Does your dog know when you are ill?

Last winter, we both came down with a nasty cold at the same time. For three days we barely left the sofa. Poppy, who normally splits her time between her favourite spots around the house, spent those three days glued to us. She lay across our feet, rested her chin on our legs, and refused to leave the room. She wasn’t hungry for extra attention. She was checking on us.

Ask any dog owner whether their dog knows when they’re ill, and you’ll get the same answer. Of course they do. But it’s not magic. There’s real science behind it, and what dogs can detect goes far beyond a runny nose.

How dogs detect illness

A dog’s sense of smell is extraordinary. The olfactory cortex in a dog’s brain is roughly 40 times larger than ours, and their nose contains up to 300 million scent receptors compared to our 6 million. They can detect chemical changes in the human body that we have absolutely no awareness of.

When you’re ill, your body chemistry changes. Infections alter the compounds in your sweat and breath. Hormonal shifts change your scent. Even subtle changes in temperature and blood flow produce chemical signals that a dog’s nose can pick up on. They don’t understand that you have a cold or a fever. But they know something is different, and that difference triggers a response.

Poppy’s nose is constantly working. She sniffs us when we come in from outside, when we’ve been near another dog, and when something about us has changed. That’s not nosiness. It’s her way of reading the world, and it’s the same mechanism that allows dogs to detect far more serious conditions than a common cold.

Dogs and cancer detection

Research into canine cancer detection has produced some remarkable findings. Studies published in journals including the BMJ and Integrative Cancer Therapies have shown that trained dogs can detect certain types of cancer, including lung, breast, bladder, and prostate cancers, by smelling breath or urine samples.

The accuracy rates in some studies have been staggering. A 2006 study published in Integrative Cancer Therapies found that trained dogs could identify lung cancer from breath samples with 99% accuracy and breast cancer with 88% accuracy. The dogs were detecting volatile organic compounds that cancer cells produce, compounds that are far too subtle for any human nose or most laboratory equipment to pick up.

This isn’t about pet dogs spontaneously diagnosing their owners (though there are anecdotal accounts of exactly that). It’s about what the canine nose is capable of when combined with training. Medical detection dogs are now used in formal programmes around the world, including Medical Detection Dogs in the UK, which trains dogs to detect the odour of human disease.

Blood sugar and diabetes alerts

Service dogs trained to assist people with Type 1 diabetes can detect when blood sugar levels are dropping dangerously low. A 2016 study published in the journal Diabetes Care found that dogs respond to isoprene, a chemical found in human breath that rises significantly during episodes of hypoglycaemia.

Humans can’t detect isoprene at all. Dogs can, and trained diabetes alert dogs will nudge, paw, or bark at their owner when they pick up the change, often before the person feels any symptoms themselves. This early warning can be life-saving, giving the person time to eat something or take glucose before a dangerous episode occurs.

Seizure and migraine detection

Some dogs develop the ability to predict seizures in people with epilepsy. What’s fascinating is that this ability often can’t be trained deliberately. Dogs placed with seizure patients frequently develop it on their own, typically within a year. Organisations working with assistance dogs report that a significant majority of dogs placed with epilepsy patients eventually learn to alert before a seizure starts, giving the person crucial time to get somewhere safe.

Migraine detection works similarly. A study conducted for the University of Pittsburgh found that 54% of migraine sufferers who owned dogs had noticed changes in their dog’s behaviour before or during a migraine. Nearly 60% of those said their dog alerted them to the onset of a headache, typically one to two hours in advance. The dogs weren’t trained for this. They picked it up through the chemical and behavioural changes that precede an attack.

Emotional awareness

Beyond physical illness, dogs are acutely sensitive to emotional states. Research from the University of Lincoln found that dogs can recognise human emotions by combining information from different senses, processing facial expressions and vocal tones together to gauge how their owner is feeling.

When you’re stressed, anxious, or upset, your body produces different chemicals. Cortisol levels rise. Your heart rate changes. Your breathing pattern shifts. Dogs pick up on all of this, both through scent and through reading your body language and behaviour.

Poppy is a textbook example. On days when one of us is feeling low or stressed, she becomes noticeably more attentive. She’ll sit closer, make more eye contact, and do that thing Lhasas do where they lean against you with their full body weight. It’s not coincidence. She’s responding to cues we’re not even conscious of giving off.

That sensitivity is part of what makes the bond with a Lhasa so strong. They’re not demonstratively affectionate in the way some breeds are. They show their care quietly, by being present, by staying close, by adjusting their behaviour to match yours. It’s subtle, but once you recognise it, you realise they’ve been tuned into you the whole time.

Why Lhasa Apsos are particularly tuned in

Lhasas were bred to live alongside monks in Tibetan monasteries. They weren’t outdoor working dogs. They lived in close quarters with their humans, constantly observing, constantly alert. That heritage means they’re wired to be exceptionally aware of the people around them.

Their independent temperament doesn’t mean they’re emotionally distant. It means they process their observations quietly rather than reacting with big dramatic displays. A Labrador might bounce around anxiously when their owner is ill. Poppy just quietly moves closer and stays there. Different approach, same awareness.

They’re also creatures of routine. When your routine changes because you’re unwell, they notice immediately. The fact that you’re not getting up at the usual time, not going through the normal morning pattern, not moving around the house the way you normally do. All of that registers. And their response is to stick close and keep watch, which is exactly what a monastery sentinel would do.

What your dog’s behaviour might be telling you

If your dog suddenly becomes more clingy, more attentive, or starts sniffing you more than usual, it’s worth paying attention. It doesn’t mean you’re seriously ill. It might just mean they’ve picked up on a minor change. But persistent changes in your dog’s behaviour around you shouldn’t be dismissed.

Dogs aren’t diagnostic tools. They can’t tell you what’s wrong. But they can tell you that something has changed, and sometimes that’s the nudge you need to pay attention to your own health.

Poppy can’t write a prescription. But she’s been our early warning system for everything from colds to bad days for over a decade. She knows when something’s off before we do, and she responds with the quiet, steady presence that makes Lhasas such brilliant companions. She can’t fix what’s wrong. But she’ll sit with you through it, and honestly, sometimes that’s exactly what you need.