Is Heartgard safe for dogs? What would cause a dog to throw up blood?

Original Question: I have a question regarding heartworm treatment. I am a foster and one of my current pets is being treated for heartworm with the slow kill version of Heartgard. He threw up a lot of blood last night, fresh bright red and it coagulated a bit, and I was wondering what could be causing this? He was taken to the vet as soon as we noticed it in the morning and we are waiting to see what they say. Being a foster and not the owner, they do not really explain much to me except for what to do as far as treatments. I understand that it could be linked to an allergic reaction to the Wolbachia bacteria inside of the heartworms, but I am a veterinary student and I'm wondering what exactly is going on inside the body. I haven't been able to find anything online that goes into depth about heartworm side effects so I was wondering if your team could possibly help. Thanks! - Elizabeth

Mar 1, 2018

Hi Elizabeth,

Thanks for this question. I’m sorry to hear you’re dealing with this.

I’m going to answer your questions in two parts.

Is Heartgard safe for dogs?

You’re right that administering Heartgard for dogs, a product with the active ingredient Ivermectin, can cause some negative reactions. First, it works to paralyze and eventually kill the heartworm parasites in the body. In doing so, it can cause allergic reactions but this is uncommon and would likely demonstrate other signs than just vomiting. Its most common drug side effect is causing a neurotoxicity which would be seen as ataxia (staggering), tremors, seizures and blindness, but again, these are rare. The exception is Collie-type breeds which are highly susceptible to these side effects and for that reason, any dog even slightly resembling a Collie-type breed should not be given this drug ever and it is contraindicated. There is a phrase that vets use for this that goes, “if the dog has white feet, don’t treat” since Collie-type breeds often have white paws (however this wouldn’t apply to Maltese, Shih Tzu, and Terrier breeds).

Heartgard is also known to cause vomiting but again, this is rare as well. Even at higher than indicated doses, vomiting is possible, but not common.

What would cause a dog to throw up blood?

The fact that your dog is throwing up blood means that there must be some degree of ulceration to the stomach or upper GI lining to cause the bleeding. So next thing to consider is if there were any other drugs being administered that could have caused the vomiting. Now some vets may prescribe an NSAID (or non-steroidal anti-inflammatory) when treating for heartworm disease by using Aspirin, Deramaxx, Rimadyl, or Metacam. Treating heartworm disease with a drug known as Melarsomine can cause muscle inflammation and the vet may have administered it in this case. If so, I would recommend any drug like this be halted and it’s unlikely that it would have any benefit in the first place unless there is another condition that I’m unaware of.

If I was treating your dog, here is what I would suggest. I would have blood work done and perform a urine test to determine if there are any negative reactions, toxic reactions or concurrent conditions. I’d consider radiographs to see if the dog swallowed anything that could cause the vomiting with blood. I’d then administer anti-vomiting medication and a drug called Sulcate, or something similar that is a liquid-like drug that coats the lining of the stomach to cover any bleeding ulcers and calm them. We would treat any dehydration with IV fluids.

I hope this has been helpful. There is an awesome website about heartworm disease, called the American Heartworm Society. It is comprehensive and you will find any answer there that you need. You can click on ‘Pet Owner Resources’ and click ‘Heartworm Basics. ’The FAQ list at the bottom of the page is great at answering many questions moving forward with this little guy. I also recommend you check out our heartworm videos series that goes through general information about heartworm in cats and dogs, heartworm prevention, and testing for heartworm that will give you more information on this topic.