mesocercarial stages of this parasite reside in tadpoles, which...

 hosts, a number of various vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s, including humans, and some invertebrates can become infected. Humans are infected, like other paratenic hosts, by ingestion of embryonated T. canis eggs. The disease caused by migrating T. canis larvae (toxocariasis
Toxocariasis
-History of discovery:Werner described a parasitic nematode in dogs in 1782 which he named Ascaris canis. Johnston determined that what Werner had described was actually a member of the genus Toxocara established by Stiles in 1905. Fữlleborn speculated that T canis larvae might cause granulomatous...

) results in two syndromes: visceralis larva migrans and ocularis larva migrans
Ocular larva migrans
Ocular larva migrans is the ocular form of the larva migrans syndrome that occurs when toxocara canis larvae invade the eye. They may be associated with visceral larva migrans...

.
Owing to transmission of the infection from bitches to puppies, preventive anthelmintic
Anthelmintic
Anthelmintics or antihelminthics are drugs that expel parasitic worms from the body, by either stunning or killing them. They may also be called vermifuges or vermicides .-Pharmaceutical classes:...

 treatment of newborn puppies is strongly recommended. Several antihelmintic drugs are effective against adult worms, for example pyrantel
Pyrantel
Pyrantel is an antinematodal thiophene. It is often prescribed by veterinarians to treat and prevent the occurrence of intestinal parasites in small animal pets.-Mechanism of Action:Pyrantel is a nicotinic receptor agonist...

, fenbendazole
Fenbendazole
Fenbendazole is a broad spectrum benzimidazole anthelmintic used against gastrointestinal parasites including roundworms, hookworms, whipworms, the taenia species of tapeworms, pinworms, aelurostrongylus, paragonimiasis, strongyles and strongyloides and can be administered to sheep, cattle,...

, selamectine etc.

Morphology

Adult T. canis have round body with spiky cranial and caudal part, covered by yellow cuticula. Cranial part of the body contains two lateral alae (length 2–2.5 mm, width 0.2 mm). Male worms measure 9–13 × 0.2–0.25 cm and female worms 10–18 × 0.25–0.3 cm. T. canis eggs have oval or spherical shape with granulated surface, thick-walled, and measures from 72 to 85 μm.

Life cycle

There are four modes of infection associated with this species. The basic form is typical to all ascaroides
Ascaridida
The order Ascaridida includes several families of parasitic roundworms with three "lips" on the anterior end. They were formerly placed in the subclass Rhabditia by some, but morphological and DNA sequence data rather unequivocally assigns them to the Spiruria...

, the egg containing the L2(the second larval developmental stage), being infective, at optimal temperature and humidity, four weeks after secreted in the faeces to the environment. After ingestion, and hatching in the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...

, the L2 travel through the portal blood stream into the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 and lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s. Such migratory route is known as entero-hepatic-pulmonar larval migration. The second molt takes place in the lungs, the now L3 returns via the trachea
Vertebrate trachea
In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...

 and into the intestines where the final two molts take place. This form of infection occurs regularly only in dogs of up to three months of age.

In older dogs, this type of migration occurs less frequently and at six months it is almost ceased. Instead, the L2 travel to a wide range of organs including the liver, lungs, brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

s, as well as to the walls of the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

. In pregnant bitch,parental infection occurs when larvae becoming mobilized at approximately, three week prior to parturition, and migrate to the lungs of the foetus where they molt into L3 just prior to birth. In the newborn pup the cycle is completed when the larva migrates through the trachea and into the intestinal lumen, where the final molts take place. Once infected, a bitch will usually harbor sufficient larvae to subsequently infect all of her litters, even if she never again encounters an infection. A certain amount of the bitch's dormant larvae penetrate into the intestinal lumen, where molting into adulthood take, yet again, place, thus leading to a new release of eggs containing L1 larvae.

The suckling pup may be infected by the presence of L3 in the milk during the first three weeks of lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

. There is no migration in the pup via this route.
L2 may also be ingested by a variety of animals where it stays in a dormant stage inside the animals tissue until the intermediate host has been eaten by a dog, when subsequent development is confined to the gastrointestinal tract.

External links

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