Toxocariasis is an infectious disease caused by the parasite Toxocara, a worm of dogs and cats. Toxocariasis is not limited to pet owners. The eggs of the parasite are passed in the stool and lie dormant in the soil. For unknown reasons, humans become infected when exposed to the eggs passed only by dogs. Infection occurs when there is purposeful or incidental ingestion of soil from hand to mouth through such activities as biting finger nails or inserting recently contaminated objects such as toys into the mouth. (Consequently, the disorder is found disproportionately among children.) Once ingested, the eggs hatch into larvae and burrow into body tissue of all types. The symptoms experienced depend on the number of eggs ingested and the person's immune status, yet a single egg has the potential of causing blindness. Everywhere the larvae travel, they cause inflammation and tissue death.
The two forms of the disease are: ocular larvae migrans (OLM) and visceral larvae migrans (VLM). Rarely are they reported to occur together. OLM is more frequently diagnosed since it affects vision. Symptoms may include a brief redness of the sclera (white of the eye) without pain, a “whitish” appearance of the pupil, visual acuity changes, or blindness of one eye. In the absence of obvious symptoms, OLM can be recognized by a qualified provider of ophthalmological health services. Symptoms of VLM may include wheezing, heart rhythm changes, seizures, headaches, and recurrent, intense abdominal and leg muscle pains. It is these later symptoms that are often misdiagnosed. In addition, VLM can be covert (hidden from view) in which case no symptoms are noticeable in the infected person. VLM is of concern because it may progress to OLM or make other illnesses, including autoimmune disease, worse.
Covert Toxicariasis can be long lasting because, if the parasite, Toxocara canis, does not complete its life cycle in humans the organism remains in body tissue. The medical literature reports cases in which long periods elapse between Covert Toxocariasis and the development of symptoms associated with Visceral or Ocular Larva Migrans.
Toxocariasis is caused by human ingestation of the eggs of a common roundworm parasite excreted by dogs (Toxocara canis) or cats (Toxocara cati). The most common source of Toxocara canis and/or cati eggs is contaminated soil. Adult roundworms are often found in the gastrointestinal tract of dogs and cats. These roundworms release a large number of eggs that are passed from the animal through the feces, leading to contamination of the soil. Toxocara canis and cati eggs can survive in the soil for several years. In one large study, 99.4% of puppies were infected with Toxocara canis at birth. Children may be infected with Toxocariasis by putting dirty fingers in their mouths, eating dirt, placing dirty toys in their mouths, biting their fingernails, and/or petting animals that have recently rolled in the dirt. In rare cases, adults may also be affected due to exposure to contaminated soil or due to initial onset of symptoms years after an apparent case of Covert Toxocariasis in which the larvae are no longer dormant.
In humans, ingested eggs hatch within the small intestine, producing larvae that penetrate the intestinal wall and spread (migrate) throughout the body via the circulatory system. When larvae reach blood vessels with small diameters that prevent their entrance, they penetrate into the surrounding tissue. Toxocara canis and/or cati larvae have been found in the liver, lungs, and, less often, the brain, heart, and eyes. The larvae leave behind pathways of inflammation and dead tissue (necrosis). Inflammatory masses of tissue consisting of granular white blood cells (eosinophilic granulomas) remain at such sites. In some cases, the larvae may die, causing an inflammatory response from the affected area of the body. In still other cases, larvae may cease their migration, temporarily becoming dormant. However, they remain within the body tissue and may resume their migration after long periods of time, up to several years after going into dormancy.
As a result of their tendency to play in dirt, toxocariasis is most often diagnosed in children. However, adults can also be infected and blindness has been reported. Toxocara eggs survive best in warm, moist climates, but have been found in all parts of the United States.
Visceral Larva Migrans (VLM) usually affects children between the ages of one to four years; Ocular Larva Migrans (OLM) usually affects older children and adults. In rare cases where VLM and OLM are both present, those affected are usually under five years of age.
Nearly all puppies are infected prior to birth. Larvae are passed from the mother dog to pups across the placenta and during nursing. The Centers for Disease Control recommends that dogs be wormed starting at two to three weeks of age. Periodic worming of older dogs is also recommended. During worming, stools should be wrapped and discarded in the garbage. (Do not flush down the toilet; eggs are resistant to sewage treatment). In addition, personal hygiene measures are necessary, such as thoroughly washing hands, cleaning under fingernails, and closely monitoring children's activities to prevent eating or sucking on dirt or dirt-contaminated objects.
Since both forms have several non-specific presentations, the diagnosis is often missed. Clinicians must rely on all clinical and laboratory data available. The ELISA titer forToxocara is the most accurate test and can be reconfirmed by a Western Blot test. Blood tests on immunoglobins have been tried but the results vary widely from one research study to another. Human toxocariasis can not be diagnosed through stool samples since neither the eggs nor the larvae are passed in the stool.
The use of anti-parasitic medications is controversial; a variety of them have been tried without notable success because the medication is effective only on active larvae. At any time, some larvae are able to enter the dormant stage and then to reactivate in a random fashion, continuing on their migratory way. If and when toxocariasis is diagnosed at an early stage, some of the symptoms may be minimized by the temporary use of steroids.
The treatment of Toxocariasis is directed toward the specific problems that are apparent in eac. individual. Since most cases of Toxocariasis are mild and self-limited, with most symptoms subsiding over a period of weeks or months, treatment often is not necessary. In severe cases, however, anti-inflammatory drugs (e.g., prednisone) may be used to improve respiratory function, anticonvulsants may be prescribed to treat seizures, and corticosteroids may be used to help minimize the symptoms and potential damage to the eye.
In many cases of Toxocariasis, treatment with medications that are destructive to worms (anthelmintic agents) may be recommended. Such anthelmintic agents may include mebendazole and diethylcarbamazine. Other anthelmintic agents, such as albendazole and thiabendazole, have been used to treat severe cases of Toxocariasis. According to the medical literature, albendazole may be used as an alternative to diethylcarbamazine. There is significant disagreement within the medical literature as to the effectiveness of antihelmintic agents, particularly thiabendazole, as a treatment for Toxocariasis. No specific method used to treat Toxocariasis has been beneficial in every case, and, since controlled clinical trials of these treatments are lacking, there is no specific, proven form of treatment for the disorder. More research and study is needed to determine the long-term safety and effectiveness of these medications for the treatment of Toxocariasis.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
For information about clinical trials sponsored by private sources, contact:
(Please note that some of these organizations may provide information concerning certain conditions potentially associated with this disorder [e.g., eye abnormalities].)
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