Parasites that affect dogs can be broadly classified as one of two types—internal and external parasites. Internal parasites reside inside your dog’s body and are rarely seen. External parasites are found on or in your dog’s skin, ears, or nose. Fleas and ticks can be seen with the naked eye. Lice and some mites can be seen with the naked eye but visualizing them with a low-powered microscope helps aid diagnosis.
Dog Internal Parasites
Internal parasites in dogs include intestinal parasites like worms and other microorganisms (such as roundworm, hookworm, whipworm, tapeworm, threadworm, giardia, coccidia, and intestinal flukes) and parasites that reside in other organs, such as heartworm and lungworm.
Roundworms are thin, light tan-colored worms that are about one to two millimeters wide (about 1/16 of an inch) and anywhere from four to fifteen centimeters long (about 1 ½ to 6 inches). One end of the worm is shaped like a spear and the other end is lightly coiled. When alive, the entire roundworm will become tightly coiled like a snake.
Most adult dogs will not exhibit any symptoms when they are infected with roundworm. Some adult dogs may vomit live roundworms or may have roundworms in their poop.
Puppies infected with roundworm will often have a pot-bellied appearance and a dull haircoat. They may have diarrhea that contains mucus. Unlike adult dogs, puppies will often vomit live roundworms and have roundworms in their poop.
Dewormers containing fenbendazole, milbemycin oxime, moxidectin, or pyrantel pamoate are all approved for treating roundworm infections. Roundworms are common parasites in dogs and especially puppies.
Whipworms are extremely thin, white worms that are about 4.5 to 7.5 centimeters long (about 2 to 3 inches). The worm resembles a leather whip (like the one used in the Indiana Jones movies). One end of the worm is thick like a whip’s handle and the other end is thin like a whip’s lash.
Dogs that have only a small number of whipworms in their small and large intestines do not typically show any symptoms of whipworm infection. Dogs that are infected with a large amount of whipworms will often exhibit weight loss and diarrhea.
Dewormers containing fenbendazole, milbemycin oxime, or moxidectin are effective for treating roundworm infections. Whipworms are common parasites in dogs and puppies.
Hookworms are off-white colored worms that are four to five millimeters wide (about ⅛ of an inch) and one to two centimeters long (about ½ to ¾ of an inch). These worms have a hook on one end that resembles a fish hook.
Dogs that have only a small number of hookworms in their small intestine do not typically show any symptoms of hookworm infection. Dogs that are infected with a large amount of hookworms may have dark, tarry stool. Left untreated, these dogs may become anemic, lethargic, and lose weight.
Puppies infected with hookworm will often have anemia that can become life-threatening. These puppies may also be thin and lethargic.
Hookworm can also cause pruritic pododermatitis (itching and inflammation of the skin of the paws). When we think of hookworm, we often think of it as being an intestinal parasite. But hookworm can also enter a dog’s body through penetration of the skin, typically the underside of the webbing between their toes. Hookworm causes an intense itching when it penetrates the skin.
Dewormers containing fenbendazole, milbemycin oxime, moxidectin, or pyrantel pamoate can be effective at eliminating adult hookworms. Only dewormers containing moxidectin are effective at eliminating fourth-stage larvae that are residing in the small intestine. See the article “How to Deworm a Dog” (WDJ Sept. 2023) for more information about treating hookworm infections.
Tapeworms are off-white colored worms that are 15 to 70 centimeters long (about 6 inches to over two feet). The part of the tapeworm that most people see in a dog’s poop or on the skin around the anus is the proglottid. The proglottid is the egg packet that a tapeworm releases into a dog’s poop. Proglottids look like small grains of rice.
Dogs rarely show any symptoms of tapeworm infection. The presence of tapeworm proglottids may cause irritation of the skin around the anus, resulting in a dog scooting his butt on the ground.
Dewormers containing praziquantel or epsiprantel are approved for treating tapeworm infections. Tapeworms, especially the species transmitted by fleas, are common in dogs and puppies.
Giardia is a microscopic protozoan parasite. Protozoans are single-celled organisms. Giardia is often described as looking like a tennis racket with eyes when viewed under a microscope.
Healthy adult dogs rarely show any symptoms of giardiasis. Puppies with giardiasis often have chronic diarrhea, weight loss, or a failure to thrive. Their poop will be soft and sometimes greasy. It is often pale in color and has a foul odor.
Fenbendazole is an effective first-line treatment for giardiasis. For cases of giardiasis that do not respond to this treatment, combination therapy with fenbendazole and metronidazole may be effective. Giardiasis is more common in puppies and in dogs that drink from outdoor, unfiltered water sources.
Coccidia is also a microscopic protozoan parasite. Coccidia is diagnosed by finding microscopic oocysts in feces.
Healthy adult dogs rarely show any symptoms of coccidiosis. Puppies with coccidiosis often have diarrhea, weight loss, or a failure to thrive. Some puppies may also exhibit vomiting, decreased appetite, and dehydration. Their poop may become bloody.
Sulfadimethoxine (Albon) is the only approved medication for treating coccidiosis. Other medications, such as ponazuril and toltrazuril, have demonstrated anecdotal success at treating coccidiosis but are not approved for this purpose. Coccidiosis is more common in puppies than adult dogs.
Heartworms are thin, white worms that are 15 to 30 centimeters long (about 6 to 12 inches). Heartworms enter a dog’s body in a larval form known as microfilaria through the bite of a mosquito. Microfilaria can only be seen with the aid of a microscope.
Symptoms of heartworm disease include cough, difficulty breathing, weight loss, ascites (fluid accumulation in the abdomen), and tiring easily or becoming winded during exercise or play. Your veterinarian may hear a heart murmur or arrhythmia or feel distension of the jugular vein in your dog’s neck on a physical exam.
Symptoms of heartworm disease may not appear until after the microfilaria have matured into adult heartworms. This can occur as early as two months after infection but most often occurs at least six months after infection. Some dogs may not show any symptoms of heartworm disease but test positive on a routine annual screening test.
Treatment for heartworm disease is a lengthy and serious process. Dogs are first started on a heartworm preventative to kill the microfilaria. Doxycycline and sometimes a steroid are prescribed during the first thirty days of treatment. This is followed by three injections of melarsomine to kill the adult heartworms. You can read more about heartworm disease and the recommended course of treatment from the American Heartworm Society.
Lungworms are small worms that range in size from 3 millimeters long to 4 centimeters long (1/10 of an inch to 1 ½ inches), depending on the lungworm species. Lungworms reside in the airways of the lungs, such as the bronchi, bronchioles, and trachea.
Dogs that have only a small number of lungworms in their airways do not typically show any symptoms of lungworm infection. Dogs that are infected with a large amount of lungworms may have a chronic cough, an increased respiratory rate and effort, or difficulty breathing.
Treatment for lungworm will vary depending on the species causing the infection but may include fenbendazole, ivermectin, selamectin, or moxidectin. Lungworms are not common in dogs or puppies.
Intestinal flukes—sometimes also known as liver flukes—cause a condition in dogs called schistosomiasis. Adult flukes are off-white colored flat worms that are 9 to 17 millimeters long (about ⅓ of an inch to ⅔ of an inch).
Symptoms of intestinal flukes include diarrhea, vomiting, decreased appetite, weight loss, decreased energy, increased thirst, and increased urination. Death may ensue if left untreated.
Treatment for intestinal flukes is not always effective, especially later in the course of the disease. Combination therapy with praziquantel and fenbendazole may be effective at clearing the infection in some dogs. Intestinal flukes are not common in dogs or puppies but may be more prevalent in certain geographic regions, such as Texas, the Southeastern United States, and some areas of southern California.
External Parasites in Dogs
External parasites include fleas, ticks, mites, and lice. These parasites reside either on top of or just within the skin. Some can also be found on the linings of the ears or nose.
Fleas are wingless blood-sucking insects that are dark brown in color and about 1 to 2 millimeters long. Although they cannot fly, they are capable of jumping on and off their host.
Fleas excrete a powdery form of the blood they ingest called frass, also known as flea dirt. Frass is used to feed flea larvae as they develop into adult fleas.
Fleas can be readily seen on most dogs by parting their fur down to the skin. You may be able to find flea dirt by running a flea comb through your dog’s fur. Flea dirt looks like fine, powdered dirt. Gently rub any dirt picked up with a flea comb on a moistened, white paper towel. Flea dirt will dissolve in water and create red streaks on the paper towel.
Symptoms of flea infestations include excessive scratching and chewing at the skin. There may be scabs on the skin where your dog has scratched or chewed, particularly on the neck, behind the ears, and at the base of the tail.
Treatment for flea infestations includes treating all of the animals in the home and treating the environment. Bathe your dog with a gentle shampoo made for dogs to reduce the flea burden. Use an oral flea treatment—such as Capstar—to help reduce the flea burden even further.
Start all of the pets in the home on a flea preventative. Some topical flea preventatives are toxic to cats and rabbits. Use only a flea preventative labeled for cats on the feline members of your family. Never use a flea preventative containing fipronil on rabbits.
It is important to treat the interior of the home for fleas. Fleas lay their eggs in the environment in which they reside and not directly on animals. This means that your carpets, the cracks between the boards of your wood floors, baseboards, and the areas under your furniture are all infested with flea eggs and larvae. Use an effective environmental treatment that kills adult fleas and contains an insect growth regulator to prevent larvae from reproducing once they become adults. You may also want to consider hiring a professional exterminator.
Ticks are actually arachnids and are in the same family as scorpions, spiders, and mites. Ticks have three life stages—larva, nymph, and adult. Larval ticks are the size of a grain of sand, nymphs are the size of a poppy seed, and adult ticks are the size of an apple seed. Ticks may be brown or black and some may have a white spot on the backside.
Ticks themselves do not cause any symptoms in dogs, but the diseases they carry and transmit can cause your dog to feel ill. It often takes several weeks or months from the time of the tick bite to develop symptoms of disease. These symptoms can include fever, profound lethargy, loss of appetite, weakness, lameness in one or more limbs, increased thirst, and increased urination.
Use one or more effective tick preventatives to prevent ticks from attaching to your dog. Minimize exposure of your dog to areas known for high tick loads, especially tall grasses in shady areas. Check your dog every day for ticks and carefully remove any that you find.
Lice are small, flat insects that are yellow in color and about 1 to 2 millimeters long. In most cases, it is not the lice that are noticed first but the nits in a dog’s coat. Nits are the eggs that lice lay and firmly attach to strands of fur. Nits are white or whitish-yellow in color and are about the size of a pinhead.
Nits look and react differently than dander. Nits are smooth and oval-shaped whereas dander is flat and flaky. You can easily comb dander from your dog’s fur. Nits remain firmly attached and cannot be removed, even with a fine-toothed comb.
Dogs with lice may rub, scratch, and chew at their skin. They may experience patchy fur loss. Puppies may develop anemia from a severe infestation.
Several flea preventatives are effective at treating lice infestations on dogs, including selamectin, fipronil, and imidacloprid. Flea/tick preventatives in the isoxazoline class of drugs—such as Nexgard and Bravecto—have also been shown to be effective against certain species of lice in dogs.
Lice are not common in dogs or puppies. Typically, lice are found on dogs and puppies that have been residing in unsanitary conditions, such as hoarding situations.
Ear mites are microscopic arachnids that are barely visible to the naked eye. Sometimes they may appear as a white speck moving across a dark background. But a definitive diagnosis of ear mites requires examination of ear exudate under a microscope.
Dogs with ear mites will have a dark brown or black discharge from one or both ears that often resembles coffee grounds. They will scratch at the infected ear, shake their head, hold one ear flap close to the head, and be sensitive about having the infected ear touched. Some dogs may have scabs along the outer base of the ear, the cheeks, or the neck.
Ear mites can be treated with flea preventatives containing selamectin and topical ear medications containing pyrethrin. Ear mites are more common in kittens and outdoor cats than they are in dogs and puppies.
Sarcoptes mites are microscopic arachnids that burrow into a dog’s skin. The condition they cause is called scabies or sarcoptic mange.
Dogs with scabies are intensely itchy and will scratch and chew at infected skin. Patchy hair loss and thickening of affected skin may develop.
Flea preventatives that contain selamectin or a combination of moxidectin and imidacloprid are approved for the use of treating sarcoptic mange in dogs. Flea/tick preventatives in the isoxazoline class of drugs—such as Nexgard and Bravecto—have also been shown to be effective against the sarcoptes mite. High-dose ivermectin was used for the treatment of scabies prior to the approval of selamectin and moxidectin/imidacloprid products and is no longer recommended due to the potential for serious side effects.
Demodex mites are microscopic arachnids that reside in the hair follicles of a dog’s skin. This is a commensal mite that is present in most dogs and puppies.
Healthy adult dogs are typically able to keep the commensal population of Demodex mites in check. Dogs that are immunocompromised and puppies whose immune systems are still developing may not be able to control the population growth of Demodex mites. The hair follicles and glands within the skin become crowded with mites, creating patchy areas of hair loss. Demodicosis can be either localized (on only a few areas of the body) or generalized (affecting large swatches of the body).
Most cases of localized demodicosis will resolve without treatment. Dogs with generalized demodicosis require treatment to resolve their condition. Treatment for generalized demodicosis is aimed at controlling the overgrowth of Demodex mites and identifying the underlying condition that allowed the overgrowth in the first place.
The only FDA-approved medication for demodicosis is a dip treatment called amitraz (brand name Mitaban). There are side effects to using amitraz and the odor of the dip is quite noxious. Other treatments for demodicosis are not FDA-approved for this purpose but have shown good efficacy in treating the condition. These treatments include ivermectin (an oral medication), milbemycin (found in several heartworm preventatives), moxidectin (found in some topical flea preventatives), and the fluralaner class of drugs (found in several oral flea/tick preventatives).
Harvest mites are microscopic orange arachnids that are also known as chiggers, red bugs, or berry bugs. Although individual mites on the skin cannot be seen with the naked eye, concentrated groups of mites on the skin may appear as small orange patches. The condition harvest mites causes is called trombiculosis.
Only the larval stage of the mite feeds on animals and people. The adult stage feeds on vegetation. Dogs with harvest mites will be intensely itchy. They may experience patchy hair loss and develop a secondary bacterial skin infection.
Flea and tick preventatives that contain fipronil or permethrin can be effective at preventing and treating trombiculosis. Harvest mites are more prevalent in the South, Southeast, and Midwestern regions of the United States.
Cheyletiella are microscopic mites that live on the surface of a dog’s skin. These mites feed on the outer layer of skin and are highly mobile. As the mites move about, they cause active and visible disruptions of skin dander. This has given Cheyletiella the common name “walking dandruff.”
Dogs with cheyletiellosis are moderately to intensely itchy. They may experience patchy hair loss. Their fur may feel rough and grainy.
The use of flea/tick or heartworm preventatives containing milbemycin oxime, moxidectin, selamectin, or fipronil have been shown to be effective at treating cheyletiellosis. Bathing with a shampoo containing pyrethrin may also be effective. Cheyletiellosis is not common but may be seen more often in dogs and puppies from high-density breeding facilities.
Nasal mites are arachnids that are light yellow in color and about 1 to 1.5 millimeters long. They infect the mucosal lining of the nasal sinuses.
Symptoms of nasal mites in dogs include sneezing, reverse sneezing, nasal discharge, bleeding from the nostrils (epistaxis), shaking of the head, and itchiness of the nose and face. Sometimes the mites may congregate around the nostril openings.
The use of flea/tick or heartworm preventatives containing milbemycin oxime, imidacloprid/moxidectin, or selamectin have been shown to be effective at treating cheyletiellosis. Ivermectin has also been shown to be an effective treatment. This is not a common parasite of dogs but may be more prevalent in large breed dogs older than three years of age.
Cuterebra is the larva of the rabbit bot fly. This larva burrows into the skin and initially creates a small swelling about one centimeter wide. As the larva grows and matures, the swelling can become large and painful and look like an abscess. The swelling will have a small hole in the middle through which the larva breathes. If you look carefully at the hole, you may see the larva moving inside!
Cuterebra is found throughout the United States. Immature larvae are grey or cream-colored, shaped like a cylinder with little ridges, and 0.5 to 1 centimeter long (about ¼ to ½ inch). The mature larvae are dark grey, can be up to three centimeters long (about 1 ¼ inches) and are cylindrically shaped with many ridges and spines.
The only treatment for cuterebra is manually extracting the larva from your dog’s skin. This procedure should only be attempted by a veterinarian. The larva must be removed whole—tearing the larva during extraction can cause anaphylaxis (a life-threatening allergic reaction). Your veterinarian may recommend giving a mild sedative to your dog and applying a numbing agent to the skin around the hole where the larva is residing.
