Brentwood Animal Hospital

3900 Rhode Island Avenue
Brentwood, MD 20722


Internal Parasites


Protect Your Pets From Internal Invaders

Some internal parasites, heartworms in particular, can be deadly if not treated.Prevention is much easier than treatment. Other internal parasites may not always be lethal but can still cause great misery for your dog or cat.


Heartworms infect both dogs and cats. These invaders are transmitted by mosquitoes when they are in a microscopic larval stage. Once inside your pet, they grow dramatically and migrate to the animal's heart and lungs. Adult heartworms can reach 7-10 inches in length. Adult heartworms can begin to cause damage to the heart and lungs as soon as they arrive. Microfiliaria circulating in the dog,in large enough numbers, can also cause damage in the organs in the body. These parasites can do tremendous damage to a dog's heart, triggering congestive heart failure and death if left untreated. While heartworms are less common in cats, they are by no means uncommon. It is estimated that 17 percent of cats in the U.S. have the disease, and it is fatal in 10-20 percent of cases. When heartworms infect a cat, they primarily affect the lungs, causing a condition known as Heartworm-Associated Respiratory Disease (HARD). They can cause permanent lung damage in both cats and dogs. Symptoms include coughing,wheezing, shortness of breath, rapid breathing, vomiting, and weight loss.Symptoms may not show up until the disease is quite advanced.

Because they are spread by mosquitoes, heartworms are a particular problem in any area where mosquitoes thrive--including Texas and the U.S. Gulf Coast. Since mosquitoes sometimes get in the home, even indoor dogs and cats are at risk.There is no treatment available for cats that will kill heartworms, but corticosteroids can help reduce inflammation of the pulmonary system. Cats may clear a heartworm infection without treatment, but they may be left with permanent damage to the lungs and related structures. Treatment to kill heartworms is available for dogs, and it can save their lives if the infection is caught in time. Even with medication, some dogs may not recover, especially if the infection is advanced. Treatment takes many months, and the dog's activities must be severely limited until treatment is completed. SNAP does nottreat ill or injured animals, so you will be referred to a full-service veterinarian if your animal is believed to be infected.

Preventingheartworms is the key. Heartworm prevention is available for both dogs and catsand kills the parasite while it is still microscopic in size--thus preventingfurther harm to the animal. Dogs can only be put on prevention if they are notalready infected, so they must be tested first. Cats can be put on prevenativeeven if they are already infected, and doing so may somewhat limit theinfection thus speeding recovery.

Prevention can be started at a puppy’s first vaccine at 6-8 weeks of age. It is given every 30 days throughout the dog’s life. Dogs 7 months of age and younger may start heartworm prevention without a heartworm test.

Heartworm prevention medications are effective if given monthly and at the correct dose.The correct dose is vital, do not under dose. If a dog is on the high edge of aweight range, it is safe enough to use the next higher weight range dose in case the dog gains weight, e.g. a 24 pound dog may be given the 25-50 pound dose of Heartgard. Heartworms may be adapting, and if medications are not given every 30 days and at an adequate dose, they may develop the ability to slip through our defenses and complete the life cycle. This means adult heartworms in the heart! Be sure to inquire about heartworm testing and prevention when you bring your animal in for a wellness exam.

Intestinal Parasites

Intestinal parasites can be difficult to eliminate and may require multiple treatments with deworming medications. This section describes the most common intestinal parasites and what can be done to free your pet from their gruesome grip.


Roundwormsaffect both cats and dogs. They can grow up to five inches in length.Roundworms are transmitted through ingestion of contaminated feces. Kittens and puppies can become infected while nursing, and puppies may be infected at birth. Roundworms attach themselves to the intestinal wall and feed on an animal's blood. During the larval stage, roundworms may pass through the lungs.This can cause coughing or gagging, especially among kittens and puppies. The larvae are coughed up, swallowed, and pass to the small intestines to mature into adult roundworms. A severe infestation can be deadly for a kitten or puppy. Roundworm eggs can be excreted in stool, and worms themselves can appear in vomit or stool. They are usually white or brown in color and may still be alive. Roundworms are zoonotic--they can be passed from animals to human.Symptoms include coughing, abdominal pain, nausea, diarrhea, blood in stool, or weight loss. This parasite also causes a bloated stomach appearance, common in puppies. When people ingest the larvae, they travel through the liver, lungs or other organs. This is called visceral larval migrans. SNAP wellness veterinarians can administer deworming agents that will kill roundworms in your pet's digestive tract.


Whipworms are a form of roundworm. Their name comes from the fact that they are thin atone end and have a thick section at the other end that resembles the handle of a whip. Whipworms primarily affect dogs and live in the cecum, which is between the small and large intestines. There is a form for whipworm that affects cats,but such infections are fortunately rare. Whipworms are transmitted by the ingestion of infective eggs. Eggs live in the environment, soil, or in animal feces. They are extremely persistent even in harsh environments. An infected animal may show no symptoms, but severe infections may cause bloody diarrhea.Because the worms feed on the animals blood supply of the large intestines,they can also cause anemia. This parasite can be difficult to kill and may require repeated treatment with deworming medication.



These parasites are named for the hook-like mouth structures they use to attach themselves to your animal's intestinal wall. Hookworms are small--less than an inch long. Like most intestinal parasites, their eggs are passed in stool and hatch into larvae. They are transmitted when another animal ingests or comes into contact with contaminated soil, water, or stool. Hookworms are more common in dogs, but affect cats as well. Once a hookworms has attached itself to the animal's intestinal wall, it will feed on the animal's blood supply. It maytherefore cause cause anemia. Hookworms, like roundworms, are zoonotic. They can be contracted when walking barefoot or while sitting on contaminatedground. SNAP wellness veterinarians can administer deworming agents that will kill hookworms. Symptoms include blood in the stool, diarrhea, fever, loss of appetite, and anemia (which can cause gums to appear pale). Hookworm larvae are infective to people also, and move within the skin causing inflammation. This is called cutaneous larva migrans.

Some of the information in this section comes from the CDC webpage on hookworms.


Tapeworms are very common among our pets and for very good reason. They are transmitted by fleas. Specifically, tapeworm larvae are present within the fleas that seek out your dog or cat. When the animal bites at the fleas, they can be ingested inadvertently. The larvae is thus introduced into the digestive tract where it attaches itself to the intestinal wall. There it can grow to be up to two feetin length. The adult tapeworm produces eggs which are contained within its body segments. These segments break off and pass out of the animal when it defecates. They can sometimes be seen in the stool or on the fur around a pet's perianal region. They are similar in appearance to a grain of rice. They may move. Symptoms include poor hair coat, abdominal pain, diarrhea, lethargy,weight loss, or itching around the anus. If you observe these symptoms or see tapeworm segments on your pet or in his or her stool, the animal should to be treated to eliminate the parasite.

Becausetapeworms are spread by fleas, canine and feline flea prevention is aneffective means of protecting your furry friend against tapeworms. SNAPwellness clinics sell several popular and effective brands.


Giardiais a parasitic disease of the intestinal tract. It is transmitted through contaminated water. This disease is zoonotic--it can be transmitted to humans. Symptoms may include diarrhea, vomiting, weight loss, or poor overall condition.


Coccidia is a parasitic disease of the intestinal tract of animals caused by coccidian protozoa. It spreads from one animal to another through infected feces or ingestion of infected tissue. Young or immunocompromised animals may suffer severe symptoms and death. The primary symptom of coccidia is diarrhea, which may become bloody.


Contrary to common belief, dogs and cats do not carry pinworms. They cannot therefore transmit them to human children (or adults).