Flea, tick and heartworm prevention: what to give and when

Most vets recommend protecting dogs and cats against fleas, ticks and, where it is present, heartworm all year round, using a vet-recommended product given on schedule without gaps. The American Heartworm Society and the Companion Animal Parasite Council (CAPC) both advise year-round heartworm prevention and an annual heartworm test for dogs. The right product depends on your pet's species, weight, health and where you live, so your vet should help you choose.

Fleas and ticks are an outdoor problem that comes indoors, while heartworm is spread by mosquitoes. Many products cover more than one parasite, which is convenient but makes it easy to double up by accident.

Why year-round prevention matters

It is tempting to stop treatments in winter, but parasites do not follow the calendar neatly. Centrally heated homes let fleas breed all year, ticks can be active on mild winter days, and mosquito seasons vary.

A flea problem that takes hold over winter can take months to clear, and missed heartworm doses can mean a test before restarting. A steady routine is usually simpler and safer than timing the seasons.

Heartworm: spread by mosquitoes

Heartworm is a parasitic worm that lives in the heart and the large blood vessels of the lungs. It is passed on by infected mosquitoes and can cause serious disease that may show no signs until advanced.

Heartworm is common in many warm and humid regions, including much of the United States, but it is not established everywhere. In the UK, for example, it is mainly a concern for pets that travel abroad, and lungworm, which dogs pick up from slugs and snails, is a more relevant local risk. Ask your vet what is present where you live and anywhere you travel.

How heartworm prevention works

Most heartworm preventives work by killing the young larvae that a pet picked up from mosquitoes over the previous weeks. That is why doses need to be given on time: a late dose can leave larvae enough time to develop beyond the stage the product can stop.

Preventives come as monthly chews, tablets or spot-ons, and in some countries as long-acting injections. Several also cover intestinal worms or fleas.

Heartworm testing for dogs

Heartworm guidance recommends a blood test for dogs before starting prevention (usually from around seven months of age), before restarting after missed doses, and once a year after that. Testing matters because giving a preventive to a dog that already has adult heartworms can cause a reaction, and because no product is perfect.

Heartworm in cats

Cats can get heartworm too, including indoor cats, since mosquitoes come indoors. Infection in cats behaves differently, can cause breathing problems that look like asthma, and there is no approved treatment to clear heartworm from cats. That makes prevention the main protection, so ask your vet whether your cat needs it where you live. The American Heartworm Society has more on both species.

Fleas: treat the pet, the home and every animal in it

The adult fleas you see on your pet are only a small part of the problem. Fleas lay eggs that drop off into bedding, carpets and furniture, where they develop into larvae and then cocooned pupae. Pupae can sit waiting for weeks or months before hatching, which is why flea problems seem to return after treatment.

To get on top of fleas:

• Treat every dog and cat in the household, not just the one scratching
• Keep treating on schedule for at least three months, so new fleas are killed as they hatch
• Wash pet bedding on a hot cycle and vacuum carpets, furniture and car seats often, emptying the vacuum outside
• Ask your vet about a household spray if the infestation is heavy

Fleas can also pass on tapeworm when a pet swallows one while grooming, and they are a common trigger of flea allergy dermatitis. Itchy, sore skin from flea bites can lead to hot spots, especially in thick-coated dogs.

Ticks: check after walks

Ticks wait in long grass and woodland, then climb onto a passing animal to feed. Depending on the region, they can transmit Lyme disease, ehrlichiosis, anaplasmosis and babesiosis, and risk generally rises the longer a tick stays attached.

• Check your pet after walks, especially around the head, ears, neck, armpits and between the toes
• Remove ticks with a tick removal tool, twisting gently without squeezing the body
• Do not burn ticks, cover them in oil or pull them off with your fingers
• Note the date and watch for lethargy, lameness, fever or loss of appetite in the weeks that follow

Product types and how they fit together

Common product types include:

• Monthly chews or tablets, which many dogs take easily with food
• Monthly spot-ons applied to the skin at the back of the neck
• Three-monthly chews or spot-ons for fleas and ticks
• Collars that release an active ingredient over several months
• Long-acting injections given by the vet, in some countries

Combination products can cover fleas, ticks, heartworm and some worms in one dose. Read what each covers so you know where the gaps are.

Dosing safely

• Dose by your pet's current weight; products are sold in weight bands, and puppies and kittens can move up a band quickly. Our guide on how often to weigh your pet can help.
• Do not use two products that contain the same type of active ingredient unless your vet has advised it
• Never split a spot-on between two pets or use a larger pet's dose on a smaller one
• Tell your vet if your pet has a history of seizures, is pregnant, nursing, very young or elderly, as some products are not suitable

Flea and tick prevention for cats

Never use a dog flea or tick product on a cat. Some dog products contain permethrin, which is highly toxic to cats even in small amounts. Cats can also be poisoned by close contact with a dog that has recently been treated with one, so keep them apart until the product has dried, or as the label says.

Use only products made for cats and dosed for your cat's weight. Indoor cats can still get fleas, which hitch a lift on people and other pets, so many vets recommend year-round cover for them too.

When to call the vet

Contact your vet for advice if you notice:

• Fleas or flea dirt that persist despite treating every pet for several weeks
• Constant scratching, hair loss, scabs or sore patches of skin
• A tick bite that becomes swollen, hot or weeping
• Lameness, fever, tiredness or poor appetite in the weeks after a tick bite
• A mild cough, tiring easily on walks or weight loss in a dog in a heartworm area

Treat these as an emergency and phone the vet or out-of-hours clinic straight away:

• Twitching, tremors, wobbliness or seizures after a parasite product is used, especially in a cat
• Heavy drooling, vomiting or collapse after application or after licking a treated pet
• Difficulty breathing or sudden breathing problems in a cat
• Pale gums, extreme weakness or collapse

If a dog product has been used on a cat, call the vet immediately, even if your cat seems fine.

How Petify helps

When you log a product in Petify, the protection timeline shows which parasites are covered and until when, so you can see gaps at a glance. Reminders let you know before the next dose is due, whether it is monthly or every three months.

Each dose sits in your pet's history alongside vaccines (see our vaccine schedule guide) and weight logs, which helps your vet decide whether a heartworm test is needed before restarting. If you are unsure about a new itch or a tick bite, the AI vet can help you think through symptoms, but it is not a substitute for an examination.

Frequently asked questions

Do dogs need heartworm prevention year round?

In areas where heartworm is present, the American Heartworm Society and CAPC recommend year-round prevention for dogs, along with an annual heartworm test. Year-round use avoids gaps caused by unpredictable mosquito seasons and late doses. Where heartworm is not established locally, your vet may only advise it for travel.

What happens if I miss a dose of heartworm prevention?

Give the dose as soon as you remember and contact your vet. Depending on how late it was and the product used, your vet may recommend a heartworm test in a few months. Do not give a double dose to catch up unless your vet tells you to.

Can I use my dog's flea treatment on my cat?

No. Some dog flea and tick products contain permethrin, which can cause tremors, seizures and death in cats. Only use products labelled for cats, and call your vet immediately if a dog product has been used on your cat.

Do indoor cats need flea treatment?

Often, yes. Fleas can come in on people and other pets, and a warm home lets them breed all year. Your vet can advise what suits your cat, including whether heartworm prevention is worthwhile locally.

How long does it take to get rid of fleas?

Usually around three months of consistent treatment for every pet in the home, plus regular cleaning, because pupae can hatch over weeks or months. A few fleas after treatment does not always mean failure, but ask your vet if it continues.