The zoonotic roundworms Toxocara canis and T. cati are not only present worldwide in their definitive hosts; they also frequently occur in other animal species, including humans. In those so-called paratenic hosts, the larvae do not develop into the adult stage, but rather migrate throughout the somatic tissue and persist as infectious L3 stage for extensive periods. Those arrested larvae may lead to severe inflammatory reactions and consequently to a wide range of pathological and clinical manifestations. However, the infected paratenic hosts also constitute a potential source of infection for the definitive hosts or humans who may also function as paratenic hosts. In the present review, current knowledge of larval migration in a variety of possible paratenic hosts is summarized including variations of migration routes and susceptibilities. Furthermore, information about the clinical and pathological changes for the presented species and possible consequences of the somatic migration of larvae, i.e. the resulting tissue damage as well as adverse host reactions to arrested larvae are reviewed. There are still many questions unanswered regarding larval behaviour in hosts other than their definitive host. Therefore, it is of great importance to continue further elaboration on the biology of Toxocara spp. to prevent further spreading of larvae in both the paratenic and the definitive host.
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