Effects of Superinfection on the Behavior of Toxocara canis Larvae in Mice

Abstract

Following the identification of the larval stage of Toxocara canis in the liver of a child in New Orleans (Beaver et al, 1952), numerous confirmatory case reports have shown that infection with larvae of this ascarid of dogs is an important cause of disease in children. Inasmuch as infection is acquired through ingesting soil previously contaminated by infected dogs, children at the dirt-eating stage of development are most frequently and heavily infected. In most of the temperate and warmer regions of the world T. canis infection in dogs is extremely common. It occurs most frequently in young dogs and is more common in the male than in the female adult (Ehrenford, 1957). It has been found prevalent among well-cared-for as well as stray dogs in New Orleans (Vaughn and Jordan, 1960). Infective eggs of T. canis have been found abundantly in door-yard soil (Headlee, 1936; Beaver, 1952). These conditions provide opportunities for young children to acquire various degrees of superinfection. Acquired resistance to nematode infection has been frequently demonstrated. As far as ascarids are concerned, the general conclusion has been that previous infection induces resistance which affects primarily the migration and survival of larvae in the tissues. Studies on the larvae of the pig ascaris have shown that fewer of them can be recovered from the liver and lungs of mice previously infected than from those given equivalent single inoculations (Wagner, 1933) ; single or multiple sublethal inoculations render guinea pigs resistant to otherwise lethal doses of infective eggs (Kerr, 1938); and in mice immunized by previous infections, the larvae are retarded in their growth and migration, remaining for longer periods in the liver before migrating to the lungs (Sprent and Chen, 1949). Comparative studies on the larval migration patterns of various ascarids have shown that Toxocara canis differs fundamentally from Ascaris lumbricoides in that its larvae are adapted to paratenic transfer, i.e., passage through a succession of intermediate cannibal or predator hosts after having reached infectivity for the final host (Sprent, 1952); therefore, the larvae of these 2 species may be affected differently by superinfection.