Parasites in dogs and cats: what every owner should know

February 14, 2024 - 9 min read
This article is not intended to be a substitute for professional veterinary advice, diagnosis, or treatment. Always seek the advice of your veterinarian with any questions you may have regarding your pet’s care, treatment, or medical conditions.
Parasites in dogs and cats

Preventative medicine is a common theme in today’s human health care scene. More than ever before, people are paying attention to their overall health to ward off disease and stay healthy. Exercise, diet, supplements—all of these have become areas of focus for us to live our best lives.

So what about our pets? Can preventative care help them live longer, healthier lives? The answer is a resounding yes.

As a loving pet parent, you’re the key to helping your pet stay healthy with preventative care. This includes keeping them at a healthy weight, feeding them high-quality food, and preventing common and communicable diseases from affecting them.

That last bit will be our main focus. We'll cover the parasites that commonly affect dogs and cats, how to treat parasitic illnesses, and how to prevent infections in the first place.

How do dogs and cats get parasites?

Dogs ear being examined

There are three main parasites that commonly attack dogs and cats, regardless of where they live: fleas, ticks, and heartworms. Dogs and cats that go outdoors frequently are more likely to come in contact with these parasites, but even indoor pets or those who don’t leave their yard are at risk.

Climate also controls the population of many of these parasites, and a good rule of thumb is that the warmer the climate, the larger the parasite population. That being said, some of these bugs are tenacious and can survive even in temperate regions with a hard frost.

Just remember, your veterinarian is an expert on the types of parasites to be worried about in your specific geographic location, but because these parasites can exist pretty much anywhere, all pet parents should be aware of what’s out there and how they can prevent infections.

Fleas and ticks are direct parasites, while heartworm is an indirect parasite and requires a mosquito host to bite a dog or cat to cause infection.

Let’s walk through the characteristics of each parasite and how they can infect your pet.

What are the main parasites in dogs and cats?


Fleas are miniscule, dark brown, jumping parasites that survive off the blood of dogs and cats. Think of them as tiny vampires.

Dogs and cats become hosts by coming into contact with live fleas. This can either be from the outdoors or, if there’s a current flea infestation, from the indoors as well (yikes!). Fleas are talented at recognizing a new source of food and will jump from their current host to a new one if the new one has more blood to offer. Fleas will also lay their eggs on dogs and cats, and can survive for quite some time if there is no intervention.


Ticks are usually contracted in the same way as fleas: from the environment. Ticks can survive in many different climates and temperatures and lurk in grasses, shrubbery, trees, and on other animals, waiting for their prey. Animals with long-hair coats are often a good target because ticks can burrow into their fur without notice, but any animal can contract ticks.

Ticks have sharp mouthpieces that bite into the pet’s skin. Once they’ve latched, it’s difficult to remove them. Ticks also live on blood and will stay attached for weeks, slowly filling themselves with your pet’s blood.


Heartworms operate much differently than fleas and ticks. Only mosquitoes can pass heartworms to a dog or cat.

Here’s how it works: When a mosquito bites a dog that’s already infected with heartworm, the mosquito becomes infected with baby heartworms (aka larvae). The infected mosquito can then transfer the larvae through its saliva after about 10–14 days have passed. If the mosquito then bites the dog, the heartworms will mature into adults and lodge in the chambers of the heart. This dog can now spread heartworm via a new mosquito bite, and on and on it goes.

Cats are a bit different. If a heartworm-infected mosquito bites a cat, they can still become ill and infected with the heartworms. They won’t spread the disease, however, because heartworms can’t mature inside a cat’s body as they’re able to in a dog’s. As a result, cats are considered an “indirect” host for heartworms, while dogs are considered a direct host.

Your skin may already be crawling thanks to this discussion of parasites. You may even be starting to panic, wondering if your four-legged friend is now host to dangerous creepy crawlies. Never fear: testing and preventive treatment are available for these parasites, and they're extremely effective.

How to treat parasites in dogs and cats

german shepherd lays on table with paws outstretched while female owner and vet comfort it

Testing is a routine part of most wellness exams. Typically, vets recommend pets have a wellness exam once a year. During the wellness exam, vets can test for heartworm and exposure to ticks with a simple blood test. All your pet will need to do is provide a few drops of blood. Some vets can do the test right at the time of the visit, while other clinics send the blood into a lab for testing. Usually, you can expect results within a week.

The test will either check for antigens (parts of the actual disease) or antibodies (molecules your pet has made to fight the disease). The results will allow your vet to determine what your pets may have been exposed to or infected with and guide them on how to get your pet the treatment they need. Ideally, your pet will test negative, and then prevention will begin (more on that soon).

If you’re worried that fleas or ticks may currently be affecting your pet, you should schedule an appointment with your vet to check them out. In other words, don’t wait until their next scheduled wellness exam if you’re concerned about an active infestation.

Symptoms of parasites in dogs and cats

Symptoms vary depending on which parasite is infecting your pet.

Signs your dog or cat has fleas

A pet with fleas typically always has one major symptom: they’re itchy! Since fleas come to the party with lots of their friends and family, most pets are truly crawling with these tiny bugs and intermittently being bitten.

Pets will scratch their sides and ears, and they will often chew on their legs and the base of their tail. This can become so excessive that pets may lose hair in certain areas. Once the fleas have been killed by an anti-flea medication, pets can develop flea bite hypersensitivity, in which they have ongoing skin issues that must be treated with steroids.

You can combat active flea infections with topical or oral medications that kill adult fleas along with any eggs or larvae. Many over-the-counter medications are available for treating fleas, but some don’t kill all life stages and may not be indicated for your pet’s age or breed. Most of these medications begin killing fleas immediately and prevent fleas for another 30–90 days. (If you’re in an area with fleas, keeping your pet on prevention at all times is ideal.)

After giving your pet flea medications, ask your vet when you can bathe your pet with an oatmeal/aloe shampoo. This will remove the parasite bodies and soothe your pet’s skin. Your vet may also want to prescribe something for itchiness. In most cases, flea treatment is easy and safe and gets your pet back to normal. In cases of severe infestation, some pets become anemic (low red blood cell counts) and may need more intensive treatments.

Signs your dog or cat has ticks

small chocolate labrador retriever puppy combats itching by scratching with its hind leg

Ticks can also be itchy, but not to the degree that fleas are.

If the tick is biting a sensitive area, pets may scratch or chew on that region of their body, trying to dislodge the parasite. Many species of ticks carry other diseases, which are transferred to your pet after the bite. These diseases include things like Lyme disease, ehrlichia, anaplasmosis, and Rocky Mountain Spotted Fever. These diseases cause your pet’s immune system to attack white and/or red blood cells within the body. Symptoms include joint pain and swelling, weakness, lethargy, fever, seizures, decreased appetite, and bleeding.

Treatment for ticks is similar to treatment for fleas. Various topical and oral medications can kill the parasites and any larvae. Many of these products treat and prevent fleas and ticks, and the combo therapy can protect your pet from both.

If your pet has become infected with a disease that the tick transferred to them, long-term antibiotics and steroids may be needed. For very sick pets, hospitalization may be necessary for the initial treatment period, especially if they have a fever or severely low red or white blood cell counts.

Signs your dog or cat has heartworm

Heartworms are tricky and are often considered one of the most dangerous parasites to affect pets. That’s because symptoms don’t begin until the heartworm has grown and matured in the dog or cat. Dogs will often begin to cough and become tired very easily. Cats may also cough, have asthma-like symptoms, vomit, lose weight, or stop eating.

Treatment for cats affected by heartworms is focused on the management of the symptoms, but unfortunately, there’s no approved medication for adult heartworms in cats. In dogs, treatment involves a course of antibiotics, heartworm oral medications, and intramuscular injections to kill the adult worms.

It’s great news that vets can treat heartworm infections in dogs. But don’t get complacent. Treatment lasts several months and is very painful. Dogs can recover well and live a normal life, but in some cases, the heart disease secondary to the parasitic infestation can cause lasting damage.

The best thing you can possibly do for your pup is to keep them from getting infected in the first place.

How to prevent parasites in dogs and cats

Tick and flea prevention for a dog stock

Prevention is simple, safe, and easy, but it needs to be done in accordance with your vet’s recommendations.

Puppies can start on heartworm prevention when they’re 6–8 weeks old. They don’t need to be tested first. Kittens are the same. But adult dogs and cats need to be tested for heartworm disease before starting prevention because the medication can cause massive death of the baby heartworms, which in turn can lead to a severe allergic reaction (aka anaphylaxis).

Vets typically recommend year-round heartworm prevention, but it’s especially important when mosquitoes are out. Mosquitoes can bite any breed of dog or cat, so chat with your vet about the best time period to keep them on the prevention list.

When it comes to flea and tick prevention, it’s extremely important to use the medication that your vet approves for your specific pet. Some products aren’t safe for certain breeds of dogs or cats or may have ingredients that could be harmful if your pet has other health conditions, such as seizures.

One more thing: If your dog or cat has an active flea or tick infection, treatment should be started immediately. And if your pet lives somewhere where re-infection is probable, they need to be kept on the prevention list year-round to avoid being attacked again.

How much is parasite medicine for dogs and cats?

Like anything in life, planning and prevention usually save time and money. For our pets, preventative medications for fleas, ticks, and heartworms will not only protect your pet from a myriad of nasty diseases, but the cost of prevention is miniscule compared to the cost of treating these diseases.

On average, monthly prevention will cost around $20–30 per pet per month. If you compare that to the average treatment for tick-borne disease or heartworm disease (about $3,000–$5,000), it’s clear that prevention will be easier on your wallet every time.

These diseases are very common, and vets recommend routine prevention to keep your pet out of harm’s way. Unfortunately, many of these infections are only recognized once they’ve become quite advanced, so preventing them in the first place will insure that you’re giving your pet the best chance at a happy, healthy, and frisky life.

If you do have financial concerns, many clinics and pet insurance companies offer wellness plans that reimburse pet parents for preventive care. These plans are billed at a monthly premium, so you can know what to be prepared for while keeping your pet safe and protected.

It's a wild and crazy world out there. But thankfully, if you take basic preventive steps, you can keep your furry friend safe.

Oneal Bogan, DVM
Veterinarian, Veterinary Writer

Dr. Oneal Bogan is a mixed animal veterinarian from Colorado. Dr. Bogan graduated from Colorado State University's College of Veterinary Medicine and Biomedical Sciences in 2013.