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Heartworm Disease in Dogs and Cats


As we mentioned in last month’s newsletter, experts believe that this should be a record year for mosquito populations.  Among the myriad diseases that mosquitoes are responsible for transmitting is Heartworm Disease, a potentially fatal disease of dogs and cats.

There was a time that Heartworm Disease was of little concern in Southern California.  Our mosquito populations were lower and it was believed that the mosquito species native to the area were simply unlikely to transmit the disease.  Unfortunately, recent climate shifts, introduction of a new mosquito species, and an influx of rescue dogs from heartworm endemic areas have resulted in a rising incidence of this parasite.

While this disease is potentially fatal, it is treatable with early detection.  Better still, it is easily prevented with safe, well-tested medications.

Heartworm Basics:

Heartworm can infect both dogs and cats, but its favored host is the dog.  In the wild, other canids such as coyotes, wolves and foxes serve as hosts.

Dogs can host heartworm throughout the entire lifespan, meaning the parasite matures, mates and produces offspring inside the dog.  Immature heartworms reach maturity in about 6 months, and can live 5-7 years in the dog.  Because these worms live inside the blood vessels, adult worms can cause severe damage to the heart, lungs and associated vascular structures.

In cats, the disease is different with few worms actually reaching adulthood.  Unfortunately, the immature stages of the parasite cause a reaction known as Heartworm Associated Respiratory Disease (H.A.R.D.) which can have severe asthma-like symptoms that last years or even a lifetime.

Transmission of Heartworm:

The mosquito is an essential part of the spread of Heartworm disease.  Microscopic baby Heartworms, known as “microfilaria” circulate in the blood stream of an infected animal.  When the mosquito takes a blood meal off of one of these animals, it picks up the microfilaria along with the blood.  These microfilaria “mature” in the mosquito until they reach the infective larval stage.  When the mosquito then takes a meal off a different animal, the baby heartworms infect this animal through the mosquito bite, and the cycle begins again.

Signs of Heartworm Disease:

Heartworm Disease in Dogs: In dogs, many of the symptoms and much of the damage are a result of the physical effects of the adult heartworms, which in later stages block the blood vessels and clog the heart.

In the early stage of this disease dogs show few clinical signs.  As the disease progresses and the dog’s vessels are filled with larger adult worms, symptoms will develop and become more severe.  The first signs are similar to those common to heart and lung disease:  exercise intolerance, fatigue, decreased appetite, weight loss and coughing.  In more progressed cases, worms will cause congestive heart failure and can even block blood flow within the heart, resulting in cardiovascular collapse (a condition known as “caval syndrome”)

Heartworm Disease in Cats: In cats few heartworms ever make it to adulthood, so disease symptoms are normally not due to heart and vessel blockage.  Instead, cats “react” to the microfilaria present in their bloodstream.  This reaction involves inflammation of the lungs and results in asthma-like symptoms including wheezing, coughing, periodic vomiting and inappetence.  In rare severe cases, cats can show even more dangerous symptoms that include difficulty walking, fainting, seizures, sudden collapse or even death.

Testing for Heartworm Disease*:

The recommendations and process for heartworm testing is different for dogs and cats.  The test requires the presence of adult heartworms, so detecting the disease in cats can be difficult and potentially misleading.  In dogs, however, the testing process is fairly straightforward and accurate.

Testing in Dogs:  Based on recommendations from the American Heartworm Society www.heartwormsociety.org, we recommend yearly heartworm screening for all dogs, whether or not they are on heartworm preventative.

Puppies:  It is recommended that puppies are tested if over 6 months of age and if they have not already been started on a heartworm preventative.  This will allow any acquired heartworms to mature and be detectable by the test.

Adults: We strongly recommend that dogs be tested yearly, even if they are on preventative medication.  While current preventatives are very effective, doses can be missed, dogs can spit out the medication and, very rarely, preventatives can fail.  Regular testing can help us detect infection early when it is most safely treated.

Testing in Cats: In general, we are currently not recommending regular heartworm testing for cats. This is because of lower infection rates and the relative inconsistency of the test results (for reasons mentioned above).

Normally, we only recommend testing cats when symptoms are present that are consistent with heartworm disease and/or H.A.R.D.  Cats require special testing, and other diagnostics such as X-rays and ultrasound may also be recommended.

*Testing recommendations for both cats and dogs can vary from region to region based on the prevalence of heartworm disease in the area.

Treatment for Heartworm Disease

In Dogs: There was a time when treatment for this terrible disease was nearly as dangerous as the disease itself.  Luckily, treatment has become more standardized and safe.  It is still in no way an “easy” treatment, but most dogs affected by heartworm disease are treated and recover well if caught early.  That being said, treatment for heartworm disease is time consuming, uncomfortable for the patient and expensive. 

Because of the disease’s effects on the body, there are often secondary symptoms that must be treated to assure the pet is stable enough to handle heartworm treatment. Once the pet is stabilized, treatment can begin.  The earlier the disease is caught the more minimal the secondary damage and the sooner the primary treatment can begin.  Unfortunately, treatment options are limited and can be expensive; but at the very least modern treatments are safer and more effective than in years past.

In Cats:  The Heartworm Disease process is much different in cats than dogs.  In cats, we are more concerned with treating the respiratory effects than the disease itself.  Cats very often “spontaneously clear” heartworm, meaning the worms do not survive to adulthood or die soon after becoming adults; therefore treatment focuses on managing the respiratory signs until the worms die off.  Affected cats should be put on Heartworm preventative medication to prevent reinfection, and should remain on preventatives throughout their lives.

Despite their limited number of adult worms, cats can have severe and lingering effects from heartworm disease.  Chronic respiratory effects are not uncommon and may require medication throughout the pet’s life.  These symptoms are difficult to distinguish from Feline Asthma, and experts believe that some cases of feline asthma may actually be feline Heartworm Disease. 

Preventing Heartworm Disease

While Heartworm Disease is scary, prevention is simple.  Readily available products, most of which have been on the market for decades, prevent the disease safely with an easy once-a-month dose.  Even better, the same products that prevent Heartworm also treats against Roundworm, Hookworm and Whipworm, three common internal parasites that can infect both animals and humans.  These products are available in combination with trusted flea preventatives, making comprehensive protection safe and convenient.

Recommended Products for Dogs*: 

We strongly recommend that all dogs, regardless of lifestyle, be maintained on Heartworm prevention all year long, as the threat of heartworm disease and other internal parasites is not seasonal.  There are many options to choose from.

Trifexis: This is our primary recommendation for dogs.  Trifexis combines the trusted Heartworm / Internal Parasite protection of milbemycin with the effective flea adulticide spinosad (the active ingredient in Comfortis).  This treatment is given once monthly with food.

Sentinel: This is our secondary recommendation for dogs. Sentinel combines milbemycin with the flea insect growth regulator lufenuron.  Lufenuron does not kill adult fleas, but alters the fleas’ eggs so they cannot hatch.  When given once monthly with food, this product effectively prevents heartworm/parasite infection while helping to control the flea population in your home.

Revolution (available through our online store, Vetsource): Revolution (Selamectin) is applied once monthly to the skin, where it is absorbed.  This product protects against Heartworm, Internal Parasites and Fleas.

Heartgard (available through our online store, Vetsource): Heartgard (Ivermectin) is a once monthly highly palatable chew which effectively prevents against Heartworm and internal parasites.  Heartgard does not treat against fleas, but has been used for decades to prevent Heartworm disease.

Interceptor (available through our online store, Vetsource): Interceptor (milbemycin) is a once monthly chewable tablet which effectively prevents against Heartworm and internal parasites.  Interceptor does not treat against fleas, but has been used for decades to prevent Heartworm disease.

*If your dog is over 6 months of age, and has not been on heartworm preventative, or has been off heartworm preventative for more than 6 months, a negative heartworm test will be required before starting any heartworm product.  A yearly exam is required for these or any other prescription product.

Recommended Products for Cats*

Because heartworm in cats is so hard to detect and symptoms can be so severe and long-lasting, we recommend that cats be on Heartworm prevention all year round.  Like dogs, treatments for cats also prevent against Roundworms, Hookworms and Whipworms.

Revolution:  This product (containing Selamectin) is applied once monthly to the skin, where it is absorbed.  Revolution protects against Heartworm, Internal Parasites and Fleas with one safe, simple dose.

Interceptor (available through our online store, Vetsource):  Although it is primarily marketed to dogs, Interceptor is FDA labeled for use in cats for prevention of Heartworm and Internal Parasite infections.  While the tablets are chewable, most cats are too finicky to eat the tablets meaning the owner must administer the tablet directly.  This treatment is very well tolerated though, and does effectively prevent the disease in cats.

*A yearly exam is required for these or any other prescription product.

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All Pets Animal Hospital


149 S El Camino Real Encinitas, CA 92024

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