Canine Parasites

Fleas, Ticks, Worms and More

Common Canine Parasites -- An Illustration.

Illustration: Hugo Lin. © The Spruce, 2018

Parasites come in many sizes, shapes, and levels of severity for your canine companion. Your dog may be affected by parasites such as fleas and ticks that infest the skin and fur. But there are also internal parasites such as heartworm and intestinal parasites affecting the digestive system. This collection of parasites are those commonly found on dogs and other species, sometimes affecting humans (called a zoonotic disease).

  • 01 of 14


    Illustrated female Mosquito.

    Anka Friedrich/Wikimedia Commons

    Heartworm disease is caused by a large roundworm parasite that lives mainly in the blood vessels of the lung and in the heart. It is transmitted by mosquitoes. While heartworm is most prevalent in the Southern states, it has been seen in every state.

    Your veterinarian will usually test for heartworm during each check-up. A dog with heartworm infestation may show no symptoms at first, but as it progresses it can be fatal. Dogs can develop a cough (including coughing up blood), become exhausted from exercise, faint, and have severe weight loss.

    Once a dog is infested with heartworm, medication is needed to kill the parasite. The dog will need complete rest during the treatment period in order to prevent dangerous complications from the dying worms.

    Fortunately, heartworm disease is easy to prevent. Any number of proven, safe preventions exist; among them, Heartgard and Heartgard Plus, which are used to prevent heartworms and several intestinal parasites when administered on a monthly basis. 

    If a mosquito bites an infected dog, the parasite can then produce larvae inside the mosquito and these can be transmitted to humans. In humans, the parasite most often causes lung lesions. Preventing the disease in your pets can help protect you, as well.

  • 02 of 14


    Detailed illustration of a flea.

    Robert Hooke/Wikimedia Commons

    Fleas can make your pet's life miserable. There is no single method or insecticide that will completely eradicate (or at least control) a flea problem. The flea life cycle is fairly complex, and understanding the various stages will make it easier to get rid of them. You will need to use a variety of methods and products to keep your pet comfortable.

    Fleas can lead to flea allergic dermatitis with itching and skin infections. Fleas are also carriers of tapeworm eggs and your dog can be infested by tapeworm after eating a flea. A large infestation with fleas can also lead to anemia.

    While dog and cat fleas don't live on humans, you can definitely feel their bites.

  • 03 of 14


    Deer tick, Ixodes scapularis, on a fingernail.

    Lauree Feldman/Getty Images

    Ticks can attach themselves to dogs as well as to humans. While you might wear protective clothing when in tick-prone areas, your dog is exposed. Early detection and removal of ticks are important as it takes some time for the tick to transmit tick-borne diseases to their hosts. Diseases spread by ticks to dogs include Lyme disease, Rocky Mountain spotted fever, babesiosis, bartonellosis, and ehrlichiosis.

  • 04 of 14

    Ear Mites

    Examining a dog's ears
    Ear check.

    Isa Foltin/Contributor/Getty Images

    Ear mites are tiny parasites found inside the ear canal. They are spread by direct contact with other animals and are more common in puppies. They can cause severe irritation and itchiness of the ears. Your dog may show head shaking, scratching at the ears, and crusty or waxy discharge that looks like coffee grounds.

    The most common cause of ear mites in cats and dogs is Otodectes cynotis, and therefore an infestation with ear mites is sometimes called otodectic mange. Your veterinarian can diagnose ear mites with an examination and give one-time or repeated treatments. Ear mites don't infect humans.

    Continue to 5 of 14 below.
  • 05 of 14

    Demodex Mite

    A dog with severe demodectic mange.

    Julie Knicely/OTRS

    Demodex mites are microscopic normal inhabitants of dog skin. In a healthy animal, the mites are few in number and do not cause skin problems. In some cases the mites can take over, leading to a condition commonly called mange or demodicosis. The dog loses hair and the bald skin can be scaly, odorous, and prone to infection.

    Puppies and young dogs usually outgrow this condition as their immune systems get stronger. In adult dogs, mange will develop due to impairment of the immune system, hormonal imbalances, or old age. Some dogs are also genetically predisposed to the condition.

    Your veterinarian will address mange differently depending on whether it is localized or if it affects many areas. There are shampoos as well as medications that can treat it. The mite is not transmissible to humans.

  • 06 of 14

    Cheyletiella Mites

    Raubmilbe, Cheyletiella.

    Kalumet/German Wikipedia

    Cheyletiella is a mite that lives on the skin, causing irritation, dandruff, and itchiness. A distinguishing feature of this mite species are the large, claw-like mouthparts. These mites can be found quite commonly on cats, dogs and rabbits, and other species. Though humans are not a natural host for this parasite, Cheyletiella mites can happily live on humans for a while, causing an itchy rash.

  • 07 of 14

    Cuterebra Parasite

    Cuterebra, a genus of botfly
    First instar larva of Cuterebra, a genus of botfly. Fly, parasite. Image courtesy CDC/Dr. George Healy.

    Smith Collection/Gado/Getty Images

    A Cuterebra parasite is an opportunistic parasite found under the skin of small mammals. This parasite is the larval stage of the Cuterebra fly (or botfly), who uses animal hosts to complete its lifecycle. The larva forms a bump just under the skin of the host, usually around the head or neck, and makes a small hole in the skin in order to breathe. The maggot then might be seen inside until it exits 30 days later. If you see this lump, it is important not to squeeze the skin or try to get it out with tweezers. If you break the larva it can produce infection or even a life-threatening anaphylactic reaction. Instead, go to the vet to have it safely removed.

  • 08 of 14


    Illustration of Giardia organism.


    Giardia lamblia is a one-celled protozoan parasite that lives in the intestinal tract of many animals. When this parasite produces a diarrheal disease in animals (including humans), it is called giardiasis.

    Your dog (and you) can pick up Giardia from contaminated water (including pristine-looking mountain streams) as it is shed by many animals in the wild. As well, the cysts are shed in the stool and your dog may pick it up from soil or groundwater after eating, drinking, or grooming.

    If your dog is sick with diarrhea, lethargy, and loss of appetite, your veterinarian can examine its stool for Giardia (which is actually quite cute under the microscope, looking like a googly-eyed kite). Medications are used to treat this illness. You should clean your home and yard of dog feces to help prevent reinfection or transmission to humans.

    Continue to 9 of 14 below.
  • 09 of 14


    Ancylostoma caninum, a type of hookworm, attached to the intestinal mucosa. Image #5205, CDC.

    CDC's Public Health Image Librabry/Wikimedia Commons

    Hookworms are small, thin worms that are less than an inch long. These intestinal parasites that are common in dogs. There are three species of hookworms that affect dogs. Some can also affect humans, migrating through the skin.

    Dogs pick up hookworms by ingesting the larvae they pick up from the environment or, in puppies, from the mom's milk. Symptoms include diarrhea, loss of appetite, anemia, weight loss, or failure to gain weight.

    A veterinarian can check for hookworm in the stool. Treatment includes deworming medications that must be used repeatedly to eliminate the larvae as they mature. Regular deworming will help prevent recurrence.

  • 10 of 14


    Whipworm egg.

    Joel Mills/Wikimedia Commons

    Whipworms are intestinal parasites (Trichuris) that are relatively common in dogs and occasionally seen in cats. These small worms have a thin, whip-like front end and a thicker back end. They attach themselves to the walls of the large intestine, feeding on blood.

    Dogs pick up whipworm eggs transmitted through the stool in the environment. A light infection has no symptoms. A heavier infection can result in weight loss, diarrhea, or anemia. A veterinarian can detect the eggs under the microscope during a stool examination.

    Whipworms are resistant to many of the usual dewormer medications, so a different medication is usually used. Keeping the environment clean of feces is the best prevention as the worm eggs take weeks to become infectious.

  • 11 of 14


    Toxocara canis adult nematode worms of dog.

    Alan R, Walker/Wikimedia Commons 

    Roundworms, including Toxocara Canis and Toxocara leonina, are intestinal parasites in dogs. They absorb nutrients from the food the dog eats, which results in less being available for the dog. Infection can occur after ingesting eggs shed in stool or from larvae transmitted during pregnancy or in the mother's milk.

    A dog may show vomiting, lethargy, weight loss, diarrhea, and a pot belly. A veterinarian can detect roundworms in the stool. Deworming medication can be used, with a number of treatments in order to clear the roundworms as they mature. A regular deworming protocol is needed to prevent reinfection.

    Roundworms can be transmitted to humans. They cause inflammation and can migrate to various tissues and organs.

  • 12 of 14


    Dipylidium caninum
    Extract from a dog gut during a surgery (found "by accident", nothing to do with the cause of the surgery). Taenia infecting dogs and cats, incidentally humans, benign. Its cycle goes through fleas. (Translatedfrom original Fr.).


    Tapeworms are flat, ribbon-like parasites that live in the intestines. Happily, they rarely cause serious disease. Dipylidium caninum is the most common one for dogs, but there are other species. They are transmitted by eating raw meat or by fleas.

    Tapeworms can cause irritation around the anus, due to the shed segments of the worm. A puppy may scoot around on the floor or lick the area. Infestation can result in poor nutrition for the dog or even an intestinal blockage in severe cases.

    A veterinarian can diagnose tapeworms by a stool exam and examination of the hair around the anus. Medication is needed to clear the infection, accompanied by good flea control and keeping the dog from eating dead prey.

    Continue to 13 of 14 below.
  • 13 of 14


    A coccidian oocyst from a fecal sample of a dog. Photo taken through a microscope at 40x on March 20, 2006.

    Joel Mills/Wikimedia Commons 

    Coccidia are single-celled protozoan parasites that are common in adult dogs without a problem but may cause illness in small puppies. They can pick up the parasite from feces in the environment. It can produce mild diarrhea, which can become more severe. It can lead to loss of appetite, weight loss, dehydration, and even death. A veterinarian can diagnose coccidia with a stool exam.

    Treatment is needed to eliminate the parasite, and a very sick puppy may need hospitalization and intravenous fluids. Keeping the environment clean of feces is essential for preventing this disease.

  • 14 of 14


    Histopathologic changes in the structure of a hair follicle, in a patient with ringworm, infected with the dermatophyte, Trichophyton tonsurans.

    Dr. Lucille K. Georg/CDC (PD-USG)


    Ringworm is a fungal infection rather than a parasite. A classic round "worm-like" lesion is seen on human skin when infected. It can be transmitted from animals to humans through contact with bedding, brushes, rugs, and furniture. In dogs, patchy areas of hair loss and scales are the common signs. Usually, these infections clear up on their own, or an anti-fungal shampoo can be used.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.