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''Toxocara canis'' (also known as dog roundworm) is a worldwide-distributed
helminth Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as ...
parasite of dogs and other
canid Canidae (; from Latin, '' canis'', " dog") is a biological family of dog-like carnivorans, colloquially referred to as dogs, and constitutes a clade. A member of this family is also called a canid (). There are three subfamilies found withi ...
s. The name is derived from the Greek word "," meaning bow or quiver, and the Latin word "caro," meaning flesh.Bassert , J., & Thomas, J. (2014). McCurnin's Clinical Textbook for Veterinary Technicians. (8th ed.). St. Louis , MO: Elsevier They live in the small intestine of the definitive host. In adult dogs, the infection is usually asymptomatic but may be characterized by diarrhea. By contrast, massive infection with ''Toxocara canis'' can be fatal in puppies, causing diarrhea, vomiting, an enlarged abdomen, flatulence, and poor growth rate. As
paratenic In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
hosts, a number of
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s, including humans, and some
invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...
can become infected. Humans are infected, like other paratenic hosts, by ingestion of embryonated ''T. canis''
eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
. The disease (
toxocariasis Toxocariasis is an illness of humans caused by the dog roundworm (''Toxocara canis'') and, less frequently, the cat roundworm ('' Toxocara cati'').https://academic.oup.com/ajcp/article/142/suppl_1/A104/1771175 Eosinophilic Pseudoleukemia Due to T ...
) caused by migrating ''T. canis'' larvae results in two syndromes: visceral larva migrans and ocular larva migrans. Owing to transmission of the infection from the mother to her puppies, preventive
anthelmintic Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host. They may ...
treatment of newborn puppies is strongly recommended. Several anthelmintic drugs are effective against adult worms, for example pyrantel,
fenbendazole Fenbendazole is a broad spectrum benzimidazole anthelmintic used against gastrointestinal parasites including: giardia, roundworms, hookworms, whipworms, the tapeworm genus '' Taenia'' (but not effective against '' Dipylidium caninum'', a comm ...
, and
selamectin Selamectin (trade names Revolution and Stronghold manufactured by Zoetis, and Revolt manufactured by Aurora Pharmaceuticals, among others) is a topical parasiticide and anthelminthic used on dogs and cats. It treats and prevents infections of ...
.


Morphology

''T. canis'' is dioecious, having morphology distinctly different between the male and female. Male worms measure 4 to 6 cm (1.5" to 2.3"), typically smaller than female worms who measure at 6.5 to 15 cm (2.6" to 5.9"). The male's posterior end is curved ventrally and the tail is bluntly pointed. The male has a single tubular testis. They also have simple spicules, which allow for direct sperm transfer. 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 chords are visible with the naked eye. No
gubernaculum The paired gubernacula (from Ancient Greek κυβερνάω = pilot, steer) also called the caudal genital ligament, are embryonic structures which begin as undifferentiated mesenchyme attaching to the caudal end of the gonads (testes in males ...
is present. In both sexes there are prominent cervical alae. The adult'' T. canis'' has a round body with spiky cranial and caudal parts, covered by yellow
cuticula A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
. ''Toxocara canis'' is
gonochoristic In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism c ...
. The cranial part of the body contains two lateral alae (length 2 to 3.5 mm, width 0.1 mm). The eggs are brownish and almost spherical.''T. canis'' eggs have oval or spherical shapes with granulated surfaces, are thick-walled, and measure from 72 to 85 μm. The eggs are very resistant to various weather and chemical conditions typically found in soil.


Life cycle

Eggs are deposited in feces of dogs, becoming infectious after 2–4 weeks. Dogs ingest infectious eggs, allowing the eggs to hatch and the larval form of the parasite to penetrate through the gut wall. In dogs under 3 months of age, the larvae hatch in the small intestine, get into the bloodstream, migrate through the liver, and enter the lungs. Once in the lungs, the larvae crawl up the trachea. The larvae are then coughed up and swallowed, leading back down to the small intestine, where they mature to adulthood. This process is called tracheal migration. In dogs older than 3 months of age, the larvae hatch in the small intestine and enter the bloodstream, where they are carried to somatic sites throughout the body (muscles, kidney, mammary glands, etc.) where they become encysted second stage larvae. This process is called somatic migration. At the height of pregnancy, the encysted eggs in an infected female dog will migrate from the mother to the developing fetus, where they will reside in the liver. After parturition, the larvae migrate from the pup's liver to the lungs to undergo tracheal migration. Alternatively, the migrating larvae in the mother may encyst within the mammary glands, becoming active during lactation and passing directly to the nursing puppy via the milk. Larvae transmitted in this manner do not migrate once they are within the small intestine of the puppy; they will develop directly into the adult stage in the small intestine. Once infected, a female dog 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 female dog's dormant larvae penetrate into the intestinal lumen, where molting into adulthood takes place again, thus leading to a new release of eggs containing L1 larvae. Another possible route of infection is the ingestion of paratenic hosts that contain encysted larvae from egg consumption, allowing the parasite to escape from the paratenic host and grow to adulthood within the small intestine of its definitive host, the dog. Four modes of infection are associated with this species. These modes of infection include direct transmission, prenatal transmission, paratenic transmission, and transmammary transmission.Johnstone, Colin Dr. "Parasites and Parasitic Diseases of Domestic Animals." University of Pennsylvania, 24 Jan. 2000. http://cal.vet.upenn.edu/projects/merial/ascarids/asc_05a.html . 22 Apr. 2014.. Transmammary transmission occurs when the suckling pup becomes infected by the presence of L3 larvae in the milk during the first three weeks of
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 naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
. There is no migration in the pup via this route. L2 larvae may also be ingested by a variety of animals like mice or rabbits, where they stay in a dormant stage inside the animals' tissue until the intermediate host has been eaten by a dog, where subsequent development is confined to the gastrointestinal tract."Toxocariasis." Parasites. Centers for Disease Control and Prevention, 10 Jan. 2013. https://www.cdc.gov/parasites/toxocariasis/treatment.html. 22 Apr. 2016. thumbnail, 270px, ''Toxocara'' eggs- the surface contour is pitted due to the mammilation of the protein layer of the shell, and because of this the eggs are murky


Transmission to humans

Consumption of eggs from feces-contaminated items is the most common method of infection for humans especially children and young adults under the age of 20 years. Although rare, being in contact with soil that contains infectious eggs can also cause human infection, especially handling soil with an open wound or accidentally swallowing contaminated soil, as well as eating undercooked or raw meat of an intermediate host of the parasite such as lamb or rabbit. Humans can be infected by this roundworm, a condition called
toxocariasis Toxocariasis is an illness of humans caused by the dog roundworm (''Toxocara canis'') and, less frequently, the cat roundworm ('' Toxocara cati'').https://academic.oup.com/ajcp/article/142/suppl_1/A104/1771175 Eosinophilic Pseudoleukemia Due to T ...
, just by stroking an infected dog's fur and accidentally ingesting infective eggs that may be present on the dog's fur. When humans ingest infective eggs, diseases like
hepatomegaly Hepatomegaly is the condition of having an enlarged liver. It is a non-specific medical sign having many causes, which can broadly be broken down into infection, hepatic tumours, or metabolic disorder. Often, hepatomegaly will present as an abdo ...
,
myocarditis Myocarditis, also known as inflammatory cardiomyopathy, is an acquired cardiomyopathy due to inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. Th ...
, respiratory failure and vision problems can result depending on where the larva are deposited in the body.. Updated 16 Feb. 2016. In humans, this parasite usually grows in the back of the eye, which can result in blindness, or in the liver or lungs."Dog stroking can transmit debilitating parasite"
'' ew Scientist'. Andy Coghlan. Published 4-23-2003. Accessed 6-01-2012.
However, a 2004 study showed, of 15 infected dogs, only seven had eggs in their
coat A coat typically is an outer garment for the upper body as worn by either gender for warmth or fashion. Coats typically have long sleeves and are open down the front and closing by means of buttons, zippers, hook-and-loop fasteners, toggles, ...
s, and no more than one egg was found on each dog. Furthermore, only 4% of those eggs were
infectious An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
. Given the low concentration of fertile eggs on infected dogs' coats (less than 0.00186% per gram), it is plausible that such eggs were transferred to the dog's coat by contact with fecal deposits in the environment, making dog coats the passive transport host vehicle. However, although the risk of being infected by petting a dog is extremely limited, a single infected puppy can produce more than 100,000 roundworm eggs per
gram The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...
of
feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a rela ...
.


Treatment

Humans suffering from visceral infection of ''T. canis'', the drugs
albendazole Albendazole (also known as albendazolum) is a broad-spectrum anthelmintic and antiprotozoal agent of the benzimidazole type. It is used for the treatment of a variety of intestinal parasite infections, including ascariasis, pinworm infection, ho ...
(preferred),
mebendazole Mebendazole (MBZ), sold under the brand name Vermox among others, is a medication used to treat a number of parasitic worm infestations. This includes ascariasis, pinworm infection, hookworm infections, guinea worm infections, hydatid disea ...
and
thiabendazole Tiabendazole (INN, BAN), also known as thiabendazole (AAN, USAN) or TBZ and the trade names Mintezol, Tresaderm, and Arbotect, is a preservative, an antifungal agent, and an antiparasitic agent. Uses Preservative Tiabendazole is used primar ...
are highly effective. For other treatments, refer to the disease pages: visceralis larva migrans and ocularis larva migrans. Anthelminthic drugs are used to treat infections in dogs and puppies for adult worms. Treatment protocol will vary based on the dog's age, production level and activity level. There are different treatment paths for puppies, pregnant bitches, lactating bitches, dogs with increased risk of infection, professional dogs, and dogs sharing homes with young children or immunocompromised individuals. Puppies: from the age of two weeks, then every 14 days up to two weeks after weaning with fenbendazole/febantel, flubendazole, pyrantel, or nitroscanate, followed by monthly treatments for up to six months of age. Pregnant bitches: to prevent transmission to the puppies, pregnant females can be given macrocyclic lactones on the 40th and 55th day of pregnancy or genbendazole daily from the 40th day of pregnancy continuing until the 14th day postpartum. Lactating bitches: should be treated concurrently with the first treatment of puppies. Dogs with increased risk of infection: i.e. those used in sports, competitions, shows, or those kept in kennels can be given two treatments 4 weeks before and 2–4 weeks after the event. Professional dogs: i.e. therapy, rescue, or police dogs: 12 times a year, if excretion of worm eggs is to be excluded. Dogs sharing homes with young children or immunocompromised individuals: 12 times a year, if excretion of worm eggs is to be excluded. ESCCAP. (2017). “Worm Control in Dogs and Cats”. ''European Scientific Counsel Companion Animal Parasites.'' https://www.esccap.org/uploads/docs/0x0o7jda_ESCCAP_Guideline_01_Third_Edition_July_2017.pdf   


Prevention

There are several ways to prevent a ''T. canis'' infection in both dogs and humans. Regular deworming by a veterinarian is important to stop canine re-infections, especially if the dog is frequently outdoors. Removing dog feces from the yard using sealed disposable bags will help control the spread of ''T. canis.'' Good practices to prevent human infections include: washing hands before eating and after disposing of animal feces, teaching children not to eat soil, and cooking meat to a safe temperature in order to kill potentially infectious eggs.


See also

* List of parasites (human) *
Toxocara cati ''Toxocara cati'', also known as the feline roundworm, is a parasite of cats and other felids. It is one of the most common nematodes of cats, infecting both wild and domestic felids worldwide. Adult worms are localised in the gut of the host. ...
*
Toxascaris leonina ''Toxascaris leonina'' is a common parasitic roundworm found in dogs, cats, foxes, and related host species. ''T. leonina'' is an ascarid nematode, a worldwide distributed helminth parasite which is in a division of eukaryotic parasites that, u ...


References


External links


CDC Parasitic Diseases Division – ToxocariasisRoundworms: Dogs and Puppies from The Pet Health Library
{{Taxonbar, from=Q133092 Ascaridida Veterinary helminthology Parasites of dogs Dog diseases Nematodes described in 1782