A Dog-Owner’s Story of Loss and Raising Awareness

This article has been reprinted with permission by Ashley Sassaman

One of our beloved dogs, Jack, a 6-year-old, chow-lab mix, died yesterday. His death was very unexpected and traumatic. Although, our dogs did not receive the same amount of attention as they did before Bobby was born, they were still loved dearly. Bob walked them religiously every single day. Jack loved, loved, loved, playing fetch. Tennis balls were his favorite. When we discovered that he had died, we began questioning what happened to our sweet Jack.

On Monday, May 7, we took both of our dogs to our vet for their annual check ups. It was a very routine visit. They were both checked for heartworms and parasites, weighed, and listened to. They were deemed normal and healthy. They both received their rabies vaccinations, and a new drug (via injection) that was recommend by our vet, ProHeart 6. Our vet simply said, “Hey, we have this new shot that we can give them for heartworm protection instead of an oral preventative. It’s easy because it’s only one dose every 6 months.” Sounded good to us, so we agreed. That was it. That was all the information we received, and—by our own fault—all we asked for.

Both of the dogs seemed fine the first week home, then Jack started acting differently. He didn’t want to go on his daily walks. He didn’t want to fetch his tennis ball. He didn’t want to eat his dog food. My husband even tried giving him some leftover steak from our dinner, and Jack wouldn’t eat it. We knew something was up, so we took him back to the vet on Monday, May 21.

The vet discovered that Jack had a fever, but could find nothing else wrong with him. He was given a steroid shot and some antibiotics in case he had an infection, and we were sent home. Over the next few days, Jack seemed to feel better. He was still a little lethargic, but was eating and perked up when we threw a ball or petted him. He seemed on the mend. We thought he must have just had a virus and was getting better. It did not even cross our minds that he was so gravely ill, so we continued with our Memorial Day plans to go to the beach.

We returned from the beach to a grim discovery—Jack dead in a pool of blood and vomit. His poor little body was crumpled up in a strange way, like he had literally just fallen over dead. We took him to an animal ER where they confirmed that he was dead, and we paid to have his body cremated. The vet at the animal ER stated that it appeared to be a heart attack or cardiopulmonary problem that killed him.

That evening, as we wrestled with grief and trying to explain death to our 3-year-old son, we began to question what on earth had happened to our previously perky puppy. Okay—he wasn’t a puppy, but he always behaved like a frisky, fun puppy, and he certainly wasn’t an old dog. We considered the possibility that he ate something bad, but his vet had all but ruled out a GI problem. We went over and over what was different, the only thing we had changed was their heartworm medicine.

We googled the shot he had received a few weeks earlier, ProHeart 6. The drug was originally made by a division of Wyeth Pharmaceuticals, but the drug company Pfizer acquired Wyeth and all of its holdings in 2009. Pfizer’s own website lists side effects of the drug as lethargy, loss of appetite, vomiting, and death. (Source: Pfizer) Jack had exhibited exactly all of those symptoms… in that order.

After doing further research, we also discovered that the drug was originally pulled off the market in 2004 for a high rate of adverse reactions to the drug. “According to an informed veterinarian, ‘Proheart caused more deaths in one year than all of the oral heartworm preventives combined did in ten years. When the FDA notified Pfizer that their drug was causing a problem, the manufacturer claimed it was due to the vaccines’ being given at the same time. The FDA looked at the data again and told them the dogs involved had been getting their vaccinations all along and the only difference was the Proheart. That’s when the FDA informed the company of their intent to pull it and the company then voluntarily took it off the market.” (Source: The Senior Dogs Project)

Pfizer was allowed to re-market the drug in 2008 with the following stipulations: Pfizer agreed to add additional warning labels to the drug packaging, and agreed to mandate that pet owners be given a drug fact sheet and be made to sign an “informed consent” document. Pfizer went even further as to mandate web-based training for veterinarians who gave the drug, and issued several “Dear Doctor” letters to all veterinarians regarding the adverse effects of the drug. (Read one of the “Dear Doctor” letters here.)

The part that bothered and alarmed us the most was this line from the drug’s website: “ProHeart 6 dog owners must be advised of the risks of ProHeart 6 and sign an Owner Consent Form prior to the first administration.” (Source: Pfizer)

We were never shown a fact sheet on the drug, never “advised of the risks”, and certainly not given an Owner Consent Form” by our vet to sign. (In hindsight, we feel that we should have asked more questions about the drug, but we trusted our veterinarian.) If the side effects had been presented to us as they have in our research, we would NOT have consented to have Pro Heart 6 administered to our dogs.

We also found some literature suggesting that ProHeart 6 should not be administered at the same time as vaccinations: “Allergic reactions, sometimes serious, have been reported when ProHeart 6 and vaccinations have been given at the same time. Talk to your veterinarian about the risks of administering ProHeart 6 at the same time as vaccinations.” (Source link here.) Our vet administered ProHeart 6 at the same time as Jack rabies vaccine, and distemper/bordetella.

The drug’s information page from Drugs.com states the following: “Owners should be advised of the potential for adverse reactions, including anaphylaxis, and be informed of the clinical signs associated with drug toxicity. Owners should be advised to contact their veterinarian immediately if signs of toxicity are observed. The vast majority of patients with drug related adverse reactions have recovered when the signs are recognized and veterinary care, if appropriate, is initiated.” (Source: Drugs.com)

When we did recognize that something was going on with our dog, we did what we thought good pet owners are supposed to do. We took him back to the vet. The vet should have recognized that he was having an adverse reaction to the ProHeart6 that she administered to him  (without our “informed” consent) only weeks earlier and treated him appropriately. If the vetrinarian had recognized his symptoms, he could have recovered. She did not, and as a result, our otherwise healthy dog died. Now, we are left waiting on the drug to work its way out of our remaining dog’s system praying that she does not fall victim to the same fate.

Stories of dogs that died as a result of being administered this drug are abundant. Check out any of the following links for more information, and please, ask for all of the facts, side effects, and more information when your vet (or doctor) recommends this drug (or any drug). ProHeart 6 provides no additional benefits or protection than safer, oral heartworm preventatives.

Additional Resources:

Letters & Data by the Drug Manufacturer from the FDA’s Website:

July 22, 2002 “Dear Doctor” Letter warning of additional “adverse reactions”

June 19, 2003 “Dear Doctor” Letter advising of label changes due to “adverse reactions”

“Risk Minimization Action Plan” for the Re-Introduction of ProHeart 6 to the Market (63 Pages which includes the newest precautions, and the actual drug label, “Client Information Sheet”, and “Owner Consent Form”—none of which we were ever given.)

May 30 UPDATE: I truly appreciate all of your condolences and support, and all of you who have shared this blog on your social media sites. It makes me feel much better knowing that we are raising consumer consciousness.

We aren’t trying to tell you what to do with your pets, just encouraging you to gather information before making any decisions.

A vet that works for Pfizer, the drug manufacturer, contacted us today to ask us if we would let them do an autopsy necropsy on Jack. (They had already called the crematorium to make sure his body had not been cremated yet.) We agreed. In fact, we felt relieved and feel that we will get some answers soon. His body is now on the way to the University of Florida to be examined by experts.

Our other dog does not seem to be exhibiting the same symptoms (so far thank goodness).

May 31 UPDATE: Pfizer called back today to let us know they were planning on reimbursing us for the money we paid to have Jack cremated.

His body is being sent to a different lab in Florida and it will be 4-6 weeks before we get a full autopsy necropsy report. They are doing tissue samples, toxicology… The whole nine yards.

Pfizer seems as interested in getting answers as we do. Which is more than we can say for our (former) vet.

The vet has not responded to our requests for answers. (I.e. Did the vet who administered the shot complete Pfizer’s mandatory training? Why were we not given the “Owner Consent Form” and drug fact sheet? Why did the vet not recognize Jack’s symptoms when we took him back in?)

I have received lots of emails and messages from others who have lost their pets. My heart goes out to you. Some have asked me to publicly name the vet. I’m not going to do that (yet). I may, or may not, do that when we receive the autopsy results. At that time, we will know who is truly culpable. In the meantime, just be sure to ask your vet (or doctor for that matter) for lots of information before consenting to anything.


Thank you to Ashley for sharing your story.  We hope that by reaching as many dog lovers as possible, additional tragedies can be avoided.

17 Responses to “Heartworm Vaccination Caution: ProHeart6”

  1. Cathy

    I too lost a beautiful lab to this shot (2002) and it may have been the cause of severe neurological damage to another of our labs who became a behavioral time bomb shortly after receiving it. I am horrified that this is still on the market as I thought it had been removed for good :( I am so very sorry for your loss, I truly do know your pain. It is something that I will never truly get over.


    • Lisa Sands

      4/26/18 this is still happening. Yesterday we lost our little yorkie to what I know believe to be pro heart. After reading for two days all the same symptoms, this has to stop. We were never given the warning either.

      I just started researching, I am sorry for everyone loss as for our own.
      If anyone has any place to direct us please let me know.

  2. Leon Beverly

    My dog had two pro heart shots. She now has congestive heart disease. I’m having to pay for meds to try and flush the drugs out of her system. I’m thinking seriously about a class action suit. My dog had all symptoms after both shots. We took her back to the vet and she said nothing about the adverse affects. We did sign an intent. And we did get the warning sheet. I voice my concerns to the vet and she assured me it was safe.

  3. Barbara Rogers

    Friends, the dogs that are not able to take Heartguard are dogs that have the MDR-1 gene. If the dog is NEGATIVE, then they could take Heartguard no problem. And MDR-1 is easy to test for! It is a test kit where you take a cheek swab and mail it in. Please go to:
    to see for yourselves (there may be others, this is the first one I pulled off the internet, as I can’t remember who I used at the time). This is what I did for my older dog, he is NEGATIVE, so he HAS had Heartguard (last month, when I ran out of Interceptor and can’t get it yet, and therefore saved it for my pup who has not yet been tested). Please, please, it is ANY collie-type dog that can have MDR-1 that you should test for.

    I am so sorry for those of you who have lost your beloved dogs due to bad info from vets. There are other new products for fleas/ticks that also are very problematic and dogs have died or gotten very sick on them. DO you research before choosing one!

    But I do want to note, that as far as Pfizer, at least there rabies vaccine from what I know is the ONLY one that I will give. My older Sheltie nearly died from rabies vaccine from one of the other manufacturers, and actually had a rabies vaccine exemption for one year as a result (which is NOT easy to get I assure you). He DID NOT have a bad reaction from the Pfizer product. Also, we NEVER give combo vaccines, never ever ever, and do rabies alone. Frankly, we never do any of the other vaccines after the initial one year of puppy shots, unless there was a bad case of kennel cough going around, I might do that one through the nose ONLY. But still never ever will do a combination of any vaccines – it is ONE at a time ever, puppy or not. YES your vet CAN do that if they care enough! And we titer every year, to test what we can.

  4. joanna

    I have had many, lengthy conversations with my vet about Heartguard and my Sheltie, and I have done some internet research. What I find on the internet is there is a gene in SOME Shelties and other long nose dogs (collies) that causes adverse reactions (death). There is a test to find out if your long nose dog has the gene incompatible with Heartguard.

    My vet told me something different. He said, if you give your long nose dog the proper dosage (by weight) you are fine. The problems came when the ‘farmers’ treated the horses, the labs and the collies with the same dose. The lab was/is fine with the dose for a horse, but the collies died in an hour. We discuss this at length every spring when it’s time for heartworm meds.

  5. Lynnel

    Words cannot describe the heart ache I am feeling for you. I don’t know which was worse, the heart guard or the vet? Our hearts are broken enough when we lose our fur babies from age, but this is inconchnable . I am so, so sorry and pray you other pup get through this ok? I’ve just come to the realization that vets guess more than they know?

  6. Julie LaPoint

    I am so very sorry for your loss, Ashley, especially one that may be unnecessary. Thank you so much for warning the rest of us. Losing a beloved pet suddenly is truly devastating. Again – so very sorry!

    MaggieMae (Julie L.)

  7. Sullivan

    I am sorry for the loss of Jack, I know you are suffering terribly. My prayers and condolences go to you and your family and I will keep continued good thoughts for the safety of the rest of your furbabies.

    I sent my beautiful Sheltie Bambi over the Rainbow Bridge 9/28/09. I guess I am going to have to show my canine ignorance here, what is the danger in HeartGuard that is particular to the Sheltie breed? Bambi had been on HeartGuard her whole 11 1/2 years and I don’t think there were any complications from that, so could someone please enlighten me so I am better informed if I decide to become a fur-mommy again. Thank you.

    • karen

      Collies, shelties ( and other herding breeds) can carry a gene (Mdr1- i believe) that gives them sensitivities to ivermectin which is found in most (if not all) heartworm preventitives other than Interceptor, which unfortunately is not being produced for the smaller weight dogs, as the Novartis plant was closed down due to a mix up with a human drug. My first two shelties were on Heartguard and never had a reaction to the Ivermectin ( I also at that point knew nothing about the sensitivities) when I got my third sheltie, a neighbor friend of mine had a collie pup – six months old that had been spayed by her vet and was also given the Proheart 6 shot (many years ago before it was taken off the market) and her puppy died that night. It was determined that the death was due to a reaction to the Proheart 6, so from that point on I switched my dogs over to Interceptor, as I did not want to take a chance. I know there is a test for the gene available that will show if your dog carries the gene. Since Interceptor is not on the market I had to switch to a heartworm preventitive that has Ivermectin in it. I knew my one sheltie would be ok as he did not react to the Heartgurard when he was on it, and the Heartworm preventitive that I got through my vet yesterday (which is a VCA heartworm pill) said that the amount of ivermectin was low, and it said on the insert that it was safe for collie type dogs. It is called Vethical, and I gave it to my dogs yesterday evening for the first time. I am still watching them closely, but so far no reaction.

  8. Liz Ramsey

    I am so sorry for your loss of Jack. Thank you for posting this alert. My vet informed me of the new heartworm med in May but I declined it as the tablets we have been using have caused no problem and if it ain’t broke don’t fix it. I have forwarded your entire alert to all my dog owning email friend and to my vet to alert to the danger of this med. I hope others will forward your alert in memory of your beloved Jack.

    Liz Ramsey, Mommy to my Shelties Beetle and Jack

  9. Ann Marie

    I am so glad I saw this. My sable Jack has his yearly appointment with the vet on Thursday (tomorrow.) I am not going to bring it up, but wait to see if they suggest this to me. Thank you so much for sharing this. If my vet, whom I trust (but your story makes me wonder why) suggested it, I would have probably gone along with it. Now, I would never agree.

  10. Dianne

    My self and my family all wish you the deepest condolences on the loss of you beloved and dear friend Jack..so awfull and neeedlessly sad.
    Such a tragic ending for this poor otherwise healthy young dog…Our heart goes out to you and your
    Also do not give Heart Guard.I informed my Vet of this,as he didn,t know… LOL

  11. Rachel

    I am so sorry for the loss of your beloved Jack. Thank you for sharing your story & for this information.

  12. Bobbie

    What a tragic, tragic story. That vet should be disbarred from EVER treating animals again! This information on this medication is very important to know. Also keep in mind that there are also certain heartworm meds shelties should NOT be given–actually, any member of the collie family should not receive certain meds, particularly HeartGuard. Do NOT let your vet give your sheltie HeartGuard!

  13. Laura

    No words can ease your pain at this senseless and preventable loss. I can only imagine how horrible this has been for you and your family. I have lost loved ones just because of “Mother Nature”. As difficult as that is, to lose Jack because of “neglect”?…..”malpractice”?……is reprehensible!….Why was this drug ever allowed back on the market?…,,Thank you for getting the word out there….My thoughts will be with your little girl that she will be OK….

    My heart goes out to you and your family.

  14. Susan L.

    I am so sorry that Jack was killed by this shot and the inadequate vet response afterwards. I deeply appreciate you letting us know about this danger.

    Your veterinarian is so unresponsive because she knows she is looking a losing malpractice suit in the eyes. She might be able to lie and say she warned you of the dangers but that signed informed consent form will be tough to fabricate. I don’t know if you are planning to sue or not but she thinks you are.

    Thanks again for generously using your loss to help prevent us from having the same awful experience.


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