Some reports estimate that 1 in 200 dogs are infected with heartworm disease each year. The adult heartworms obviously occupy the dog’s heart and as you can imagine, cause lots of problems–from heart valve damage to ventricular wall damage to just plain blockage. Left untreated, they can kill your dog. Prevention is therefore very important especially in locations where the disease is found in high numbers.  The above map shows relative incidence (the darker the state, the higher the incidence of heartworm disease).

The Cause of Heartworms

The organism, Dirofilaria immitis (D immitis), a blood-borne parasite, is the cause of heartworm disease. It is spread to the dog by the bite of an infected mosquito. The mosquito is considered an intermediate host as it is not affected by this organism but only carries it to another, definitive host, usually the dog. Cats, ferrets and wild canines are also known to be affected. The cycle goes like this. A mosquito bites an infected animal and ingests the microfilariae (a neonatal larval stage of D immitis) which then progresses through three larval stages within a matter of 1-4 weeks. This length is dependent upon the environmental temperature and humidity. With ideal temperatures ( > 81 degrees) and relative humidity ( around 80%), it takes only 10-14 days to complete this part of the life cycle. The third larval stage (L3) is the infective stage for the dog and migrates into the bite wound when the mosquito bites the dog. Then the L3 develops into the 4th larval stage (L4) in 3-12 days and stays floating around in the bloodstream to become a young adult and then find a home in the lungs or heart within 7-9 months after the original bite occurs. An infected dog may also pass her heartworms to the young in utero if she has floating larvae in her bloodstream.

Model picture showing heartworms within the heart.

Adult female heartworms (can be 10-12 inches long) mate with male heartworms (can be 4-6 inches long) to produce microfilariae to swim in the bloodstream waiting to infect more mosquitoes that may bite this affected dog. These microfilariae can live for about 2 years within the bloodstream. Adults can live 5-7 years in the dog’s heart.

Lifecycle of heartworms.

Heartworm disease is most common along the Atlantic and Gulf Coast and along the Mississippi River and it’s tributaries. But, probably due in part to vacationing dogs, it has been reported in all 50 states, Canada, Australia, Latin America and southern Europe.

Symptoms of Heartworm Disease

Once the infected mosquito bites the dog, as discussed earlier, it takes several months to become an adult. During it’s different larval stages, these larvae are migrating around and often get lodged within the lung vessels to cause damage to the lung tissue. Once the adult worms find their home within the heart, obviously, heart function can become compromised. The valves and therefore pumping action of the heart may be disrupted. Over time, the heart enlarges and heart failure or chronic heart failure (CHF) signs can be evident. Coughing, exersize intolerance (becoming very tired with mild exersize) and fatigue are seen. The worm burden, or number of worms inside the heart determines how quick and how severe these symptoms are. The average number of worms is about 15, but the range is reported to be 1-250! Can you imagine? 250 (the record) 6-12 inch pieces of “spaghetti looking” worms within the dog’s heart? Nasty!

Diagnosis of Heartworm

It takes about 5 months after a bite with an infected mosquito to test positive on most of the heartworm antigen test kits available today.

One type of test kit for detecting heartworms.

Usually testing is not done on puppies under 7 months of age, they are simply started on a prevention (discussed below).

Heartworm Treatment Options

Once a dog is tested positive, a decision must be made as to a treatment protocol. Your veterinarian will need to determine a few things before proceeding. In general, there are 4 classes of heartworm disease:

  • Class 1 No symptoms or very mild symptoms.
  • Class 2 Mild to moderate symptoms.
  • Class 3 Loss of body condition, persistent cough, exercise intolerance, trouble breathing, other common signs of CHF and chest x-ray signs.
  • Class 4 Caval syndrome which occurs with a high worm burden that causes an overflow of worms out of the heart. They have to move out of the heart into the adjacent artery or vein, generally the vena cava (hence the name). These dogs are often jaundiced and in acute liver failure which is often life-threatening, even with emergency surgery to remove these worms ( I actually saw this in a 2-year-old German Shepherd dog in veterinary school that died).

Your veterinarian will determine which class your dog is in and probably do other diagnostics such as CBC and Chemistry Profile to check on other organ function prior to starting a treatment program. Most veterinarians follow the protocol put forth by the American Heartworm Society (AHS) as it is the safest treatment available. The only FDA (Food and Drug Administration) approved treatment is melarsomine dihydrochloride ( 2 companies manufacture this under trade names Immiticide and Diroban).

One type of heartworm treatment.

This drug contains arsenic and is given deep into the epaxial muscles (these are along the spine at the lumbar level). The main potential side effects of treatment and killing of these worms are blood clots which can be fatal if they lodge in critical areas of the body (lungs or brain).

How to Prevent Heartworms

As I said before, prevention is key. Why would we let this happen to our pet if it could be prevented? There are several products on the market to prevent heartworms and puppies can be started on one of them as early as 8 weeks of age (I usually don’t start until 12 weeks as I have seen some side effects in young puppies!). Among these products are:

  • Oral pills–Interceptor, Interceptor Plus, Sentinel, Trifexis, Heartgard, Triheart, Iverheart
  • Topicals–Advantage Multi, Revolution
  • Injectable–Program (lasts for 6 months)

Some of these products also incorporate flea and/or tick medicine in their formulations.

Different types of medicines for heartworm prevention.

These products will kill the larval stages floating within the bloodstream but not the young adults in the bloodstream or adults in the heart. Depending on which product is used, what part of the country you are in and the size of your dog, the cost for monthly prevention can run from $5.00 to $25.00.

What kind of product you need should be discussed with your local veterinarian. They should be able to answer questions for your area or your vacation spots around the country. And, in case you are curious, I found reports that humans can get heartworms, but since they are not the usual hosts, they react differently. The worms tend to migrate to the lungs and cause round lesions (coin lesions) which mimic the look of lung cancer lesions! Another reason to use mosquito repellent for us as well as our dogs when we go camping, fishing, boating or swimming along coastal regions!