Cat and Dog Parasites: Fleas, Ticks and Worms

This poodle mix takes medication to prevent parasites, including heartworm.

Parasites can present a big challenge when it comes to your dog’s or cat’s overall health. They can affect your animal’s skin, digestive tract, heart — almost any organ, for that matter.

Pet parasites

Which parasites you have to worry about depends greatly on your pet’s lifestyle and where you live. For instance, a Persian cat in a New York City apartment has much less parasite concern than an adventurous, ever-hungry Labrador in Louisiana. Ultimately, what type of parasite control you do for your pet should be discussed with your veterinarian, who can tailor a program that fits your pet’s needs, your personal belief system and the particular parasites present in your area or places where you travel with your pet.

The following are some broad guidelines to keep in mind when thinking about parasite control. While this discussion is broken down into three major categories of parasites — ectoparasites, intestinal parasites and heartworm — there are certainly other categories. However, those tend to be less common or specific to certain geographic regions.

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Ectoparasites are the ones you can see, such as fleas, ticks and mosquitoes, so they can have a significant “ick” factor. These types of parasites not only cause discomfort in the form of itchiness and pain, they can also transmit some very serious diseases, like mycoplasma haemophilus (from fleas), Lyme disease and Rocky Mountain spotted fever (from ticks), and heartworm disease (from mosquitoes).

Twenty years ago, these parasites were very difficult to control. Over the last 10–15 years, though, very good products have been developed that can prevent infestation by these parasites. Most of them are topical applications that are done once a month and have minimal toxicity to animals. The one exception: Some of the products (mostly over-the-counter ones, which tend to contain permethrins) can be very toxic to cats. With that said, any animal can have a toxic reaction to any product. If you are worried that your pet has had a bad reaction, let your veterinarian know, and you may want to stay away from that product in the future.

People often say they do not like putting these “poisons” on their pets. While they are technically poisons, they are poisonous to the insects. What often has to be weighed against the concern of toxicity is the discomfort and disease status of your pet. If your dog or cat has a very bad flea allergy, and no known reaction to the topical flea products, it is almost cruel to not try and relieve your pet of the discomfort of being constantly bitten by fleas.

Intestinal parasites

Intestinal parasites can cause diarrhea, a poor hair coat (due to not absorbing nutrients), weight loss, vomiting and loss of appetite. The most common intestinal parasites are roundworms, hookworms and whipworms. Tapeworms are also very common — not to mention very gross — but they tend not to cause problems in dogs and cats.

Most veterinarians will recommend a pretty aggressive deworming protocol for young dogs and cats, as they are very prone to picking up these infections. They normally pick them up from the soil where other animals have defecated. Most of these deworming protocols involve using a dewormer such as pyrantel every two weeks from age 4 weeks to 16 weeks. In some areas of the country, it is recommended that you continue to deworm your dog or cat every month for the rest of his/her life. Indoor cats are not likely to pick up worms, so this protocol applies more to cats allowed outdoors.

Being diligent with a deworming program benefits not only the health of pets, but the health of people as well. Many of these intestinal parasites are zoonotic, which means they are transmissible from animals to people. For most healthy adults, this is not a big problem, but young people, elderly people or anyone with a weak immune system (such as those with cancer or HIV) can get serious infections from some of these parasites. In fact, infants infected with roundworms can go blind. The best defense against picking up these infections is to maintain a deworming protocol for your pets (if appropriate for their lifestyle) and to wash your hands after you have touched animal feces or the fur on the hind end of your pet.


Heartworms are transmitted by mosquitoes. And yes, when the heartworms are allowed to mature in your pet, they do reside in the heart (actually, the pulmonary artery that leaves the heart). The mosquitoes transmit an immature larval stage of the heartworm and then these larvae go through four more stages to become adults. If they become adults, the result can be heart failure and severe lung disease. Heartworms can infect both dogs and cats, but cats tend to have a lower incidence than dogs.

The good news is that there are a lot of options for prevention of heartworm disease. The preventatives work by killing the larval stages that have already been injected into the dog or cat. So a dose of preventative works retroactively, killing the larval stages of heartworms for 45 days. For that reason, you can get away with only giving most preventatives every 45 days. Veterinarians recommend dosing every 30 days because it’s easier to remember to give preventatives at the same time every month. The other benefit of most heartworm preventatives is that they also contain an intestinal dewormer.

There is also a topical product that acts as a heartworm preventative, but some veterinarians don’t like using it for this purpose because it is harder to verify that the animal has absorbed the appropriate dose. You can tell when the topical products work for fleas and ticks because you no longer see the fleas and ticks. However, there is no way to know if the topical heartworm preventative was absorbed without doing heartworm testing — and by then, it is too late. (A treatment for heartworm disease exists, but it has a lot of side effects, is expensive, and is not always available.) To be fair, without doing testing, it is also hard to know if the oral medication worked, but you at least know your pet received it.

Read more about heartworm disease

Heartworm disease testing

On the topic of testing for heartworm disease, different veterinarians have different philosophies about whether to test yearly or every other year. There is no hard and fast rule on this. Some veterinarians are willing to put a dog on preventative without testing if you need to, but it is just not ideal. You will want to know if your animal has heartworms. Also, in some areas, such as the southeastern U.S., you will want to keep your animal on heartworm prevention all year. In other areas, such as the most northern states, it is not necessary to keep pets on preventative in the middle of winter (unless you travel with your pets), since there aren’t any mosquitoes buzzing around during a Wisconsin January.

Anti-parasitic plan for dogs and cats

To maintain the health of your pet, it is very important to have an anti-parasitic plan in place. This plan should be worked out with your veterinarian, who can give you the pros and cons of different medications and let you know what types of risks you might face if you choose not to use a preventative.

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