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Toxocara

Toxocara is a helminth worm of the phyla roundworms (nematodes) and is the causative agent of a number of diseases, including Toxocariasis in humans. Toxocara canis is the dog roundworm, and is a worldwide parasite of dogs and other canids. In adult dogs, the infection is usually asymptomatic but it can be fatal in puppies. Humans, usually children, can also become infected by this roundworm, which may result in significant morbidity, causing damage to a range of organs, including the lungs, liver, or eyes.

Toxocara Background

Toxocara is classified under the super-family Ascaridoidea, and includes four species, namely Toxocara canis, T. cati, T. malaysiensis, and T. vitulorum. T. canis are unsegmented roundworms, which contain a body cavity, a digestive tract and have both male and female sexes. Male worms measure 4-6 cm, and are typically smaller than female worms who measure 6.5-15 cm. When a dog or cat ingests infectious Toxocara eggs, the larvae penetrate through the gut wall and mature into adult worms in the intestines. In dogs under 3 months of age larvae follow a process called tracheal migration; they hatch in the small intestine, move into the bloodstream, migrate through the liver, and enter the lungs. Once in the lungs, the larvae crawl up the trachea, where they are coughed up and swallowed, leading back down to the small intestine, to mature to adulthood. In dogs older than 3 months of age, larvae follow a process known as somatic migration. 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.), eventually become encysted second stage larvae. In an infected female dog during pregnancy, the encysted eggs 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. Eggs are deposited in the dogs feces and are very resistant to various weather and chemical conditions typically found in soil, becoming infectious after 2–4 weeks (Chen et al., 2018).

Toxocara is a good example of a parasite moving from wild canids to their domestic counterparts and then to humans. The first human infection was reported in 1950, and since then it has been reported in almost 100 countries with most cases occurring in France, Austria, India, Japan, Korea, China, USA, and Brazil. In the UK, urban and rural foxes are the primary source of eggs and infections to humans. The human transmission cycle begins when they ingest infectious Toxocara eggs in soil contaminated by dog or cat feces. Other routes are possible, including stroking an infected dog and ingesting eggs that are present on the dog’s fur. Once inside the human intestine, Toxocara larvae hatch but do not undergo further maturation into adult worms; instead, the larvae burrow through the intestinal wall into blood vessels and then migrate to various organs in the body. Infected humans cannot transmit toxocariasis to others because humans do not excrete eggs i.e., they are paratenic hosts. The disease (toxocariasis) caused by migrating T. canis larvae results in two syndromes: visceral larva migrans (affecting lungs, liver, and central nervous system) and ocular larva migrans, where the larvae enter the posterior segment of the eye (Woodhall & Fiore, 2014).

Toxocariasis has a significant socioeconomic impact, particularly on impoverished communities around the world. Over the last few years, toxocariasis has gained an increasing international attention and was listed among the five most neglected parasitic infections according to the US Centers for Disease Control and Prevention (CDC). There is no anti-toxocariasis vaccine and anthelminthic drugs are used to treat infections in dogs and humans for adult worms. However, chemotherapy in humans varies, depending on symptoms and location of larvae (Rostami et al., 2019). Other methods used to decrease the risk of ingesting Toxocara eggs include regularly deworming pets, removing pet waste, promoting good hand hygiene and teaching children the dangers of eating soil.

References

  • Chen et al. (2018). Toxocariasis: a silent threat with a progressive public health impact. Infect Dis Poverty. 7: 59.
  • Rostami et al. (2019). Human toxocariasis – A look at a neglected disease through an epidemiological ‘prism’. Infect Genet Evol. 74:104002.
  • Woodhall & Fiore (2014). Toxocariasis: A Review for Pediatricians. Journal of the Pediatric Infectious Diseases Society, Volume 3, Issue 2.

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