Transmission of the parasite is through a fecal-oral route. A cat that has ingested the eggs or larval (stage 2) from an infected cat (or other small mammal) becomes infected. While it is true that if an older cat is shedding eggs, transmission could occur in the litterbox, which seems to be a rare event. Furthermore, it is likely that eggs will be spread through the house from the cat's paws. Last, your mom's cat probably had the infection as a kitten and has encysted larvae, which are dormant since this is such a common problem, so transmission of a new infection to your mom's cat is unlikely.

My recommendation in this case is to repeat three dewormings at three-week intervals and continue to have the feces checked over the next couple of months for evidence of the parasite. If she catches mice, it would be good to deworm her on a monthly basis and this can be achieved by putting her on heartworm prevention, which will also protect her from this disease. Last, be certain to have her tested for feline leukemia virus (FeLV) now and in eight weeks, and feline immunodeficiency virus (FIV) after 6 months of age.

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