Heartworm Prevention

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Spotlight on Parasitology: Heartworm Preventative

Last entry we talked about what heartworm disease, how it is spread, and how it is treated.  We will continue our look at heartworm disease by taking a closer look at prevention.  The American Heartworm Society recommends heartworm prevention year round and dogs being tested annually.

Why Prevent?

As discussed in our last entry, heartworm disease is treatable; however, it is expensive and complicated.  Veterinarians walk a fine line of giving enough adulticide to kill adult heartworms and not harm the dog.  Restricted activity is necessary and could last several weeks to even months, to prevent dangerous complications in treatment.  Even after treatment the pet may still suffer from cardiovascular and respiratory disorders from damage done by the heartworms.  Preventatives interrupt heartworm development by killing the adolescent heartworms before they mature (AHS, 2012).

“To everything there is a season”

In the past there was “heartworm season” between April and October.  During these six months of the year dogs would receive their monthly heartworm prevention.  Then every spring the dogs return to their veterinarian for the annual heartworm test and refill.  Research into heartworm disease and mosquitoes has shown us that the heartworm season is much bigger than we used to think.  Heartworm season is all year around!

Those pesky mosquitoes

Mosquitoes thrive in warm, wet places.  They lay eggs in standing water, but will avoid running water such as fast moving streams.  Some people in urban areas feel the threat from mosquitoes is lower than those living in rural areas.  That is simply not true; mosquitoes are adaptable and will use any standing water to lay eggs.  Urban mosquitoes will find old tin cans, old tires, watered lawns, sewer systems, and they love bird baths.  Even in times of drought man-made breeding areas exist.

While in the mosquito, heartworm development is affected by temperature (CAPC, 2009).   Their growth will slow and even stop in cold weather, and will resume once temperatures increase.  In Michigan our cooler temperatures won’t completely stop development of heart worm larvae.

Another common myth is that mosquitoes cannot bite an animal through thick coats of fur.  Hair may slow a mosquito down but it will not stop them completely.  Even dogs and cats with thick, double coats have thin areas on their face and legs.

Tying it all together

Getting the timing just right to start and stop preventative with the season is very difficult, especially in Michigan given our sporadic temperature ranges from one day to the next.  Using our heartworm season schedule from the past as an example, preventative is stopped in early October.  The infective lifecycle of the heartworm develops in the tissue for up to 70 days before entering the bloodstream where preventative will kill it.  If infected in October that adolescent heartworm can enter the bloodstream in November or even December, that worm has about 5 months to develop into an adult before preventative is started in April.  Heartworm tests cannot detect a heartworm infection until 7 months after exposure.  This leaves the possibility of the heartworm not even being detected for a whole year and a half!  Putting your pet on a monthly routine is simple and dramatically cuts down on risk of infection (Think 12, AHS, 2012).

Where you get your preventative matters

Into today’s market there are many online retailers selling heartworm preventatives at discount rates.  However, there are pros to spending a little more at your veterinarian’s office.  First of all it keeps your business local and supports your community.  If there is a problem with your pet’s medication you have a direct line to help fix it.  Secondly, most pharmaceutical companies offer rebates on their products when you purchase them at your veterinarian.  Considering this, preventative is the same cost or less than the online retailer.  Finally most manufactures offer guarantees on their product.  They may cover costs of treatment if your pet gets heartworm or intestinal parasites while taking their preventative year round, IF you bought it through your veterinarian.  Rebates and guarantees are only offered through your veterinarian because pharmaceutical companies trust the veterinarian more than an on-line pharmacies.  Manufactures sell directly to your veterinarian’s office, this guarantees that the product is stored properly and not compromised during shipping.

Types of preventative

There are several brands and types of heartworm preventative on the market today. They include monthly flavored oral preventatives, such as Heartgard Plus, these are chewable treats that make medicating a breeze.  Topical solutions, such as Revolution, are a great alternative for dogs with food allergies and also protects against fleas.  Finally there is an injectable preventative called ProHeart6, which lasts for 6 months.  Many preventatives protect against internal parasites as well.  At this time there are no “natural” or herbal therapies shown to be safe and effective for the prevention or treatment of heartworm disease (AHS, 2012).

In closing…

“An ounce of prevention is worth a pound of cure.” – Benjamin Franklin



Works Cited

AHS. (2012, January). American Heartworm Society. Retrieved February 8, 2013, from Canine Guidelines Summary:

CAPC. (2009, November). Canine Heartworm CAPC Vet. Retrieved February 8, 2013, from Companion Animal Parasite Council:

Think 12, AHS. (2012). Think 12. Retrieved February 8, 2013, from Truth About Treatment, Mosquitos in Winter, Don’t Try to Predict-Protect:



Heartworm Disease

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Spotlight On Parasitology: Heartworm Disease

Everyone knows their dog needs yearly heartworm preventative, but do they really know what heartworm is or why we give preventatives?  Heartworm disease is caused by parasitic worms that make their home in the arteries leading to the lungs and sometimes in the right side of the heart.  These parasites can infect dogs, cats, wolves, foxes, ferrets, and sea lions (American Heartworm Society, 2013).  Though rare, humans can also be infected by heartworm larva.  The immature heartworms don’t survive long in humans, but they end up in the lungs causing nodules to form (CAPC, 2009).  These nodules can be mistaken for tumors or tuberculosis!

Life Cycle and Stages

            The lifecycle of the heartworm involves numerous steps before they mature into adult heartworm.   Adult heartworms live the arteries between the heart and lungs, where females give birth to live young, called microfilaria.  Microfilaria must be ingested by a mosquito before they become infectious.  This means heartworm cannot be from pet-to-pet contact, it can only be transmitted by mosquitoes.  Dogs are highly susceptible to infection, virtually 100% of dogs exposed become infected, whereas 61-90% of cats become infected (American Heartworm Society, 2013).  After entering the pet the adolescent worms spend up to 70 days migrating through the tissues of the body before they make their way into the bloodstream.

Where is it found?

            Heartworm disease is found worldwide and in all 50 states. It is found in higher numbers in warm, moist climates.  The Southeastern United States and the Mississippi River Valley have the greatest number of reported cases in the US (CAPC, 2009).  Michigan comes in with an average of 6-25 cases per clinic.  Please keep in mind the actual number may be higher as these numbers are reported cases, and not all veterinary clinics report positive cases.

Clinical Signs and Testing

            In the early stages of heartworm disease the clinical signs are mild and can even be absent. Dogs suffering from severe heartworm disease may have a persistent cough, exercise intolerance, abnormal lung sounds, fluid in the chest cavity, episodes of passing out, and/or weight loss.  Dogs can harbor 30 or more adult heartworms; this large number of worms can alter blood flow through the heart and potentially interfere the normal function of the heart.  This can cause the heart to work harder to pump blood throughout the body, and can lead to murmurs, severe heart disease, anemia, and more (CAPC, 2009).

Testing for dogs is very simple and looks for the presence of a protein found only on the surface of female heartworms.  For dogs that return positive a confirmatory test is performed to rule out any potential false positive results.  If both tests are positive the dog is considered heartworm positive.

Cats with heartworm disease display symptoms similar to feline asthma, bronchitis, or can have no symptoms at all (American Heartworm Society, 2013).  Symptoms include vomiting, coughing, difficulty breathing, diarrhea, and not eating.  Anyone who has owned a cat can tell you that not eating and vomiting can be a regular occurrence in their cat, thus making the heartworm diagnosis more difficult.

Testing for cats is different because cats have smaller hearts than dogs therefore usually harbor only 1-2 adult heartworms. Heartworm test for cats looks for the body’s immune system response to the heartworms rather than an antigen testing used in dogs.


Heartworm disease is treatable in dogs; however it can be costly and recovery is very lengthy.  Treatment is designed to kill off circulating microfilaria (immature heartworm) and adult heartworms while minimizing complications caused by worm die off and treatment.  The dog undergoing treatment should be as healthy as possible before undergoing treatment (AHS, 2013).  Treatment consists of three injections of melarsomine, derived from arsenic, over 31 days (Think 12, AHS, 2012).  To reduce side effects and complications of treatment an antibiotics, steroids, and medications to control heart disease may also be prescribed.  Dogs with severe heartworm disease may require surgical removal of the heartworms.

During and after treatment, dogs are on a very strict exercise regimen.  All excitement and exercise (beyond slow walking for bathroom breaks) should be restricted.  As the worms die off and begin to decompose they can become lodged in arteries and capillaries around the heart and lungs.  Increased activity causes more blood to flow to these blocked vessels and increase risk of cardiovascular side effects including heart failure (AHS, 2013).

Unfortunately there is no approved treatment for heartworm disease in cats (American Heartworm Society, 2013).  Cats react severely when the worms begin to die off.  This reaction can cause shock and even death.  In some cases surgical removal of the adult heartworms may be attempted.

Next time we will look at heartworm preventatives.


Works Cited

AHS. (2013). Canine Guidelines. Retrieved February 8, 2013, from American Heartworm Society:

American Heartworm Society. (2013). What is Heartworm Disease? Retrieved February 8, 2013, from

CAPC. (2009, November). Canine Heartworm CAPC Vet. Retrieved February 8, 2013, from Companion Animal Parasite Council:

Think 12, AHS. (2012). Think 12. Retrieved February 8, 2013, from Truth About Treatment, Mosquitos in Winter, Don’t Try to Predict-Protect:




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Summer is over and the fall season is approaching but that doesn’t mean the possibility of flea infection is fading. In fact, we see more flea infestation when the cool weather breaks than during the entire summer. The reason? Well, in Michigan, the risk of flea infestation is at its highest in the fall and here is why;

In the early spring, when temperatures rise, the flea comes out of dormancy. They find a host (wildlife or your pet), to get a blood meal and begin to lay eggs – rapidly. These eggs hatch and begin to lay more eggs. The vicious circle continues and by Fall, trillions of fleas are produced and looking for new hosts.

Here’s the fact; if you aren’t using flea protection, your pet and home are more likely to experience flea infestation now than any other time of the year because trillions of fleas are looking for warmth and blood meals. If you’ve ever experienced flea infestation, you know that the costs and frustration of treating your pets as well as your home far outweigh the cost of monthly flea preventative and, ease and efficacy has come a long way. In addition, fleas are responsible for flea allergy dermatitis and tapeworm.

According to experts, fleas cannot survive exposure to temperatures below 30.2 degrees for more than 5 consecutive days. If you’re not using flea prevention all year round and you’re not battling flea infestation, we recommend two hard freezes before stopping your flea preventative.