Answer: In the case of one or two worms, it is correct that they can infect humans, usually children because their immunity is less.
The worm which has received most attention is the roundworm, Toxocara cati. In its life cycle the infective larva, still inside the egg-shell in which has developed, may be eaten by another species, often a rodent, which in turn my be consumed by a hunting cat. At times, however, these infective eggs (i.e. containing the larva) may be eaten by children. A number of the factors contribute:
1 The outside of the microscopic eggs are sticky, so that they readily adhere to a cat’s coat, at least until it groom itself, and to human fingers.
2 Young children, especially toddlers, have a habit of putting of their hands and various objects, even contaminated soil, into their mouths.
3 The infective eggs can survive for two years or longer in the ground and will withstand all disinfectants, long-term freezing and even short periods in boiling water. Only the use of horticultural flame-guns on concrete runs effectively destroys the eggs.
4 The effects of wind, rain and human activity can spread the eggs over a wide area.
In human the larvae hatch from the eggs, penetrate the wall of the intestine disperse to the liver, kidneys, brain and eyes. They do not develop further but remain in those organs, and can cause damage leading sometimes to liver enlargement, blindness or convulsion. This disease, caused by the migration or penetration of the larvae, is called visceral larva migrans, and it chiefly affects children between one and half and three years old. It cannot be considered a common disease by any means, although probably more children become infected then ever show signs. Good hygiene and regular de-worming of your cat are important in eliminating the possibility of human infection taking place. At present there is no evidence that Toxascaris leonine can infect humans similarly.
It is also possible fro a person, again usually a child, who swallows a flea infected with the intermediate stage of the tapeworm Dipylidium caninum to become infected with an adult tapeworm in their intestine, although this very rarely happens.
The eggs of the very small (½ cm long) tapeworm Echinococcus multilocularis may at times infect humans. This parasite can occur in cats in North America and some parts of Europe and Asia. Man plays the role of an intermediate host and the egg develops into a tumour-like structure in the liver. It is of course unlikely that this organ will over be eaten by a cat which would be necessary if the worm were over to complete its life cycle. Only recently has this worm being distinguished from Echinococcus granulosus, which at one time was believed to occur in the cat although not able to produce eggs, so the true situation is not yet clear. Echinococcus granulosus certainly does occur in dogs, and its eggs, when eaten by man, can develop into large cystic structures in the liver, lungs or other organs, resulting in the severe, and sometime fatal illness, hydatidosis.
