Ivermectin Poisoning from Parasite Prevention Drugs in Dogs

By isak, December 28, 2009

This toxic reaction occurs especially in dogs that are genetically hypersensitive to ivermectin, an anti-parasite medication most commonly used for heartworm prevention, or to treat ear and hair mites, which can lead to mange. Ivermectin prevents or kills parasites by causing neurological damage to the parasite, resulting in paralysis and death for the parasite. But dogs genetically sensitive to the medication have an anomaly that allows the ivermectin to pass the dog’s blood-brain barrier and into its central nervous system, which can be lethal for the animal.
About Ivermectin »

While the sensitivity to this type of medication is not always guaranteed, the following breeds are most likely to be affected (although not every “sensitive” animal in the breed is affected):

  • Old English Sheepdog
  • English Sheepdog
  • Shetland Sheepdog (Sheltie)
  • Australian Shepherd
  • German Shepherd
  • Long-haired Whippet
  • Silken Windhound
  • Skye Terrier
  • Collie

Washington State University Veterinary School developed a DNA test for a mutant gene which produces adverse reaction to ivermectin and other medications in some dogs. They have discovered that 75% of Collies, 25% of Border Collies, 8% of Old English Sheep dogs, mixed dogs of these breeds, plus other breeds have this mutant gene.

It is also seen in mixed-breed dogs, older dogs that have experienced a blow to the head, puppies, dogs that have overdosed on similar types of drugs, or other central nervous system diseases that affects the blood-brain barrier. Treating dogs that are susceptible to ivermectin toxicity with parasitic medication should be only be done under a veterinarian’s supervision and with great caution.

For breeds predisposed to the disease, an autosomal recessive trait (MDR-1) gene has been identified that causes a defect in the p-glycoprotein multidrug transporter to the blood brain barrier. This allows the ivermectin to pass into the brain at low dosages thus causing toxicity. This trait may also cause toxicity from other related drugs and should also be avoided or used in lower doses in susceptible breeds.

Toxicity can occur in cats, although it is uncommon.

Symptoms for the dog may be acute or mild. Acute signs will become apparent within 4 to 12 hours of the drug’s administration. In mild cases, symptoms will occur between 48 to 96 hours after your dog has been treated. Such symptoms include:

  • Lethargy
  • Depression
  • Drooling
  • Vomiting
  • Dilation of the pupil
  • Loss of appetite (anorexia)
  • Difficulty controlling voluntary movement
  • Disorientation
  • Tremors/Seizures
  • Inability to stand
  • Blindness
  • Slow heartbeat
  • Respiratory distress
  • Coma

Unfortunately, ivermectin toxicity cannot be reversed. Therefore, it is best to make your pet comfortable and treat the symptoms to the best of your ability. If exposure has occurred within the past four to six hours, induce vomiting and/or administer activated charcoal for the purposes of minimizing absorption. Be on the lookout for signs of secondary complications.

Some or all of the following measures may also be recommended by your veterinarian:

  • Intravenous fluid therapy
  • Keeping electrolytes in balance
  • Intravenous nutritional support
  • Turn the dog over frequently
  • Appropriate bedding
  • Physical therapy
  • Ocular lubricants
  • Ventilator in case of respiratory distress
  • Heat support if body temperature is low
  • Fans if body temperature is high
  • If your dog can not stand up, urinary catheters may be needed
  • Medication for seizures if appropriate

Much will depend on the severity of the dog’s reaction, along with its initial overall health. It may take several weeks of dedicated care before the dog fully recovers.

There is a test available to check sensitivity to ivermectin. If your dog is one of the breeds that is prone to ivermectin toxicity, you might consider testing for it. If you decide not to have the testing done, be cautious about using ivermectin to prevent heartworm disease or for the treatment of mites.

In dogs with MDR 1 gene mutation, the following drugs should be avoided or used with caution:

  • Ivermectin
  • Selamectin
  • Milbemycin
  • Moxidectin
  • Loperamide
  • Acepromazine
  • Butorphanol
  • Vincristine
  • Vinblastine
  • Doxorubicin

I include all this scary stuff so you know what can go wrong when using ivermectin on your own to treat your pets. It’s the same ingredient used in the popular heartworm meds you can buy for cats and dogs, but I know someone whose dog died when she administered Zimecterin Gold, a horse wormer, to her dogs. I don’t know the particulars in her case — what breed her dog was, how much she dosed, etc. I offer this info so you know what can go wrong and are as informed as possible before going this route. I assume no responsibility. Zimecterin Gold contains ivermectin and praziquantel. The praziquantel kills tapeworms. I have used this product for several years on my cats and dogs with no ill effects (knock on wood). My dogs are mutts. But be aware of the hazards if you have any of the breeds mentioned above. Having several cats and dogs, this is a more affordable alternative for me. There are several generic versions of heartworm preventative on the market. I used to order them from PetShed out of Australia. That worked fine, too. However, I have a number of cats and dogs and even the generic version became quite expensive.

Animals at highest risk are very young dogs, multi-drug sensitive breeds, dogs that may be exposed to ivermectin-containing products for large animals and/or the feces of large animals (horses, cows, pigs) treated with ivermectin.

Zemecterin Gold comes in an oversized syringe with a dosage appropriate for horses. The plunger on the syringe is marked at 50 lb intervals. This is how I determine the appropriate dosage. One syringe treats approximately 1300 pounds and costs about $15.

You can also buy the cattle version of just plain 1% ivomec (without the praziquantel), but it requires a larger dosage ivermectin (1/10 of 1 cc per 10 pounds) and it tastes really nasty. So you need to mix it in something like orange juice or yogurt.

Note: Cats can experience ivermectin toxicity. A small percentage of cats (less than dogs) have a sensitivity  to it and will react adversely to Ivermectin. It’s very easy to overdose them. Safe dewormers for cats are not that much more expensive than ivermectin. Praziquantel and pyrantel pamoate are 2 that can be used with less risk and the most common ones you’ll find sold for cats.


  • Parasitic diseases are common in animals. Parasites can affect the skin, ears, stomach and intestines, and the internal organs including the heart, lungs and liver. Several drugs have been developed to kill or prevent parasites such as fleas, ticks, mites and worms. Ivermectin and related drugs are among the most effective of these.
  • Ivermectin is a parasite control drug. Ivermectin causes neurologic damage to the parasite, resulting in paralysis and death.
  • Ivermectin has been used to prevent parasite infections, as with heartworm prevention, and to treat infections, as with ear mites.

Ivermectin as a Heartworm Preventive Medication in Dogs
Ivermectin is approved by the United States Food and Drug Administration (FDA) to prevent heartworm infections in dogs. Products containing Ivermectin are normally administered monthly for heartworm prevention and include medications such as Heartgard®, Iverhart®, Tri-Heart® and many other generic ivermectin-based heartworm preventive medications.

When used as a heartworm preventive medicine, ivermectin is used at much lower dosages than when used for other purposes. When used at the lower heartworm prevention dosages, ivermectin has a much lower potential for side effects than when used at higher dosages and this is the reason that ivermectin is approved for use as a heartworm preventive medication but must be used off-label (in a fashion not approved by the FDA) for many other purposes.

Read more at Suite101: Ivermectin for Dogs: Usages, Safety and Side Effects of Ivermectin in Dogs

Brand Names and Other Names

  • This drug is registered for use in animals only.
  • Human formulations: None
  • Veterinary formulations: Ivomec® (Merial), Zimectrin® (Farnam), Eqvalan® (Merial), Heartgard® (Merial), Iverhart® (Virbac) and various generic preparations

Uses of Ivermectin

  • Ivermectin is used to control skin parasites, gastrointestinal parasites and parasites within the bloodstream.
  • Ivermectin prevents development of heartworm disease in dogs and cats.
  • Ivermectin can be used in an extra-label manner to kill microfilaria (microscopic offspring) in heartworm infected dogs.
  • Ivermectin is not effective against tapeworms and liver flukes.

How Ivermectin Is Supplied

  • Ivermectin is available in 10 mg/ml and 2.7 mg/ml injectable form; 0.153 percent and 1.87 percent paste form; 10 mg/ml liquid oral form and 68 mcg, 136 mcg and 272 mcg tablets.
  • Heartgard Plus® and Iverhart® are available in various concentrations of ivermectin combined with pyrantel pamoate.

Dosing Information

  • Medication should never be administered without first consulting your veterinarian. The dose for ivermectin varies from species to species and also depends on the intent of treatment. General dosing guidelines follow.
  • For dogs: Dose is 0.0015 to 0.003 mg per pound (0.003 to 0.006 mg/kg) once a month for heartworm prevention; 0.15 mg per pound (0.3 mg/kg) once, then repeat in 14 days for skin parasites; and 0.1 mg per pound (0.2 mg/kg) once for gastrointestinal parasites.
  • For cats: Dose is 0.012 mg per pound (0.024 mg/kg) once monthly for heartworm prevention.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse or prevent the development of resistance.
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  1. Paramita Purakayastha says:

    I administered half a tablet of ivermectin to my 4 year old male cat weighing 3-4 kilos a little over 12 hours ago. Has dilated pupils, cold body temperature and is very dizzy. The vet suggested to feed icecream and liver tonic over the phone. No vet available before tomorrow morning. What should I do? Please help.

    • isak says:

      Certainly sounds like ivermectin toxicity. Ivermectin can affect the central nervous system. So, when an animal gets ivermectin toxicity they can have tremors, seizures and other neurological symptoms. The condition can be fatal.

      You are very limited in what you can do at home. If an animal is going to recover then it can sometimes take several days for this to happen.

      Here are some things that you can do to help, but ultimately if there is a way to get him to a veterinary clinic this would be his best chance at survival.

      Keep him warm. If you have a thermometer at home, the best way to take his temperature is rectally using some vaseline or KY jelly as a lubricant on the thermometer. Or, if you prefer not to do that you can put the thermometer in his armpit although this is not as accurate. Normal temperature for a cat is between 38-39 C or 100-102 F. To keep your cat warm you can wrap him in a warm blanket if he will tolerate this, or fill up a large bottle with warm water, cover it with a blanket and place him next to it. Be careful that it is not too hot against his skin.

      If he is unresponsive, then turning him over every hour or so will help.

      If he is consciousness then you can try getting small amounts of water into him using an oral syringe or turkey baster. It is important for him to stay hydrated. But don’t force water into him if he is unconscious.

      Good luck.

  2. Davon says:

    My 8week old pitbull puppy was given horse dewormer today for the first time which had her sleeping alot and very weak. That brought alarm to me! Im in a tough situation that i cant afford the vet.. Im praying she makes it.

    • isak says:

      Yes, that is too young for horse wormer. Follow the measures above and keep an eye out for secondary complications. If she seems to be worsening, you should see a vet. It could be a lifesaving measure.

      Recovery is slow and treatment is primarily supportive in nature. Supportive care may include keeping the patient’s body temperature in the normal range, preferably with circulating warm water or air blankets since many patients are not alert enough to know when to move away from a heat source. Dogs that are semi-conscious or unconscious should have their bodies rotated every four to eight hours. You should have your vet check him out. He may also need fluids.

      The good news is most dogs respond well to treatment for ivermectin toxicosis, and the prognosis is very good if supportive care is given early and aggressively. Good luck.

  3. Brittany woodall says:

    I have about an eight week old puppy and me and my husband have always use horse wormer on our older dogs and were great with it but the pup isn’t doing to hot about it shes not eating drinking or walking on her over night but this morning she was doing a little better and got a little more strength in her legs and is peeing but not popping and is sleeping a lot what should I do because im greatly concerned

    • isak says:

      Some dogs do not react well to horse wormer and sometimes it’s hard to dose down for puppies. From what you are saying, it sounds like she may be experiencing ivermectin poisoning.

      Dogs most at risk are very young animals with immature blood-brain barriers; multi-drug sensitive breeds including the Border Collie, Australian Shepherd, long-haired Whippet, Silken Windhound, Rough- and Smooth-Coated Collies and associated mixed breeds; dogs that may be exposed to ivermectin-containing products for large animals and/or the feces of large animals (horses, cows, pigs) treated with ivermectin.

      Symptoms of ivermectin toxicosis include loss of coordination, prolonged abnormal pupil dilation, reduced alertness that can progress to stupor or coma, excessive saliva production, vomiting, blindness, tremors and seizures.

      Again IF this is ivermectin poisoning and not something else, recovery is slow and treatment is primarily supportive in nature. Supportive care may include keeping the patient’s body temperature in the normal range, preferably with circulating warm water or air blankets since many patients are not alert enough to know when to move away from a heat source. Dogs that are semi-conscious or unconscious should have their bodies rotated every four to eight hours. You should have your vet check him out. He may also need fluids.

      The good news is most dogs respond well to treatment for ivermectin toxicosis, and the prognosis is very good if supportive care is given early and aggressively. Good luck.

  4. David says:

    I’m on a very tight budget yet have three dogs that are 50 – 80 lbs.. They eat better than I do!! I bought this Zimecterin as an alternative heard worm prevention told to me from a friend, but after reading I don’t think I should. What would you recommend as a Heart Worm Prevention for dogs that would save me some money so I can eat too? 🙂 Buying it from the vet runs around $500/yr.


    • isak says:

      I have never had any problems using Zimecterin Gold for my dogs in over 10 years using it — all mine are mixed breeds, so not necessarily the breeds that are sensitive to ivermectin. But I know a woman whose elderly dog had a reaction and died. There are certain breeds that cannot tolerate it. An affordable alternative is Valuheart Heartworm Tablets. You can find them online. In your case, it would cost you about $20 for 6 tablets for dogs 45-88 lbs.

      I have used these over the years, too. However, they contain the same active ingredient as Heartgard tablets, ivermectin. If you are trying to avoid ivermectin, you might try something like Sentinel or Advantage Multi or Revolution. You could buy the one for the largest dogs and dose it down to what you need. Here’s info on doing that. This will save you some money. I do this for my cats.

  5. Keith says:

    Hi has anyone heard of rotties crosses becoming ill from ivermectin. I have a 4 year old hitch who has been ill. She has been in heat for 4 weeks plus so I took her to the vets as she wasn’t, right. He took bloods xray and scanned her body but can’t find nothing wrong. My dog is bleeding\leaking from her vag and has a dark runny stool she’s not eating and seems down so was wondering if ivermectin from harkers that was used on the chickens next door could bemaking her ill

    • isak says:

      If she eats the chicken poop, she could ingest more than a safe dosage of ivermectin. I’m not sure how your neighbors administer the ivermectin — whether by shots or in their food. But if in their food and your dog is eating their food as well, again she may be getting too much.

      Ivermectin poisoning can look like many things, but a staggered walk, disorientation, inability to stand are symptoms. You haven’t mentioned them, so maybe that is not the problem.

      How long ago did you see your vet? It could be that a couple things are happening that are unrelated — perhaps she has eaten something that isn’t agreeing with her, so she isn’t eating, is lethargic and has a dark runny stool.

      As for the discharge: during a dog’s heat, the cervix is relaxed and bacteria can easily have access and flourish on the thickened uterus walls. If this is the case, it will likely require a course of antibiotics to correct. I recommend you get the vet involved so they can prescribe the correct antibiotic for the particular bacteria she has, if this is what’s happening.

      Good luck.

  6. Sabrina Andrews says:

    I have a Labrador mix with a form of Shepard it sounds like Mandy your dog may have the genetic mutation called MDR-1 you can look up Ivermectin poisoning and you will find what I am talking about once a dog has the toxin hit its brain the dog may never recover! I have been studying this for sometime now to see if I could treat my dogs that are all rescues ..Unfortunately there are many dogs they do not list one being Labradors almost all Shepard’s are listed but I have noticed that Labradors and mixed labs are also effected by Ivermectin poisoning and a multitude of other medications! Flea,tick,and wormer medications actually have a great deal of things as well as heart worm medicines! Please do research as much as possible before using Ivermectin or anything else on your dogs mixed or not! Some vets honestly these days seem to be less and less caring about our wonderful fur babies! Becareful about what vet you take your animals to you get what you pay for! Please research and ask around for the best possible vet!

  7. Jeremy says:

    I recently adopted lab/shepherd mix puppy from a shelter. He has demodective mange. The vet prescribed ivermectin and have administered for 4 days in small does with slight increases each day. Gave him a .69ml dose last night before bed. Woke up to him making odd noises in his kennel and he wouldn’t come out. I pulled him out to go to the bathroom and he couldn’t walk and kept falling. Brought him back in to give him some water and he kept walking in to the wall and collapsing. Appears to be blind and twitching.

    Took him to the vet immediately and they’re saying he is showing no improvement. Why would they give this to a puppy with such a harsh side effect and not tell me?

    Will he recover??

    • isak says:

      It’s pretty much up to your pup how well he will recover and how long it will take. Recovery is a slow process, so don’t give up on him too soon. Hopefully the slowly increasing dosage and the fact that you caught it quickly will be to his advantage. I had a dog that took ivermectin for several months years ago with no problem, but I hear so many stories about what people have been through. I will never risk it again. It’s a bit like Russian Roulette and certainly not worth the consequences. Best to your baby.

  8. Sheila says:

    My horse was highly allergic to zemectrin gold. My vet said that it is not uncommon with this wormer. The horses tongue and mouth became incredibly swollen. There are so many other choices which I now use and alternate with every 3 months.

  9. sahil godara says:

    its too late! :'(
    bt thks fr the replies.

  10. sahil godara says:

    No! this was second time.
    2.5 mg was given by vet1 because he was having scabies.
    after one day he stared vomiting.
    first it was white in colour nd after ine day it was reddish in colour.
    I think it was blood vomiting.
    vet2 said that due to overdose of ivermectin the walls of stomach are infected which makes him to vomit blood as well. something he eats, comes out within a hours or 2.
    but now after injecting for 3 days these :
    metronidazole intravenous infusion Ip
    injecting glucose bottle
    adreonochrome monosemicarbazone 1 mg
    metoclopramide hydrochloride injection 2 mg
    mecabalmun, pyridoxine hcl & nicotinamide injection
    ranitidine injection

    vomiting is stopped for now
    but he still have paralyses symptoms.
    white tongue
    Dilation of the pupil
    Loss of appetite
    Difficulty controlling voluntary
    Inability to stand
    1)now please tell me what should I start giving him so that walls of stomach recovers and what should I give to eat so that walls remains unharmed?
    2) ice cream would help?
    3) any suggestions? or instructions?

    • isak says:

      Geez, this poor guy is having a rough time. I wonder if he is having some reactions to these injections as well.

      Give him some plain yogurt or maybe some cottage cheese. The ice cream may have too much sugar in it for him. A dog with a mild upset stomach can benefit from a gastric coating of soothing preparations such as Pepto-Bismol, but given the other meds he is receiving from your vets, you should check with them first before administering Pepto-Bismol. Generally, it is best to withhold food, but not water. Withholding food will allow the stomach to rest. Bland diets such as chicken and rice can be fed once the stomach has settled down. Rather than allowing him to drink a lot at one time, water should be given often and in small amounts.

      When it is time to feed him a bland diet, some good suggestions are:
      • white rice
      • boiled chicken breast
      • cooked potatoes
      • yogurt
      • strained beef (get all the fat out)
      • baby food

      You want to feed your dog the most bland food possible. If he doesn’t like the rice, try mixing it in with the yogurt. Pouring baby food over the rice is also an option. You can also try blending the food altogether in a food processor.

      Make sure he stays hydrated. You can give him children’s Pedialyte or even Gatorade.

      Also, if his eyes are dilated, keep him away from bright light.

  11. sahil godara says:

    6 years of age nd normal weight!

  12. sahil godara says:

    please tell me in
    question(1)how many weeks does ivermectin toxicity would be finish in a Labrador??
    2.5 mg was given by vet.
    (2) does it causes stomach infection??
    (3) what should I give to my dog to eat? so that it can be easily digested?

    • isak says:

      1. I can’t say how long it lasts. It depends on the dog — their health, the dosage vs. their weight and such. But it does take time and is not always a compete recovery.
      2. I don’t know what you mean by stomach infection. Anything a dog eats can affect their digestion and possibly give them an upset stomach.
      3. If your dog has an upset stomach with diarrhea, try something bland like boiled chicken or beef with cooked plain rice. Add some powdered acidophyllus to it to add good bacteria back into their gut.

      I hope this helps. Was this the first time your dog was given Ivermectin?

  13. Mandy says:

    Isak, yes, I spoke with my vet, and he did confirm that ivermectin could contribute to Buck’s ticking (jerking) motions. he also told me to stop giving it to him which I already had. Buck has distemper, and I know that some of his symptoms are due to the distemper. However, ivermectin poisoning and the distemper symptoms are very much alike. Buck has always maintained a heavy appetite but he’s extremely picky – another symptom of distemper.

    As for his mange, the vet will give me Bravecto in a few weeks, once the ivermectin clears his system. One pill is all it takes and within three weeks, he should be so much better. It works incredibly fast.

    I’ll keep you posted on his recovery and thank you for your input 🙂

    • isak says:

      Wow, haven’t heard of a case of distemper in a while. I didn’t realize the similar in the ivermectin poisoning and distemper symptoms. Thanks for sharing. And good on you for sharing your home with Buck!

      Keep us posted. And add a photo. I’d love to see him!

  14. Mandy says:

    I have been fostering a dog since June 9, 2015. He’s been on the Ivermectin for over two months for demodex mange. He was also on steroids, which interfered with the meds he was on, therefore, he still has mange. I came home two days ago and Buck was not twitching whatsoever! Later that night I gave him his Ivermectin and the twitching started shortly thereafter. It seems worse than before now. I stopped giving him the Ivermectin and will never give it him again. I am wondering if I need to do anything for him. I’m making him get up and walk be he seems so frail and think. He seems to drink but I am beginning to give him PediaLite. Do you think there’s a chance he will recover?? I need any information you can give me.


    • isak says:

      Only time will tell on his recovery. It’s important that he drinks and eats. Have you contacted his vet for their input? They know him best. Did he have any other ailments that may be contributing factors?

      As for the mange, if you have Buck confined, you might try some oil on his mange to suffocate the mites. If he is able to lick the area, use a food grade oil like coconut oil or cooking oil.

What do you think?

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