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Pet Bucket Blog

The Side Effects of Heartgard Plus Worming Treatment

 by simone on 13 May 2014 |
19 Comment(s)

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The old adage that prevention is better than a cure is never more true, or more important, than with your pet and worm treatments. Roundworm, hookworm and heartworm are easily transferred via mosquitoes or faeces and therefore commonly affect our pets. If left untreated, worms can spread and grow, creating serious complications to your pet’s intestinal and respiratory systems. A worm infestation can lead to severe digestive problems, heart failure and even death for your pet. 

Worms can also be transferred to humans so it is essential that you have an effective worming care plan. Protection against infection needs to be carried out regularly. Luckily Heartgard Plus is a monthly, easy to administer, very safe and proven treatment for worm control. It prevents worms developing to the dangerous adult stage and is available for both dogs for cats. 

It’s highly likely that every puppy or dog has been exposed to worms. Puppies need to be wormed every two weeks until 12 weeks of age and lactating females should also be treated. Dogs over 12 weeks old require frequent treatment for the rest of their lives. 

Heartgard Plus is available in different dosages for animals of different weights. It comes as a chewable and deliciously tasty tablet (or so I’ve observed). The dosage is designed to be chewed, rather than swallowed whole so you can always break it up and add it to your pet’s food if needed. You’ll need to make sure your pet consumes the entire dose, so observe them during administration and for a short time afterwards.

It is very important that you talk to your vet before beginning any treatment for heartworm. Not just to verify the correct dosage, but there can be severe reactions, similar to toxic shock, in animals who already have heartworm infections and are then given heartworm medication. Your dog will need to be tested, and if infected, adult heartworms and larvae (microfilariae) treated before beginning any course of worm control.

If you’re currently enjoying a nice meal, it’s best you put your meal aside for now as we take a closer look at these ghastly, villainous little worms.

Roundworms (ascarids) are the most common worms affecting dogs and cats. 
Virtually every puppy is infected with roundworms at birth or become infected via their mother’s milk. 

Adult worms are found in the intestine and can grow 2-3mm wide and up to 20cm long. Roundworms cause diarrhoea and vomiting, weight loss, poor growth, a pot-bellied appearance and lethargy. If left untreated or in severe cases, pneumonia and obstruction of the intestine or digestive system can occur.

In infected animals, the adult worms lay eggs that are then expelled in faeces. Other animals become infected by ingesting contaminated soil or eating other infected animals such as mice, rodents or birds. The eggs then hatch in the animal’s stomach, feeding on content found here and develop into egg-laying adults. Roundworm larvae and adults can also be found in other organs such as the liver and lungs but generally remain in the digestive system. 

Hookworms are another intestinal parasite that most commonly affect young dogs and cats. The worms attach to the host animal's intestinal lining and feed on blood, tissue and fluid. As with roundworm, larvae and adult egg-laying worms live in the intestine. Some larvae attach to other organs and can be particularly problematic for your pet if they migrate through the respiratory system. 

Hookworm eggs are expelled in the animal’s faeces. Dogs and cats can be infected by ingesting contaminated soil or from larvae penetrating the skin. Puppies can also be infected through their mother’s milk.


Bloody diarrhoea is a common symptom of hookworm. Other signs of infection are intestinal upset, pale gums, weight loss, stunted growth and poor coat condition. Serious loss of blood and anaemia can result from large numbers of worms being present or in puppies. In extreme cases this can lead to death.

Heartworm is the deadliest of all parasites and although infection is more common in dogs, cats are also susceptible. The worms inhabit the animal's lungs, heart and pulmonary arteries and can cause heart failure or death. 
Puppies should begin heartworm prevention by at least 12 weeks of age. 

Primarily it is actually the lungs that are affected by heartworm but the heart, circulatory system and even liver and kidneys may also be damaged. The symptoms indicating heartworm infection include coughing, shortness of breath, respiratory sounds, fainting, lethargy, weight loss, blood present in the saliva or mucus, abdominal swelling, vomiting and loss of appetite.

Heartworms are transmitted by mosquitoes and when an infected mosquito bites another animal, the heatworm larvae (microfilariae) are transferred. These larvae migrate through tissue and the host animal's circulatory system to the lungs, heart and pulmonary arteries, feed on blood and grow to maturity, some worms can reach 30cm in length and 2cm in width. Heartworm larvae can survive in the bloodstream of a host animal for two years. 

Heartworms will lead to the inflammation of tissue, blood clotting and the thickening of blood vessel walls. The animal’s blood pressure rises and cardiac strain is increased. 

There are very serious side effects if a dog is treated with prevention medicine and is already infected with heartworm.

Side Effects
As with any medication, there is the potential for unpleasant side effects and the possibility of individual sensitivities for some pets. However, tests show that Heartgard Plus is safe and well tolerated when used correctly, having a very low incidence of digestive and neurological side effects. 

Heartgard Plus uses ivermectin which causes paralysis and death to the parasitic worms. The dosage of ivermectin contained in Heartgard is relatively low, with little risk of side effects. As mentioned, it is vital that you do not start a worming program prior to having your pet checked for the presence of heartworm. 

Check the dosage required for your pet as serious side effects often result from a larger dose being administered than recommended. If the dosage given is correct and you notice any of the side effects listed below take your pet to the vet immediately.

  • Digestive Problems. The most common side effect associated with Heartgard Plus is digestive problems such as nausea, vomiting and diarrhoea. In clinical field trials, vomiting or diarrhoea within 24 hours of dosing was observed in only 1.1% of cases.
  • Hypersalivation
  • Depression and Lethargy. Depression, lethargy, lack of appetite and loss of interest in activities has been noticed in some pets.
  • Lack of Coordination (Ataxia), Disorientation and Hind Leg Paralysis
  • Stumbling, running into walls and furniture or paralysis could occur.
  • Dilated Pupils (Mydriasis) or Blindness
  • Crying, Agitation
  • Low Body Temperature and Hypothermia
  • Seizures and Muscle Tremors. Seizures, tremors and convulsions are very rare and may mean your pet has a particular sensitivity, the dosage is too high or that heartworm was already present. 

Susceptible Breeds
Some dog breeds have a mutation in the multi-drug resistance gene (MDR1) making them more sensitive to ivermectin at very elevated dose levels (16 times more than the recommended level). In trials, no adverse reactions were observed in dogs at ten times the Heartgard recommended dose.

This gene mutation is known to occur most commonly in breeds such as Collies, Australian Shepherds, Shelties and Long-haired Whippets. The level of ivermectin used for heartworm prevention is generally safe and at a low level suitable for these dogs. You can talk to your vet about testing your dog for the gene mutation.

When dosage and treatment instructions are followed, Heartgard Plus is a safe and effective worming care plan. It is approved for animals as young as 6 weeks and pregnant or lactating cats and dogs. Administering the medication is as simple as giving your pet a chewable treat each month. The packaging also has a calendar so you will never lose track of where you are up to. Peace of mind has never been so easy. 


Melody - Comment
Melody20 Jan 2018Reply
Our dog was positive on his heartworm test but they gave him prednisone an muscle relaxers plus heartguard....we thought he was dying lastnight because all he did was lay there eyes were dialated and he would cough and had this mucous and foam type drooling....we thought he would die during night for sure but today he is getting up some an nodrool.....happening.....he reallying scared us???????????????
Katy - Comment
Katy20 Jan 2018Reply
I am not an expert but I am a breeder. I have lost a breeding dog with HeartGard before, and my other dogs have had serious side effects, vomiting every day, and constantly being tired and depressed. It took about a week for them to heal. I did some research and it is because they're pesticides that some dogs can't handle. Most dogs can handle it with only minor side effects, but others have very serious side effects, and, in some cases, death. I have even had some clients who told me they lost their dog to this medecine. (HeartGard) The best solution is to avoid HeartGard and other pesticide medicines. Always go for the organic and natural ones!
Hope your dog(s) feels better soon!
Amanda Dalton  - Comment
Amanda Dalton 20 Jan 2018Reply
Thank you, I'm currently in the vet with a very sick baby 2 days after heartgaurd they keep telling me it couldn't be the heartgaurd. But she is having bloody stool so bad she needs IV and throwing up
Sheree' Welch - Comment
Sheree' Welch03 Apr 2018Reply
My dog Molly was treated for heartworms in July last year & was heartworm free at her 6 month check up. As suggested, I've continued to give her heartguard monthly. However, for the past 2 minths, I've noticed her coughing the day after it's given & the cough lasts a day or two. Suggestions?
Steve - Comment
Steve03 Apr 2018Reply
My dog has the same issues. Everytime we give him heartgard plus he coughs for the next several days.
Shirley - Comment
Shirley21 Apr 2018Reply
Might gave my 8 year old dog heartguard plus this morning. Tonight her back legs are not working. Is there anything I can do for her right now and will this pass
Marcy - Comment
Marcy21 Apr 2018Reply
Did your dog get better? I gave two of my dogs ther Heartguard Plus last night. One is perfectly fine the other is having trouble with her back legs. She doesn’t seem to be in any pain and she will get up to go to go outside however she struggles with getting her back legs to walk.
Natalie Wood - Comment
Natalie Wood13 Sep 2018Reply
My dogs hind legs are having problems too. How are yalls dogs now?
Jody - Comment
Jody13 Sep 2018Reply
Did your dogs leg issues get better? I gave all 4 of mine Heartguard Sunday and Tuesday one of my dogs started having back leg pain. She still seems like it’s bothering her.
Jody Creighton  - Comment
Jody Creighton 28 Oct 2018Reply
I gave my Lab/Basset her Heartguard dose Sunday and since the following Tuesday her back left leg has hurt her. I’ve been researching hip dysphasia symptoms and joint pain in senior dogs but thought I’d check to see if MAYBE any other dogs had leg pain after their heart worm meds and found this thread. Did your dogs leg issues clear up?
Troy - Comment
Troy06 Feb 2019Reply
I have a 2 year old shepherd we adopted. They agency said she was Heartworm free. We did a Heartworm test after we had her and we had been giving heart heart guard( I have a lab too which has been on Heartgard no issue) she was diagnosed last year with micro flare and we started the slow kill treatment with heart guard plus. Last year she had a spell we’re she just payed down and like she passed out. We rushed her to dr and they gave her a Benadryl and she was fine in about and hour. This was during the slow kill treatment. We had her tested last year after the slow kill and test showed Heartworm free. She had another spell 3 weeks ago and it was worse. I picked her up and carried her to Dr and she just payed there with trashy clinches. He ran blood work ( liver enzymes a little escalated) and exrays and everything was normal. He felt she had a seizure. He told us to bring her back (today) and he tested her again and liver was fine. He did test for Heartworm again and now she is back micro flare positive, not sure how we should proceed?
Sharon - Comment
Sharon01 Apr 2019Reply
My mixed breed adopted pup, 16 months old, had an issue back in the early winter with her hind legs to the point she ended up on steroids. During the rest of the winter I was a slacker when it came to giving the Heartgard Plus and her leg pain had resolved. I restarted the med on Saturday and Saturday evening she was limping so badly I had to help her into the bed. (She acted like her knee was painful). By Sunday morning she was barely limping, but even today the limp is still noticeable. I'm starting to wonder if there is a correlation.
Sharon  - Comment
Sharon 01 Apr 2019Reply
I gave My mixed breed adopted pup, 16 months old, had an issue back in the early winter with her hind legs to the point she ended up on steroids. During the rest of the winter I was a slacker when it came to giving the Heartgard Plus and her leg pain had resolved. I restarted the med on Saturday and Saturday evening she was limping so badly I had to help her into the bed. (She acted like her knee was painful). By Sunday morning she was barely limping, but even today the limp is still noticeable. I'm starting to wonder if there is a correlation.
Carla - Comment
Carla13 Apr 2020Reply
My German Shepherd has the Heartgard injection on Thursday, by Saturday night he could use his back legs. Still unable to stand this morning (Sun). Will it get better? How long dies it take?
Nicole - Comment
Nicole11 Sep 2020Reply
I gave my dog heart guard..I thought it was something else then again and same thing shaking leg pain ..I think it's the heart guard cause see the long does it last
Arlene - Comment
Arlene03 Jan 2021Reply
I gave my 8 week old puppy 1st dose of heartguard and had seizures the same day. I saw that convulsions can be a side effect. Brought her to the vet and liver enzymes were ok but still having seizure 1-2 x a day. Gave the medicine Dec 15. Vet said it’s just a coincidence because the med is safe for puppies. I’m worried. Help!
Lyss - Comment
Lyss03 Jan 2021Reply
Do 8 week old puppies need heartgard? I thought they get dewormed at a very early age.
Angela Cote - Comment
Angela Cote28 Jan 2021Reply
I’ve given my dog Heartgard for over 4 years without issue but this month she has an episode where she lost control over her limbs and she was scared! She couldn’t walk and was trembling! A couple hours later she was totally fine. I can’t help but wonder if it was the Heartgard?
Pat Wester - Comment
Pat Wester18 Feb 2021Reply
My miniature schnauzer started having trembling in his back legs about a year ago. In June, 2020 he had an episode of stumbling and loosing control of his hind legs. I've had him to the vet and we've gone through a ton of tests including a consult with a neurologist and nothing. His last episode was in Nov. I have always taken him off Heartgard and Frontline during the cold months. It's been 14 weeks since he had an episode of problems in his back legs. I started researching the effects of Heartgard and Frontline and have become concerned that these drugs are the cause of his problems. He's 12 1/2 yrs. old. I'm not going to put him back on either of those drugs.
Richard Coughlin  - Comment
Richard Coughlin 06 Jun 2021Reply
Gave my lab heart guard plus ever since it’s had digest promblems
Terry - Comment
Terry06 Jun 2021Reply
Did your dog get better with appetite and how long did it take?
Kristi Burton  - Comment
Kristi Burton 29 Jun 2021Reply
3 outta my 4 dogs that have been giving heartgard had episodes of vomiting and diarrhea 2 to 3 days after each dosage. Living in Louisiana it is hard not to give preventative even though my dogs are inside dogs. But it has been 5 days since I gave another dose and now my french bulldog is having leg tremors and is very lethargic. I usually have healthy, happy dogs unless it is the mosquito time of the year. Everyone might as well spray the yard with pesticides instead of giving it as a treat, What is the difference? Heart worms will kill a dog but we need a better preventative solution. I DONT WANT MY DOG TO SUFFER from a medication or a parasite. I know this is why my dogs are acting this way because there is no other explanation. I gave my 2 boys imodium for 2 days and the diarrhea finally stopped, but had to use the adult pill the children's imodium did nothing. As for my girl, she is resting because I gave her a pain pill. Only thing I can do is wait for this stuff get out of their system and don't give it again. Off to the vet in the morning. Good luck everyone.
Mon - Comment
Mon29 Jun 2021Reply
Our coonhound is allergic to one of the ingredients in heartguard and had terrible diarrhea after each dose. We've switched to interceptor and no more issues. Doesn't taste as good, but a little peanut butter does the trick.  - Comment
Same exact thing is happening with my pomeranian. Such a happy dog until I gave him heartgard. I'm suing. Contact me. (Email removed. We can not publish email addresses.)
Kristi Burton  - Comment
Kristi Burton 08 Jul 2021Reply
thank you, we will try that.
Al Bendorf - Comment
Al Bendorf30 Jul 2021Reply
Gave my chiweenie Heartgard, had bruising, platelet count went down to 8K. After discontinuing, platelets back up to 200K. Dangerous medication!!
Barb - Comment
Barb02 Jul 2022Reply
My lab is on heart guard plus year round. Does she need to be dewormed also
sinan - Comment
sinan02 Jul 2022Reply
Yes, you need a dewormer like Drontal for intestinal worms.
Robin - Comment
Robin12 Sep 2022Reply
My golden retriever is so tried after taking heartgard plus. How long will this last?
He is 8

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