Ascariasis is a very common infectious disease caused by roundworm, the ascaris. It often goes unrecognized or ignored not only by the infected person and his family, but also by medical workers. Yet the complications of an ascariasis infection can be serious and even fatal.
Roundworms may be minute or sometimes nearly 30 cm in length. They are parasitic in the human intestine and in horses and swine. Among the grave consequences of ascariasis are surgical complications, liver abscess and bile-duct obstruction. In several regions the commonest cause of intestinal obstructions in children in ascaris infection; these obstructions can be fatal.
The large roundworms (ascaroid nematodes) of dogs and cats are commonly encountered, especially in puppies and kittens. Of the three species, Toxocara canis, T. leonina, and T. cati, the most important in T. canis, not only because its larvae may migrate in man, but because fatal infections may occur in young pups. T. leonina occurs most often in adult dogs, less often in cats. T. cati distribution varies around the world. In domestic animals, infective eggs may ingested directly from other animals or their faeces, or through eating infected animals (such as rats). These species also occur in wild carnivores who are probably infected by eating other infected animals.
Ascariasis is found throughout the world, but the degree of its seriousness varies. Ascaris is prominent parasite in both temperate and tropical countries; it is most common in warm countries where sanitation is poor or lax. Approximately 900 million of the world's population are infected. Although ascaris occurs at all ages, it is mostly found among children - who are more frequently exposed to contaminated soil than adults - and slightly more often among males. Prevalence rates vary greatly, even within the same country. In the early 1980s some of the highest prevalences reported included: Philippines 85-90%, Malaysia 82%, Thailand 70%, Indonesia 83%, Taiwan 50%, Brazil 58%, Colombia 59%, Costa Rica 40%, Nigeria 30%, India 20%, Republic of Korea 58%, Vietnam 45%, Iran 98%, Ethiopia 58%, South Pacific countries 35%.