Heartworm Prevention for Dogs in Los Angeles: What South Pasadena and Alhambra Pet Owners Need to Know

We hear it regularly: "We're in California — do we really need heartworm prevention?" The answer is yes. Heartworm disease is present in Southern California. The risk is moderate compared to high-transmission regions like the Gulf Coast or Southeast, but it is real, it is increasing, and it is entirely preventable with a medication that costs far less per year than treating an active infection.

This post covers what heartworm disease is, why Los Angeles pet owners should care, what the disease looks like as it progresses, your prevention options, and what cat owners specifically need to know.

What Is Heartworm Disease?

Heartworm disease is caused by Dirofilaria immitis, a parasitic roundworm transmitted exclusively through the bite of an infected mosquito. The parasite is not spread directly between animals — a mosquito must first feed on an infected host (typically a dog, coyote, or fox), pick up the larval stage of the parasite (microfilariae), and then transmit those larvae to a new host through a subsequent bite.

Once transmitted, the larvae migrate through the tissues over a period of several months, eventually reaching the heart and pulmonary arteries, where they mature into adult worms. Adult heartworms can reach 4–12 inches in length. In a heavily infected dog, hundreds of worms may be present, physically obstructing blood flow through the heart and lungs.

The entire lifecycle from mosquito bite to detectable adult worm takes approximately 6 months — which is why a dog can be infected for half a year before a standard antigen test will return positive, and why clinical signs often don't appear until the disease is already significantly advanced.

The Risk in Los Angeles and the San Gabriel Valley

Southern California has historically been considered a lower-risk region compared to the Mississippi River valley or Gulf Coast. But "lower risk" is not "no risk," and the situation is changing. Several factors are relevant for SGV pet owners:

• Mosquitoes are active year-round. Unlike colder climates where mosquito season has a defined end, LA's mild winters mean mosquitoes remain active — and capable of transmitting heartworm — throughout the year. There is no off-season here.
• The invasive Aedes mosquitoes are now established in the region. Aedes aegypti and Aedes albopictus, which arrived in Southern California within the last decade, are highly efficient biters and breed in tiny amounts of standing water. They're found throughout LA County.
• Wildlife reservoir hosts are common. Coyotes — which carry heartworm and are abundant in foothill and park areas throughout the SGV — are the primary wildlife reservoir in our region. Dogs that visit parks, trails, or areas where coyotes are present have higher exposure risk.
• Rescued and rehomed dogs are often from high-prevalence areas. Many dogs adopted through LA rescue organizations were transported from the Southeast or Gulf states, where heartworm prevalence is dramatically higher. Any dog with an unknown history should be tested before starting prevention.

Signs of Heartworm Disease by Stage

Heartworm disease is classified into four stages based on severity. This matters because treatment options and prognosis differ significantly depending on what stage a dog is in at diagnosis.

Class I — Mild Disease

No clinical signs. The dog looks and acts completely healthy. The only indication is a positive antigen test. This is the best stage at which to catch the disease — treatment at this point is straightforward and outcomes are very good. The reason we recommend annual testing even for dogs on prevention is precisely to catch Class I disease before it advances.

Class II — Mild to Moderate Disease

Occasional mild cough, fatigue after moderate exercise. The dog may seem slightly less energetic than usual — easy to attribute to age, heat, or "just being tired." Some owners don't notice anything wrong at this stage. Chest radiographs typically show some pulmonary vessel changes.

Class III — Moderate to Severe Disease

Persistent cough, difficulty breathing, exercise intolerance, and weight loss become noticeable. The dog may tire after very little exertion, or cough consistently rather than occasionally. At this stage, echocardiography often reveals worms in the pulmonary arteries and right side of the heart. Treatment is still possible but carries higher risk, and the dog requires pre-treatment stabilization.

Class IV — Caval Syndrome

The worm burden has become so massive that the right side of the heart and the vena cava are physically obstructed. This is a life-threatening emergency. The dog may collapse, have labored breathing, dark urine (from red blood cell destruction), and pale or bluish gums. Standard melarsomine treatment cannot be safely administered at this stage — surgical removal of worms via jugular catheter is the only option, and prognosis is guarded even with intervention.

Why Prevention Beats Treatment Every Time

Treatment for active heartworm disease — Class I through III — involves a series of deep intramuscular injections of melarsomine (an arsenic compound) administered by a veterinarian, often over a multi-day protocol. The treatment itself is painful. After injection, dying adult worms fragment and can cause pulmonary thromboembolism (clots in the lung vessels) if the dog exerts itself. This means strict rest — essentially kennel confinement — for 6–8 weeks. No walks, no play, no excitement. For most dogs and their owners, this is an extraordinarily difficult period.

Heartworm Prevention Options

All heartworm preventives work by eliminating the larval stages of the parasite that have been transmitted in the previous 30 days (or longer, for extended-release products). They do not kill adult heartworms — which is why it's critical to test before starting or restarting prevention after any lapse.

• Monthly oral preventives: Products containing ivermectin or milbemycin oxime are the standard for monthly heartworm prevention. Most also protect against intestinal parasites (roundworms, hookworms, and in some formulations, whipworms). Safe, effective, and convenient for most dogs.
• Injectable preventive (ProHeart 6 / ProHeart 12): Administered by a veterinarian, providing 6 or 12 months of continuous protection. An excellent option for owners who find monthly dosing difficult to maintain, or for dogs that resist taking oral medications. Given at a clinic visit — no missed doses at home.

We do not recommend one product over another without knowing your dog — different dogs have different health histories, and there are clinical reasons to prefer one approach in some cases. Call us or contact us online to schedule a consultation and find out what works best for your situation.

Heartworm Disease in Cats: A Different Story

Cats can be infected with heartworm, though they are "atypical hosts" — most larvae don't survive to adulthood, and adult worm numbers are typically very low (often just 1–3 worms). This might sound reassuring, but it isn't. In cats, even a single worm causes serious inflammatory disease in the lungs.

Heartworm-associated respiratory disease in cats — called HARD — presents as episodes of coughing, wheezing, open-mouth breathing, or sudden respiratory distress that closely resembles asthma or bronchitis. Many cats with HARD are misdiagnosed with feline asthma for months before the underlying cause is identified, if it's identified at all. Some infected cats die suddenly with no prior clinical signs.

When to Test Your Dog

The American Heartworm Society recommends annual testing for dogs, even those on continuous prevention. This is the protocol we follow at our clinic. Specific situations that require testing before starting or adjusting prevention:

• Before starting heartworm prevention for the first time
• If there has been any lapse in prevention, even a month or two
• When adopting a dog of unknown history, especially if the dog came from a high-prevalence region
• 6 months after any known missed doses
• Dogs 7 months of age or older who have never been on prevention

Administering a preventive to a dog with an existing adult heartworm infection can trigger a severe reaction from the sudden death of microfilariae in the bloodstream. Testing first removes this risk.

Annual heartworm testing is typically included as part of a wellness exam. See our services page for more details, or contact us to schedule your dog's annual wellness visit at our Alhambra location.