Toxocara canis also known as dog roundworm is a worldwide-distributed helminth parasite of dogs and other canids. The name is derived from the Greek word "toxon," meaning bow or quiver, and the Latin word "caro," meaning flesh. 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.
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Toxocariasis in humans is caused by infection with larvae of Toxocara spp. Confirmed zoonotic species include the dog roundworm T. It is not known whether other closely-related Toxocara species can infect humans e. Toxocara canis infects essentially all wild and domestic canids; patent infections are more prevalent among puppies than older dogs.
Paratenic host ranges for both species encompass numerous species of mammals and birds. Livestock are important paratenic hosts; some human cases have been linked to consumption of undercooked beef, lamb, chicken, and duck meat particularly liver. Cockroaches and earthworms have been experimentally infected, and could possibly serve as paratenic or transport hosts. Toxocara canis and T. While common globally, prevalence in both animals and people is highest in developing countries.
In developed countries, more infections are detected among persons in lower socioeconomic strata. The main clinical presentations of toxocariasis are visceral larva migrans VLM and ocular larva migrans OLM , although most infections are asymptomatic. In VLM, which occurs mostly in preschool children, the larvae invade multiple tissues commonly liver, lung, skeletal muscle, occasionally heart and cause various nonspecific symptoms e.
Migration to the central nervous system neurotoxocariasis or neural larva migrans NLM is uncommon and can cause eosinophilic meningoencephalitis. Death can occur in instances of severe cardiac, pulmonary, or neurologic involvement. Involvement is typically unilateral affecting one eye and associated visual impairment usually presents with uveitis, retinitis, or endophthalmitis; permanent visual damage or blindness can occur.
Associated larval granulomas have in some cases have been misdiagnosed as retinoblastoma. OLM most often occurs in older children or young adults, who uncommonly have visceral manifestations. Eggs are not clinically diagnostic for human cases as humans are incapable of harboring adult worms that pass eggs.
Humans are paratenic hosts for Toxocara spp. Toxocara eggs are golden in color, spherical to slightly pear shaped, thick-shelled, and have a pitted surface. The size range for different species varies slightly; T. Toxocara sp. Toxocara spp. Adult Toxocara spp. They also possess large, spear-shaped cervical alae, which are broader in T.
In tissue, Toxocara spp. Two large excretory columns are present and single-pointed lateral alae are conspicuous along most of the body length. Diagnosis of toxocariasis relies mostly on indirect means, particularly serology, since larvae are trapped in tissues and not readily detected morphologically. While visualization of larvae in histologic sections provides unequivocal diagnosis, the probability of capturing a larva in a small biopsy specimen is low.
Since the larvae do not develop into adults in humans, a stool examination would not detect any Toxocara eggs. Antibody detection tests are the only means of confirmation of a clinical diagnosis of visceral larva migrans VLM and ocular larva migrans OLM , clinical syndromes associated with Toxocara infections.
The currently recommended serologic test for toxocariasis is enzyme immunoassay EIA with larval stage antigens extracted from embryonated eggs or released in vitro by cultured third-stage larvae.
The latter, Toxocara excretory-secretory TES antigens, are preferable to larval extracts because they are convenient to produce and an absorption-purification step is not required for obtaining maximum specificity.
Further confirmation of serologic diagnosis of OLM can be obtained by testing aqueous or vitreous humor samples for antibodies.
Moreira, G. Human toxocariasis: current advances in diagnostics, treatment, and interventions. Trends in Parasitology , 30 9 , pp. Fillaux, J. Laboratory diagnosis of human toxocariasis. Veterinary Parasitology , 4 , pp. Fisher, M. Toxocara cati : an underestimated zoonotic agent. Trends in Parasitology , 19 4 , pp. DPDx is an educational resource designed for health professionals and laboratory scientists.
For an overview including prevention, control, and treatment visit www. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Section Navigation. Minus Related Pages. Life Cycle View Larger. Figure A: Toxocara sp. The worm was never identified, but the egg size is most consistent with T. Figure B: Toxocara sp. Figure C: Toxocara sp. Toxocara canis larva hatching. Figure A: Toxocara canis larva beginning to hatch. Figure B: T.
Figure C: T. Adult Toxocara sp. Figure A: Toxocara canis adult male and female. Figure B: Close-up of the anterior end of Toxocara canis , showing the three lips characteristic of ascarid worms. Figure C: Close-up of the anterior end of Toxocara cati , showing the three lips characteristic of ascarid worms. Figure E: Close-up of the posterior end of T. Figure F: Close-up of the posterior end of Toxocara sp. Figure A: Cross-section of Toxocara sp. Figure B: Longitudinal section of a Toxocara sp.
Figure C: Longitudinal section of a Toxocara sp. Figure D. Toxocara canis longitudinal section arrow in the liver of an infected monkey.
Note the prominent granulomatous reaction and host infiltrate around the larva 20x magnification. Figure E: Cross-section of a larva of T. Laboratory Diagnosis Diagnosis of toxocariasis relies mostly on indirect means, particularly serology, since larvae are trapped in tissues and not readily detected morphologically. Antibody Detection Antibody detection tests are the only means of confirmation of a clinical diagnosis of visceral larva migrans VLM and ocular larva migrans OLM , clinical syndromes associated with Toxocara infections.
Special considerations: The assay detects infections caused by both T. Evaluation of the true sensitivity and specificity of serologic tests for toxocariasis in human populations is not possible because of the lack of feasible parasitological methods to detect migrating Toxocara larvae in tissue. These inherent problems result in underestimations of sensitivity and specificity.
When interpreting serologic findings, clinicians must be aware that a measurable titer does not necessarily indicate current clinical Toxocara infection.
In most human populations, a small number of those tested have positive EIA titers that apparently reflect the prevalence of asymptomatic toxocariasis, or past infections that have since been cleared. Paired serum samples demonstrating a significant rise in antibody level over time may be useful to confirm active infection.
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