Heartworm disease is a serious, potentially fatal condition caused by parasitic worms that live inside a dog’s heart and the blood vessels of the lungs. These worms, spread exclusively through mosquito bites, can grow up to 12 inches long and survive inside a dog for five to seven years. Left untreated, heartworms cause lasting damage to the heart, lungs, and arteries.
How Dogs Get Heartworms
Dogs cannot catch heartworms from other dogs directly. The only way a dog becomes infected is through the bite of an infected mosquito. Here’s how it works: when a mosquito feeds on a dog that already has heartworms, it picks up microscopic baby worms (called microfilariae) circulating in that dog’s blood. Inside the mosquito, these tiny organisms develop over a couple of weeks into an infective larval stage. When the mosquito bites another dog, it deposits those larvae into the skin through the bite wound.
Once inside a dog, the larvae spend roughly six months migrating through tissue, molting twice, and growing into adult worms. The adults settle into the pulmonary arteries, the large blood vessels connecting the heart to the lungs, and sometimes into the right side of the heart itself. Female worms then release new microfilariae into the bloodstream, completing the cycle when the next mosquito comes along.
Where Heartworm Is Most Common
Heartworm exists in all 50 U.S. states, but infection rates vary dramatically by region. The highest concentrations are in the lower Mississippi Delta, where warm, humid conditions support year-round mosquito activity and large populations of untreated dogs and wildlife carry the parasite. The five states with the highest density of diagnosed cases are Mississippi, Louisiana, Texas, Alabama, and Arkansas.
The disease is also expanding into areas once considered low-risk. A nationwide survey reflecting 2022 data showed unexpected increases in states like Washington, Oregon, North Dakota, Massachusetts, and Connecticut. Cities including Seattle, Bismarck, and Tucson all saw significant jumps in heartworm rates. This geographic spread means heartworm prevention matters regardless of where you live.
What Adult Heartworms Look Like
Adult heartworms are long, thin, spaghetti-like parasites. Males reach about 5 to 6 inches, while females grow to 10 to 12 inches. A single dog can harbor dozens of worms at once. Because they live inside blood vessels and the heart, they physically obstruct blood flow and trigger inflammation in the vessel walls. Over time, this damages the lining of the arteries and forces the heart to work harder to pump blood through the lungs.
Symptoms by Stage
Heartworm disease progresses through four recognized stages, and early infection often produces no visible symptoms at all.
- Stage 1: No symptoms, or only an occasional cough. Most dogs appear completely normal. This is why routine testing catches infections that owners would otherwise miss.
- Stage 2: An occasional cough and noticeable tiredness after moderate exercise. Your dog might slow down on walks or seem winded more easily than before.
- Stage 3: A persistent cough, fatigue after even mild activity, and a generally sickly appearance. Difficulty breathing and signs of heart failure become common. Chest X-rays at this stage typically reveal visible changes to the heart and lungs.
- Stage 4: Known as caval syndrome, this is a medical emergency. The worm burden is so heavy that a mass of worms physically blocks blood from flowing back into the heart. Emergency surgical removal of the worms is the only option, and even with surgery, most dogs at this stage do not survive.
The jump from stage 1 to stage 3 can happen over months or years depending on how many worms are present and how the dog’s body responds to the infection. Because the early stages are silent, annual testing is the only reliable way to catch the disease before serious damage occurs.
How Heartworm Is Diagnosed
Veterinarians use two primary blood tests to diagnose heartworm. The first is an antigen test, which detects proteins released by adult female worms. The second looks for microfilariae circulating in the bloodstream. The Companion Animal Parasite Council recommends all dogs be tested annually using both methods, even dogs already on preventive medication. In areas with heavy mosquito exposure, twice-yearly testing may be appropriate.
One important limitation: these tests only detect mature infections that are at least six months old and include at least one female worm. That six-month gap between the mosquito bite and a detectable infection is why timing matters. A puppy bitten in June may not test positive until December or later. For dogs with positive results or symptoms, veterinarians may also use chest X-rays, ultrasound of the heart, or other imaging to assess how much damage the worms have caused.
How Treatment Works
Treating heartworm is far more complex, expensive, and risky than preventing it. The goal is to kill the adult worms living in the heart and pulmonary arteries, but the process has to be managed carefully. As worms die, their bodies break apart and can lodge in smaller blood vessels in the lungs, causing dangerous blockages and inflammation. Dogs undergoing treatment need strict exercise restriction, sometimes for months, to reduce the risk of these complications.
An important part of the treatment protocol involves an antibiotic given before the worm-killing phase. Heartworms harbor a type of bacteria called Wolbachia inside their bodies, and these bacteria contribute to the inflammatory damage the worms cause. Eliminating the bacteria first weakens the worms and reduces the severity of the immune reaction when the worms eventually die. This discovery significantly improved treatment outcomes.
Treatment typically spans several months from start to finish, including the antibiotic phase, injections to kill the adult worms, and a lengthy recovery period of restricted activity. Dogs in the early stages of disease generally respond well. Dogs with advanced disease face higher risks and may have permanent heart or lung damage even after the worms are eliminated.
Preventing Heartworm
Prevention is straightforward and highly effective. Preventive medications work by killing the larval stages of heartworm before they mature into adults. They come in three forms:
- Monthly oral tablets or chews: The most common option. These are given once a month year-round.
- Monthly topical treatments: Applied to the skin, typically between the shoulder blades.
- Injectable prevention: Administered by a veterinarian every 6 or 12 months, removing the need to remember a monthly dose.
All preventives require a prescription, and your dog should be tested for heartworm before starting any preventive program. Giving a preventive to a dog that already has an active infection can cause serious, sometimes fatal reactions. This is especially true if the dog has microfilariae in its bloodstream, because a sudden die-off of large numbers of these organisms can trigger shock.
Year-round prevention is recommended even in northern climates. Mosquito seasons are unpredictable, and a single missed month can leave a gap in protection. The cost of monthly prevention is a fraction of what treatment runs, and it spares your dog from a disease that causes real, lasting harm to their cardiovascular system.