Managing Parasites in Dogs & Cats: Recognize, Treat & Prevent

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Parasites are more than just an annoyance—fleas, ticks, worms and heartworms threaten your pet’s comfort and their life. Recognizing infestations early, treating them appropriately and preventing future problems will keep your dog or cat healthy and protect your family.

Why parasite control matters

Internal and external parasites steal nutrients, cause skin irritation, transmit disease and, in the case of heartworms, can be fatal. Roundworms, hookworms, whipworms, tapeworms, coccidia and Giardia inhabit the digestive tract of dogs and cats. Fleas, ticks, mites and mosquitoes feed on the skin or blood and can transmit tapeworms, Lyme disease, Rocky Mountain spotted fever and heartworm disease. Some parasites are zoonotic, meaning they can infect people. Because pets may show few or no symptoms, routine testing and year‑round parasite control are essential.

Common internal parasites

Roundworms

Roundworms are tubular worms that live in the small intestine. Pets become infected by ingesting eggs in contaminated soil, water or prey animals. Puppies and kittens often acquire roundworms from their mother before birth or through milk. The adults steal nutrients and can cause poor growth, diarrhea and pneumonia in young animals. Because roundworms are zoonotic and can damage human organs and eyes, strict hygiene and prompt deworming are important.

Hookworms

Hookworms attach to the intestinal lining and suck blood. Dogs and cats are infected by ingesting larvae or through skin contact. Severe infections cause weakness, diarrhea and potentially life‑threatening anemia in puppies and kittens. Larvae can penetrate human skin, causing itchy “creeping eruptions”, so wear shoes and pick up feces promptly.

Whipworms

Whipworms thread themselves into the large intestine, feeding on blood and tissue. Dogs are far more commonly affected than cats. Signs may include weight loss, diarrhea or anemia, but some dogs show no symptoms. Although whipworms rarely infect people, they can persist in soil for years, making regular deworming and fecal checks important.

Tapeworms

Pets usually get tapeworms by swallowing infected fleas or lice while grooming. Most tapeworm species live in the small intestine and cause few clinical signs, but owners often notice rice‑like segments around the rear end or in bedding. Humans can be infected by accidentally ingesting fleas, highlighting the importance of flea control.

Coccidia & Giardia

Coccidia (Cystoisospora) and Giardia are microscopic protozoa transmitted via contaminated soil, water or feces. They damage the intestinal lining, causing diarrhea, abdominal pain, weight loss and dehydration. Young animals are particularly vulnerable because they lack immunity. These parasites rarely infect humans, but prompt veterinary treatment is still necessary.

External parasites

Fleas

Fleas thrive in warm, humid environments and infest pets directly or via contaminated bedding, carpets and wildlife. After jumping onto a host, female fleas lay up to 50 eggs per day. Eggs fall into the environment, hatch into larvae and pupate, making reinfestation likely unless both the pet and its environment are treated. Signs include itchiness, flea dirt (black droppings) and, in heavy infestations, anemia or allergy‐induced “hot spots”. Fleas can transmit tapeworms to pets and children.

Ticks

Ticks inhabit wooded or grassy areas and feed on animal or human blood. They are often found around a dog’s neck, ears, toes and underbelly or on a cat’s face. Tick bites cause irritation and anemia, and ticks can transmit Lyme disease, Rocky Mountain spotted fever and other serious illnesses. Some species produce toxins that cause paralysis in pets.

To remove a tick, grasp it with tweezers as close to the skin as possible and pull gently without twisting or crushing. Avoid petroleum jelly, alcohol or matches, which may cause the tick to inject saliva. After returning from tick‑prone areas, examine your pet and yourself thoroughly and remove any ticks; keep lawns trimmed and brush cleared to reduce tick habitat.

Ear mites

Ear mites are microscopic parasites common in young cats but also seen in dogs. They cause intense ear itching, head shaking and dark discharge; severe scratching can lead to bleeding or blood blisters. A veterinarian will confirm the diagnosis via ear swab and prescribe ear cleaning and mite‑killing medication.

Mange mites

Sarcoptic mange mites (scabies) burrow through a dog’s skin, causing intense itching, hair loss and thickened skin. They are highly contagious and may cause an itchy rash in people. Demodectic mites normally live on dogs and cats but can cause localized or generalized hair loss and scaly skin if the animal’s immune system is compromised. Treatment includes medications to kill the mites and addressing any underlying health issues.

Heartworm disease

Heartworm disease is spread by mosquitoes and is potentially fatal. When a mosquito bites an infected dog or wild canid, it ingests microscopic larvae; after 10–14 days those larvae become infective and are injected into a new host. In dogs, the larvae mature into foot‑long worms living in the heart and pulmonary arteries; a single dog can harbour dozens of worms, which cause lasting damage to the heart and lungs. Cats are atypical hosts and usually carry only one to three worms, but even immature worms can cause heartworm‐associated respiratory disease.

Signs in dogs may include a persistent cough, exercise intolerance, fatigue, weight loss and, in advanced cases, heart failure or sudden collapse. Cats may show coughing, asthma‑like attacks, vomiting, weight loss or sudden death. Because heartworm disease has been diagnosed in all 50 states and risk is unpredictable, the American Heartworm Society’s “Think 12” initiative recommends testing dogs annually and giving heartworm preventive year‑round. There is no approved heartworm treatment for cats, so prevention is the only protection.

Diagnosis & treatment

Because pets can harbour multiple parasites without obvious signs, routine fecal examinations—twice yearly for healthy adult pets and four times yearly for puppies and kittens—are essential. Veterinarians diagnose intestinal parasites by examining a stool sample for eggs or using antigen, DNA or AI‑assisted tests. External parasites are usually detected by combing the coat or taking skin scrapings. Heartworm infection is detected via a blood test.

If parasites are found, your veterinarian will prescribe an appropriate treatment. Deworming for roundworms, hookworms and whipworms often begins at two weeks of age and is repeated every two weeks until monthly broad‑spectrum preventives are started. Some monthly heartworm preventives also protect against intestinal worms and fleas. Protozoal parasites like coccidia and Giardia require specific medications. Ear mites and mange mites are treated with topical or oral medications and environmental cleaning.

The following over‑the‑counter products may help manage parasites. Always consult your veterinarian before starting any treatment, and never use dog products on cats:

Flea & tick collars (dogs): View on Amazon
Topical flea & tick medication (dogs): View on Amazon
Dewormer (dogs): View on Amazon
Tapeworm dewormer (cats): View on Amazon

Prevention & home hygiene

Year‑round prevention is the cornerstone of parasite control. The Companion Animal Parasite Council and AAHA recommend administering broad‑spectrum products that protect against heartworm, intestinal worms, fleas and ticks throughout a pet’s life. Puppies and kittens should receive anthelmintic treatment starting at two weeks old and repeating every two weeks until monthly preventives begin. Heartworm preventive should start as early as eight weeks old and continue year‑round.

Regular fecal exams help verify the effectiveness of preventives and detect new infections. Dogs should be tested annually for heartworm, and screening for tick‑borne diseases is advised in endemic areas. Cats should be tested before starting heartworm preventives and re‑tested as recommended by your veterinarian. Because some tick‑borne diseases are increasing in prevalence, consistent ectoparasite control and environmental management are important.

In addition to medications, take the following steps to reduce exposure:

Keep pets on high‑quality commercial or cooked diets and provide clean water.
Prevent roaming: leash dogs, use fences and keep cats indoors to limit contact with wildlife and contaminated soil.
Pick up feces promptly during walks and clean your yard daily to reduce environmental contamination.
Cover sandboxes and avoid letting pets drink from puddles or eat rodents.
Vacuum carpets and wash pet bedding frequently to remove flea eggs and larvae.
Trim grass, bushes and brush to discourage ticks and perform “tick checks” after outdoor activities.
Wash your hands after handling pets or soil, wear shoes outside and teach children not to eat dirt.

A veterinarian can tailor a prevention program based on your pet’s species, age, health and environment. Remember that some products contain ingredients toxic to cats; always read labels and follow veterinary guidance.

When to see a veterinarian

Contact your veterinarian promptly if your pet shows signs of parasitic disease—persistent scratching, hair loss, vomiting, diarrhea, coughing, lethargy, weight loss or the presence of worms or segments in feces. Early diagnosis improves outcomes and reduces the risk of zoonotic transmission. Avoid over‑the‑counter treatments without veterinary guidance, especially for heartworm or protozoal infections, as improper use can be ineffective or harmful.