As far as I know, heartworms have to be present in the animal for a substantially long period of time to cause death. Not sure about roundworms, but I know you can get wormer for those anywhere. I believe hookworms, though, can cause anemia and death... but not sure how long it takes. I doubt it was parvo if there were no signs of the disease. At any rate, it's a shame the little one didn't make it.