Border collie suddenly aggressive

“Border collie suddenly aggressive” is a very highly searched term on Google and unfortunately aggressive behaviour is a common problem within the breed. There are many reasons why a border collie would suddenly become aggressive, and techniques for helping the dog to overcome its aggressive behaviour will vary according to what is causing it. It’s important to find out what initially caused, and what is maintaining, the behaviour because only by understanding these motivating factors can we create successful, long-lasting behaviour modification plans that will help you and your collie. Asking for advice on the internet will result in a string of different answers, some of which may help, many of which will not. And often this leads people to try a training method for a while then shift onto another one, then another. This means that you are not being consistent or giving any one method long enough to have an effect because you’re not sure if it’s the right method to use or not!

Dog body language

Before looking at factors that can cause your border collie to suddenly become aggressive, and what you can do about it, it’s important to ensure that everyone stays safe. Understanding canine body language is a great way to start. If you can understand your dog’s attempts to say that they don’t feel ok, you can take action immediately to prevent them from feeling that they need to resort to aggressive behaviour. The ladder of aggression is very helpful in visualising the signals our dogs give us when they start to feel anxious.

border collie suddenly aggressive

The ladder of aggression shows how a dog’s body language changes from lip licking, yawning and nose licking when they first become worried about a particular trigger or situation, through a range of different behaviours until they finally resort to biting. Recognising these signals and taking action to remove the dog from a situation can be extremely helpful in preventing aggression.

Muzzle training

I also recommend that every dog is taught to enjoy wearing a muzzle. This isn’t as difficult as it sounds. If we take muzzle training slowly using a well-designed training plan, dogs do actually enjoy wearing muzzles! If the worst happens and they are involved in an accident or are injured in some way, having an unfamiliar item stuck on their face can make a scary situation much worse. It’s much better if they are already trained and familiar with muzzles.

Obviously, with dogs that can be aggressive, the muzzle is extra important and has a number of benefits:
1. It keeps people, dogs and other animals safe.
2. It may stop people from approaching your dog which is helpful if your dog is very fearful.
3. It may also stop people from letting their dogs run over to say hello which takes a lot of pressure off if your dog is reactive with other dogs.
Always use a basket muzzle – these allow your dog to have space to pant and drink water.

Stairgates and crate training

If your dog is likely to be aggressive at home, then using stair gates or a crate can help ensure that the dog still feels part of the household and is able to get used to visitors without feeling threatened or forced to interact. It also ensures that visitors are kept safe.

Why is my border collie suddenly aggressive?


border collie suddenly aggressive

Pain is a major cause of sudden aggressive behaviour in border collies. Any dog that is in pain will have a much lower threshold for reaction, meaning that it will take less to tip the dog from being ok to being totally overwhelmed. A dog that is overwhelmed is more likely to bite, either from panic, fear, frustration or sometimes just from sheer over-excitement. The pain may be caused by an obvious injury that needs veterinary treatment, and the dog may be aggressive in these situations because the injury really hurts, and they don’t want anyone to hurt them even more.

However, think about how we feel when we have a headache. We are much less likely to tolerate loud noises or children arguing, for example, and we are much more likely to snap at our loved ones than usual. If a dog has a medical condition that is causing pain or discomfort they are more likely to become aggressive. Even itchy skin conditions can have the same effect. Daniel Mills’ excellent paper, “Pain and Problem Behaviour in Cats and Dogs”, written in 2020, shows just how much pain or discomfort can cause problem behaviours, including aggression.

So, if your previously very loving, social border collie becomes suddenly aggressive with known or unknown people, or other dogs, a vet check is the first thing to do, to rule out any medical issues.

One-off incident

This is one of the most common causes of border collie aggression. They are very sensitive dogs, and were not bred to be social butterflies, unlike some other breeds that have been bred more extensively as pets or to work alongside lots of other dogs or people. Border collies were bred to work with one person in the peace and quiet of the countryside. Famers did not breed dogs that had wonderful, social temperaments: they didn’t need them. This is often why our collies struggle when put into social situations, which they can find overwhelming and difficult to cope with.

A dog attack is the most likely reason why collies suddenly become aggressive towards other dogs. When they are afraid, their fight or flight reflex kicks in, and if they are unable, or choose not to “flight”, they may “fight”, in the hope that their aggressive response will drive the other dog away. When this starts to work, either because the other dog retreats, or because we pull them away from the dog using the lead, the dog gets the result that he was hoping for and the behaviour is reinforced. These dogs can often sound very aggressive even though internally they are very fearful. As well as being attacked by another dog, any event in relation to other dogs that your dog finds stressful, such as being chased (even in play), attempts by other dogs to mount your collie, dog that won’t leave them alone on walks, can all induce aggressive behaviour if your collie it upset.border collie suddenly aggressive

Similarly, other events involving people that may seem very minor to us can also provoke a fear response in a dog. For instance, one of the dogs I am helping became fearful of people after someone ducked under a fence right next to her. As far as she was concerned, the person suddenly lunged towards her for no reason and scared her. From that point on, for a while, she would growl and bark at people to make sure it didn’t happen again. Another dog I am seeing was very traumatised when his owner slipped on a road in the dark in the snow. A car was just approaching, lights on, and pulled up to help the owner get up. The next day the dog was too scared to leave the house despite being quite a relaxed very confident dog prior to this event. It’s important to remember that even very little, seemingly irrelevant events can be terrifying to our collies.

If this happens, a behaviourist can help you to guide your dog through its trauma to ensure that the dog is rehabilitated properly without leaving any lasting unwanted behaviours, such as aggression.

Genetics & early learning experiences

border collie puppy suddenly aggressive

Some dogs are prone to being more fearful or more frustrated – it’s inherited from their parents. Similarly, puppies whose mothers are fearful may have a tendency to be more fearful in certain situations, and puppies can even be affected genetically before they are even born to be more likely to have a fearful temperament if their mother was subjected to a lot of stress during her pregnancy.

These dogs may cope well generally until something happens that really scares them, or hurts them, or if they are subjected to an unusually severe amount of stress or trauma. If this happens, these dogs are less likely to cope than dogs with more confident personalities. And fear can often provoke aggression in certain situations.


adolescent border collie

Border collies can often be very social puppies but suddenly develop a mild fear of people and/or other dogs at around 10-14 months when they hit adolescence.  Adolescence could last until they are 36 months. If you notice signs of your previously social collie starting to become worried in social situations, make sure you read their body language and get them out of the situation as soon as possible, before they feel the need to defend themselves.

Behaviour modification processes such as desensitisation and counterconditioning will help them to cope better and if they are not put under any pressure to interact with triggers during this phase, and are not able to practise any aggressive behaviour, they should come through the phase without any lasting effects. However, if your collie starts to become fearful at this age, contact a behaviourist as soon as possible. This is important to ensure that you guide them through this stage successfully. If it is not handled correctly, your dog could develop a more permanent phobia about certain triggers or situations.

Resource guarding

border collie suddenly resource guarding

Dogs that are prone to resource guarding may not have had any reason to practise the behaviour, if their owners have brought them up to be relaxed around food, without taking food or chews away once the dog has been given them. However, if the family suddenly adopt a new dog or puppy, or a baby arrives, that grows into a moving toddler, dogs can suddenly become very defensive of their food, toys and other items. It’s important to introduce the new family member carefully, watch for any issues, and contact a behaviourist straight away if you see any concerning behaviour. The more a behaviour is practised, the more it will become that dog’s way of dealing with that situation, and the more difficult the behaviour will be to change.

The same could happen if you take treats or toys out on walks with you and your dog starts to resent other dogs approaching in case they take their toys/food away. This can become worse if you routinely feed other dogs treats while out with your own dog.

Resource guarding is a common collie complaint, but it is a difficult behaviour to change, especially if it has been well practised, so get in touch as soon as you see this behaviour in your puppy or adult dog.


Dogs that are rehomed can take a while to settle in. It may be a while before they feel relaxed or confident enough to practise behaviours that may have been usual for them prior to rehoming. If you rehome a border collie, take advice from the rescue organisation about how to integrate your dog happily into your family life. A dog that suddenly becomes aggressive after rehoming may have been aggressive in their previous home. However, rehoming is a very difficult time for a dog and the huge change in every aspect of their life could initiate fears and frustrations that can lower a dog’s threshold for reaction, causing an aggressive response to certain triggers. Because the history of these dogs is often not known, helping rescue dogs can be more complicated so always contact the rescue centre or a behaviourist for help if your rescue collie is suddenly aggressive.

Change in circumstances

Border collie change in circumstances

Because border collies are so sensitive, a change in circumstances, such as a new family member, new pet, moving house, losing a loved one, going on holiday, or anything that is permanent or semi-permanent can lower the dog’s threshold for reaction. They can become extremely anxious or frustrated about things that we may not even have considered, and now behaviours, including aggression, can appear as if from nowhere.

A process known as stress-induced dishabituation may be involved in such situations. When this occurs, a dog that was previously well habituated to, for example, strangers, other dogs, busy areas or house visitors, can suddenly, due to stress, become very anxious in these contexts. And if no one is listening to his or her anxiety, they may feel the need to escalate their behaviour to aggression. Always take care to watch your dog carefully for signs of stress and do a stress audit, to check for things that may be stressing your dog, and taking appropriate action to alleviate the stress, can help prevent further anxiety.

Dogs can become frustrated in a range of different situations, but for a dog to suddenly become frustrated enough to become aggressive would require quite specific circumstances.


frustrated border collie puppy

Redirected aggression can occur if a dog becomes frustrated but has no outlet upon which to direct their frustration. So, for instance, if your dog suddenly starts to become frustrated they can redirect this frustration to owners, visitors, or other dogs, known or unknown. Situations in which a dog is able to watch other dogs or children passing his house, or if your garden is directly next to a pathway and other dogs and people are passing by frequently are good examples of how aggression could occur if people try to move the dog, or distract it when it is very overexcited or aroused.

Similarly, giving dogs hugs to “comfort” them or allowing children to put their arms around dogs can also result in redirected frustration if the dog is busy doing something else and arousal is already high.

Attention seeking and/or to get what they wantborder collie suddenly aggressive

Surprisingly, some dogs have learned that by being aggressive, they get the attention of their owner, or they are given treats to move them away from a trigger. These dogs may have been aggressive for one of the above reasons initially, but their aggressive behaviour has become so well practised that they are very confident about using it, and do so with very little emotion. And they may also have learned that if they are aggressive, their owner may scatter some treats away from the other dog so that they can move them away. Or they may have learned that if you are grooming them and they snap, you stop grooming them. Alternatively, they may have learned that if they snap at a guest, you pop them into another room and give them a chew to keep them quiet.

When dogs practise aggressive behaviour and seem to recover very quickly, it is likely that there is no longer too much negative emotion involved and the dog has simply learned that being aggressive is in their interest. This is called habitual responding – the behaviour has become a well-reinforced habit and is very difficult to change. It’s difficult to determine if your dog is responding in this way or if there is more emotion behind the aggression that needs to be resolved, so always contact a behaviourist if you suspect that your dog could be responding habitually.

Herding instinct or Predatory Drive

border collie suddenly nipping

Finally, is your border collie suddenly aggressive because they have a very strong herding instinct?  The border collie herding instinct is part of the predatory drive, but usually with the catch and kill sequences missed off, due to careful breeding.  This means that collies have a strong drive to control movement.  If people, other dogs or items such as cyclists, lawnmowers or wheelbarrows, don’t stop moving, or move in the way that the border collie thinks they should be moving, then they may nip to try to control movement.  It is very difficult to determine whether your collie is behaving aggressively or trying to herd, so if your collie is nipping, always ask a breed specialist to determine if it is due to the herding drive or if it is aggression.  The way that both are treated is very different, so it’s important to recognise what is causing the nipping.

To summarise

Hopefully, this has given you an idea of some of the reasons why so many people Google “border collie suddenly aggressive”, and helped you to understand your dog a little more. Understanding why dogs are being aggressive can often help us to empathise with how they are feeling, and can give us the motivation needed to help them to feel better about everything so that they don’t need to resort to aggression. Dogs will very rarely choose to attack a person or other dog – there is a risk involved and they could come off worse. In most cases, we need to find out why they are practising the aggressive behaviour, and then help them to feel better about the situation or trigger that is causing it, thus reducing their need to be aggressive.

For further help and advice, please feel free to get in touch!

8 thoughts on “Border collie suddenly aggressive”

  1. Great article. I’ve been working with a personal trainer for mollies reactivity to other dogs, especially on a lead. We’re really getting there now and it’s no longer difficult to take her to the pub and much better in walks. She loves people as long as they don’t get in her face so to speak. And good with children. The training is as much about training me as anything. Lol.
    I’m sure this started when I took her to puppy training where she was so good until an older 10 month dachshund kept trying to mount her, all the other pups were around 12/14 weeks. So great article and appreciated. Thanks. Always learning. Although I previously had a colly inherited from my dad at 8 years and as soft as they come I’ve never had a puppy but loving it now. She is 2 years old on Saturday

    • Hi Clive, I have a 3 year old collie who was also badly affected by a dog that kept trying to mount her at a working trial. It only happened a couple of times, but it was enough to convice her that she doesn’t want other dogs to approach her. Sometimes events that we don’t see as anything that bad can be seriously traumatic for our dogs. Well done for working through things with your dog.

  2. HELP! Our 3yr old BC mix has suddenly started attacking our 10yr old BC. Both are spade females. The 3yr old has been with the older dog since she was 6wks old. She has no problem with her male sibling nor does the 10 yr old. We”re at our wits end! She normally is a very loving cuddly dog.

    • Hi Mary, I’m so sorry that I didn’t see this until now. How are things now? If it is still happening, then please get your older dog vet checked – this often happens when the dog being attacked has an illness or something not quite right medically. I hope that this helps. Please get back to me if you are still having issues and your older dog has been vet checked and I can offer further advice.

  3. Great read & good to know we are not alone. Unfortunately, our Collie was attacked by another dog when he was about 8/9 months old which has led to him being very uncertain on road walks (perfectly fine when given free reign!) We have been seeing a behaviourist but I do have one question…..he is really not food orientated & when he fixates on something it is so hard to break the stare by distraction/ leading him away so he feels safe & doesn’t react, any other hints or tips for this would be welcomed. Thank you.

    • Hi Naomi, I’m so sorry to hear this – it’s so awful how such experiences can have life long consequences for our dogs.
      Have you tried lots of different foods – chicken, liver, beef, cheese? If you are very close to another dog, then he is unlikely to be able to take treats because the neurochemicals in his brain will be screaming to him that food isn’t important – escape is! So if you have more distance between him and the trigger, then that could help.
      In rare cases, in order to carry out the training, then it may be necessary to change the feeding regime – so if you usually just leave food out all day and your dog helps himself, then having set feeding times, after which if he hasn’t eaten, the food is removed, can help to increase the value of the food. Also, taking him for training sessions before he has eaten can make him more likely to tae food.
      If all else fails, and the dog is very anxious, then it may be necessary to use anti-anixety medication prescribed by a vet. This will help to lower your dog’s arousal and help him to relax a bit more, and he may therefore start taking food.
      Good luck with him!

  4. I really need help with my 5 month old Border. He’s become aggressive towards my older Jack Russell (she’s 10) in the last 3 weeks. We’ve had him since he was 14 was & they got on famously when we first introduced him. As he’s got bigger his play has become a lot more boisterous to the point that she reacted by growling, and he then attacked her. It’s happened 3 times now and once he’s got her he won’t let go. I got bitten by pricing his jaws apart – it was the only way to get him off her. We now have to keep them apart – when the Jack Russell sees him she starts growling and that sets him off. When they’re not separated I have his lead and harness on so I can keep control. When I take them both out and throw a ball they’ll both chase it but then he let’s her have it. I don’t want to give up on him but need to know what to do to resolve this. Please help!

    • Hi Sally,
      I’m so sorry I didn’t see this sooner – are you still having the problems?
      At 5 months old, your pup is starting to mature and it’s not unusual for them to start trying to control their environment using their teeth. After any such interaction, both dogs involvewd can start to become fearful of each other and this can cause an increase in anxiety, leading to more issues.
      If it happens again, keep them separate for at least 3 days – alternativing them to spend time in a crate with enrichment can help, or in different rooms. They need time for their cortisol levels to return to normal. Then when they are back together, monitor interactions very carefully. Have your pup on a house line so that you can move him away before any fights break out. Get used to using an interrupter word – I use “ah ah” and it tells the dog that I’m going to be moving him away imminently. In time they hear the noise and move away themselves.
      It might also be worth getting your older girl vet checked as sometimes an illness or injury can make older dogs smell different, triggering an attack.
      I hope that this helps a little. If it keeps happening, then please get in touch for futher advice.


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