Heartworm Disease in Dogs and Cats: Why Florida Pets Need Year-Round Protection

Heartworm disease is one of the most dangerous parasitic infections your pet can face, and living in Florida puts dogs and cats at higher risk than almost anywhere else in the country. At Community Animal Hospitals in St. Petersburg, our veterinary team — Dr. Leslie Block, Dr. Craig Mullenax, and Dr. Mark Williamson — diagnoses and manages heartworm cases regularly. Here is what every pet owner in the Tampa Bay area needs to know.

What Causes Heartworm Disease

Heartworm disease is caused by Dirofilaria immitis, a parasitic roundworm that lives in the heart, lungs, and associated blood vessels of infected animals. The worms are transmitted exclusively through mosquito bites. When a mosquito feeds on an infected animal, it picks up microscopic heartworm larvae called microfilariae. Those larvae develop inside the mosquito over about two weeks, then get deposited into the next animal the mosquito bites.

Once inside your pet, the larvae migrate through tissue and eventually reach the heart and pulmonary arteries. In dogs, adult heartworms can grow up to 12 inches long and live five to seven years. A single dog can harbor hundreds of worms. In cats, the worms are fewer — typically one to three — but even a small number can cause serious damage.

Why Florida Is a Heartworm Hotspot

Florida consistently ranks among the highest-risk states for heartworm disease. The reason is simple: mosquitoes. Florida’s warm, humid climate supports mosquito populations year-round. There is no true “mosquito season” here the way there is in northern states. That means your pet faces potential exposure every single day of the year.

The St. Petersburg and Pinellas County area is especially problematic. Standing water from afternoon thunderstorms, coastal humidity, and mild winters create ideal breeding conditions for more than 80 mosquito species found in the state. Even indoor pets are at risk — mosquitoes get inside homes regularly.

Heartworm Symptoms in Dogs

Dogs are the natural host for heartworms, meaning the parasites complete their full lifecycle inside a dog’s body. Early-stage infections often show no symptoms at all, which is why routine testing matters so much.

As the disease progresses, watch for these signs:

• A mild, persistent cough
• Reluctance to exercise or tiring quickly on walks
• Decreased appetite and weight loss
• A swollen belly from fluid accumulation

In advanced cases, dogs can develop a life-threatening condition called caval syndrome, where the worm burden becomes so heavy it physically blocks blood flow through the heart. Without emergency surgical removal of the worms, caval syndrome is almost always fatal.

Heartworm Symptoms in Cats

Cats present a different and often more confusing picture. Because cats are atypical hosts, the worms don’t always mature fully — but even immature worms trigger a severe inflammatory response in the lungs called heartworm-associated respiratory disease (HARD).

Symptoms in cats may include:

• Coughing or asthma-like attacks
• Periodic vomiting unrelated to eating
• Decreased appetite or weight loss
• Difficulty walking or sudden collapse

In some cases, the first sign of heartworm disease in a cat is sudden death. The disease is often misdiagnosed as asthma or bronchitis, making awareness critical for cat owners.

How We Diagnose Heartworm Disease

At Community Animal Hospitals, we use several diagnostic tools depending on whether we are testing a dog or cat.

For dogs, the primary screening tool is an antigen test — a simple blood test that detects proteins produced by adult female heartworms. We recommend annual testing for all dogs, even those on prevention. If the antigen test comes back positive, we confirm with a microfilaria test to check for circulating larvae in the bloodstream.

For cats, diagnosis is more complicated. Antigen tests are less reliable because cats typically carry fewer worms. We often combine antibody testing (which shows exposure to heartworm larvae) with chest X-rays and sometimes ultrasound to look for characteristic changes in the heart and lungs.

Treatment: Difficult for Dogs, Unavailable for Cats

Treating heartworm disease in dogs is possible but expensive, painful, and risky. The FDA-approved treatment involves a series of deep intramuscular injections of melarsomine (brand name Immiticide) given over several months. During treatment, dogs must be strictly exercise-restricted — sometimes for months — because as the worms die, fragments can lodge in the lungs and cause dangerous blood clots.

The full treatment protocol typically includes:

• Stabilization with antibiotics (doxycycline) and heartworm preventive
• A series of three melarsomine injections spread over several months
• Strict cage rest and activity restriction throughout treatment
• Follow-up antigen testing six months after treatment completion

Total treatment cost often runs between $1,000 and $3,000 or more, depending on the severity of infection and the size of the dog.

For cats, there is no approved drug treatment for heartworm disease. The medication used in dogs is toxic to cats. Management focuses on supportive care — controlling symptoms with corticosteroids and monitoring the cat until the worms die naturally, which can take two to three years. In severe cases, surgical extraction of worms may be attempted, but it carries significant risk.

Prevention Is the Only Real Answer

Given the difficulty and expense of treatment — and the complete lack of treatment options for cats — monthly heartworm prevention is not optional for Florida pets. It is essential.

Modern heartworm preventives are safe, effective, and affordable. Options include:

• Monthly chewable tablets (such as Heartgard or Interceptor)
• Monthly topical treatments (such as Revolution or Advantage Multi)
• Injectable preventive (ProHeart 6 or ProHeart 12, given at the vet’s office every 6 or 12 months)

These medications work by killing heartworm larvae before they can mature into adults. They must be given consistently — even one missed dose during Florida’s year-round mosquito season creates a window for infection.

We also recommend annual heartworm testing even for pets on prevention. No preventive is 100% effective if a dose is late, spit out, or vomited up. Early detection makes a huge difference in treatment outcomes for dogs and management options for cats.

Schedule a Heartworm Test Today

If your dog or cat is not currently on heartworm prevention, or if it has been more than a year since their last test, now is the time to act. The team at Community Animal Hospitals is here to help you choose the right prevention plan for your pet and your budget.

Call us at (727) 592-1816 to schedule a heartworm screening or to ask about our parasite prevention options. Protecting your pet from heartworm disease is one of the most important things you can do as a pet owner in Florida — and it starts with a simple monthly preventive.