Biology of Toxocara canis

The definitive hosts for T. canis (Werner 1782) are dogs, foxes, and less frequently other canids, and the paratenic hosts are mammals and birds. The life cycle of T. canis is very complex in the definitive hosts (Lloyd 1998). The paratenic hosts acquire the infection by ingesting eggs excreted by infected canids. The eggs are usually found in soil, but as they are very sticky, they may adhere to fingers, foodstuffs, insects, etc. Thus, risk factors include geophagia (pica); contact with dogs, particularly pups; poor hygiene and contaminated food, for example, raw snails (Romeu et al. 1991). In food animals, which also act as paratenic hosts, larvae have been found in all tissues, with the highest number in liver especially after superinfection and/or reinfection. Consumption of raw liver (Nagakura et al. 1989; Salem and Schantz 1992), rabbit giblets and 'lightly grilled' meat (Struchler et al. 1990) and raw cattle meat (Espana et al. 1993) have been related to the infection.

Following ingestion of infected eggs, the larvae hatch in the small intestine, penetrate the mucosa, and within a week after infection migrate through the liver and lungs to other tissues; the larvae (350-450 x 14-21^m) can be found in virtually all organs. Some of the larvae may remain in the liver. The larvae do not mature during the migration, even if they make their way back into the intestine. Most larvae survive for many months or even years, although their distribution in the tissue may continue to shift. Eggs (90 x 75^m) of T. canis are not embryonated when passed in the stool of the canids and are thus not directly infective for humans. To develop, the eggs need a temperature above 10°C and at a temperature of 15-25°C, the infective larvae will develop within 2-7 weeks. In most cold conditions, the eggs can survive for years, even over extremely cold winters (Ghadirian et al. 1976). It has also been shown that T. canis eggs resist both the anaerobic and aerobic conditions of sewage processing, and that the surviving eggs are viable (Black et al. 1982). Eggs stored for years in sludge also remained both viable and infective (O'Donnell et al. 1994). Ozone treatment has no effect on the viability of the embryonated second stage larvae (Ooi et al. 1998) but heat, more than 35°C, desiccation, and microwaves (Bouchet et al. 1986) will kill the larva within the eggs.

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