651-224-3038   •   1140 Grand Avenue, St. Paul, MN 55105

Minnesota State Hitchhiker

May 1, 2012

It’s springtime in Minnesota, full of beautiful days and growing things. Alongside our leafing trees, emerging flowers, and singing birds, another familiar Minnesota resident is making its spring appearance: The mosquito. Since ticks and the diseases they can carry have had their day here at Grand Tails, let’s give the mosquitoes and their very own hitchhiker equal time in the spotlight; in other words, it’s time to talk about heartworm.


Welcome to Heartworm 101

Heartworm disease is caused by the heartworm, Dirofilaria immitis. Adult heartworms can grow up to 14 inches long and live in the heart and pulmonary artery (large vessel carrying blood from the heart to the lungs) of dogs and related canines such as coyotes. Canines are called the definitive hosts for heartworms, meaning they are the animals heartworms are adapted to and normally infect. A few other animals, including cats, can be aberrant hosts, meaning that if they are exposed to heartworm larvae a few worms may mature but not usually in significant numbers; however, this article will be limited to discussing heartworms and heartworm disease in dogs.

Terminology can be confusing when it comes to heartworm, because the termsheartworm, heartworm infection, and heartworm disease refer to different things but are sometimes mistakenly used interchangeably. Heartworm simply refers to the worm itself. Heartworm infection means that a dog has been infected with one or more life stages of the heartworm, but may or may not be sick.Heartworm disease is when a dog is sick due to a mature heartworm infection consisting of adult worms in the heart and/or pulmonary arteries that are causing symptoms.


The Life and Times of Dirofilaria immitis

If adult male and female heartworms are both present in a dog, they may mate and produce offspring. If only females or only males are present, they will eventually die of old age without breeding. Unlike many parasitic worms, heartworms give birth to live young called microfilariae (also called L1 or first stage larvae). These microfilariae circulate in the blood of an infected dog, who now serves as a reservoir (a source of infection) for other dogs. Microfilariae can be ingested by a mosquito along with a blood meal and passed to another dog.

Microfilariae can live up to two years in their original host, but cannot ever mature past the L1 stage without being ingested by a mosquito. If they are not ingested by a mosquito, they will die of “old age” even though they are an immature stage of the worm.

When a mosquito ingests microfilariae, the L1 microfilariae develop into L2 and then L3 (second and third stage) larvae within the mosquito. This process takes varying lengths of time depending on the weather; the warmer the weather, the faster the larvae mature, but generally it takes a few weeks for microfilariae to mature to the L3 stage. The outside temperature must be over at least 57 degrees F throughout this time period for the microfilariae to mature. It is the L3 larvae that are able to infect new dogs.

When an uninfected dog is bitten by a mosquito carrying L3 larvae, the larvae are deposited on the skin next to the wound in a drop of saliva and must swim into the bite wound. Once this happens, the L3 develops through the L4 and L5 stages in the dog’s skin. At the L5 (young adult) stage, the worms are ready to migrate to the heart, where they mature to the final adult stage. 5-7 months after first entering its host, the worm is mature and may mate and produce offspring of its own.


An Ounce of Prevention… can be confusing.

There are several medications, most given once a month, that are referred to asheartworm pills or heartworm preventatives. (A discussion of specific products is outside the scope of this article and can be discussed with one of our veterinarians.) Heartworm prevention can be confusing for two reasons. First, the terminology can be confusing as the preventatives do not repel heartworm larvae or prevent larvae from actually entering the body. Rather, they kill larvae before they can mature into adult heartworms. That is, they prevent heartworm diseasefrom developing after exposure to larvae.

Secondly, the preventative does not remain in the body and kill larvae that the dog is exposed to after receiving the pill. Instead, it kills “backwards”; that is, it kills L3 larvae that a dog has been exposed to in the month previous to receiving the pill, as well as any L4 larvae that have developed during that time. (Very inconsistently, some preventatives may kill some L5 larvae, but this is not at all reliable.) This means, for example, that a heartworm pill given on June 1st will kill any L3 larvae that a dog has been infected with since roughly May 1st, but it will not kill any L3 larvae the dog is infected with between June 1st and July 1st when the next pill is given.

It takes an average of about 50-70 days for L1 larvae to mature to L4 larvae, but under the right conditions this can happen as early as 40 days; therefore it is recommended to give most heartworm preventatives monthly to ensure that these “early bloomers” are killed before maturing past the L4 stage.


Testing, Schmesting?

Testing for heartworm is recommended for all dogs, even those who consistently receive their monthly preventative. This is because breakthrough infections, infections that occur even though a dog is on heartworm preventative, may occur. Although rare, they may occur if a dog vomits or spits out a pill where an owner does not see, if a dose is forgotten, or simply because no medication is 100% effective. In some areas of the country, there are some reports of resistant strains of heartworms that are not affected by the commonly used preventatives; however, this has not been reported in Minnesota.

The sooner we know about a breakthrough infection, the better; the earlier an infection is detected and treated, the easier the experience is for the dog and the better the prognosis.

Several of the heartworm preventative manufacturers have a guarantee under which they will reimburse some of the costs for treatment for a dog who tests positive for heartworm while on preventative if you have a purchase history with a veterinarian, so it is well worth keeping up with testing. Testing is recommended every other year for dogs receiving monthly heartworm prevention year-round.


Why year-round preventative? Mosquitoes in winter??!

We used to recommend giving heartworm preventative seasonally in Minnesota. Due to several factors, we have changed that recommendation to year round. First, there is some evidence that receiving monthly heartworm preventative year round may increase its overall efficacy, although the mechanism for this is not known. Secondly, it is getting harder and harder to predict weather patterns and, by extension, how soon mosquitoes may emerge in the spring and how long they will last in the fall. Thirdly, several heartworm preventatives also contain an ingredient to kill intestinal parasites; while traditionally intestinal parasites have been a relatively minor problem in Minnesota compared to warmer states, due to warmer weather and the trend of rescue groups and shelters bringing dogs (and their intestinal parasites) from the Southern US to Minnesota, the risk and incidence has increased here in recent years. Fourth, more and more people are traveling with their dogs, and giving preventative year-round helps ensure that it is not forgotten during vacations to warmer climates.


What about my collie?

There is a genetic mutation in dogs called MDR1, most commonly found in herding breeds such as collies, Australian shepherds, and many others. There is normally a barrier between the bloodstream and the brain called the blood-brain barrier which prevents substances, including most drugs, from ever reaching the brain itself. The MDR1 mutation allows several drugs, including all commonly used heartworm preventatives, that normally would not be able to cross this barrier to do so. At the doses used for heartworm prevention, these drugs are safe and should not cause symptoms. At higher doses, or if a dog is accidentally overdosed, they may cause problems ranging from stomach upset to serious neurological signs. There is a test for the MDR1 mutation available through Washington State University, but the bottoms line is that at normal heartworm preventative doses, these drugs are safe even for affected dogs. See the following links for more information:


Heartworm Disease: The Nuts and Bolts

Symptoms of heartworm disease are caused by the physical presence of worms, which damage the walls of the heart and pulmonary artery and block normal blood flow. In the early stages a dog may not show any signs at all, which is part of why routine testing is so important. Later in the course of disease, the most common symptom is a chronic, soft cough. Later, dogs may tire easily and/or have difficulty breathing. Late-stage heartworm disease can cause congestive heart failure, a condition which causes fluid to build up in the lungs and severe breathing difficulty. Chronic stimulation of the immune system can also cause inflammation in many tissues of the body and put dogs at risk for developing fatal blood clots.

There are different tests available to diagnose heartworm disease. Blood tests look for the presence of adult worms by either detecting antibodies (immune proteins produced by the body in response to infection) or antigens (proteins or other parts of the worm themselves). A negative heartworm test is very, very reliable – that is, the test very rarely gives a false negative result. However, false positive results may uncommonly occur, so it is generally recommended to confirm a positive heartworm test with additional testing.

Once a dog is diagnosed with heartworm disease, a process called staging is done. This puts a dog into one of four classes of severity based on symptoms, overall health, and results of general blood chemistries and chest x-rays. The class helps determine the exact treatment regimen and prognosis for an individual dog. The specifics of treatment are beyond the scope of this article, but in general include an adulticide (a drug that kills the adult heartworms), a microfilaricide (a drug to kill any circulating microfilariae), and possibly (depending on the class) additional drugs such as anti-inflammatories or antibiotics. The higher the dog’s class, the more risky treatment is and the greater the chance for side effects.


The Bottom Line, Here at the Bottom

If you have made it this far, congratulations for hanging in there. At the end of the day, the take away message about heartworm disease includes:

  • Heartworm disease is caused by a worm that is spread from dog to dog by mosquitoes and goes through several stages of development before causing disease
  • Heartworm disease is easily prevented by giving a monthly pill which kills larval stages of heartworms before they can mature into adults and cause disease
  • Routine testing is recommended (every two years for dogs receiving year round heartworm preventative) to catch any breakthrough infections as early as possible
  • At recommended doses, heartworm preventatives are safe even for breeds at risk for the MDR1 mutation
  • Heartworm disease can have serious symptoms and consequences, and can be risky and costly to treat.

If you have any further questions about heartworm disease, please feel free to call the clinic to speak to one of our doctors.


If you haven’t had enough… More Resources:

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