One year old stray heartworm positive?

Is it normal for a dog so young to get Heartworm..He is my foster dog (who was pulled from pound last week), and after the vet visit found he was HW positive. Is this something maybe ped from his mom? We (all those that rescued him) feel awful that this poor guy has this and will have a long battle with the treatment..We dont know the severity yet, but found it odd for a dog to test positive so young...Thanks for any info..Also he is part collie and I heard they are sensitve to HW treatment. Anyone have this experience?