The outcome of Toxocara canis
infections in the canine host depends on the migratory pathway of parasite larvae (somatic or tracheal) which is considered to be related to the host's age and its immune status. However, field studies attest high prevalences of patent T. canis
infections in adult animals. The controlled induction of patent
infections with low doses of embryonated eggs was investigated in 18 beagles in a 7-month study until their 16th life month. The animals were assigned to three groups, each consisting of three vertically infected dogs (with a short patent
infection as pups before
anthelmintic treatment) and three helminth-free dogs. At study days 10 and 40, the animals of groups 1 and 3 were given each 100 embryonated T. canis eggs. In each case, group 1 was treated 10 days post-
infection with Milbemax, while dogs of group 3 remained untreated. Control group 2 was not experimentally infected but treated as group 1. Two weeks after first egg administration, a sharp increase of specific antibody reactions in ELISA and increased eosinophilic counts indicated larval invasion in all infected dogs. 42-56 days following first
infection, patent
infections were detected coproscopically in all animals of group 3, but in none of the uninfected dogs (group 2) or the infected and treated dogs (group 1). Following a 3-month observation period, all animals of the three groups were treated with
piperazine citrate to eliminate intestinal
infections and all were administered 100 embryonated eggs. Subsequently, patent
infections developed in animals of all groups: in one of the infected and treated animals of group 1, in five of the so far not infected control group 2 and in four of the dogs with previous patent
infections (group 3). Susceptibility to patent
infections was not significantly altered in T. canis-free dogs compared to dogs with previous patent
infection (vertically acquired or experimentally induced). However, dogs of group 1 treated with Milbemax after repeated egg administration developed a significantly increased resistance to patent
infections as compared to control dogs (group 2). Observed prepatency periods were between 40 and 56 days and did not differ in the three groups. Even in urban areas, facing high
infection pressure with Toxocara eggs maintained by a high dog and fox population, dogs of all ages are at risk to develop patent T. canis
infections.