Affects: Cats, Dogs
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.
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.
Transmission: Toxocara is usually transmitted to humans 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.
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.
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 surgically removed, or laser photocoagulation and cryoretinopexy can be used to destroy ocular granulomas.
Visceral toxocariasis in humans can be treated with antiparasitic drugs such as albendazole or mebendazole, tiabendazole or diethylcarbamazine usually in combination with anti-inflammatory medications. Steroids have been utilized with some positive results. Anti-helminthic therapy is reserved for severe infections (lungs, brain) because therapy may induce, due to massive larval killing, a strong inflammatory response. Ocular toxocariasis is more difficult to treat and usually consists of measures to prevent progressive damage to the eye.
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 US poverty, because of its prevalence in Appalachia, the southern U.S., inner city settings, and minority populations.
There is currently no vaccine available or under development.