Parasitic Infections

Dogs/Cats have many parasites, both internal and external. Internal parasites are those which live in the various organs inside the dog’s body. Some of the internal parasites migrate through a number of different organs. Intestinal parasites are internal parasites that live in the small or large intestine (colon). External parasites, such as fleas, ticks, and mites, live on the outside of the body. Click on a topic header to read more about each type of parasitic infection. 

Roundworms represent a common intestinal parasite of the dog and cat. They are also sometimes called ascarids. Typically, they are large, heavy-bodied worms and average 3-5 inches in length. They live in the intestines, consuming partially digested food. In contrast to the behavior of the hookworm, the roundworm does not attach to the intestinal wall; rather, this worm literally swims in it’s food.

A majority of newborn puppies have roundworms while a lesser numbers of adult dogs are infected. The most common roundworm in dogs is Toxocara canis, Toxocara cati & Toxascaris leonini in the cat.

Puppies acquire the majority of roundworm larvae from the placenta before they are born; transmission through the mother’s milk does occur but is less important for puppies than for kittens.

Additionally, dogs may become infected by swallowing roundworm eggs that contain infective larvae. The larvae hatch in the dog’s stomach and small intestine and migrate through the muscle, liver, and lungs. After several weeks, the larvae make their way back to the intestine to mature. When these worms begin to reproduce, new eggs will pass in the stool, and the life cycle of the parasite is completed.

Obviously, roundworm eggs passed in one animal’s stool may be infectious to other dogs. Interestingly, a large number of other animal species have been found to harbor roundworm eggs and represent potential sources of infection for our pets – these include cockroaches, earthworms, chickens, and rodents.

Clinical Signs
They are not highly pathogenic (harmful) to adult dogs until they occur in large numbers. Then they can cause liver damage, pneumonia, weight loss and a pot-bellied appearance in out pets. Decreased appetite, vomiting or diarrhea will be observed on occasion. Young animals may die with serious roundworm infection.

Roundworm infection is diagnosed by microscopic examination of your pet’s stool. They pass a moderate number of eggs, so examination of more than one stool sample may be necessary to find them. Occasionally, the mature worms can be found in your pet’s stool or vomit.

Treatment is quite simple. Several very safe and effective drugs are available to kill roundworms in the intestine. Several are available which temporarily anesthetize the worms so they pass out of your pet with a normal bowel movement. The live or dead worms are found in the bowel movement. Because of their large size, the worms are easily seen. Two or three treatments may be needed; they are typically performed at 2-4
weeks intervals. None of these treatments will kill the immature forms of the worm or the migrating larvae. Some types of canine heartworm preventives contain medication that will help in controlling roundworm, hookworm, and whipworm infections in dogs.

The prognosis is better for mature dogs/cats than for youngsters. Extremely debilitated pets may succumb when large numbers of worms and/or severe clinical signs are present.

Transmission to Humans
The roundworms of dogs and cats pose a health risk for humans. As many as 10,000 cases of roundworm infection in humans have been reported in one year. Children, in particular, are at risk for significant health problems should they become infected. A variety of organs may be affected as the larvae migrate through the body. In suitable environments, the eggs may remain infective to humans (and to cats and dogs) for

1. Pregnant cats and dogs should be dewormed in late pregnancy to reduce potential contamination of the environment for newborn animals.
2. Dogs with predatory habits should have a fecal examination several times a year. Rodent control is desirable since rodents may serve as a source of roundworm infection for pets.
3. The eggs are highly resistant to most commonly used disinfectants and, and even to harsh environmental conditions. Runs and kennels may be treated with a 1% solution of household bleach. Although bleach does not kill the eggs, it will remove the sticky outer coating of the eggs, making it easier to rinse them away. Remember the obvious limitations about where bleach may be safely applied.

Hookworms are dangerous parasites that live in a cat and dog’s small intestine and represent a common intestinal parasite. Hookworms “graze” on the lining of the intestine, leaving multiple bloody holes in their wake. These can cause anemia and may even be fatal in some cases. In humans, hookworms migrate through tissue close to the skin, causing painful, itchy rashes.

Various species of hookworms are found on a worldwide basis. The most common hookworm of dogs and cats in warm (tropical or subtropical) climates is called Ancylostoma. The canine hookworm found in cooler climates is called Uncinaria.

Pets may become infected with hookworms by four routes: orally, through the skin, through the mother’s placenta, and through the mother’s milk. The oral and transcolostral (through the mother’s milk) routes are the most common. The latter is of particular importance for puppies.

It is important to note that if a pregnant dog/cat has hookworms, the pregnancy may reactivate larvae. These larvae will enter the female’s circulation and pass to the youngster through the placental blood flow.

Clinical Signs
Hookworms received their name because of the characteristic hook-like mouthparts that they use to attach to the lining of the intestinal wall. Because they are very small (about 1/8” in diameter), you have to be looking very carefully to see the adult worm. Despite this, they are one of the most pathogenic worms found in dogs because they suck blood from the tiny vessels in the intestinal wall. A large number of hookworms can cause anemia; this occurs especially in puppies, but it may even occur in some adult dogs/cats.

A blood transfusion may be needed in some dogs/cats because of the rather severe anemia that can be produced by hookworms. The presence of pale gums, weakness, and diarrhea (possibly with blood) might suggest the need to specifically determine the dog’s red blood cell count. Iron deficiency anemia secondary to hookworm infection can be a life-threatening problem for young pets.

Skin irritation and itching can be one of the common signs of a heavily infested environment. The larvae burrow into the skin and cause the dog a great deal of discomfort. The most common hookworm of dogs does not appear to have this type of burrowing behavior.

Finding hookworm eggs during microscopic examination of a stool sample is diagnostic for the parasite. Because eggs are produced on a daily basis, they are usually plentiful in number. The adult female hookworm is reported to produce as many as 20,000 eggs a day!

In puppies/kittens, adequate numbers of worms may not be present for production of ova. For this reason, fecal examination may be less reliable in puppies than in adult dogs.

There are a number of effective drugs that will kill hookworms. These are given by injection or orally and have few, if any, side effects. However, these drugs only kill the adult hookworms. Therefore, it is necessary to treat again in about 3-4 weeks to kill any newly formed adult worms that were larvae at the time of the first treatment.

Since the dog’s environment can be laden with hookworm eggs and larvae, it may be necessary to treat it with a chemical to kill them. There are several available products that are safe to use on grass.

Hookworms can be life threatening for young and adult pets lacking immunity to the parasite. Young animals who suffer severe hookworm infection may be stunted and never reach their full growth potential. In general, the prognosis is good for adult pets.

Transmission to Humans
Adult hookworms do not infect humans internally. However, the larvae can burrow into human skin. This causes itching, commonly called “ground itch,” but the worms do not mature into adults. Direct contact of human skin to moist, hookworm infested soil is required. Fortunately, this does not occur very often if normal hygiene practices are observed.

In rare instances, the canine hookworm will penetrate into deeper tissues and partially mature in the human intestine. A few reports of hookworm enterocolitis (small and large intestinal inflammation) have surfaced in the recent past.

Whipworms are intestinal parasites of the dog and cat. In North America the feline whip worm is uncommon. They parasitize the lower intestine (cecum and colon) and cause signs related to intestinal irritation. Infection with whipworms can be significant because they are one of the more pathogenic intestinal parasites found in the dog.

They are small, about 1/4″ (45 to 75 mm) long, and have a characteristic “whip” or “lash” at one end. After the worm burrows into the intestine, it is this “whip” that causes damage to the lining of the bowel. The tip of the “lash” is able to slash and shred tissue. The worm then feeds on the blood and damaged lining of the intestine.

Contributing Factors
A dog is more likely to become infected with whipworms if it resides in an environment contaminated with whipworm eggs. The eggs are very resistant to destruction and can remain viable (infective) in the soil for months or even years.

The canine whipworm is found throughout North America, although it is most common in the eastern and southern United States. In other countries, different species of whipworms can exist.

Whipworms pass microscopic eggs in the stool which must be swallowed to infect the dog. Once the eggs mature into adult worms, eggs are passed in the dog’s stool and can infect another dog.

Clinical Signs
The most significant clinical sign of whipworm infection is bloody and/or mucoid (mucous) stool. Overt diarrhea may or may not be present. The dog may exhibit pain during defecation and attempt frequent bowel movements. In dogs with chronic whipworm infection, debilitation may develop and the dog will lose weight and possibly become anemic.

Whipworm infection is diagnosed by finding the characteristic eggs during a microscopic examination of the stool. Several samples may be required because these parasites pass small numbers of eggs on an irregular basis. Any dog with chronic diarrhea can be reasonably suspected to have whipworms, regardless of several negative stool examinations. It is advisable to treat for whipworms based on assumption of infection when
chronic or refractory diarrhea is present. Response to treatment is an indication that whipworms were present but could not be detected on fecal examination.

Treatment and Prevention
Several available drugs are very effective against whipworms. Two treatments are needed at a 3-4 week interval. Since reinfection is such a problem, it is advisable to treat again every 3-4 months or to put the dog on a heartworm preventive product that also protects against other parasites. Whipworms are not nearly as common now because of widespread use of the heartworm preventives that help to control whipworms.

The prognosis for full recovery is good if the entire course of treatment is completed. In contaminated environments, reinfection may occur.

Transmission to Humans
There is a very small risk of human infection by canine whipworms. Eggs of this parasite have occasionally been observed in stool samples of humans. These should be noted as rare events, however. The overall health risk to humans is considered very small.

The most common tapeworm of dogs and cats is called Dipylidium & Taenia. This parasite attaches to the small intestinal wall by hook-like mouthparts. Adult tapeworms may reach 8 inches (20 cm) in length. The adult worm is actually made up of many small segments about 1/8 inch (3 mm) long. As the tail end of the worm matures, the terminal segments break off and pass into the stool. Occasionally, the mobile segments
can be seen crawling near the anus or on the surface of a fresh bowel movement. These segments look like grains of rice and contain tapeworm eggs; the eggs are released into the environment when the segment dries. The dried segments are small (about 1/16″, or 2 mm), hard and golden in color. These dried segments can sometimes be seen stuck to the hair around the anus.

Means of Infection
First, tapeworms eggs must be swallowed by flea larvae (an immature stage of the flea). Contact between flea larvae and tapeworm eggs is thought to occur most frequently in contaminated bedding or carpet. The life cycle of the tapeworm cannot be completed unless the flea swallows tapeworm larvae. Next, the pet chews or licks its skin as a flea bites; the flea is then swallowed. As the flea is digested within the pet’s intestine, the tapeworm hatches and anchors itself to the intestinal lining.

Clinical Signs
Tapeworms are not highly pathogenic (harmful) to your pet. They may cause debilitation and weight loss when they occur in large numbers. Sometimes, the pet will scoot or drag its anus across the ground or carpet because the segments are irritating to the skin in this area. The adult worm is generally not seen, but the white segments which break away from the tapeworm and pass outside the body rarely fail to get an
owner’s attention!

Occasionally, a tapeworm will release its attachment in the intestines and move into the stomach. This irritates the stomach, causing the dog to vomit the worm. When this happens, a worm several inches in length will be seen.

Tapeworm infection is usually diagnosed when the white, mobile segments are seen crawling on your pet or in the stool. Tapeworms are not usually detected by the routine fecal examination performed by the veterinarian. Because of this, veterinarians depend on the owner to notify them of possible tapeworm infection in your pet.

Treatment is simple and, fortunately, very effective. A drug that kills tapeworms is given, either orally, topically or by injection. It causes the tapeworm to dissolve within the intestines. Since the worm is usually digested before it passes, it is not visible in your dog’s stool. These drugs should not cause vomiting, diarrhea, or any other adverse side effects.

Control of fleas is very important in the management and prevention of tapeworm infection. Flea control involves treatment of your dog, the indoor environment and the outdoor environment where the dog resides. If the dog lives in a flea-infested environment, reinfection with tapeworms may occur in as little as two weeks. Because the medication that treats tapeworm infection is so effective, return of the tapeworms is
almost always due to reinfection from the environment.

Tapeworms and pinworms look very similar. However, contrary to popular belief, pinworms do not infect dogs or cats. Any worm segments seen associated with dogs are due to tapeworms. Children who get pinworms do not get them from dogs or cats.

Contagion to Humans
It s possible for humans to become infected with tapeworms, although infection is not common or likely because humans are not the natural host of the dog’s tapeworms. A flea must be ingested for humans to become infected with the most common tapeworm of dogs. Most reported cases have involved children, individuals whose immune system is not fully functioning. The most effective way to prevent human infection is through aggressive, thorough flea control. The risk for infection with this tapeworm in humans is quite small but does exist.

One less common group of tapeworms, called Echinococcus, is of particular concern as a threat to human health. These tapeworms cause very serious disease when humans become infected. This parasite is harder to diagnose than the tapeworm caused by fleas because the segments are small and not readily seen. Hunters and trappers in the north central United States and south central Canada may be at risk for infection by this worm if strict hygiene is not observed. Foxes and coyotes (and the wild rodents upon which they prey) are important in the life cycle of this parasite. Dogs and cats may also become infected if they eat rodents carrying the parasite. When eggs of Echinococcus are passed in the feces of the dog and cat, humans are at risk for infection. Free-roaming cats and dogs may need to be periodically treated with tapeworm medication. Rodent control and good hygiene are important in preventing the spread of this disease to humans. As with the more common tapeworm, infection with Echinococcus is infrequent but possible.

Intestinal coccidiosis is an infection with a one-celled organism; these organisms are classified as protozoa and are called coccidia. Coccidia are not worms; they are microscopic parasites that live within cells of the intestinal lining. Because they live in the intestinal tract and commonly cause diarrhea, they are often confused with worms.

Contributing Factors
Conditions of crowding, poor sanitation, and stress contribute to infections with coccidia.

Coccidiosis is a relatively common finding in young kittens and puppies especially if they are strays or have come from a crowded environment with poor hygiene (pet store, puppy “mill,” etc.).

Oocysts (immature coccidia; “eggs”) are passed in the stool of the dog. They lie in the environment and eventually sporulate (mature; hatch) into a more developed oocyst that can infect the dog again. Other dogs, cats, or mice may also become infected. This process can occur in as little as 6 hours, but it usually takes 7-10 days. If the sporulated oocysts are swallowed, they mature in the dog’s intestine to complete the life cycle. If the oocysts are swallowed by a mouse, the dog may also become infected by eating the mouse.

Clinical Signs
Most dogs with coccidia have subclinical infections, meaning that they do not have diarrhea or any other clinical signs. When the eggs (oocysts) are found in the stool of a pet without diarrhea, they are generally considered a transient, insignificant finding. However, in young pets and debilitated adult pets, they may cause severe, watery diarrhea, dehydration, abdominal distress, and vomiting. In severe cases, death may occur.

Coccidiosis is diagnosed by performing a microscopic examination of a stool sample. Since the oocysts are much smaller than the eggs of the intestinal worms, a very careful study must be made. Infection with some of the less common coccidial parasites is confirmed with additional tests that require specialized laboratory facilities.

The most common drug used to eliminate coccidia is a sulfa-type antibiotic. It is given for 10-14 days. The medication is sweet-tasting and objection to the taste is usually not a problem. If the sulfa-type drug is not effective, others are available. Additional medication may be needed if diarrhea and dehydration occur.

In most cases, a good response to treatment is expected and most dogs recover. The presence of concurrent diseases or immune suppression can make the prognosis less certain.

Transmission to Humans
The most common coccidia found in dogs do not have any affect on humans. However, less common types of coccidia are potentially infectious to humans. One parasite, called Cryptosporidium, may be carried by dogs or cats and may possibly be transmitted to people. This parasite has also been found in public water supplies in some major cites. Another coccidial organism, Toxoplasma, is carried by cats and is of particular concern to pregnant women because of the potential to cause birth defects in newborns. This organism is only able to complete its life cycle in the cat.

These two coccidial parasites pose a health risk for immunosuppressed humans (i.e., AIDS patients, those taking immune suppressant drugs, cancer patients, the elderly). Good hygiene and proper disposal of dog and cat feces are important in minimizing risk of transmission of all parasites to humans. Although there is risk of pets transmitting parasites to humans, it does not warrant removing the dog from the household
except in very rare instances.

Reinfection of dogs is common so environmental disinfection is important. The use of chlorine bleach, one cup in a gallon of water, is effective if the surfaces and premises can be safely treated with it. Because coccidiosis tends to be a problem in areas of poor sanitation, the need for environmental decontamination cannot be emphasized enough.

Giardia Infection

Giardia are sometimes confused with worms because they invade the gastrointestinal tract and can cause diarrhea. They are not worms; instead, they are one-celled parasites classified as protozoa. Most pets that are infected with Giardia do not have diarrhea or any other signs of illness. When the eggs (cysts) are found in the stool without diarrhea, they are generally considered a transient, insignificant finding. However, in young and debilitated adult pets, they may cause severe, watery diarrhea that may be fatal.

A pet becomes infected with Giardia when it swallows the cyst stage of the parasite. Once inside the intestine, the cyst goes through several stages of maturation. Eventually infective cysts pass in the stool. These cysts lie in the environment and can infect others. They may also be transmitted through drinking infected water.

Giardiasis is sometimes diagnosed by performing a microscopic examination of a stool sample. The cysts are quite small and usually require a special floatation medium for detection, so they are not normally found on routine fecal examinations. Occasionally, the parasites may be seen on a direct smear of the feces. A blood test is also available for detection of antigens (cell proteins) of Giardia in the blood. This test is probably more accurate than the stool exam, but it requires several days to get a result from the laboratory performing the test.

None of the tests for giardiasis are completely accurate. Therefore, some veterinarians choose to treat the pet  with one of the safe and very effective medications. If successful, this approach will eliminate the parasite even though a confirmed diagnosis is not made.

The most common drug used to kill Giardia is metronidazole, an antibiotic-type drug. It is given for 5-7 days. Other drugs are also used if diarrhea and dehydration occur. If metronidazole is not effective, others are available.

Transmission to Humans
Giardia can also cause diarrhea in humans. Therefore, environmental disinfection is important. The use of chlorine bleach, one cup in a gallon (500 ml in 4 liters) of water, is effective if the surfaces and premises can be safely treated with it.

Parasite Prevention

  1. All new youngsters should be treated by 2-3 weeks of age. To effectively break the life cycle of the most common intestinal parasites, and should be dewormed on the schedule recommended by your veterinarian.
  2. Prompt deworming should be given when parasites are detected; periodic deworming may be appropriate for pets at high risk for reinfection.
  3. Prompt disposal of all pet feces, especially in yards, playgrounds, and public parks.
  4. Strict hygiene is especially important for children. Do not allow children to play in potentially contaminated environments. Be mindful of the risk posed by public parks and non-covered sandboxes. Sandboxes that have fitted covers are popular and are recommended to prevent infection of children with intestinal parasites.
  5. Nursing females should be treated concurrently with their youngsters; nursing may reactivate infection in the female.
  6. Use of a heartworm preventive that contains medication for prevention/control of roundworm and hookworms is advisable.