Toxocariasis is an accidental human infection with the cat and dog Ascaris (Toxocara cati and Toxocara canis). The eggs of these species are infective two to three weeks after being passed, and if ingested by man the second stage larvae emerge. These penetrate the intestinal wall and reach the liver. The majority remain in the liver but others migrate to other organs, particularly the brain and the eye. Rarely they complete their cycle of development in man and produce adult worms in the bowel. Children are particularly susceptible because of more frequent soiling of the fingers and habits of playing with puppies, in which the incidence of infection is very high. Initially reported from the southern United States, these parasites are common in dogs and cats in many parts of the world. Although many reports have come from North America and Europe, it is likely that this syndrome also occurs in other parts of the world.
The migrating larvae produce “eosinophilic trails” and tissue inflammation in the affected organ. Usually a granuloma forms with epithelial cells, fibroblasts, lymphocytes, plasma cells, and occasional giant cells. A fibrous capsule eventually encloses the larva, which may remain alive for months. The granulomas can be seen microscopically as small grayish-white spots. Such granulomas have been found in the lungs, eyes, brain, heart, kidney, and striated muscle.
The most common clinical form is that of a patient with a mild fever and tender hepatomegaly. Routine investigation reveals a marked eosinophilia (50 to 60 per cent), and further questioning often brings to light a history of contact with dogs. These signs and symptoms may persist for 18 months. The serum globulins may be elevated and anti-r globulins are found. Another clinical form is as an endophthalmitis with a space-occupying granuloma visibly distorting the contour of the retina on funduscopy. In the past such granulomas were often mistaken for retinoblastomas, and the eye was removed. Several such series of “retinoblastomas” have been examined, and many of the tumors were found to consist of granulomas containing Toxocara larvae. Many mild infections are asymptomatic.
The syndrome of tender hepatomegaly and eo-sinophilia must be distinguished from invasive schistosomiasis or fascioliasis. A variety of nematodes can produce visceral larva migrans in special circumstances, among them Ascaris lum-bricoides, Necator americanus, and Strongyloides stercoralis. Other nonhuman nematodes such as Gnathostoma, Capillaria, Hepaticola, and Diro-filaria may be involved in granuloma formation in the liver. Sarcoidosis and periarteritis nodosa may mimic this disease.
Blind liver biopsy is seldom helpful, but where facilities exist, direct visualization of surface granulomas of the liver with a peritoneoscope may enable biopsy of a granuloma to be made. A definitive diagnosis can often be made by examination of this granuloma. A diagnosis of second-stage rhabdoid Toxocara larva can be made on a section at mid-gut level, showing a maximal width of 12 to 20 /x and lateral alae. A variety of serologic tests are available but lack specificity, as they cross-react with other helminthic infections. However, in areas where such infections are rarely encountered, a fluorescent antibody test or an indirect hemagglutination test may be helpful.
Treatment of Toxocariasis.
Diethylcarbamazine (vide infra, Bancroftian Filariasis) has been used and does kill some larvae in the tissues of infected mice. A resolution of symptoms has followed the use of thiabendazole in one case in a dose of 25 mg. per kilogram twice daily for seven days. The prognosis is good if the source of infection is removed by treating the dog with piperazine. Care must be taken to worm pets regularly, especially puppies, if they are in contact with children.