Heartworm Prevention, Why Vets Say Year-Round and Test Yearly

Heartworm prevention is one of the first prescriptions a new dog owner gets, and one of the easiest to let slide once the weather cools and mosquitoes seem to disappear. The U.S. Food and Drug Administration (FDA) and the American Heartworm Society (AHS) both make the same case for not stopping: prevention is far easier, safer, and cheaper than treatment, and for cats there is no approved treatment at all.

Here is what heartworm disease is, how it spreads, and what the official guidance says about prevention and testing.

1. What heartworm disease is

FDA describes heartworm disease as a serious illness that results in severe lung disease, heart failure, other organ damage, and death, mainly in dogs, cats, and ferrets. It is caused by a parasitic worm, Dirofilaria immitis. The adults live in the heart, lungs, and associated blood vessels, which is where the name comes from.

FDA says adult heartworms look like strands of cooked spaghetti. Males reach about 4 to 6 inches and females about 10 to 12 inches. Inside a dog, they can live 5 to 7 years. AHS notes untreated dogs have been known to harbor several hundred worms.

2. How it spreads: only through mosquitoes

Heartworm is not contagious between pets. FDA says a dog cannot catch it from being near an infected dog; it spreads only through the bite of a mosquito. AHS adds that wild animals such as foxes and coyotes living near urban areas are considered important carriers.

The cycle, according to FDA and AHS:

1. An infected animal has baby worms, called microfilariae, circulating in its blood.
2. A mosquito bites it and picks them up.
3. Over 10 to 14 days, under the right conditions, they develop into infective larvae inside the mosquito.
4. The mosquito bites another animal and deposits the larvae, which enter through the bite wound.
5. In a dog, it takes about 6 to 7 months for the larvae to mature into adult heartworms.

FDA says that in the United States, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.

3. Why prevention, and why year-round

FDA's heading says it plainly: the best treatment is prevention. Treatment for dogs exists, but FDA describes it as hard on the dog, potentially toxic, able to cause serious complications such as life-threatening blood clots in the lungs, and expensive because it requires multiple vet visits, blood tests, x-rays, hospitalization, and a series of injections.

Prevention options, according to FDA:

• Many products are FDA-approved for dogs, and all require a veterinarian's prescription.
• Most are given monthly, as a topical liquid or an oral tablet.
• One product is an injection given every 6 or 12 months, only by a veterinarian.
• Some also control intestinal worms, fleas, ticks, or ear mites.

FDA states that year-round prevention is best, and cites the AHS message to "Think 12": give dogs 12 months of heartworm prevention and test them every 12 months.

4. Why dogs are tested before starting prevention

This surprises many owners. FDA says dogs 7 months of age and older should be tested before starting heartworm prevention, for two reasons:

• Preventives do not kill adult heartworms. A dog that is already infected stays infected until it gets sick enough to show symptoms.
• It can be dangerous. If microfilariae are in the blood, FDA says a preventive may make them die suddenly, triggering a shock-like reaction and possibly death.

After that, FDA recommends annual testing of all dogs on prevention. Other factors can change the timing, such as a missed dose and for how long, switching products, recent travel to an area where heartworm is more common, and the length of the local heartworm season. Your vet will decide when to retest.

How soon can a test find an infection? FDA says the earliest an antigen test can detect heartworm proteins is about 5 months after the infective bite, and microfilariae appear in the blood about 6 months after.

5. Signs in dogs

FDA and AHS say many dogs show few or no symptoms early on, especially if they have few worms or are not very active. As the disease progresses, signs may include a mild persistent cough, reluctance to exercise, tiredness after moderate activity, decreased appetite, and weight loss. A severe form called caval syndrome, where worms physically block blood flow, is life-threatening and requires prompt surgery.

6. Cats are different, and prevention matters even more

FDA says cats can get heartworms but are not as susceptible as dogs, and the worms do not thrive as well in them. Usually a cat has only one or two worms, but because a cat's body is small, even a few worms count as a heavy infection. FDA says both indoor and outdoor cats are at risk.

Key differences, from FDA and AHS:

• Infections are harder to detect in cats, and a negative test does not rule one out.
• Immature worms can cause heartworm associated respiratory disease (HARD), with breathing trouble and coughing that may look like asthma.
• Some infected cats die suddenly without ever seeming sick.
• There is no FDA-approved drug to treat heartworm disease in cats, and AHS notes the dog treatment cannot be used in cats.

FDA lists several FDA-approved monthly preventives for cats, topical and oral, all by prescription, and again says year-round prevention is best.

7. Key takeaways

• Heartworms spread only through mosquito bites and have been reported in dogs in all 50 states.
• Preventives require a prescription; most are monthly, and one injectable lasts 6 or 12 months.
• FDA and AHS urge year-round prevention and yearly testing: "Think 12."
• Dogs 7 months and older should be tested before starting prevention, because preventives do not kill adult worms and can trigger a dangerous reaction.
• Cats get fewer worms but have no approved treatment, so prevention is the only protection.