Vomiting and unexplained weight loss are signs a cat has roundworms, tapeworms, hookworms or heartworms, but not all cats with parasitic worms show signs of illness, which is why veterinarians recommend regular testing, says veterinarian Amanda McNabb. Keeping cats indoors, feeding them a cooked diet and using a prescription antiparasitic medication prevents infections, say Dr. McNabb and veterinarian Travis Arndt.