Key Takeaways
- Texas currently leads the nation in heartworm incidence, with Gulf Coast and Deep South states — Mississippi, Louisiana, Arkansas, and Alabama — consistently ranking among the most affected.
- Heartworm is now found in all 50 states, with risk expanding into cooler regions like the Pacific Northwest, New England, and the Great Plains — no pet is completely off the hook.
- Early symptoms are easy to miss — a mild cough or slight fatigue can be the only warning signs before the disease progresses significantly.
- Year-round prevention combined with annual testing is the most effective strategy recommended by the American Heartworm Society (AHS), regardless of where a pet lives.
- Resources like FleaTickFix help pet owners stay informed about parasite risks and understand what proactive protection actually looks like in practice.
Heartworm disease is one of the most serious — and most preventable — threats facing dogs and cats in the United States. Transmitted by a single mosquito bite, the disease often causes no visible symptoms until significant damage has already occurred. Knowing where risk is highest, what to watch for, and how to protect pets year-round can make a real difference in an animal’s long-term health.
Texas and the Gulf Coast Lead the Nation in Heartworm Cases
According to the American Heartworm Society’s 2026 survey — based on 2025 data — Texas currently leads the entire United States in canine heartworm incidence. That’s not a title anyone wants to hold, but it reflects a long-standing reality for the Gulf Coast region: warm temperatures, high humidity, abundant standing water, and mosquito seasons that stretch most of the year.
The Gulf Coast and Southeastern states have dominated heartworm statistics for decades. The conditions there are essentially ideal for Dirofilaria immitis, the parasitic worm responsible for the disease. Mosquitoes need warm temperatures — typically above 57°F — to complete the transmission cycle, and in states like Texas, Louisiana, and Mississippi, those conditions persist far longer than anywhere else in the country.
What makes this especially concerning for pet owners is that the disease progresses quietly. A dog can be infected for months before any noticeable signs emerge. By the time a persistent cough or exercise intolerance becomes obvious, there may already be real damage to the heart, lungs, and blood vessels. That gap between infection and visible symptoms is exactly why prevention and early testing matter so much.
The Highest-Risk States, Ranked
Not all high-risk states look the same. Some have been entrenched hotspots for over two decades. Others are climbing the list more recently, driven by shifting climates, wildlife movement, and the transport of untreated rescue animals.
The Persistent Core: Gulf Coast and Deep South
The states with the deepest, longest-running heartworm problems share a common geography: the lower Mississippi Delta and Gulf Coast corridor. Based on data from both the AHS and the Companion Animal Parasite Council (CAPC), the most consistently affected states include:
- Mississippi — The highest infection rate in the country, with approximately 1 in 15 dogs testing positive according to 2024 CAPC data.
- Louisiana — Consistently near the top, with roughly 1 in 30 dogs testing positive.
- Arkansas — Mirrors Louisiana’s infection rates and has appeared in the top 10 since AHS surveys began in 2001.
- Alabama — About 1 in 30 positive, with year-round mosquito pressure in much of the state.
- Texas — Currently leads in total case volume, particularly in eastern and coastal regions.
- Tennessee, Georgia, South Carolina, North Carolina — High incidence throughout, especially in lower-elevation and coastal areas.
- Florida — Year-round risk, particularly in northern and inland regions where the humid climate sustains mosquito populations in every season.
The AHS specifically notes that Mississippi, Louisiana, Arkansas, Texas, and Tennessee have appeared in the top 10 heartworm states in every survey since 2001 — a sign of persistent, deep endemicity rather than a passing trend. For pets in these states, the question isn’t really whether mosquito exposure will happen; it’s a matter of when.
Emerging Hot Spots Beyond the South
For years, heartworm was considered primarily a Southern problem. That’s no longer accurate. The AHS confirms heartworm cases are now reported in all 50 U.S. states, and several regions outside the Deep South are seeing notable increases:
- Pacific Northwest — New hotspots have been documented in parts of Washington and Oregon, with increases noted in central Washington and northern Idaho.
- Great Plains — North Dakota has seen higher-than-expected heartworm rates, with new infection pockets identified across the region.
- New England — Southern Maine has been flagged as a newly expanded moderate-risk zone, and case numbers are being tracked across the broader Northeast.
- Southwest — Southern California and southwest Colorado have newly identified moderate infection areas.
- Midwest — Southern Illinois has been identified as a new high-density hotspot, with significant risk also present in river-valley regions of Oklahoma.
Climate shifts play a role in this expansion, but so does the movement of pets. Rescue transport — especially after natural disasters like hurricanes — moves heartworm-positive dogs from high-incidence Southern states into lower-risk regions, introducing the disease to new mosquito populations. A dog adopted from a Gulf Coast shelter and brought to the Pacific Northwest doesn’t leave the risk behind.
Category States Highest, persistent core Texas, Louisiana, Mississippi, Arkansas, Alabama, Tennessee, South Carolina Broader high-risk South Georgia, Florida, North Carolina, additional parts of Tennessee Emerging hot spots Washington, Oregon, North Dakota, southern Maine, parts of CA, CO, southern Illinois, Oklahoma
Why These States Breed So Much Heartworm
Heartworm transmission has one requirement: a mosquito. But not just any conditions will do. The parasite’s larvae — called microfilariae — need specific environmental thresholds to develop inside the mosquito to an infective stage. That process requires sustained temperatures above roughly 57°F and takes about two weeks to complete. In Gulf Coast states, those conditions exist for eight to twelve months of the year. In some parts of South Florida and coastal Louisiana, mosquito season is essentially year-round.
High humidity and standing water are equally important. Mosquitoes breed in stagnant water — birdbaths, clogged gutters, low-lying yards, drainage ditches, retention ponds. The Mississippi Delta region, with its river flooding, warm summers, and dense vegetation, functions as a near-constant incubator. It’s no coincidence that Mississippi holds the country’s highest infection rate alongside some of the most intense mosquito pressure in the nation.
Beyond climate, the population density of unprotected or stray animals matters. Heartworm spreads when a mosquito bites an infected animal and then bites a healthy one. In regions with large populations of unprotected dogs — including strays, working dogs, and shelter populations — the cycle sustains itself efficiently. That’s one reason high-endemicity states remain high even when prevention products are widely available: it’s a compliance problem as much as a climate one.
Precautions for Pet Owners in High-Risk States
Living in a high-risk state doesn’t mean heartworm is inevitable — it means the margin for error is thinner. The combination of prescription prevention and regular testing closes that gap significantly. A few additional environmental steps can reduce exposure even further.
1. Use Year-Round Prescription Preventives
The American Heartworm Society recommends 12-month, year-round use of an FDA-approved heartworm preventive for all dogs — and especially for those in high-risk states. These products work by eliminating heartworm larvae before they can mature into adults. Missing even a single dose can create a window of vulnerability, particularly in regions where mosquito exposure is constant.
There are several FDA-approved options available through veterinarians:
- Monthly oral chewables — Ivermectin-based products like Heartgard Plus are widely used and palatable for most dogs.
- Monthly topical treatments — Selamectin-based options like Revolution work through skin absorption.
- Injectable preventives — ProHeart 6 and ProHeart 12, administered by a vet every 6 or 12 months, are particularly useful for owners who struggle with monthly compliance.
One important note: MDR1-sensitive breeds — including Collies, Shetland Sheepdogs, Australian Shepherds, and related herding breeds — can have sensitivity to ivermectin at higher doses. At standard labeled preventive doses, these products have been tested and are generally considered safe, but discussing alternatives like milbemycin oxime (Interceptor) or selamectin (Revolution) with a veterinarian is worthwhile for these breeds.
In high-risk states like Texas, Louisiana, and Mississippi, stopping prevention in winter is one of the most common mistakes pet owners make. Even when mosquitoes seem less active, warm-season species can persist through mild Southern winters, and the risk doesn’t fully disappear.
2. Test Annually — Even on Prevention
No preventive is 100% effective, and real-world compliance is rarely perfect. A single missed dose, a late refill, or a dog that spits out a chewable unnoticed can leave a brief but meaningful window of vulnerability. That’s why the AHS follows what it calls the Think 12 principle: get a heartworm test every 12 months, and give preventives every 12 months of the year.
Annual testing is especially relevant for:
- Dogs that have had any lapse in prevention
- Newly adopted or rescue dogs, especially those from high-incidence states
- Dogs that travel to or from high-risk regions
- Any pet starting a prevention regimen for the first time — preventives don’t kill adult worms, so a dog should be tested before starting
For cats, the situation is more complicated. There is no approved treatment for feline heartworm disease, which makes prevention the only real option. Cats in high-risk areas should be on feline-appropriate preventives and evaluated by a veterinarian, even if they live primarily indoors — mosquitoes do get inside.
3. Reduce Mosquito Breeding Sites at Home (Supplementary)
Mosquito control around the home doesn’t replace prescription prevention, but it’s a smart supplementary layer — especially in high-density transmission states. Reducing standing water eliminates breeding habitat directly:
- Empty and refresh birdbaths at least once a week
- Clear clogged gutters and check for pooling near downspouts
- Remove or overturn old buckets, flowerpots, and tarps that collect rainwater
- Check for low spots in the yard where water gathers after rain
- Maintain screens on windows and doors to limit indoor mosquito entry
Community-level mosquito abatement programs — such as municipal larvicide treatments or neighborhood cleanups after flooding — can further reduce local mosquito density, particularly in river-adjacent areas common in the Mississippi Delta region.
4. Limit Peak-Hour Outdoor Exposure (Supplementary)
Mosquito activity peaks at dawn and dusk. Keeping pets indoors during those windows — particularly in summer and fall in high-risk states — reduces bite frequency without requiring any additional product use. It’s a simple adjustment that adds up meaningfully over time.
When outdoor time during peak hours is unavoidable, pet-safe mosquito repellents can provide an additional buffer. Products containing picaridin are generally considered safer for pets than DEET-based options. Human insect repellents containing DEET are toxic to dogs and should never be applied to pets — a veterinary warning worth repeating. Some pet-specific formulations use essential oils like lemongrass as alternatives, but effectiveness varies, and none should replace prescription prevention.
Early Heartworm Symptoms Are Easy to Miss
Heartworm larvae take approximately 6 to 8 months to mature into adult worms after initial infection. During that entire period, most pets show no symptoms at all — or signs so subtle they’re easily attributed to something else. By the time clear symptoms appear, the disease has typically advanced well beyond the early stages, making treatment longer, more expensive, and harder on the animal.
This is what makes annual testing so critical. Symptoms alone are not a reliable way to catch heartworm early.
Warning Signs in Dogs
In dogs, heartworm primarily affects the heart and pulmonary arteries. Early-stage signs are mild and easy to dismiss:
- Mild, persistent dry cough — often worse after exercise or excitement; sometimes mistaken for allergies or a minor respiratory issue
- Decreased stamina — tiring sooner on walks, reluctance to run or play at the same intensity
- Subtle lethargy — sleeping more than usual, slightly less engaged
- Gradual weight loss — slow changes in body condition that develop over weeks
As the disease progresses into moderate and severe stages, signs become much harder to ignore:
- More pronounced coughing and labored or rapid breathing
- Fainting or dizziness after mild exertion
- Distended belly from fluid accumulation — a sign of right-sided heart failure
- Pronounced muscle wasting and poor coat condition
In its most severe form — Caval Syndrome (Class 4) — the worm burden physically obstructs blood flow back to the heart. Signs include sudden labored breathing, pale or bluish gums, and dark, coffee-colored urine from red blood cell destruction. This stage requires emergency surgical removal of worms and is often fatal even with immediate intervention.
Warning Signs in Cats
Cats are an atypical host for heartworm — the worms don’t survive as long or reach the same numbers as in dogs. But that doesn’t make the disease less dangerous. In fact, it can be more unpredictable.
The condition in cats is sometimes called Heartworm Associated Respiratory Disease (HARD), and early signs are frequently mistaken for feline asthma or bronchitis:
- Intermittent coughing or wheezing — often misdiagnosed as asthma
- Rapid or difficult breathing, including open-mouth breathing under stress
- Intermittent vomiting — often unrelated to meals
- Lethargy and hiding — decreased activity or social withdrawal
- Gradual weight loss
What makes heartworm in cats particularly alarming is that the first visible sign can be sudden collapse or death — even in cats that appeared completely healthy. This occurs when worms die naturally in the lungs and trigger a severe inflammatory response. There is no approved treatment for feline heartworm disease, which means prevention isn’t optional — it’s the only real defense.
Symptom Area Dogs Cats Primary organ affected Heart and pulmonary arteries Lungs primarily Early warning signs Cough, exercise intolerance Respiratory distress Typical worm burden Can harbor 30+ worms Usually only 1 to 3 worms Diagnosis Relatively straightforward Difficult — requires multiple tests Approved treatment Yes No approved treatment Risk of sudden death In advanced stages Even with low worm burden
Year-Round Prevention and Annual Testing Work Best Together
The central message from veterinary and parasitology experts is consistent: prevention and early detection are two halves of the same strategy. Neither works as well without the other.
Prevention stops the disease from establishing in the first place — but only if it’s given consistently, on schedule, every month of the year. The AHS’s Think 12 framework puts it plainly: 12 months of prevention, 12-month testing cycle. Surveys of veterinarians identify poor owner compliance — missed doses, stopping medication in the off-season, or forgetting to refill — as one of the main reasons heartworm cases keep rising even in regions where prevention products are widely available and well understood.
Annual testing catches what prevention misses. A blood test detects heartworm proteins (antigens) released by adult female worms — the only reliable way to identify an infection before visible symptoms appear. For pet owners in high-risk states, the combination of year-round prevention plus annual testing isn’t an abundance of caution; it’s the standard of care.
A few practical habits help with long-term compliance:
- Sync all pets in the household to the same dosing day
- Set a recurring calendar reminder tied to a consistent date each month
- Use auto-ship or pharmacy refill reminders to avoid lapses between doses
- Schedule the annual heartworm test at the same time as the yearly wellness visit — it’s a simple add-on that takes minutes
Treatment for an established heartworm infection is costly, lengthy, and physically demanding on the pet. It involves strict exercise restriction for months, multiple veterinary visits, and a series of injections. Prevention, by comparison, is straightforward and far less expensive. The math is clear — and so is the recommendation.
For pet owners looking to stay ahead of parasite risks and access trusted information on protection options, FleaTickFix is a resource built to help pet owners navigate flea, tick, and heartworm prevention with confidence.
