toxocariasis toxocariasis toxocariasis is an illness of humans caused by the dog roundworm (toxocara canis) and, less frequently, the cat roundworm (toxocara cati). these are the most common intestinal roundworms of dogs, coyotes, wolves and foxes and domestic cats, respectively. humans are among the many "accidental" or paratenic hosts of these roundworms. toxocariasisspecialtyparasitology, infectious diseasesymptomsfever, enlarged lymph nodes, cough, bronchospasm, wheezing, abdominal pain, headaches, shortness of breath, visual impairmentcomplicationsrespiratory failure, meningitis, encephalitis, epileptic seizures, myocarditis, blindnesscausesdog roundworm (toxocara canis), cat roundworm (toxocara catidiagnostic methodserologic testing (serum total ige, elisa to detect specific anti-toxocara ige)differential diagnosisbaylisascaris procyonis, filariasispreventioncontrol of parasites in domestic dogs and cats, proper disposal of pet feces, hand hygienetreatmentanthelmintic medicationsmedicationalbendazole, diethylcarbamazine while this zoonotic infection is usually asymptomatic, it may cause severe disease. there are three distinct syndromes of toxocariasis: covert toxocariasis is a relatively mild illness very similar to löffler's syndrome. it is characterized by fever, eosinophilia, urticaria, enlarged lymph nodes, cough, bronchospasm, wheezing, abdominal pain, headaches, and/or hepatosplenomegaly. visceral larva migrans (vlm) is a more severe form of the disease; signs and symptoms depend on the specific organ system(s) involved. lung involvement may manifest as shortness of breath, interstitial lung disease, pleural effusion, and even respiratory failure. brain involvement may manifest as meningitis, encephalitis, or epileptic seizures. cardiac involvement may manifest as myocarditis. ocular larva migrans (olm) is the third syndrome, manifesting as uveitis, endophthalmitis, visual impairment or even blindness in the affected eye. signs and symptoms physiological reactions to toxocara infection depend on the host's immune response and the parasitic load. most cases of toxocara infection are asymptomatic, especially in adults. when symptoms do occur, they are the result of migration of second-stage toxocara larvae through the body. covert toxocariasis is the least serious of the three syndromes and is believed to be due to chronic exposure. signs and symptoms of covert toxocariasis are coughing, fever, abdominal pain, headaches, and changes in behavior and ability to sleep. upon medical examination, wheezing, hepatomegaly, and lymphadenitis are often noted. high parasitic loads or repeated infection can lead to visceral larva migrans (vlm). vlm is primarily diagnosed in young children, because they are more prone to exposure and ingestion of infective eggs. toxocara infection commonly resolves itself within weeks, but chronic eosinophilia may result. in vlm, larvae migration incites inflammation of internal organs and sometimes the central nervous system. symptoms depend on the organs affected. patients can present with pallor, fatigue, weight loss, anorexia, fever, headache, skin rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. sometimes the subcutaneous migration tracks of the larvae can be seen. patients are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (igm, igg, and ige classes), leukocytosis, and elevated anti-a and anti-b isohaemagglutinins. severe cases have occurred in people who are hypersensitive to allergens; in rare cases, epilepsy, inflammation of the heart, pleural effusion, respiratory failure, and death have resulted from vlm. retinal granuloma in ocular toxocariasis ocular larva migrans (olm) is rare compared with vlm. a light toxocara burden is thought to induce a low immune response, allowing a larva to enter the host's eye. although there have been cases of concurrent olm and vlm, these are extremely exceptional. olm often occurs in just one eye and from a single larva migrating into and encysting within the orbit. loss of vision occurs over days or weeks. other signs and symptoms are red eye, white pupil, fixed pupil, retinal fibrosis, retinal detachment, inflammation of the eye tissues, retinal granulomas, and strabismus. ocular granulomas resulting from olm are frequently misdiagnosed as retinoblastomas. toxocara damage in the eye is permanent and can result in blindness. a case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with toxocara. in this study, the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive elisa for t. canis. cause transmission transmission of toxocara to humans is usually through ingestion of infective eggs. t. canis can lay around 200,000 eggs per day. these eggs are passed in cat or dog feces, but the defecation habits of dogs cause t. canis transmission to be more common than that of t. cati. both toxocara canis and toxocara cati eggs require a several week incubation period in moist, humid weather outside a host before becoming infective, so fresh eggs cannot cause toxocariasis. many objects and surfaces can become contaminated with infectious toxocara eggs. flies that feed on feces can spread toxocara eggs to surfaces or foods. young children who put contaminated objects in their mouths or eat dirt (pica) are at risk of developing symptoms. humans can also contaminate foods by not washing their hands before eating. humans are not the only accidental hosts of toxocara. eating undercooked rabbit, chicken, or sheep can lead to infection; encysted larvae in the meat can become reactivated and migrate through a human host, causing toxocariasis. special attention should be paid to thoroughly cooking giblets and liver to avoid transmission. incubation period the incubation period for toxocara canis and cati eggs depends on temperature and humidity. t. canis females, specifically, are capable of producing up to 200,000 eggs a day that require 2–6 weeks minimum up to a couple months before full development into the infectious stage. under ideal summer conditions, eggs can mature to the infective stage after two weeks outside of a host. provided sufficient oxygen and moisture availability, toxocara eggs can remain infectious for years, as their resistant outer shell enables the protection from most environmental threats. however, as identified in a case study presented within the journal of helminthology, the second stage of larvae development poses strict vulnerabilities to certain environmental elements. high temperatures and low moisture levels will quickly degrade the larvae during this stage of growth. toxocariasis scientific classification kingdom: animalia phylum: nematoda class: chromadorea order: ascaridida family: toxocaridae genus: toxocara species species include: toxocara canis toxocara cati toxocara malaysiensis toxocara tanuki toxocara vitulorum reservoir puppies are the major source of environmental contamination with roundworm eggs dogs and foxes are the reservoir for toxocara canis, but puppies and cubs pose the greatest risk of spreading the infection to humans. infection in most adult dogs is characterized by encysted second stage larvae. however, these larvae can become reactivated in pregnant females and cross the placental barrier to infect the pups. vertical transmission can also occur through breastmilk. infectious mothers, and puppies under five weeks old, pass eggs in their feces. approximately 50% of puppies and 20% of adult dogs are infected with t. canis. cats are the reservoir for toxocara cati. as with t. canis, encysted second stage larvae in pregnant or lactating cats become reactivated. however, vertical transmission can only occur through breastfeeding. flies can act as mechanical vectors for toxocara, but most infections occur without a vector. most incidents with toxocariasis result from prokaryotic expression vectors and their transmission through direct physical contact with feces that results in the contraction of the illness. morphology both species produce eggs that are brown and pitted. t. canis eggs measure 75-90 µm and are spherical in shape, whereas the eggs of t. cati are 65-70 µm in diameter and oblong. second stage larvae hatch from these eggs and are approximately 0.5mm long and 0.02mm wide. adults of both species have complete digestive systems and three lips, each composed of a dentigerous ridge. adult t. canis are found only within dogs and foxes and the males are 4–6 cm in length, with a curved posterior end. the males each have spicules and one “tubular testis.” females can be as long as 15 cm, with the vulva stretching one third of their bodylength. the females do not curve at the posterior end. t. cati adult females are approximately 10 cm long, while males are typically 6 cm or less. the t. cati adults only occur within cats, and male t. cati are curved at the posterior end. life cycle cats, dogs and foxes can become infected with toxocara through the ingestion of eggs or by transmission of the larvae from a mother to her offspring. transmission to cats and dogs can also occur by ingestion of infected accidental hosts, such as earthworms, cockroaches, rodents, rabbits, chickens, or sheep. eggs hatch as second stage larvae in the intestines of the cat, dog or fox host (for consistency, this article will assume that second stage larvae emerge from toxocara eggs, although there is debate as to whether larvae are truly in their second or third stage of development). larvae enter the bloodstream and migrate to the lungs, where they are coughed up and swallowed. the larvae mature into adults within the small intestine of a cat, dog or fox, where mating and egg laying occurs. eggs are passed in the feces and only become infective after three weeks outside of a host. during this incubation period, molting from first to second (and possibly third) stage larva takes place within the egg. in most adult dogs, cats and foxes, the full lifecycle does not occur, but instead second stage larvae encyst after a period of migration through the body. reactivation of the larvae is common only in pregnant or lactating cats, dogs and foxes. the full lifecycle usually only occurs in these females and their offspring. second stage larvae will also hatch in the small intestine of an accidental host, such as a human, after ingestion of infective eggs. the larvae will then migrate through the organs and tissues of the accidental host, most commonly the lungs, liver, eyes, and brain. since l2 larvae cannot mature in accidental hosts, after this period of migration, toxocara larvae will encyst as second stage larvae. diagnosis finding toxocara larvae within a patient is the only definitive diagnosis for toxocariasis; however, biopsies to look for second stage larvae in humans are generally not very effective. pcr, elisa, and serological testing are more commonly used to diagnose toxocara infection. serological tests are dependent on the number of larvae within the patient, and are unfortunately not very specific. elisas are much more reliable and currently have a 78% sensitivity and a 90% specificity. a 2007 study announced an elisa specific to toxocara canis, which will minimize false positives from cross reactions with similar roundworms and will help distinguish if a patient is infected with t. canis or t. cati. olm is often diagnosed after a clinical examination. granulomas can be found throughout the body and can be visualized using ultrasound, mri, and ct technologies. prevention actively involving veterinarians and pet owners is important for controlling the transmission of toxocara from pets to humans. a group very actively involved in promoting a reduction of infections in dogs in the united states is the companion animal parasite council -- capc. since pregnant or lactating dogs and cats and their offspring have the highest, active parasitic load, these animals should be placed on a deworming program. pet feces should be picked up and disposed of or buried, as they may contain toxocara eggs. practicing this measure in public areas, such as parks and beaches, is especially essential for decreasing transmission. up to 20% of soil samples of u.s. playgrounds have found roundworm eggs. also, sandboxes should be covered when not in use to prevent cats from using them as litter boxes. hand washing before eating and after playing with pets, as well as after handling dirt will reduce the chances of ingesting toxocara eggs. washing all fruits and vegetables, keeping pets out of gardens and thoroughly cooking meats can also prevent transmission. finally, teaching children not to place nonfood items, especially dirt, in their mouths will drastically reduce the chances of infection. toxocariasis has been named one of the neglected diseases of u.s. poverty, because of its prevalence in appalachia, the southern u.s., inner city settings, and minority populations. unfortunately, there is currently no vaccine available or under development. however, the mitochondrial genomes of both t. cati and t. canis have recently been sequenced, which could lead to breakthroughs in treatment and prevention. treatment toxocariasis will often resolve itself, because the toxocara larvae cannot mature within human hosts. corticosteroids are prescribed in severe cases of vlm or if the patient is diagnosed with olm. either albendazole (preferred) or mebendazole (“second line therapy”) may be prescribed. granulomas can be