Heartworm 101. What Every Pet Parent in Seattle Should Know
Happy April! It’s Heartworm Awareness Month! BoosterPet’s team is here to answer the question that has probably popped up in your mind “Do I really need to worry about heartworm in Seattle?”
It's a fair question, and for a long time, the honest answer was "not really." The Pacific Northwest has historically been low-risk compared to the South and Midwest, and for years, many veterinarians in our region rarely saw cases. That has started to change. We are seeing more positive cases in the PNW year over year, and the trend is moving in the wrong direction. If your pet was born in or came from a higher-risk state, that adds another layer of concern.
In honor of Heartworm Awareness Month, we want to give you a clear picture of what heartworm disease actually is, why it matters here, and what you can do about it.
If you take nothing else from this post, take this: Heartworm is one of the most dangerous parasites dogs and cats face in the US; the treatment is costly and hard on your pet, and there is no cure for cats. Prevention is key!
Overview
What is heartworm disease?
Does heartworm disease affect dogs and cats the same way?
How do pets get heartworms?
Signs of heartworm disease in dogs and cats
How is heartworm diagnosed and treated?
If my pet is on parasite prevention, why do they still need an annual test?
How much does heartworm prevention and treatment cost?
Get a free heartworm test
What is heartworm disease?
The name is pretty literal. Heartworm disease is caused by a parasitic worm, Dirofilaria immitis, that lives in and around your pet's heart, lungs, and blood vessels. Adult worms can reach up to a foot long, and in dogs they are capable of fully maturing, reproducing, and living in the body for five to seven years. Some infected dogs have been found carrying hundreds of worms. Even when treatment is successful, the damage to the heart and lungs is often permanent to some degree. This is not a mild or easily reversible condition.
Does heartworm disease affect dogs and cats the same way?
Not at all, and this is important to understand, especially if you have cats.
Dogs are the ideal host for heartworm. The worms thrive in them, complete their full life cycle, and cause progressive, cumulative damage over time, the longer an infection goes untreated.
Cats are a different story. They are considered an atypical host, meaning the worms rarely reach adulthood in cats and cannot reproduce. That might sound like good news, but there's a significant catch: because cats don't show the same symptoms dogs do, infections frequently go undiagnosed. Even immature worms cause serious inflammation and lung damage, or a condition called Heartworm Associated Respiratory Disease (HARD). And while there are approved medications to treat heartworm in dogs, there are none for cats. Treatment in cats is focused on managing symptoms and reducing inflammation, not eliminating the infection. For cats, prevention isn't just the best option…it's really the only one.
How do pets get heartworms?
Mosquito bites are the only known route of transmission. A mosquito bites an infected animal, picks up microscopic immature worms called microfilaria, and carries them through a two-week maturation process. Once those larvae reach an infective stage, the next animal that mosquitoe bites can become infected. From there, it takes roughly six months for the larvae to develop into adult worms and reproduce.
In Seattle, our rainy winters have kept mosquito populations in check, which is a big reason heartworm hasn't been on most local pet owners' radar. But spring changes things fast. As temperatures come up, mosquitoes multiply quickly, and our wet climate actually works in their favor — standing water from rain, gutters, birdbaths, and planters all become breeding grounds right when we and our pets are spending the most time outside. Add to that the broader shifts in mosquito life cycles driven by climate change, and the old assumption that heartworm "just isn't a Seattle problem" doesn't hold up quite the way it used to.
Signs of heartworm disease in dogs and cats
Unfortunately, the early stages of heartworm infection usually do not produce noticeable symptoms. By the time something seems off, the disease has often been progressing for a while.
In dogs, early warning signs include a persistent mild cough, lower energy levels, less interest in exercise, and gradual weight loss. As the infection advances, fluid can build up in the abdomen. In severe cases, a life-threatening complication called caval syndrome can develop suddenly, characterized by labored breathing, pale gums, and dark or bloody urine. This is an emergency that requires immediate surgery.
In cats, the picture is harder to read. Some cats show no signs whatsoever. Others may have episodes that look like asthma, trouble walking, fainting, or seizures. In some cases, the first sign is sudden collapse. There is no reliable way to predict how an individual cat will respond, which is yet another reason prevention is so critical for them.
How is heartworm diagnosed and treated?
In dogs, we start with an in-clinic blood antigen test and look for microfilaria. If the test comes back positive, we run a second confirmatory test before moving forward, because the treatment process is significant and we want to be sure. One thing many pet owners don't realize is that it can take about 6-7 months for microfilaria to mature to adult worms and show up on a test, so diagnosis can be delayed.
Treatment in dogs involves strict exercise restriction for an extended period while the worms are eliminated. The reason for this is serious: as worms die off, we need to minimize the risk of them causing blockages in the heart, lungs, or brain. Most dogs come through it, but many will have some lasting damage to the heart and pulmonary arteries. The earlier it's caught, the better the outcome. Mild cases involve oral medications for up to 2 months to kill the immature stage of worms, and then injections to kill the adult worms.
For cats, we submit an antigen and antibody test because they tend to have few adult worms, and a more extensive workup is done. As mentioned, there is no drug approved to eliminate heartworm in cats. Hospitalization, anti-inflammatory medications, and supportive care to manage symptoms are the tools we have.
If my pet is on parasite prevention, why do they still need an annual test?
This is one of the most common follow-up questions we get.
Heartworm preventives are highly effective — but they work by targeting larvae at a specific early stage of development. They do not kill adult worms. So if a dose was ever given late, skipped, or not properly absorbed, there's a window where larvae could have matured beyond the point where prevention can stop them. An annual test is how we make sure nothing slipped through undetected.
It's also very easy to let prevention slide a bit in the winter when mosquitoes aren't around. The problem is that it's just as easy to forget that first spring dose, or to have a warm stretch arrive before you've restarted. A consistent monthly routine year-round, backed up by an annual test, is the most reliable way to keep your pet protected.
How much does heartworm prevention and treatment cost?
Our heartworm tests cost around $70. Parasite prevention pricing depends on patient weight, but for instance, for a 10lb pound cat, a supply of parasite prevention for six months would be ~$160 and for a 50lb dog it would be ~$223.
Heartworm treatment for a dog is also dependent on the weight, it could be with vet visits, meds, work-up around 2-3k. Unfortunately, the injection we have to give is painful, and it has arsenic in it.
Get a free Heartworm Test!
We offer a free heartworm test with the purchase of a six-month supply of parasite prevention!
Have more questions? Come see us.
If we haven’t seen you in over a year or if you are a new patient, schedule a wellness exam, and we can get you sorted on parasite prevention. If we’ve seen you within the year and would like to get a test, give us a call!
If you have any other questions about your pet's risk, their current prevention, or whether they're due for a test, give us a call, email, or text.
Note: This article is not intended as a substitute for veterinary care by a licensed veterinarian. Contact your veterinarian for any health-related advice concerning your specific pet.