Heartworm Prevention Basics: CDC and AVMA Facts for 2026

Mosquito season does not end on a calendar sticker. The CDC and veterinary organizations such as the AVMA publish owner-facing explanations of heartworm disease: it is caused by *Dirofilaria immitis*, transmitted by mosquitoes, and primarily a concern for dogs, with cats also able to be infected. Year-round prevention is the public-health style recommendation you will see from these bodies because mosquitoes persist in many U.S. regions and because prevention is far simpler than treating established infection. That is the seasonal peg for a September 2026 explainer: people stop preventives after Labor Day. The parasite does not.

We are the Pet Standings editorial team. We are not diagnosing your dog. We will not rank chewables. There is no verified heartworm-product table on this site yet.

What heartworm infection is

Larvae enter when a mosquito that previously fed on an infected animal bites your pet. In dogs, worms can mature in the heart and pulmonary arteries, leading to lung disease, heart failure, and other complications described in veterinary references. In cats, even a small number of worms can cause serious respiratory disease; cats are not a natural reservoir in the same way dogs are, but “cats cannot get heartworm” is false.

Heartworm is not contagious dog-to-dog without mosquitoes. That does not make an indoor dog magically unbiteable. Mosquitoes enter houses.

Prevention versus treatment (do not mix the stories)

Prevention uses medications that kill larval stages on a schedule (monthly products are common; some clinics use other protocols). Treatment of adult heartworm disease in dogs is a separate, involved veterinary process that can include stabilization, adulticide therapy, and strict exercise restriction. It is not a weekend DIY. The CDC’s owner page emphasizes prevention because treatment is difficult. We will not invent a national treatment-cost number.

A dog who is already infected may still be placed on a preventive as part of a veterinarian’s plan; that is clinic-directed. Giving random leftover preventives to a coughing dog is not a protocol.

Testing

Veterinarians commonly recommend antigen testing in dogs on a schedule, including before starting or restarting prevention, according to clinic protocol and American Heartworm Society-style guidance that clinics follow. If you skipped summer doses, tell the veterinarian the truth. A test is not an accusation.

Cats are tested differently and less routinely; that is a conversation with the veterinarian, not a blog algorithm.

Year-round, including the North

People in colder states often hear “we don’t have mosquitoes in January.” Indoor mosquitoes, travel to endemic areas, and shoulder seasons still exist. CDC-style messaging supports year-round prevention rather than a homemade frost calendar. If your veterinarian has a regional protocol, follow that veterinarian. Do not use this paragraph to override a clinic that knows your county.

Fall travel with a dog — hunting trips, football weekends, hurricane evacuations — moves animals between mosquito densities. Bring enough doses. Keep them in original packaging.

Safety and species

Use products labeled for the species and weight. Some ingredients safe for dogs are not for cats. Share-the-dose math is how cats get poisoned. If a dose is vomited, call the clinic; do not double automatically.

Heartworm preventives may also have effects on other parasites depending on the product. That is a bonus of some labels, not a reason to skip fecal exams.

Environment is not enough

You cannot mosquito-proof a suburb with citronella folklore. Source reduction (standing water) helps people and pets. It does not replace veterinary prevention.

Criteria for a clinic visit this month

1. When was the last heartworm test?
2. How many doses were missed in 2026?
3. Indoor-only or travel?
4. Cats in the home?
5. What product and interval does this clinic actually want on the record?

What we will not do

No “top ten chewables.” No invented outbreak. If local public-health authorities report a change, that would be a sourced peg; we did not fabricate one.

Disclosure

No insurance-style pick cards. Pet Standings’ only verified ranking in health right now is pet insurance. Heartworm care is a veterinarian-and-label problem. Ask the clinic, then set a reminder that survives the first frost.

Dogs who “never go outside”

Balconies, screened porches that are not actually sealed, and mosquitoes that ride in on people still create exposure. Indoor lifestyle lowers some risks and does not zero heartworm risk. CDC materials on heartworm describe mosquito transmission without a “indoors is fine” carve-out you can take to the bank.

Cats, again, because the myth is stubborn

Cats can develop heartworm-associated respiratory disease. Prevention discussions for cats are clinic-specific. Do not use a dog’s chewable for a cat. If a cat in the house is on a combination flea-heartworm product, tell whoever is also dosing the dog so you do not stack ingredients accidentally.

Travel and relocation

Moving from a lower-prevalence area to the South or Mississippi River corridor is a reason to restart the conversation, not to panic-buy a random box. Take the last test date with you to the new clinic.

Missed doses

If you missed one monthly dose, call the clinic. Do not run a search-engine protocol. Restarting after a gap may require testing. Honesty lets the veterinarian pick a path.

Questions to take to the appointment

• Last antigen test date and result.
• Product name, dose, and whether it is monthly or otherwise.
• What to do if the dog vomits the chewable.
• Cat protocol if applicable.
• Record of missed 2026 doses.

Sources and limits

CDC heartworm pages and AVMA pet-care heartworm material are the public sources we are willing to name. Clinic protocols follow groups such as the American Heartworm Society; your veterinarian applies them. We did not invent prevalence maps. We may earn commissions on verified insurance rankings. This page still has no product table.

Indoor dogs, missed doses, and storage

Indoor dogs still meet mosquitoes. Year-round prevention matches CDC-style messaging better than a homemade frost calendar. If you skipped months, tell the veterinarian and test as directed. Store chewables as labeled, not in a hot car. Use species-correct products; a dog chewable is not a cat dose. All dogs in a house belong in the same prevention conversation. Travel between regions is a reason to brief the new clinic. American Heartworm Society protocols are applied by veterinarians; we are not your protocol. No chewable ranking.

Treatment is not prevention with extra steps

Adult heartworm treatment in dogs can mean activity restriction and clinic-directed adulticide protocols. It is hard on the animal and the calendar. That is why CDC-style pages push prevention. Do not start a coughing dog on leftover preventives and hope. Do not exercise a treating dog like a fall athlete. Cats with heartworm-associated respiratory signs need a veterinarian, not a copied dog protocol. Keep this page bookmarked next to your reminder app, not next to a shopping table we refused to invent.

September action list

1. Find the last test date. 2. Count remaining chewables or topicals. 3. Book a test if you missed doses. 4. Put a recurring reminder through winter. 5. Tell the clinic about planned travel. 6. Separate dog and cat products on the shelf. That is the whole program. Public CDC and AVMA pages will still be there when marketing emails tell you the season ended.

What a mosquito season actually is

Standing water, warm nights, and travel are the practical inputs. Empty trays under pots. Repair screens. These steps help and still do not replace a labeled preventive. If you foster, ask the rescue who pays for prevention and testing. If you hunt, pack doses like you pack ammunition licenses: in the kit, labeled, not loose in a pocket. We remain not a product ranker on this topic. The veterinarian names the product; you name the reminder.

Heartworm prevention is boring on purpose. The dramatic story is treatment, exercise restriction, and a very sick dog. September 2026 is a good month to be boring: test if you skipped, refill if you did not, keep cats on cat-labeled products, and ignore any social post that says frost ended mosquitoes. CDC and AVMA pages will match that boredom. Our insurance ranking is the only verified product podium in health; it does not replace a heartworm test.

Set the reminder now, label the box, and take the last test record to the fall wellness visit.