Why Your Dog Does Not Need A Heartworm Test Every Year

What is Heartworm and Why is My Dog Tested?

Heartworm is a large internal parasite that is spread through the mosquito. Dogs with a case of heartworm disease start out with mild symptoms like coughing and exercise intolerance but later develop full blown cardiomyopathy (with swelling in the limbs, fluid buildup in the lungs, and reluctance to move as the dog becomes quickly exhausted).

Testing dogs from the shelter, older dogs that have never been on preventative, and dogs that have been off of preventative for a long time is really needed. Annual testing of dogs that are on heartworm preventative is not necessary, however. It is not harmful to the dog, only to your wallet, so unless you feel the need to test you should just keep your dog on the preventative.

Why Do Dogs Need to be Tested Yearly?

Throughout my years in practice, including several years in some of the areas with the highest endemic heartworm populations the US (Mississippi valley), I never saw one dog test positive that was already on monthly heartworm preventative. I have never met a colleague who found a positive heartworm dog that was being treated with preventative.

So is heartworm testing a scam? Why are dogs already on preventative required to be tested? Several of the sites I found on a web search made the sort of recommendation we always gave in small animal practice. Dogs were required to be tested each year before their heartworm medication could be refilled.

(The stated reason for this was that dogs who received a dose of preventative while infected with heartworms could suffer a severe and potentially fatal reaction. This was the case when older preventatives were used, medications that were popular in the 1970s. If anyone has ever seen and documented a case like that I would like him to leave a comment.)

Vets make a profit on each test, certainly a lot more than in just dispensing the medicine. It does not harm the dog, just the owner, so there has been no outcry to stop the procedure.

Can I Keep My Dog on Preventative Without Testing?

Looking to avoid this issue and get a prescription so you can get your medication online? Dogs that did not have a current test could not receive a prescription from the vet. Most dog owners (at least those who do not own a pack) will find they are better off buying ivermectin tablets online. There are several sources of nonprescription medications available. Nuheart and Valuheart are generic ivermectin and when purchased through Canada do not need a prescription. They are available at .

If you do own a lot of dogs and want to use the ivermectin available in feed stores I have to warn you that it is concentrated for use with large animals and a dog dose for heartworm prevention is very low.

Several sources on the internet give doses for ivermectin preventative that are absurd but the label on a common heartworm preventative is only 0.006 mg/kg, so a 20 kg dog will only be receiving 0.32mg. Since Ivomec is sold at 1%, or 10mg/cc, the 20 kg dog only needs less than 0.05cc.

The only way to get close to this dose is by using an insulin syringe; preferably one sold that holds only 0.5cc. For smaller dogs there is really no way to dose that low. (

One of my professors recommended diluting that tiny dose with sterile water and then putting it onto a biscuit before giving it to the dog--that way you are sure he is going to consume the medication)

That being said, ivermectin is used at a much higher dose when treating demodectic mange (0.3-0.6 mg/kg) and for a long time, an average of 3-8 months. It is quite safe even at those levels.

Can Ivermectin Cause Problems?

Owners of Collies, Shelties, OES, Aussies, and some other mixed breed dogs might be sensitive to ivermectin. If you are concerned in any way you can test for the mutation on the MDR1 gene that causes sensitivity. Testing is available though the Washington State University College of Veterinary Medicine, . Of course these dogs can be given heartworm preventatives that do not utilize ivermectin (Revolution (selamectin) and Advantage multi (moxidectin) are both topical so treat fleas as well as preventing heartworm disease).

The drug companies who marketed the preventatives also recommended we tell clients not to buy a large dog dose and split it, as the medication might not be adequately mixed in the tablet and one of the dogs would not be medicated. (If this were the case it would be dangerous to prescribe one-half tablets of any medication.)

Is Heartworm The Most Important Health Issue?

If you choose not to go in for a heartworm test each year do not forget about your dogĀ“s teeth. Many dogs die each year from the secondary effects of periodontal disease.

If you live in an area where heartworms are a danger to your dog, and do not have the income to spend on annual testing, at least spend what is necessary to prevent this disease in your dog.

Prevention is simple and inexpensive and untreated cases are terrible to observe.

My dog and her sister on the beach.
My dog and her sister on the beach. | Source

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Comments 33 comments

DrMark1961 profile image

DrMark1961 5 weeks ago from The Beach of Brazil Author

Thanks for reading, other DVM. As you are probably aware, the major funders for are the big pharmas that manufacture and sell the heartworm preventatives (Merial, for example.) I am sure you can see the danger in that.

Many of the families I have come across in practice over the years cannot afford yearly testing and medications. They may be spending their money on children, rent, food, and other things, and I do not think they have to give up their pets because they cannot afford everything they are told to do. If they cannot afford yearly testing, they probably can afford to buy heartworm preventative from an online pharmacy,or maybe ivermectin sheep drench, which does not need to be diluted and can treat many dogs for less than a box of heartworm tablets.

Thank you again for taking the time to share your views.

otherdvm 5 weeks ago

I encourage everyone who owns a dog or cat and also reads my comments to visit

Heartworm disease is one of the most studied diseases in veterinary medicine, one of only a handful with a dedicated society of professionals and for which detailed guidelines for prevention, diagnosis, and treatment are available to the public.

As a veterinarian, I have seen dogs who are on prevention test positive for Heartworm disease. Should you be one of the unfortunate few, testing annually allows you to catch it early in it's course, when treatment is much safer and the heart, arteries, and lungs have not been damaged. When your dog begins to show signs of illness due to Heartworm disease, it is in an advanced stage.

Also, if you choose to purchase your own ivermectin and make your own solution, ivermectin does not mix with water, necessitating an alternate diluent, such as propylene glycol.

I wish Dr. Mark the best in his professional endeavors, including this forum, just speaking out as one professional having a different perspective on educating the public regarding Heartworm disease.

Buster's mom 6 weeks ago

Hi My vet also charged for an office visi of $22 on top of the heart worm test! My dog has been going there for 3 years. Does anyone else's vets do this?

Doug S. 2 months ago

An important issue here is owner compliance. I will often ask if the patient has been on preventatives. We had a client recently who admitted they gave it once years ago, it didn't work, so they haven't given it since. Usually, they aren't as upfront. We also look at the Owner's body language when we ask. If they avert their eyes, hang their head, etc. can be signs of lying.

DrMark1961 profile image

DrMark1961 7 months ago from The Beach of Brazil Author

dawnmha--your dog is more at risk because of where you live. Yes, the links are valid, but you should read the link about "How to avoid and prevent resistant heartworm" and consider another preventative instead of ivermectin. Leave me a not if you have any questions I can help you with.

Dawnmha 7 months ago

Hi, I do live in the Mississippi Valley. My vet is trying to do that to me and my prior vet did not. The link you provide for heartworm meds is legit right? I believe everything you say, but of course I am concerned about the safety of buying these pills for my dogs. Hope you can confirm their safety and effectiveness. Thanks

DrMark1961 profile image

DrMark1961 20 months ago from The Beach of Brazil Author

I appreciate your sharing this info on your Facebook. Ivermectin has been used since the 1980s, and is safe at the doses listed above. It is no change for most people since it is the active ingredient in heartgard, just a lot cheaper than the product sold through veterinary clinics. Here is some more info on ivermectin if you want to read more:

Thanks again.

Trena 20 months ago

Thanks for this info! It would be helpful if you could provide references, though...for example, where can the factual info be found to support how ivermectin works etc. I don't disbelieve you, but I like to check out facts before making major changes affecting my pets' or family's health and it would save us a LOT of time if you already had the info referenced at the bottom of your articles. I posted to Facebook for you, as either way it's good information to provoke thought and inquiry rather than blindly trusting healthcare professionals who may not stay on top of the latest/best studies, and conversely get much of their information from drug reps! Thanks again!! :)

DrMark1961 profile image

DrMark1961 21 months ago from The Beach of Brazil Author

Nicole, I do not want to pay Merial hundreds of dollars a year so that my dog can suffer through a painful heartworm treatment.

No, testing is not necessary before starting ivermectin. Most of the vets and techs that tell you that your dog will die without testing are using the example of Filaribits. Those old scare tactics are morally reprehensible.

Any drug can be fatal if given in the wrong amount. The proper doses are given, and people can administer it without worrying about paying a drug company for advertising.

Yes, heartworm prevention is safe and affordable. Just not purchased from big Pharma.

Nicole 21 months ago

Actually the reason for the testing is the makers of the product. If an animal test positive and the owners have been using the product as directed than they will pay for the treatment. There is also an issue of compliance, owners may have purchased the heart worm prevention but that doesn't mean they are giving it correctly. Also there are some products if they are used and the animal is heart worm positive they can potentially kill the animal. Oral ivermectin can be fatal if not given the right amount, or if not given enough you are not doing any good. Heart worm prevention is actually very affordable and safe. If you are consistent with you giving your dog heart worm prevention, ask you veterinarian if you can do a heart worm test every 3 years, most are willing.

DrMark1961 profile image

DrMark1961 2 years ago from The Beach of Brazil Author

Health Reports--your dogs are probably totally safe in Georgia, because as far as I know the resistant strain has only been found there one time. That being said, no one can guarantee your dogs will not become infected with the resistant strain since you live down south.

If you want to use an alternative product, and can afford the extra cost, you should go ahead, but this is really your decision.

I hope your dogs stay healthy! Let me know if I can be of any other assistance, but you should really read the Auburn info on the internet about this too.

Health Reports profile image

Health Reports 2 years ago from North America

Ah - we are in Georgia - I'll go find your other article. Thanks for the response!

DrMark1961 profile image

DrMark1961 2 years ago from The Beach of Brazil Author

Health Reports

Yes, there have been a few reports in isolated areas of the US--lower Mississippi and Louisiana, and a case from Georgia. (There are details in an article I wrote on "How to Avoid and Prevent Resistant Heartworm in Dogs".) If you live anywhere else, I really do not think it is even worth worrying about.

As you pointed out in your article on HP, there are a lot of advantages to giving ivermectin year round as a slow kill method anyway. If you are treating with the slow kill method, and preventing Heartworm year round, the test is always going to be negative.

Most people have limited money, and we spend what we can on our pets. I think the best thing to do is spend a little on sheep drench (ivermectin) and give it year round.

Thanks for taking the time to read this.

Health Reports profile image

Health Reports 2 years ago from North America

Great article - thanks for posting it. My last vet tried to insist on a test before selling me the Heartgaurd. Her reasoning was that the FDA had recently detected that heartworms were becoming immune to preventatives. This was a couple of years ago, and I still have not heard of immunity becoming a problem in protecting dogs from heartworms. Have you heard anything like that?

DrMark1961 profile image

DrMark1961 2 years ago from The Beach of Brazil Author

Hi bodylevive, since rabies shots are required by law you still have to take them in at the recommended time (1 or 3 years, depending on your state). The vax actually last a lot longer than that but if one of your dogs bit someone you could get in a lot of trouble if the rabies vax were not current.

bodylevive profile image

bodylevive 2 years ago from Loachapoka, Alabama

Thank you for all this information, I find it to be useful because I have two dogs that really put a dent into our pockets when we take them to the vet. We will be following your advice on some of the fixes for our dogs. I do agree that some preventative measures through the vet is really a rip off and I appreciate your honesty in what can be done at home. We still have to take them for rabbies shots?

gail641 profile image

gail641 3 years ago from Mason City

Heartworm is a terrible disease. My mother went with someone who'se dog had heartworm and the dog had to be put to sleep. The dog had wolf in him and he died a long time ago. I thought that the dog was on preventative medication, but he got the disease anyway and it was too late for him. I voted up.

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

Thanks for leaving that comment! I was worried about that high dose!!! I just read a study from Auburn and they recommend slightly higher ivermectin doses, due to the development of some resistance. 0.4 cc would still be safe and provide an addittional 80micrograms of ivermectin.

Thank you again for the comment. I appreciate your visit.

Shelly 4 years ago

Thanks for catching my typo on the dosage for Ivermectin for my Aussie, Dr. Mark. (One of the perils of my iPad) I should have typed .03ml dose:-(. I purchased a syringe without needle that will deliver that dose so don't worry about my middle aged eyesight tripping me up when my first purchase of sheep drench arrives this week!

I am going to read your article about teeth brushing next since my guy has some issues.

Shelly and Akilah's Sweetheart Porter aka Mr. P

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

Yes, it is really unfortuante that veterinary practices feel the need to take advantage of those people who take the time to learn about the disease. Since heartworm preventative (ivermectin) works as a treatment, anyway, that refusal to refil the meds has absolutely no medical basis.

Your Australian Shepherd only needs 0.3-0.4 ml of sheep drench, which is 240-320 micrograms. I put the doses in my article "heartworm prevention for less than 10 dollars a year"; it is so cheap that it is easy to keep a dog on year-round and never worry about this disease. 3.0 is a lot of ivermectin! I found a lot of recommendations on the internet forums but most of them were a lot higher than the amount of ivermectin contained in Heartgard.

As long as the 3.0 ml dose is not hurting him, though, there is nothing wrong with it except that he is taking in a lot of insecticide each month. If he has any symptoms of ivermectin toxicity, like incoordination, you should give him less. Thanks you for your great comment. I apprectiate your visit.

Shelly 4 years ago

Hi Dr. Mark,

Thanks for the information. I wanted to point out that you can buy .08% (diluted) Ivermectin (Sheep Drench) from several places on the internet including My 51 - 100 lb Australian Shepherd dog then needs a 3ml dose according to other forums. Yes, I did have him tested for sensitivity a few years ago. I am just very angry and disappointed with the vet for flat out refusing to re-fill the meds unless I have him tested (we live in Southern California - not a hotbed for heartworm) even though I have been buying the Heartgard (at a 100% mark-up) from them. For information, I went to the FDA's site for a neutral perspective. Since the test will only reveal the presence of adult heartworm, which takes 7 to 8 months to achieve, isn't it true that the "preventative" which is actually a poison that kills the larvae when already present, should keep a dog free of heartworm?

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

Thank you all for taking the time to read and be concerned!

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

According to the Heartworm society, yes. If there are any mosquitoes in your area it is theoretically possible, although it may be unlikely.

Daughter of Maat, for safety sake she should be tested before starting the meds again, but the preventative should be started anyway, no matter what the test results show

Millionaire Tips profile image

Millionaire Tips 4 years ago from USA

This is handy advice. Thanks! My vet doesn't require the test, but we don't live in the Mississippi valley. Should I still be giving her the heartworm treatment?

Daughter Of Maat profile image

Daughter Of Maat 4 years ago from Rural Central Florida

Fantastic hub. This is probably a stupid question, but should you give heartworm medication to a dog who has not been tested and who has been off heartworm medication for about a year and a half especially if the dog lives in Florida?

Voted up and Shared all over!

Angela Brummer profile image

Angela Brummer 4 years ago from Lincoln, Nebraska

I am going to share this again with all the sources!

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

Im glad it is useful for everyone. Just some more info--that bottle from Amazon will last about 3 years for a medium sized dog, so for anyone that needs it that is less than 10 dollars a year for heartworm protection

catgypsy profile image

catgypsy 4 years ago from the South

Very interesting article. I have wondered why the testing was necessary too. Thanks for the information!

rebeccamealey profile image

rebeccamealey 4 years ago from Northeastern Georgia, USA

Thanks so much for that website for the heart worm meds w/o a prescription.This is so useful for so many people, including me. Voted very useful and shared!

Angela Brummer profile image

Angela Brummer 4 years ago from Lincoln, Nebraska

This is really an amazing hub. I looks very professional. I have gotten this shared. Also, you should comment to the others and share their hubs and leave the ones you would like shared.

travel_man1971 profile image

travel_man1971 4 years ago from Bicol, Philippines

Here in the Philippines, the military officials with the help of health department administer such kind of dog treatment.

In the farflung barangays or counties, stray dogs outnumber handful vet doctors in the nearest city.

Although, animal rights is extensively promoted here and responsible pet ownership as well, still, the scenario on pet health care is not fully implemented yet.

Dog treatment is always a case-to-case basis and it's for the owner who can afford such medication.

DrMark1961 profile image

DrMark1961 4 years ago from The Beach of Brazil Author

That is somewhat akin to guaranteeing that your home in Iowa will never be struck by a tsunami.

Dragonrain profile image

Dragonrain 4 years ago

Interesting hub. I know someone who buys Heartgard online from Australia and has never had any issues doing so. I'm too paranoid to do anything like that myself. Plus, most heartworm preventative companies will refund your vet costs if your pet gets heartworm while on their product, but they apparently only do so if you bought your preventative meds directly from a vet.

Some animal shelters and low cost clinics will offer heartworm testing for a fraction of the cost that you'd pay going to a regular vet. That way you can save some money, but still get a prescription for heartworm meds so that you can buy them from a vet.

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