Heartworm is exactly what it sounds like: foot-long worms living in the heart and lung vessels of infected dogs. It's transmitted by a single mosquito bite, progresses silently for months, and eventually causes coughing, exercise intolerance, and heart failure.
Treatment exists for dogs, but it's costly, takes months of strict rest, and carries real risk — while prevention is a simple monthly chewable. We test annually (required for safe prescribing) and keep your dog protected twelve months a year.
Heartworm program
- Annual blood testing
- Monthly chewable or topical prevention
- Year-round protection — mosquitoes don't respect seasons here
- Combination products that also cover intestinal parasites
- Treatment coordination for heartworm-positive dogs
What to expect
Annual visits include a quick blood test (results in minutes) and a year of prevention — a monthly chewable most dogs take as a treat. For heartworm-positive dogs, we stage the disease and manage the months-long treatment protocol with strict-rest coaching and support throughout.
Heartworm Symptoms: The Four Stages of Heartworm Disease in Dogs
Heartworm disease in dogs is generally divided into four stages based on the severity of the infection and the symptoms the dog is experiencing. The stages are not always clearly defined and symptoms can overlap. However, understanding the four general stages can help pet owners recognize how heartworm disease can progress.
Stage 1: Few or No Symptoms
During the first stage of heartworm disease, dogs typically show few or no noticeable symptoms. The infection is present, but the disease has not yet progressed enough to cause significant outward signs.
Possible signs may include:
- No noticeable symptoms
- Occasional mild cough
- Normal activity and energy levels
Because early heartworm disease can be difficult for pet owners to recognize, routine heartworm testing is important.
Stage 2: Mild to Moderate Symptoms
As heartworm disease progresses, symptoms generally become easier to notice.
Common symptoms may include:
- Persistent or lingering cough
- Exercise intolerance
- Fatigue after physical activity
- Decreased stamina
At this stage, diagnostic testing may identify evidence of heartworm infection.
Stage 3: Severe Symptoms
By Stage 3, heartworm disease can have a significant impact on a dog's health and quality of life.
Symptoms may include:
- Frequent or persistent coughing
- Significant exercise intolerance
- Fatigue after minimal activity
- Reluctance to exercise
- Difficulty breathing
- Possible coughing up blood
Diagnostic imaging, including X-rays and ultrasound, may reveal changes involving the heart, lungs, and major blood vessels.
Stage 4: Severe or Life-Threatening Heartworm Disease
Stage 4 represents the most severe form of heartworm disease. Dogs at this stage can be extremely ill and may have significant damage to the heart, lungs, and blood vessels.
Symptoms may include:
- Severe difficulty breathing
- Persistent coughing
- Extreme fatigue
- Weakness or collapse
- Severe exercise intolerance
- Abnormal heart or lung sounds
- Enlarged liver
- Signs of heart failure
Advanced heartworm disease can result in serious complications and can be fatal.
What Determines the Severity of Heartworm Disease?
The severity of heartworm disease can depend on several factors, including:
- The number of heartworms present
- How long the dog has been infected
- The dog's individual response to the infection
- The extent of damage to the heart, lungs, and blood vessels
As heartworm disease progresses, treatment can become more complicated. Early detection gives the veterinarian an opportunity to diagnose and address the disease before severe complications develop.
Other Signs of Advanced Heartworm Disease
Additional signs associated with advanced heartworm disease may include:
- Anemia
- Fainting or collapse
- Right-sided congestive heart failure
- Rapid heartbeat
- Respiratory distress
When to See a Veterinarian
Pet owners should not wait for severe symptoms before having their dog tested for heartworm disease. Dogs with early heartworm infections may have few or no obvious symptoms.
If a dog develops persistent coughing, exercise intolerance, unusual fatigue, difficulty breathing, fainting, or other concerning symptoms, the owner should contact a veterinarian promptly.
Frequently asked questions
Why test annually if my dog never misses a dose?
Because no prevention is perfect — a vomited dose, a missed month, or rare product failure can let an infection through, and giving prevention to an infected dog can cause a dangerous reaction. The annual test is the safety check that keeps prevention safe.
Is heartworm actually a risk in Los Angeles?
Yes — lower than the Gulf states, but real and rising, with local mosquito species fully capable of transmission and infected dogs seeding the cycle. Prevention costs a fraction of the months-long, high-risk treatment.
What happens if my dog tests positive?
Staged treatment: confirming tests, imaging, then a protocol of injections that kill adult worms over several months — with strict exercise restriction, because dying worms make exertion dangerous. Dogs come through it well with careful management, but it's an experience worth a lifetime of prevention to avoid.
What Is Heartworm Disease?
Heartworm disease in dogs is a blood-borne parasitic nematode (roundworm) known as Dirofilaria immitis transmitted by mosquitoes. Upwards of 30 species of mosquitoes can act as heartworm transmitters. Mosquitoes ingest immature heartworm larvae, called microfilariae, by feeding on either an infected cat or dog (or coyote). The microfilariae develop further for 10 to 30 days in the mosquito's gut and then enter parts of the mosquito's mouth. When an infected mosquito bites a dog, it injects larvae into the dog. The larvae then mature over a period of several months, eventually ending up in the right side of the heart and the pulmonary arteries. Once this occurs, they mature into adult heartworms in dogs, and can begin to reproduce about six months from the time of invasion. At approximately eight months after the invasion, heartworm in dogs begin to produce a new crop of microfilariae that will live in the dog's blood for about one month. By the time this occurs, most dogs are showing significant heartworm symptoms, and their lives are in danger.
Heartworm Prevention For Dogs
Heartworm prevention is given in the form of a monthly chewable. The chewables are readily accepted by most dogs as a treat. It is very important that the chewable be given once a month on the same day of the month to make sure that your dog is adequately protected. Side effects of the chewable medication are rare. However, as with any medication, please call if you notice any changes in your dog's health or behavior.
Ready to schedule?
Request an appointment at our Glendale clinic or call our team — we serve pets and their people across the surrounding communities.