Pretty sure it can't be transmitted by fleas, and for kittens: No. Heartworm larvae are transmitted from one animal to another only by the bite of a mosquito carrying the infective stage of the larvae. Occasionally, microfilariae circulating in the blood of a pregnant dog can cross the placenta and get into the blood of puppies in utero, but this has never been reported in cats. And even when this occurs in dogs, it is of no consequence, as the microfilariae do not develop further in the puppies. So, they die in the puppies within a year unless they are taken up in the bite of a mosquito.
Unless you live in a very endemic area for heartworm (anywhere where there are mosquitoes all year round), I don't see a reason to place a cat on preventatives... I guess you could test for it, to make sure. Franlky I've worked in 3 vet clinics and none of them really bothers with feline heartworm, but I live in Quebec so we have mosquitoes like 4-5 months a year.
“Your birth is a mistake you'll spend your whole life trying to correct.” Chuck Palahniuk
I love pus but I hate people.
I can say words like undifferentiated gonads now!