Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Understanding parasite life cycles explains why prevention is year-round and why some treatments need repeating. Heartworm preventives kill young larvae from the previous month. Flea treatments must continue for months because eggs and pupae in the home keep hatching. Deworming is repeated because many medications kill adult worms but not all developing stages.
Heartworm
- A mosquito carrying larvae bites a dog
- Larvae mature over about 6 months into adult worms in the heart and lungs
- Monthly preventives kill larvae picked up during the previous month, so doses must be on time
- Tests detect adult worms, so infections may not show until about 6–7 months after infection
Fleas
- Adult fleas live on the dog and lay eggs that fall into the environment
- Eggs become larvae, then pupae, which can wait weeks before hatching
- Most of a flea problem is in the home, not on the pet
- Consistent treatment of all pets for at least 3 months usually breaks the cycle
Roundworms and hookworms
- Puppies can be infected before birth or through nursing
- Puppies are dewormed every 2 weeks early in life to catch worms as they mature
- Monthly preventives protect adult dogs
Tapeworms
Dogs get the common tapeworm by swallowing infected fleas, so flea control prevents it.
Sources
- Companion Animal Parasite Council (CAPC): General guidelines for dogs
- American Heartworm Society: Canine heartworm guidelines
- Merck Veterinary Manual: Fleas and flea allergy dermatitis
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.