Flea, Tick & Heartworm Prevention for Dogs: Basics

CAPC’s general guidelines call for year-round broad-spectrum parasite control with efficacy against heartworm, intestinal parasites, fleas, and ticks for every pet¹. The reasons differ a little by parasite:

• Heartworm. CAPC says all dogs, regardless of lifestyle, should be on year-round heartworm prevention². The AHS adds that preventives work on heartworm infections picked up in the past month or longer, so they need to be given on time, every time, and dogs do not need protection only in warm months³.
• Ticks. CAPC supports year-round tick control because tick activity varies by year and region, and brown dog ticks can infest homes every month of the year. Starting only when ticks appear often means the product goes on after transmission has begun⁴.
• Fleas. CAPC recommends year-round flea control for dogs, notes that an established infestation can take months to bring under control, and says every pet in the home needs treatment⁶.

CAPC also says that if year-round control truly cannot be kept up, your vet can suggest a fallback plan¹. That is a conversation to have with your vet, not a plan to make on your own.

Heartworm disease is caused by Dirofilaria immitis. It spreads through the bite of an infected mosquito. CAPC notes that rare cases of spread from mother to puppies before birth or through blood transfusion have been reported. A mosquito picks up microfilariae (baby worms) from an infected animal, the larvae develop inside the mosquito for about two weeks, and they are then deposited on a dog’s skin during a later bite². CAPC says development in the mosquito slows in cooler weather and stops below 57°F, and that transmission can occur any time infected mosquitoes are active and feeding². The AHS notes that any area with mosquitoes, even a few, can have heartworm disease³.

The larvae mature over about two months into young adult worms that travel to the heart and pulmonary arteries². Clinical signs, when they appear, can include cough, trouble breathing, weight loss, and tiring easily with exercise². The AHS says treatment can be costly and difficult, requiring multiple vet visits and months of exercise restriction³. Heartworm disease is also not limited to dogs; cats and ferrets can be infected³.

CAPC recommends annual heartworm testing for all dogs, even those on preventives. Annual testing makes sure an infected dog is found and treated as early as possible, and in areas with a lot of heartworm your vet may advise testing twice a year². CAPC also recommends annual testing for tick-transmitted infections, especially where those pathogens are common or emerging¹. Your vet will tell you which tests fit your dog and your region.

Tick species and the diseases they carry vary across the U.S. According to CAPC⁴:

• Black-legged (deer) ticks can transmit the bacteria that cause Lyme disease and anaplasmosis.
• The lone star tick can transmit ehrlichiosis, among other pathogens.
• The American dog tick can transmit Rocky Mountain spotted fever and tularemia.
• The brown dog tick can transmit several organisms, including Ehrlichia canis, and can establish itself indoors.

For Lyme disease specifically, CAPC says only black-legged ticks have been shown to transmit it to dogs in North America. It recommends daily tick checks, avoiding tick habitat, year-round tick control, and, in areas where Lyme is common or emerging, vaccination⁵. Whether your dog should be vaccinated is a decision for your vet.

Tick control products reduce risk but do not remove it. CAPC notes that ticks are sometimes found on pets that are routinely treated, often because of heavy tick numbers in the environment⁴. CAPC lists daily tick checks as part of Lyme prevention⁵. After walks or time in grass, brush, or woods:

1. Run your hands over your dog’s whole body, including ears, face, armpits, belly, groin, and between the toes.
2. If you find an attached tick, remove it promptly. CAPC advises using forceps or a commercial tick-removal device, avoiding contact with the tick’s contents (ideally wearing gloves), and washing your hands afterward⁵.
3. Tell your vet if your dog becomes unwell in the weeks afterward.

Ticks that bite dogs also bite people and can ride into the house on pets⁴, so check yourself and your family as well.

Heartworm preventives are available only by prescription from veterinarians, and the AHS says some protect against heartworm only while others also cover intestinal parasites, fleas, ticks, and mites. Because vets know which parasites are common locally, the AHS tells owners to ask their vet which product is best³. CAPC likewise says control programs should be matched to local parasite prevalence and each pet’s lifestyle¹. The AHS also says there is no such thing as a “natural” heartworm preventive³.

Do not mix species. The safety issue documented in the sources we reviewed runs from dog products to cats. Permethrin is an insecticide commonly used in topical flea and tick products for dogs, and cats are sensitive to it, especially at the concentrations in dog products. Exposure can cause unsteadiness, muscle tremors, and in severe cases seizures. The ASPCA advises reading labels, especially in homes with both dogs and cats, and not using these products on cats unless they are clearly labeled for them⁷. Call your vet or an animal poison control line right away if a cat is exposed. In general, follow the label and your vet’s directions exactly, and do not use a product made for another species on your dog without asking your vet.

Call your vet if your dog has a persistent cough, tires easily, loses weight, or has trouble breathing, since these can go with heartworm disease². Call after a tick bite if your dog develops fever, limping, loss of appetite, or lethargy; these are general warning signs, and your vet can decide whether testing is needed.