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Do I Need To Test My Dog For Heartworms Every Year?

| Published on August 17, 2015

We hopefully all know how important heartworm preventative is, and I often get questions about my recommendation for yearly testing for heartworms. Heartworms kill dogs. But people wonder why they need to test every year and it seems like there are those on the internet that want to make veterinarians into the villains, making ridiculous and unnecessary suggestions.

The American Heartworm Society is a group that helps veterinarians make unified recommendations to promote eradication of this damaging parasite and they recommend at least once yearly testing, but their recommendation is not the only reason.

My clients explain that they faithfully give the heart pills and so they know the test will be negative. Their faith in these products is not unjustified, but blind faith in anything does not take into account any situation that falls outside the norms. We are human. We make mistakes and we sometimes forget things and nothing is 100%.

I can absolutely understand why people would not want to waste their money if this test was not a really good idea. I get it. So I am going to explain to you why I give the pills (or an injectable heartworm preventative) as directed to my personal dogs AND I test my dogs yearly and sometimes more. It is not as simple as giving a pill each month in this case and here is why:

The Way These Medications Work

Heartworm Preventive medication stops infection by killing the worms before they mature, reproduce and cause damage. There is no drug that can stop the mosquito from carrying the larvae to your dog, so we have to stop them before they become damaging adults and make the infection worse with their offspring. It is important to know that these drugs are effective by retroactive protection and work only against certain larval stages of the heartworm. For example, if a dog is dosed on November 1, the dose covers any susceptible larvae at that time (likely ones he was exposed to in October). Any larvae that are present, but too young to be susceptible will not be covered until the next dose (December in this example), but any larvae that are too old and past the susceptible larval stages will be able to grow into adults and there is no medication to stop them here. So you can see that any lapse could result in a larva slipping through and making it to adulthood where no preventive medication can kill him and you will not know that adult worm is there at all without testing.

The Limitations of Available Testing

No commercially available test will detect the babies until they become adults, so there is a limbo period where the heartworms are neither susceptible nor detectable. If we are giving the pills and running the tests every year, we have a chance to catch any larvae that break through before they cause extensive damage to the vessels of the heart and lungs, but if we skip testing entirely as a routine preventive test, we will not realize our dog has heartworm disease until he is showing clinical signs of cardiac impairment. Once signs of heart failure show themselves, irreversible damage has occurred.

The Consequences of Missed Infections

The treatment to kill heartworm adults is expensive and complex, requiring strict activity restriction (not fun for a dog) for several months and multiple visits to the veterinary clinic. Your infected dog will need “staging” to see how much damage has already occurred, comprised of blood testing, radiographs and medical evaluation. The entire treatment protocol is going to extend out for up to 4 months, with a series of oral medications at home followed by injections at the veterinary facility. It will require some hospitalization and the only labeled treatment is Immiticide (melarsomine), an arsenic based compound that is not without possible side effects. A copy of the actual suggested protocol can be found here.

Resistant worms

Resistance is developing to our preventatives whether we like it or not. The Companion Animal Parasite Council changed its guidelines in 2013 based on American Association of Veterinary Parasitologists Conference findings. Researchers have identified heartworms from the Mississippi Delta area that can even develop in dogs that are receiving heartworm preventatives. Because the preventatives are our best hope (and are still effective against the strains outside the Delta region… for now) we MUST continue to give them exactly as directed, only we have to be sure that we are doing the suggested testing too, so that we will know if and when these resistant worms are moving into our area.

Changes to the way we use and recommend the drugs

For a long time, if someone came in with a heartworm positive dog and did not want to treat the adult infection, veterinarians would put that infected dog on preventive medications. The rationale for this choice was that we could at least keep the infection from getting worse (by killing new larvae as they reached susceptible stages) and maybe eventually the existing adults would die off. This “slow kill” was considered a valid option in the face of a positive test when adulticide treatment was not an option.

It is no longer recommended to give preventatives to infected dogs. The presence of our heartworm preventative compounds where there are already adult heartworms is what has caused the worms to adapt and develop resistance, much like inadequate dosing with antibiotics has encouraged resistance among bacteria, endangering us all. As a responsible pet owner, you must KNOW (with testing) if your dog has become infected with adults for his safety and the safety of everyone’s beloved dogs. We have to test before filling or refilling a preventative prescription.

Economics and Insurance

The last reason is more about economics than medicine. Heartworm manufacturers offer product guarantees. If their product was purchased from a veterinarian (that knows you and your pet and was prescribed for your dog), your record shows that you have purchased doses without lapse, and you have done the recommended yearly testing, then any heartworm infection in your dog will be paid for by the company. But you have to have the negative test on record each year for the guarantee to apply.

So you can see why testing is important for your own dog and everyone’s dogs. The recommendations vary from region to region so ask you own veterinarian what they are for you, but please follow their guidelines this important step in your dog’s preventive care regimen.


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