Skip to main content

Dog Parasites


Free-ranging animals usually carry some parasites, but they may not show symptoms as nature provides many plants capable of eradicating internal parasites.

The ingestion of grass blades is well known in carnivores and acts as a means of stimulating regurgitation of fur and other undigested material, but it may also remove and purge intestinal parasites. The action is mechanical and is possibly controlled by some toxic compounds. On the home front, domestic dogs occasionally chew grass with two effects. One is emetic (stimulating vomiting) while the other, a purgative scour (expel through the feces). Some dog species discriminate between different grasses for different medicinal functions, using hairy grasses for emetics and couch grass as a purgative.

Most dog owners believe that if a dog is found to have parasites in his stool, then he must be suffering from a disease state. This is not necessarily the case. Most dogs are infested at one time or another with intestinal parasites. Some are born with them and others acquire them later in life. When they recover, they develop a certain amount of immunity. This helps to keep the worms in check. One should distinguish a disease state from the presence of a parasite. If worms are causing disease, there will likely be some change in the appearance of the dog. In turn you would note decreased appetite, weight loss, upset stomach, anemia, mucus and/or blood in the stool.

Immunosuppressive drugs such as cortisone have been shown to activate large numbers of hookworm larvae lying dormant in the dog's tissue. Stressful events such as trauma, surgery, severe disease, and emotional upsets also can activate dormant larvae. This leads to the appearance of parasites in the stool. During pregnancy, round and hookworm larvae are activated and migrate to the unborn puppies. Accordingly, a heavy parasite problem may appear in a litter even when the mother was effectively dewormed. This can happen because none of the deworming agents are effective against larvae encysted in the tissue. If parasites are a concern for you, discuss an individualized deworming program for your dog with your veterinarian.

Common internal parasites in the dog include nematodes, cestodes and protozoan parasites.

Roundworms (Ascarids)
Intestinal parasites are the most common type of internal parasite found in dogs. The four most commonly seen types are roundworms, hookworms, whipworms and tapeworms. Ascarids are a nematode parasite and are among the most common type of gastrointestinal parasite in dogs. Adult parasites live and reproduce in the small intestine and inactive encysted larvae can be found in the muscle and other tissues. Adult parasites are large, between 1 to 7 inches long and have the appearance of strands of spaghetti. The most common species to infect dogs is Toxocara canis. It is estimated that approximately 75 percent of puppies and young dogs are infected with Toxocara canis.

Dogs can contract ascarids from four possible routes: transplacental, transmammary, ingestion of eggs from the environment or predation of infected animals such as birds and mice. The major routes of transmission to dogs are transplacental and the ingestion of eggs. The majority of puppies are born with Toxocara canis and begin shedding eggs of this parasite in their feces by the time they are 3 weeks old.

Dogs that are older than three months develop partial immunity to ascarids, so they rarely are infected with the intestinal form of this parasite. However, when eggs are ingested from the environment, some dogs may develop a short-term infection and shed large numbers of eggs in the feces. This often occurs in lactating females when they lick the anus of their puppies or consume puppies fecal matter when cleaning up after the litter. Another way in which adults can contract short-term, self-limiting intestinal ascarids is through ingesting infected wild animals such as rodents, birds and some insects.

Infection with ascarids is rarely a health problem in adult dogs but can be very serious in young puppies. If large numbers of larvae are passed to the fetuses transplacentally, stillbirths and early neonatal death can occur as a result of a heavy larval infestation in the liver and the lungs. In newborn puppies, developing adult parasites in the intestine causes malnutrition, impaired growth, and emaciation. In severe cases, ascarids can cause death within the first few weeks of life. The typical ascarid-ridden puppy has a pot bellied appearance, with a rough haircoat and poor muscle development. Vomiting and diarrhea occur and adult worms may be seen in the vomit or feces.

Hookworms
Adult hookworms are relatively small in size compared to ascarids. They are gray in color and between ½ and ¾ of an inch in length. The anterior end of the worm has a slight hook with cutting plates for attaching to the lining of the intestine and ingesting blood. Hookworms reside primarily in the small intestine but may be found in the colon and cecum in heavily infested dogs. Several species can infect the domestic dog. These include Ancylostoma canium, Ancylostoma braziniense, and Unicaria stenocephala. Ancylostoma canium is by far the most important and most pathogenic hookworm parasite that infects dogs. Ancylostoma species are blood-ingesting parasites and can lead to serious blood loss when large numbers occur. Unicaria stenocephala commonly is referred to as the "fox hookworm" because of its prevalence in that species. Unlike Ancylostoma, species, Unicaria species are not blood-ingesting parasites and so do not cause severe health problems in dogs.

Two common routes of hookworm infection are transmammary and infection with third-stage larvae from the environment. Larvae can either be consumed orally from the soil or can enter the dog's body by penetrating the skin on the pads of the feet. Adult dogs often have acquired some immunity to hookworms. As with ascarids, encysted forms of hookworm remain dormant indefinitely in male dogs, spayed females and females who are never bred. However, in pregnant females, encysted hookworm larvae deposited in the muscle become active during pregnancy, travel to mammary tissue and infect newborn puppies through the mother's milk. Approximately 60 percent of the larvae transmitted through milk do so during the first week of nursing. The larvae travel directly to the intestine and have matured to adult parasites by the time the puppies are 3 weeks old. Ancylostoma species cause moderate to severe blood loss through feeding and damage to the intestinal wall. Anemia occurs rapidly in young puppies and is characterized by weakness, depression, lethargy and pale mucous membranes. Hookworms typically cause chronic diarrhea, usually containing blood and mucous. If heavily infected, newborn puppies may die as early as 8 days following exposure. As dogs grow older, most develop immunity to hookworms. However, adult dogs that have compromised immune systems, are housed in a highly contaminated environment or are experiencing malnutrition can develop chronic hookworm infections. In these cases, mild clinical signs that include diarrhea and vomiting are seen.

Whipworms
Trichuriasis or whipworm infection is caused by nematode parasites of the genus Trichuris vulpis is the species that commonly infests dogs. Adult parasites are 2 to 3 inches in length and the anterior three-quarters of the body is much thinner than the posterior, giving the parasite an appearance similar to a whip. Whipworms are blood-ingesting parasites and inhabit the cecum and colon of their host.

Adult whipworms attach firmly to the mucosa of the cecum and proximal colon where they feed on blood. Clinical signs of trichuriasis vary with the numbers of parasites that are present, the individual dog's susceptibility and the extent to which adult worms penetrate the intestinal mucosa. Adult dogs do not appear to develop immunity to this intestinal parasite as they age, so they are susceptible to repeat infections throughout life. Clinical signs include diarrhea, vomiting, and weight loss. Light infections may show no diarrhea but may be associated with gradual weight loss. Heavy infestations may be associated with chronic bloody diarrhea, dehydration, and anemia. Because infective whipworm larvae persist for very long periods in the environment, re-infection after treatment is very common.

Tapeworms
Tapeworms, also known as cestodes, are highly adaptive internal parasites that can be found in all groups of vertebrate animals. At least 14 different species of tapeworm infect dogs in North America, of which Dipylidium caninum and Taenia species are the most common.

The life cycle of the tapeworm depends on the species of the parasite. All tapeworms have one or more intermediate hosts. The intermediate host ingests gravid proglottids (gravid proglottid refers to segments of the tapeworm that are sexually mature and contain eggs) from the environment and dogs are subsequently infected when they ingest the tissue of the intermediate host. Proglottids containing eggs appear in feces or around the tail and anus of an infected dog.

The species Dipylidium caninum is carried by fleas and lice and will infect dogs that swallow one of these external parasites.Taenia species include Taenia hydatigena, Taenia multiceps, Taenia ovis, Taenia pisiformis.

Taenia hydatigena has an indirect life cycle, with sheep, goats, cattle, pigs or other mammals as intermediate hosts. The disease produced in dogs by this tapeworm is less severe than that produced in sheep, goats, and cattle. Parasite loads of these tapeworms in the dog are usually harmless, but large numbers may cause gastrointestinal inflammation which causes diarrhea. Many anthelmintics effective against adult tapeworms are available. Uncooked ruminant meat should be deep-frozen for at least 10 days before feeding to dogs to eliminate potential parasites.

Taenia multiceps intermediate hosts are sheep, goats, cattle horses, other ungulates, and rodents. Ingestion of tissue from intermediate hosts by dogs results in the development of adult tapeworms. Ingestion of brains from infected sheep, goats, cattle or other possible intermediate hosts should be avoided. Effective medication is available to treat infected dogs.

Taenia ovis (sheep measles) is a tapeworm which can be found in organs and muscles. It is of little significance to the dog. Proglottids containing eggs may appear in feces. Sheep meat should be deep frozen for at least 10 days prior to feeding.

Taenia pisiformis intermediate hosts are rodents, mainly rabbits, and hares. Taenia pisiformis is not often a threat to dogs; only intermediate hosts are seriously compromised. Proglottids may appear in feces. Anthelmintics (antiparasitic agents) will cure infections with tapeworms.

Tapeworm infection rarely causes health problems in dogs, except in heavy infections. Over time, an infected dog may develop a dull, lusterless coat and show a decreased appetite or a slight weight loss. However, most cases are asymptomatic. Generally, any raw meat should be deep-frozen for at least 10 days before feeding to dogs to eliminate tapeworms.

Coccidiosis
Coccidiosis is a parasitic disease that is caused by a protozoan, which is a microscopic, single-celled organism. There are at least 12 species of the coccidia protozoa that can affect dogs, but the most common are Isospora canis. Coccidiosis is widespread throughout the domestic dog population. Clinical disease is most often seen in puppies and is highly contagious within litters. Infections can be serious in young puppies and can cause death from dehydration and malnourishment. Infected adult dogs often are asymptomatic or show only mild clinical signs. Adults can become carriers and will shed infective oocysts in their feces, serving as an important source of transmission to other dogs. In puppies and young dogs, coccidiosis is characterized by severe diarrhea, which may be mucoid and bloody; dehydration; weight loss; and anemia. Some puppies develop slight upper respiratory infections characterized by a cough and nasal and eye discharges. Coccidiosis usually occurs in connection with poor sanitation or overcrowded conditions.

Giardia
Giardia is a common flagellar protozoan, using a hairlike tail to motivate. It is found in many vertebrate hosts and many species have been identified. The most common species to infect humans and domestic animals are Giardia lamblia. Many water sources are contaminated with giardia from feces of infected wild animals, domestic animals or humans. The most common route of infection to dogs is through the ingestion of contaminated outdoor water sources. Infection with giardia causes mild enteritis and chronic, intermittent diarrhea. Signs are generally more severe in puppies.

Trichinella (spiralis)
Trichinella is a parasitic roundworm. The life cycle of the parasite begins when the infectious cysts are consumed with the flesh of any meat eating animal. The cysts are digested and the larvae invade the small intestine. The female hatches larvae that are carried by the blood and lymph to the muscles. Natural infections occur in wild carnivores; trichinellosis has been found in hogs, bear, horses, rats, beavers, opossums, walruses and meat-eating birds. Most infections in domestic and wild animals go undiagnosed. In humans, heavy infections may produce serious illness with three clinical phases (intestinal, muscle invasion, and convalescent) and occasionally death. Treatment is generally impractical in animals. The objective is to prevent ingestion by any animal. Freezing effectively kills trichinella in pork, but cannot be relied upon to kill trichinae in other meats. The freezing requirements differ with the size of the meat. Pieces not exceeding 6 inches in thickness require 20 days at -15C, 10 days at -23C, and 6 days at -30C.

Toxoplasma gondii
Toxoplasma gondii is one of the most common protozoan parasites in the world. T.gondii is an obligate intracellular parasite - which means that it is only able to reproduce inside cells. Humans are primarily infected with T.gondii either by the ingestion of undercooked or raw meat containing tissue cysts or by contaminated water and food from oocysts shed by infected cats. Another mode of human infection is congenital infection when a pregnant woman becomes infected and transmits the parasite to her fetus. T. gondii has three forms: The cysts in tissues of infected individuals; the oocysts in the intestine of members of the cat family; the invasive form, the tachyzoites, which is responsible for acute disease - toxoplasmosis. (Tachyzoites are rapidly-dividing organisms found in tissues during the acute phase of infection).

Cats are the only animals known to shed oocysts. Sheep and goats can be infected by the organism. Other animals that can be infected include birds, horses, game animals (e.g. deer), mice and rats, marsupials and dogs. The role of dogs in the spread of the disease has generally been considered as of secondary importance. Intimate contact between man and dogs has led to many studies highlighting the importance of dogs in the epidemiological chain of the disease through the habit of ingesting and rolling in cat feces, thereby permitting transmission of oocysts by contact of the contaminated hide.

Infected meat from cattle is not consequential but pork is considered to be significant. Sheep, chicken, rabbit, horse and goat meat can be infected and unpasteurized goat milk has also been implicated as a source of disease. Vegetables which may be contaminated with soil are another potential source, particularly from gardens where there may be cats. Freezing raw meat at -20C for 54 hours destroys T.gondii.

Nanophyetus salmincola (Salmon poisoning disease)
Salmon poisoning disease is an acute, infectious disease of dogs in which the infective agent is transmitted through the various stages of a fluke in a snail-fish-dog life cycle. N.salmincola is a trematodes parasite of the small intestines of dogs, cats and wild carnivores. N.salmincola is a host for Neorickettsia helminthoeca which is the causative organism for "salmon poisoning" in canines. Salmon poisoning is characterized by appetite loss, depression, fever, lymph node enlargement and severe hemorrhagic gastro-enteritis and can be fatal to 80% of untreated dogs. Currently, the only means of prevention is to avoid ingestion of uncooked salmon, trout, steelhead, and similar Pacific Ocean freshwater fish.