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11 Parasites in Dogs You Need to Know

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If you’ve noticed your dog scratching, coughing, vomiting, having diarrhea, or losing weight, there may be a parasite to blame.

What is a parasite? Parasites are organisms that live off of other organisms, called hosts, in order to survive. There are two main types of parasites in dogs: internal (such as worms) and external (fleas, ticks, mites, and lice).

Protection: While it’s impossible to prevent your dog from encountering parasites, you can protect them easily by using a suitable product on a regular basis. 

Veterinarians recommend broad-spectrum parasite protection, which essentially refers to using products that protect against multiple parasitic organisms at once. 

For instance, Interceptor® Plus (milbemycin oxime/praziquantel) is a monthly chewable that protects dogs against heartworm disease in addition to adult hookworm, roundworm, whipworm, and tapeworm infections. Credelio® (lotilaner), which kills adult fleas, treats and prevents flea infestations, and treats and controls tick infestations, is another example of a monthly product that can be used as part of a broad-spectrum protection plan. 

See important safety information below for Interceptor® Plus and Credelio®.

According to the American Veterinary Medical Association (AVMA) and American Animal Hospital Association (AAHA), and the Companion Animal Parasite Council (CAPC), every dog should receive year-round, broad-spectrum parasite control against both internal and external parasites.

Read on to learn about the parasites you should know as a pet parent, as well as important broad-spectrum protection plans.

Internal Parasites

As mentioned earlier internal parasites are organisms that live inside a dog’s body and can be exhibited through a variety of different symptoms, including diarrhea, vomiting, weight loss, coughing, lethargy, or a distended belly. 


What is it? Roundworms are very common in dogs. They live in a dog’s intestines and feed off food particles. There are two main types of roundworms: Toxascaris leonina and Toxocara canis. The latter is a more serious zoonotic parasite, which means it can be spread to humans.

How do dogs get roundworms? Most puppies are born with roundworms, contracted through their mother’s placenta or passed along through nursing. It can also be transmitted through eggs in the soil that were deposited from the feces of infected dogs, or by eating smaller infected animals. Low level infections in adult dogs can result in further environmental contamination as they shed eggs in their feces, putting others at an even greater risk for infection.

What are the symptoms? Typically symptoms of roundworm include a pot-bellied appearance, weight loss due to malnourishment, coughing, vomiting and diarrhea. Dead worms may be evident in the feces or vomited after treatment. 

How dangerous is it? Roundworm can lead to poor development in puppies if left untreated, whereas there are usually no clinical symptoms in adult dogs. 


3D illustration of hookworms

What is it? These common parasites use their mouthparts, which contain either teeth or cutting plates, to dig into a dog’s intestinal lining. They’re called hookworms because the front end of the body bends into a hook shape. They’re usually 1-2 centimeters long and feed off of blood and the intestinal lining, causing significant damage.

How do dogs get hookworm? Puppies can contract hookworm from their mothers through nursing. Adult dogs can get hookworm by ingesting contaminated water, soil or feces, or through the skin in infected environments. Larval hookworms can penetrate the skin of both dogs and people.

What are the symptoms? Symptoms of hookworm infection include bloody diarrhea, tarry stools, anemia, pale gums, weight loss, itchy paws and stunted growth.

How dangerous is it? Since hookworms feed on blood, they can cause severe anemia and even death in puppies. Skin issues like redness, itching, and blister-like lesions may appear after hookworms burrow through a dog’s skin.


What is it? Upon entering a dog’s bloodstream, larval heartworms from mosquitoes travel to the heart and lungs. In the right ventricle, and nearby blood vessels, heartworms can reach lengths of nearly 1 foot (25 cm) in just a few months. They reproduce, creating millions of baby heartworms that circulate in the dog’s blood, waiting to be ingested by a mosquito during a blood meal.

How do dogs get heartworm? Dogs do not transmit heartworm directly to other dogs. Rather, heartworm spreads via an intermediary mosquito, passing heartworm from an infected dog to its new host via a mosquito bite. 

What are heartworm disease symptoms? General malaise, fatigue, and coughing are all possible signs of heartworm.

How dangerous is it? Heartworm disease is potentially deadly. In severe cases, it can lead to heart failure and organ damage. While treatment is available, it is painful, long and arduous. 


What is it? Adult tapeworms are made up of many small segments, which break off and end up in a dog’s stool. These segments contain tapeworm eggs. Different types of tapeworms can affect dogs: the flea tapeworm (Dipylidium caninum), species of Taenia, and species of Echinococcus. While some adult tapeworms can be several meters long, adult Echinococcus tapeworms are less than 1 centimeter long inside a dog’s intestine and can be deadly to humans. They are so small, they may be missed by the naked eye. 

How do dogs get tapeworms? The infective stages of the tapeworm are found in an an intermediate host. Rabbits, rodents, livestock, wildlife, and fleas are all potential intermediate hosts, depending on the species of tapeworm. A dog might eat the small host carrying these infective stages, which can then grow into an adult tapeworm inside a dog’s intestines. An infected dog can not transmit a tapeworm directly to another dog or a human (with the exception of some species of Echinococcus, under certain circumstances).

What are tapeworm symptoms in dogs? In most cases, dogs can be infected with tapeworms and show no symptoms. With Dipylidium and Taenia tapeworms, you might see small segments of the worm around an infected dog’s anus or in the stool. However, segments are not normally visible for Echinococcus tapeworm, as they are too small to be noticed. Tapeworms are very easy to miss, even with normal veterinary diagnostic tools, if a dog is not actively passing tapeworm segments at the time of an exam. 

How dangerous is it? It depends on the type of tapeworm. Most tapeworm infections are easy to treat with medication once detected. The vast majority of animals will experience no symptoms. In rare instances, it can lead to more serious symptoms, such as intestinal blockage, if left untreated. However, Echinococcus tapeworms in people, and sometimes in dogs, can cause severe disease, can be difficult to treat, and can be fatal. 


What is it? Whipworms are more common in adult dogs than roundworms. These whip-shaped parasites, which can reach 4 to 7 centimeters in length, live in a dog’s large intestine. 

How do dogs get whipworm? A whipworm lays eggs in its host, then those microscopic eggs are passed to the outside environment via stool. The eggs can survive harsh conditions in the soil for many years. A dog contracts whipworm by ingesting infected stool or soil. 

What are the symptoms? Diarrhea, bloody stool, and weight loss are common symptoms of whipworms in dogs. 

How dangerous is it? Regular deworming is best, since whipworm can easily reinfect a host. Eggs shed are very hardy and live in the environment for years.

Coccidia and Giardia

3D illustration of giardia

What are they? Coccidia and Giardia are microscopic parasites that can live in our dogs’ intestines and cause gastrointestinal problems. They are not worms. 

How do dogs get coccidia and giardia? These parasites are commonly transmitted when dogs ingest infected feces, soil, or water. 

What are the symptoms? Both of these parasites can cause diarrhea, bloody stool, and vomiting in dogs. However, they may not cause symptoms in adult dogs.

How dangerous are they? Cases of coccidiosis (which almost always exclusively infect puppies) and giardia in dogs are challenging to fully treat, because these two parasites are very hardy in the environment. Reinfection is common, making them difficult to get rid of. Steps pet parents can take to help prevent reinfection include bathing all household pets, thoroughly cleaning the pets’ environment, regularly washing food and water bowls, and limiting contact with fecal matter as much as possible.

External Parasites

At some point in every dog’s life they will come into contact with an external parasite. These parasites can cause mild symptoms such as irritated skin or carry diseases that result in potentially fatal medical problems. 

Broad-spectrum protection and treatment medications are both common methods for dealing with these external parasites. For ticks and fleas, your veterinarian may prescribe chewable Credelio® (lotilaner) tablets that begin killing ticks (l. rincus) and fleas within a few hours. 

See important safety information below for Credelio®.


What are they? A flea is an external parasite that feeds on a dog’s blood. Adult fleas hop onto a host from a contaminated environment, then generally stay on that host. They do not move from animal to animal unless they are in direct contact. 

How do dogs get fleas? Flea eggs hatch into larvae, which then pupate. Pupae (cocoons) can remain dormant in your home for months. Adult fleas emerge from the pupa and wait for a host to walk by to jump onto. Fleas on people almost always come from the contaminated environment and are looking for a host. The flea life cycle is faster when the weather is warm and humid, so flea numbers increase rapidly in warm, humid locations.

What are the symptoms? You may see “flea dirt” (tiny black droppings) on your dog’s coat before you see fleas. Though you may notice excessive itching, chewing, hot spots, or other signs of general discomfort. 

How dangerous are fleas? Some dogs develop mild to severe health problems from scratching or skin conditions such as Flea Allergy Dermatitis (FAD). Fleas may also be carriers of internal parasites or bacterial infections. Severe infestations can lead to anemia and even death due to excessive blood loss.


removal of tick from dog

What are they? Ticks are parasites that attach themselves to hosts and feed on their blood. There are hundreds of species of ticks in the world.

How do dogs get ticks? Your dog doesn’t need to venture into the woods to come into contact with a tick—wild mammals and birds can drop ticks around homes, yards, and gardens. Ticks often reside in brush or long grass. Unlike fleas, ticks don’t jump. They attach to their hosts by crawling up on blades of grass or bushes (or the walls of your home), waiting, and grabbing on when an animal or person brushes up against them. Ticks are more active during warm months, but can be a year-round threat as some species like cooler weather. Brown dog ticks have adapted to live indoors and can infest homes and buildings in large numbers.

What are the symptoms? Your dog may scratch irritated skin caused by tick bites. Ticks can usually be seen or felt, usually around your dog’s face, ears, under the front legs, in the groin area, and between the toes.

How dangerous are ticks? There are many varieties of ticks that carry serious diseases including Lyme Disease. If you see a tick on your dog, carefully remove it with tweezers as soon as it is discovered. Some ticks spread potentially fatal diseases to both pets and people. Keep the tick in a Ziploc bag or a closed container and bring it to your veterinarian for evaluation. Dogs should be on year-round tick protection to help prevent the spread of disease. 


What are they? Mites are tiny parasites that burrow into a dog’s skin. There are three common types of mites dogs are susceptible to: Ear mites, sarcoptic mange mites, and demodectic mange mites. 

How do dogs get mites? Ear mites and sarcoptic mange mites can be transmitted between dogs. Demodectic mange mites are usually only transmitted from mother to puppy and only cause disease when present in large numbers. Demodectic mange is not contagious.

What are the symptoms? Look for irritated, scratchy skin or hair loss. The location of the itch on your dog’s body will help you identify the type of mite. Although ear mites are very difficult to see by the human eye, an infection produces brown ear wax in dogs that looks like coffee grounds. Your veterinarian can determine if your dog has ear mites or some other type of ear infection.

How dangerous are mites? All skin problems caused by mites can usually be cleared up through proper treatment. Keep in mind that sarcoptic mites can bite humans and cause a rash.

Dog Lice

closeup of dog lice

What is it? Dog lice are a species-specific variation, different from the lice humans get. Of the two types of dog lice, chewing lice stay alive by eating a dog’s skin particles, and sucking lice feed on a dog’s blood.

How do dogs get lice? Lice spread through contact with another louse-infested dog.

What are the symptoms? Itchy fur, dry or inflamed skin, and hair loss are common symptoms of lice on dogs.

How dangerous are lice? Prevention of lice is the best method. While it’s not life threatening on its own, lice infestation in dogs is not easy to eradicate.

After learning all about parasites and their symptoms, it’s easy to feel overwhelmed or anxious that your dog will be exposed to something unpleasant. But there’s no need to panic. Through regular veterinary check-ups and broad-spectrum parasite protection, you can safely enjoy quality time and adventures with your dog.

Credelio Indications

Credelio kills adult fleas and is indicated for the treatment and prevention of flea infestations, treatment and control of tick infestations (lone star tick, American dog tick, black-legged tick, and brown dog tick) for one month in dogs and puppies 8 weeks and older and 4.4 pounds or greater.

Credelio Important Safety Information

Lotilaner is a member of the isoxazoline class of drugs.  This class has been associated with neurologic adverse reactions including tremors, incoordination, and seizures. Seizures have been reported in dogs receiving this class of drugs, even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders. The safe use of Credelio in breeding, pregnant or lactating dogs has not been evaluated. The most frequently reported adverse reactions are weight loss, elevated blood urea nitrogen, increased urination, and diarrhea.  For complete safety information, please see Credelio product label or ask your veterinarian.

Interceptor Plus Indications

Interceptor Plus prevents heartworm disease and treats and controls adult roundworm, hookworm, whipworm, and tapeworm infections in dogs and puppies 6 weeks or older and 2 pounds or greater.

Interceptor Plus Important Safety Information

Treatment with fewer than 6 monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. Prior to administration of Interceptor Plus, dogs should be tested for existing heartworm infections. The safety of Interceptor Plus has not been evaluated in dogs used for breeding or in lactating females. The following adverse reactions have been reported in dogs after administration of milbemycin oxime or praziquantel: vomiting, diarrhea, decreased activity, incoordination, weight loss, convulsions, weakness, and salivation. For complete safety information, please see Interceptor Plus product label or ask your veterinarian.

Disclaimer: The author received compensation from Elanco US Inc., the maker of Interceptor Plus and Credelio, for her services in writing this article. 

Credelio and Interceptor are trademarks of Elanco or its affiliates. 

© 2020 Elanco.  PM-US-19-2123

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