Lyme disease may cause sudden aggression in dogs

Written by Betsy Sommers of: Peppertree Rescue, Albany, NY, USA.

28th May 2004: We recently had a very strange event that I think we should share around the rescue community.

A Young (~2 years) Lab mix, male, came into our program with a “questionable” background. He may have been aggressive toward some children, maybe not. We kept him for a good long while, months of fostering in our premier foster home, with no problem and placed him carefully, with a single mid-age man who absolutely adored him. We also, as we do with all our dogs, tested him for Lyme. He had it so we treated it; case closed we thought.

Everything went very well after adoption. He was the star of his obedience classes, a frequent alumni visitor to our clinics for over a year, and truly adored by his adopter. Then, over a year after placement, Mojo became suddenly, erratically, and seriously aggressive, literally attacked visitors to the home, people at the vet’s waiting room, etc. Terrifying. Very sudden. Totally inexplicable. He was returned to us with genuine heartbreak from a very loving adopter.

Mojo then went to our regular vets and was a totally different dog, bared teeth and growling to anyone who approached his kennel, lunging at other dogs when being walked, etc. We figured that whatever was happening with him, he had become implacable and started a TDC (Tough Decisions Committee – something we “convene” and that is open to anyone with an interest in the dog when we think that euthanasia might be an option).

However, someone at the vet’s office said that perhaps we should test him for Lyme. Huh? They had had a regular client of theirs come in recently with similar, out of the blue aggression, and it turned out that was the problem, it puzzled them, but seemed to be the case. Okay, we’ll try anything, so we had him tested. He was high positive! Fine, we started treatment while we continued to figure out what to do with him via the TDC. Almost immediately, however, once the antibiotics were begun, the Mojo we knew came back! He was himself again – bouncy, happy, a bit neurotic, but not at *all* aggressive! The staff at the vet’s was amazed, but all confirmed this change.

We didn’t believe it; and the veterinarians didn’t believe it BUT a thorough search of the Internet turned up a number of studies and anecdotal observations indicating that in some dogs (and in some humans) the primary symptom of their Lyme Disease can be sudden, irrational and serious aggression. Well, we’ve known for a while to check the thyroid levels of dogs that show aggression that just “doesn’t fit”. Now we have added testing for Lyme as well. And we have – results not yet in – another dog that we placed over a year ago who has been returned because of out-of-the-blue aggression and he has also tested high positive for Lyme! We’ve started treatment and will be monitoring his response.

So plug this in to your protocols. It’s worth checking out. I spent the day today with Mojo and he truly is just the same dog we placed over a year ago. We’ve let his original adopter know because he vowed that it had to be ‘something’ causing this behavior. But he cannot take Mojo back because his roommate, one of the people attacked, won’t even consider it. For the record, there were no skin-breaking contacts in any of these attacks, but plenty of fear and we consider them as serious as if they were full-fledged bites. We actually have additional insight into this because one of our volunteers (human) has had Lyme Disease. It took many months for her to be diagnosed, and once she was, she learned that it’s a VERY nasty bug that really remains around permanently, waiting for a chance to “crop up” again. When we place Mojo again (and our TDC unanimously agrees that we should do this), we’re going to explain the background, these amazing events, and require that the adopters have him tested every six months, whether or not he’s showing symptoms. We have no idea whether that will work or be sufficient, we’re rather flying blind in this, but it seems a rational approach.

I’ll post again if we learn more from the second dog (also a Lab mix). But based on what we know now, it is a real possibility: Lyme *can*, in a few rare cases, cause aggression, aggression that can be reversed.

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  1. I wish I had read this sooner. I just made the most difficult decision of my life and put my Tank down because of out of the blue aggression. He bit 3 people in the last few months and that was completely out of character for him. We spend a lot of time at camp during the summer and it never once crossed my mind to think of ticks and lyme disease. I’ve searched and searched about dog aggression and not once did lyme disease come up. Now this morning during a moment of huge sadness, we put him down yesterday, and still needing to know ‘why’ i thought of ticks. There needs to be more info about this published so people can get their pets tested and treated!

  2. I have had the same reaction in a dog I train once with Lyme and once with Anaplasmosis. In this case the dog mostly showed aggression to other dogs- especially the dog he lives with and adores. He is being treated for the second time and we are hoping the fights are behind us. These tick diseases are bad news. We have no idea how the dogs feel while affected.

  3. I just put my female pit bull to sleep last week!! She became aggressive towards me and my grand babies. She had Lyme but my vet insisted it wasn’t from Lyme . We found out in August 2014 she had Lyme, his whole personality changed!!! I’m devastated over putting her down but I couldn’t risk her hurting my grand babies!!!! More studies need to be done on this Lyme in dogs!! I’m sorry for your loss too.

  4. I adopted a dog knowing she had Lyme disease. I was told she might live a good long life, or might become symptomatic in a physical sense: swollen joints, pain. Aggression was not raised as a possibility or l would not have adopted her. She has been a great companion for my first dog. I have only had her 4 months. Last month she became aggressive with a neighbor’s dog. Last week, at her doggie day care, she and another dog mixed it up and my dog bit the worker who intervened. Today, completely unprovoked, she went after my first dog. I have NEVER seen anything like this before and now l am afraid to leave them alone unsupervised. I will speak to my vet this week, but l am scared. It was helpful to read about the (possible) cause, but I am not sure how knowing the cause will alter her behavior.

  5. My son was just attacked by a dog of a friend who had been diagnosed and treated for lyme disease. It was completely out of character and unexpected. The dog gave no warning, just lunged and bit. In doing research I came across this article about Mojo written in 2004 that gives hope of treatment, but as I continued to search the web I found a follow up article written by by Betsy Sommers in 2015 from the Peppertree Rescue. It appears that Mojo continued to show spastic aggressive behavior and a note that once the disease reaches the nervous system the aggressive behavior can not be reversed. Very sad and very scary.

  6. This sounds very similar to what we are experiencing. We rescued a dog who turned out to be Lyme positive. His titre was high. He was treated in the spring with 30 days of antibiotics and he has been just a wonderful boy. A couple of months ago he attacked our neighbours dog, completely unprovoked. They were sitting next to each other and he just grabbed him, almost ripping his ear off. Then just Christmas morning we were at my moms and he grabbed my moms dog, as one was going outside and one was coming in, shook him in the air before my husband intervened and we got him to let go, causing two puncture holes. Then last night he turned on our dog we’ve had for 4 years, just out of nowhere, something just caught his eye and he went for her neck as they were coming in from outside. We are heartbroken about all of it and are thinking we should put him down before there is another attack, God forbid he attacks a person. Very sad.

  7. Hi there.
    We have a 4 year old black lab who we adopted/rehomed 2 years ago.Lovely nature.
    We look after dogs in our home and she is absolutely wonderful with them…BUT..when we go on walks, she has just started to show aggression to some other dogs.
    It is really horrible.
    I now have to walk her on a lead. Could this be Lymes?

  8. Have this checked by the veterinarian, but typically there would be other signs as well including lethargy and or sore/stiff joints.

  9. It seems that once a dog gets “Lyme rage” there’s no turning back. I rescued a young male Lab mix who had a very bad case of Lyme, it took almost a year to get his blood levels near normal. He was the PERFECT dog until one day when he was about 4 years old he curled his lips at me, showed his teeth and attacked my leg. I was shocked. My vet put him back on antibiotics for 30 days and he was like his old self. He would do well for a few months but then the rage would rear it’s ugly head. When he had episodes it was as if he wasn’t my dog. His face would contort and his eyes glazed over- then he would attack. The antibiotics began losing their effectiveness. I could no longer trust him around people and especially children. I too was becoming afraid of him. After consulting with my vet and also a neurologist we made the decision to let him go. It was horribly sad but the humane thing to do.

  10. OMG! My husband and I have both had Lyme with Erlichilosis. We just had to out our 2 yr old beLOVED Border Collie destroyed in front of us bcz over this summer she became increasingly irritable and began taking chunks out of us. WHY did I not think of this after all we have been thru? I am beside myself sad!!! My vet thought inbreeding. My breeder (who is a judge of the breed) thought brain tumor but I bet not either one!!! …FOLKS EDUCATE YOURSELVES ABOUT LYME! My cat had it first and the vet never check for lyme but he had lame-leg syndrome and “cats are not supposed to get lyme”. EDUCATE, BE INFORMED, AND INSIST ON LYME TESTING FOR UR ANIMALS! TREATMENT IS SIMPLE! :'(

  11. I adopted a Great Pyr mix at 4 months ol and he started acting aggressively at 8-10 months old. It has gotten worse as he has gotten older (now 1 1/2 yrs old). It was recently suggested to me that this could be a response to the Lyme vaccine (antigens in the vaccine). I vaccinated him last fall (at 5 months old) and this fall without having that information. Has anyone else had the experience of dog aggression after getting the Lyme vaccine?

  12. Our 5 1/2 month old sweet goofy German Shepherd puppy started getting aggressive over his food. Sometimes he was fine, other times growled. We tried several things to correct the behavior but it seemed erratic. This only happened with his food. One day he was in noticeable pain and not his bubbly self. Next day, he had problems going up and down the stairs. He seemed better later in the day but still not himself. After a 3rd day of this, I took him to the veterinarian and he tested positive for Lyme and Anaplasmosis. He is now on antibiotics and pain medicine. His bubbly personality is returning and I hope the food aggression goes away.

  13. I had my pit since he was a baby an all of a sudden out of no where he bit my husband , I guess I should get him checked for lyme disease ..

  14. I have to put a dog down today because of Lyme. All of a sudden because out of control and attacks. And its like no stopping it. And it doesnt stop. There is not a cure. I goes to the disease it did damage to his wiring. I have gone to vets and vets

  15. I went to put my dog down today because he had Lyme disease and he is aggressive all of a sudden. And they said maybe just putting him up for a rescue because Lyme disease doesnt cause agression. So one vet says it does and the other says it doesnt.

  16. I would like to learn more. Can you provide some direction with regard to:

    “a thorough search of the Internet turned up a number of studies and anecdotal observations indicating that in some dogs (and in some humans) the primary symptom of their Lyme Disease can be sudden, irrational and serious aggression.”

    I’ve done a preliminary search for studies and haven’t found anything. Could you help me source?


  17. Hey! Just letting you know in humans once it reaches the chronic stage just doxycycline or ONE antibiotic for several months will produce relapse ALWAYS. To get a human in remission 2-3 antibiotics have to be used at the same time and alternated every four-6 months to prevent resistance. If the dog also has bartonella (some ticks have that infection too) it needs another class of antibiotics entirely than the lyme ones. Treatment time in humans is usually four months to two years of antibiotics! However I’ve heard that dogs need a shorter time period. I encourage you to contact ILADS they are the best source on human lyme experts and can likely direct you to a veterinarian that is lyme literate in your area.

  18. My dog has been diagnosed with Lyme disease but has been doing well(physically). The vet recommended Frontline, along with what she was already taking(Revolution). We give them 2 weeks apart. We gave her the Frontline on Friday and since then she has been acting very strange. She won’t go in certain rooms and is afraid to walk on the floor in the livingroom she jumps from one couch to another to get across the room(she is a St. Bernard). She has been taking Revolution for a long time but just started the Frontline a month ago. My question is- Do you think this behavior is caused by the Frontline or is it a symptom of Lyme Disease?? She has also had IBD for 5 years which is under control with a proper diet. She is 8 years old now, otherwise healthy(130 pounds) eats well and is active. She is just acting very kooky.

  19. My advice to you is keep a close watch on her behavior. June 2nd, 6 days ago, my sisters 130 lb. beloved dog turned on her family. Yes, there was a non family member on the property. A very close friend. Suddenly he was attacked. My sisters husband jumped to defend our friend…the dog attacked, viciously his own papa. Two ambulances later, the humane society, and multiple police vehicles, the tragedy was over. My sisters best friend put down. Dusty had been on meds for Lyme Disease in the past. There are studies that suggest aggressive behavior can occur, not only in dogs, but humans as well. My sister has also undergone the treatment. Tissue samples have been taken by the vet, maybe we will find an answer in the next few weeks. Research is on going. Thoughts and prayers…

  20. I know a couple people who have Lyme disease also . My question is, is there any chance that these symptoms of aggression can be the same for towards humans too??

  21. I just put my sweet standard poodle down last week because of a sudden, terrifying display of aggression that looked just like rabies. The emergency vet ruled out injury, but did not test for Lyme. I’m in Indiana. I wonder if others may be experiencing this, too, He was on a flea and tick preventive.

  22. that is what is going on with me..after 2 years never a fight he attacked her and the neighbors dog and I took him for a checkup and he has lymes disease…treating now and keeping them separated I hope to God that his lymes disease goes away and he loves her again…not sure what to do…

  23. I know this article is a bit old, but I had a dog with Lyme who was suddenly fearful of loud sounds and never in her life had she been afraid before. She was never aggressive. Thunder, loud trucks, a shooting range several miles away could sometimes (rarely) be heard and she would be scared. We knew she had Lyme (limping suddenly, swelling in areas and we had her tested). But Lyme seems to mess with dogs neurologically and can cause fear or aggression.

  24. My new rescue puppy, Dallas was diagnosed with Lyme Disease. He is very aggressive and does sever biting. This week the vet will do a full panel blood work and body scan to rule out a medical condition. Trying to research to see if Lyme Disease cause sever aggression and biting. My vet said Lyme disease does not affect the brain, but I am not convinced. If we can’t get the aggression fixed, he will have to be put to sleep and that will break my heart. I just got him on 28 January and I don’t think the previous owner was honest with me. Dallas has viciously bit me almost 10 times despite being very attached to me. After the second bite, I hired a trainer who will take him next week to her home for a doggie boot camp for a week. His first family was Hispanic who had him for 6 months, but the family was deported and they left Easton (I renamed Dallas) at the boarder of Chula Vista, CA. He was in a rescue for 24 hours, when the Delgado family got him and they eventually got another dog Hank. While living in CA, both dogs were kept outdoors most of the day. Last August 2018, both dogs were transported by van to PA on a 8 day journey, and Dallas cried until he was put in the cage with Hank. Upon arriving in PA, Dallas was given to an Aunt hoping to help with her cancer therapy but she couldn’t handle him? and when the Delgado family got him back in November, they said he began to be aggressive with her three boys?? Mrs. Delgado said Dallas only bonded with her and Hank and that she didn’t have time for Dallas that he demanded much attention. I’ve recently tried to get more information about Dallas’s past, but she has not responded.I love him so much but can’t risk any more serious bites.

  25. My ex wife’s dog acted just like this I think I got Lyme after staying over a couple of times when we first met. My exwife had strange symptoms from before we met; however, was never tested for it. I was tested about a year and a half after we broke up and was pos for lyme. I wonder if the dog can be reservoir. Would make sense.

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