Heartworm Disease in Dogs and Cats: What Every Florida Pet Owner Should Know

If you live in Florida and share your home with a dog or cat, heartworm disease should be on your radar year-round. Unlike many parts of the country that get a seasonal break from mosquitoes, our warm, humid climate keeps these pests active 365 days a year — and every bite carries the potential to transmit a dangerous parasite.

What Is Heartworm Disease?

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm that lives in the heart, lungs, and associated blood vessels of infected animals. Adult heartworms can grow up to 12 inches long, and a single dog can harbor hundreds of them. The damage they cause to the heart, lungs, and blood vessels can be severe and permanent — even after treatment.

How Heartworms Are Transmitted

Heartworms are spread exclusively through mosquito bites. Here’s how the lifecycle works:

• A mosquito bites an infected animal and picks up microscopic heartworm larvae called microfilariae.
• Over the next 10 to 14 days, the larvae develop inside the mosquito into an infective stage.
• When that mosquito bites your dog or cat, the larvae enter through the bite wound.
• Over the next six months, the larvae migrate through the body, eventually reaching the heart and pulmonary arteries where they mature into adults.
• Adult worms reproduce, releasing new microfilariae into the bloodstream — and the cycle continues.

There is no way to tell which mosquitoes carry heartworm larvae. It only takes one bite from one infected mosquito.

Why Florida Is a Heartworm Hotspot

The American Heartworm Society consistently ranks Florida among the highest-risk states in the country. The reasons are straightforward:

• Year-round mosquito activity. Our mild winters never get cold enough to eliminate mosquito populations.
• High humidity and standing water. Florida’s landscape provides ideal mosquito breeding grounds everywhere — from retention ponds to your backyard birdbath.
• Wildlife reservoirs. Coyotes, foxes, and feral dogs carry heartworms and serve as a constant source of infection for local mosquito populations.
• Indoor pets are not safe. Mosquitoes easily get inside homes. Indoor-only cats and dogs are still at risk.

Heartworm Symptoms in Dogs

In the early stages, most dogs show no symptoms at all. That’s what makes this disease so dangerous — by the time you notice something is wrong, significant damage may already be done.

As the disease progresses, watch for:

• A mild, persistent cough
• Reluctance to exercise or tiring easily during activity
• Decreased appetite and weight loss
• A swollen belly from fluid accumulation

In severe cases, dogs can develop a life-threatening condition called caval syndrome, where a large mass of worms blocks blood flow through the heart. Without emergency surgical removal of the worms, caval syndrome is usually fatal.

Heartworm Symptoms in Cats

Cats are atypical hosts for heartworms, which means the disease presents very differently than in dogs. Even one or two worms can cause serious illness in a cat.

Common signs include:

• Coughing or asthma-like attacks
• Periodic vomiting (often unrelated to eating)
• Lack of appetite and weight loss
• Difficulty walking or sudden collapse

In some cats, the first and only sign of heartworm disease is sudden death. The disease is frequently misdiagnosed as asthma or bronchitis because the symptoms overlap so closely.

How Heartworm Disease Is Diagnosed

For dogs, diagnosis typically starts with a simple antigen blood test that detects proteins produced by adult female heartworms. This test can be run right in our clinic during your visit. If the test is positive, we may recommend additional diagnostics including:

• A microfilaria test to confirm the presence of circulating larvae
• Chest X-rays to evaluate the heart and lungs
• Blood work to assess organ function before starting treatment

Diagnosing heartworm in cats is more challenging. Antigen tests are less reliable because cats typically carry fewer worms. We often use a combination of antigen and antibody testing, along with X-rays and sometimes ultrasound, to piece together the diagnosis.

Treatment: Expensive, Risky, and Not Available for Cats

Treating heartworm disease in dogs is possible, but it’s a long, expensive, and physically demanding process for your pet. The FDA-approved treatment involves a series of injections of melarsomine (Immiticide) given deep into the back muscles over several months. During treatment, dogs must be kept on strict exercise restriction — sometimes for months — because as the worms die, fragments can block blood vessels in the lungs.

The full cost of heartworm treatment typically ranges from $1,000 to $3,000 or more, depending on the severity of the disease and the size of the dog. Compare that to the cost of monthly prevention, which runs roughly $10 to $25 per month.

For cats, there is no approved treatment to kill adult heartworms. Veterinary care focuses on managing symptoms and supporting the cat while hoping the worms eventually die on their own — which can take two to three years. The drugs used to treat dogs are toxic to cats and cannot be safely administered.

Why Monthly Prevention Is Non-Negotiable in Florida

Given how common heartworm-positive mosquitoes are in our area, and how serious the consequences of infection can be, monthly heartworm prevention is one of the most important things you can do for your pet’s health.

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

• Monthly chewable tablets (many also protect against intestinal parasites)
• Monthly topical treatments (applied to the skin)
• Injectable preventive (given by your veterinarian every 6 or 12 months)

These medications work by killing heartworm larvae before they can mature into adults. They must be given consistently and on schedule — a single missed dose during Florida’s year-round mosquito season can leave your pet vulnerable.

We also recommend annual heartworm testing for dogs, even those on prevention. No preventive is 100% effective, and early detection is always better than finding out after damage has occurred.

Protect Your Pet — Schedule a Heartworm Test Today

At Dunedin Animal Medical Center, heartworm testing and prevention are a routine part of every wellness visit. If your dog or cat isn’t currently on heartworm prevention, or if you’ve missed doses, give us a call to schedule a check-up. Getting your pet tested and started on prevention is one of the simplest, most cost-effective ways to protect their health.

Call us at (727) 733-9351 or stop by our clinic in Dunedin to get your pet’s heartworm prevention plan started.