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Fleas, ticks, heartworms, and intestinal worms are some of the most common parasites that threaten dogs and cats in the US and Europe. These pests may be tiny, but the health risks they pose range from skin irritation to life-threatening disease. For pet owners, knowing which parasites are common in your region—and how to prevent them—is an essential part of keeping pets healthy.

This guide covers the most important parasites, what symptoms to watch for, and the best ways to protect your pets year-round.

Fleas

Fleas are bloodsucking insects that live on pets and reproduce in the environment, making them notoriously hard to control. Eggs are often found in soft fabrics like carpets, pet beds, and blankets—so even after killing adult fleas on your pet, reinfestation can happen quickly.

Signs of fleas:

• Scratching or biting at the skin
• Flea dirt (black specks that turn red when wet—actually digested blood)
• Visible fleas, often near the rump in dogs or under the chin in cats

Prevention & treatment products:

• Dogs: NexGard, NexGard Spectra, Bravecto, Simparica, Simparica Trio, Credelio Plus
• Cats: Bravecto, NexGard Combo, Advantage, Advocate, Revolution, Revolution Plus

Ticks

Ticks vary by region, and the risks they pose depend on the diseases they carry.

• United States:
• Deer tick (blacklegged tick): Spreads Lyme disease
• American dog tick: Can transmit Rocky Mountain spotted fever
• Lone star tick: Linked to ehrlichiosis and “red meat allergy” (alpha-gal syndrome)
• Europe:
• Castor bean tick (Ixodes ricinus): Spreads Lyme disease and babesiosis
• Dermacentor ticks: Can transmit tick-borne encephalitis
• Brown dog tick (Rhipicephalus sanguineus): Common in warmer parts of southern Europe; transmits ehrlichiosis and babesiosis

What to do if you find a tick on your pet:

• Remove it promptly with tweezers or a tick tool
• Monitor for signs of illness (fever, lethargy, joint pain, appetite loss)
• Contact your vet—especially if your pet shows neurological or respiratory changes

Preventatives for dogs and cats are the same as for fleas (many products cover both).

Heartworm

Heartworm disease, spread by mosquitoes, is a major concern in the US (particularly the South, Midwest, and along the Mississippi River) and parts of southern Europe.

When an infected mosquito bites your dog, it injects microscopic larvae that mature in the heart and lungs. Heartworm disease is often fatal if untreated, and treatment is risky and expensive.

Cats can also be infected, though the disease looks different and can be harder to diagnose.

Symptoms in dogs may include:

• Persistent cough
• Exercise intolerance
• Weight loss
• Weakness or collapse

Prevention is critical:

• Dogs: Interceptor, Milbemax, NexGard Spectra, Simparica Trio, Advocate, Credelio Plus, or an annual injection from your veterinarian
• Cats: Bravecto Plus, NexGard Combo, Advocate, Revolution, Revolution Plus

Intestinal Worms

Dogs and cats can pick up intestinal worms from infected animals, contaminated soil, water, or food.

Types of worms in the US & Europe include:

• Roundworms (common in puppies/kittens, can spread to humans)
• Hookworms (cause anemia and intestinal bleeding)
• Whipworms (chronic diarrhea in dogs)
• Tapeworms (contracted from fleas or hunting prey)

Symptoms:

• Diarrhea (sometimes bloody)
• Worms in stool or around anus
• Pot-bellied appearance
• Vomiting
• Weight loss
• Weakness or anemia

Effective treatments:

• Dogs: Milbemax, Drontal, Interceptor
• Cats: Milbemax, Drontal, Profender

Mites

Mites are less common but can cause intense itching, hair loss, or ear irritation.

• US & Europe common mites include:
• Demodex (mange): Often affects puppies or pets with weak immune systems
• Sarcoptes (scabies): Highly contagious, causes severe itching
• Otodectes (ear mites): Itchy, inflamed ears, often with dark discharge

Products that treat mites:

• Dogs: Bravecto, Simparica, Simparica Trio, Advocate, NexGard, NexGard Spectra, Credelio Plus
• Cats: Bravecto, Advocate, Revolution, Revolution Plus

Parasite prevention isn’t one-size-fits-all—it depends on your pet’s lifestyle, your region, and local risks. In the US and Europe, fleas, ticks, heartworm, intestinal worms, and mites are all threats that require year-round protection.

By working with your veterinarian and sticking to a consistent prevention plan, you’ll keep your pet safe from parasites and the diseases they spread.