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Giardia....the parasite with fangs which is everywhere and often overlooked as an underlying contributor to behavioural issues amongst other things!

Updated: 3 days ago

I won't do pictures for this one.....I really don't want to gross you out too much! I'll also try to keep this short, its not exactly something I want to think about too much...any bugs make me itch!

Quick note if you don't want to read everything: If you decide to test, or have tested previously, once you have your results I’d be grateful if you could share your results via this extremely short anonymous survey.

Your vet can test for you or you can do it independently via Vet Laboratory for £30 who have a very quick turnaround.


I’m not a vet, let me lead with that! I'm not going to pretend to have superior knowledge on this nasty either! I definitely do not, nor do I want to.....yuck! However as a behaviour professional the behavioural red flags of this little nasty are affecting so many dogs and I want to raise awareness. This nasty critter is impacting dog society and it needs to be addressed. It is massively impacting so may dogs and carer's lives, and awareness needs to be raised. There are many dogs exhibiting extreme behaviours who have been diagnosed with Giardia, and with treatment it has made the behavioural journey much easier to address!

It amazes me we treat regularly for worms and fleas, without testing whether they are present, and yet me don’t do occasional screening for this little nasty which can cause big problems!  Why? Its literally everywhere, in water, in parks, on the pavements. It is rife! So why aren't we checking for it?

What is it?

There was a time it was associated with puppy farms and areas with poor hygiene…..I’m being polite in case you’re eating or squeamish!  However now the beastie is prevalent everywhere and more worryingly it seems to be carried without presenting the typical clinical symptoms we would expect but manifests in behavioural issues.

Giardia is a parasite that lives in the small intestines of several vertebrates.  There are several species which affect animals and humans. They pick it up by ingesting or coming into contact with contaminated soil, food, or water tainted by the poop of another carrier. You cannot spot it with the naked eye. I compare it to a flattened tadpole with fangs!


In humans?

Physical symptoms in humans may appear 2 days after infection and can include diarrhoea, gas, stomach cramps and nausea.  It can cause dehydration.  And typically  a mild infection may resolve on its own between 2-6 weeks.  However it can last longer and be more severe.  Carriers can also then transmit to others.  It can lead to malabsorption and even lactose intolerance!


In dogs?

Clinical signs

This is when it is more obvious and its most likely to be tested when you talk to your vet about the smells and squits!  It is more frequently seen in young dogs of less than a year, however I have encountered several cases where dogs are 2 years plus.  The most common clinical symptoms associated with this parasite are malabsorption (under weight), smelly poop and the runs, weight loss or lack of weight gain.  However many do not have these signs!

Whilst it is zoonotic (can pass to other species), recent data suggests the cat version is of greater concern than the dog version, and most Giardia infections are host specific!  However, I’d speak to your GP if your dogs get a positive result especially if you are pregnant or have young children!

Behavioural signs

Over the last 4 years I have seen an increasing number of dogs diagnosed with this nasty who lack the clinical symptoms above.  It is a topic which we as behaviourists are becoming increasingly aware of and in some cases we suggest testing for it. 

I have seen the behavioural signs below, individually and some in combination:

·        Sudden onset of aggression

·        Resource guarding

·        Pica (Eating anything, food or non-food items) (excessively, including stones, materials, dirt, clothing)

·        Often slow to get up in the morning

·        Reluctance to eat

·        Excessive licking of floors and walls or different surfaces.

·        Reluctance to go on walks

·        Increased reactivity

·        Hyperactivity

·        Compulsive behaviours (e.g. guarding areas or food, reflection chasing, self mutilation/nibbling/biting themselves)

·        Aggression relating to being lifted

·        Aggression relating to being approached

·        Sometimes can walk differently (unusual gait)

·        Unwarranted escalation of aggression

·        Excessive vocalization

Some of these dogs have been particularly lean, but many are ‘normal’ weight.


Whilst I am sure if you’ve any of those behavioural issues listed above you’ve sought out professional support and the behaviourist has probably suggested certain health tests to rule out underlying issues, giardia is often missed where it lacks those clinical symptoms. It may be the behavioural symptoms are a coincidence, however if Giardia is possible it will most certainly exacerbate many of these issues.


How can they have a parasite and no clinical symptoms?

Well in honestly, there’s not much research on it in dogs, and nothing academic on its impact on behaviour that I can see. There are mentions of some behavioural changes in humans including aggression.  But, anecdotally, if these nasties are sticking their vampiric fangs into the dogs intestine when it eats it has to be uncomfortable.  If they are stealing the vitamins and nutrients from their body that will make the best of us grumpy.  And, if that discomfort starts just after they eat, its quite easy to imagine why they don’t want to eat. And anyone who is hungry is cranky!

But here’s what's interesting.  The research in humans - According to Hallie and Buret (2013) Giardia can cause eyesight issues, reactive arthritis, allergies, muscular issues, nutrition issues,  cognitive dysfunction, chronic fatigue, gastrointestinal issues such as IBS and irritable bowel syndrome, amongst other things. I've seen similar in dogs. Some may have been sent to the vets for these issues, could the underlying cause have been Giardia?


How can I test?

You can speak to your vet if you think Giardia may be a possible contributor to the clinical or behavioural symptoms or you can order the test yourself. If you are seeing any of the issues described above, clinical or behavioural, you really should be having a chat with your vet to rule out underlying medical issues anyhow.

But, if you really don’t want to do that, or you want to do spot checks just to monitor you can order the test for £30 from Vet Laboratory (no, I don’t get any handbacks).  If you have a positive result you will need to see your vet.  Some vets may want to repeat the test themselves, so do take a sample with you.  They will be able to administer the appropriate medication and monitor your pet going forwards.


It’s a sod to be honest.  There isn’t a specific treatment but it is likely to include an intensive course of specific parasite treatments, normal worming won’t cover it, some vets may also use antibiotics.  BUT, it can take a while to resolve and eliminate the cysts.   So treating the environment, creating an appropriate management plan and careful monitoring is key.

Regularly cleaning up poop, and decontaminating hard surfaces, promoting gut health with probiotics and high fiber diet are all recommendations made to support treatment.  Giardia cysts are infective as soon as they are passed into the environment so pick up the poop and dispose of it properly promptly.  It is suggested by several sources regular bathing should be carried out to remove cysts from the coat. 

As with most things, the young, vulnerable or geriatric are at increased risk of complications, however generally the prognosis is good. 

A word of warning….I have had several clients say their dogs behaviour became very erratic and hyper active…..quote “it’s like hes on speed!” when given the antiparasitic medication at night.  It may be worth thinking about the timing of these meds if you want to sleep through the night.


My thoughts

Every few years there’s a new “thing” that we discover is linked, in the 90’s it was the thyroid which was linked to aggressive issues.  And over the last 2 years its Giardia.  It first came onto my radar beyond the 'typical' environments its seen 4 years ago.

It is a problem.  Many vets who have treated my clients have commented on how “rife” it is in that area.  These area’s seem to span the UK! And yet, awareness is low.

Over three quarters of the dogs I have suggested testing for have come back with a positive result.  These were cases which lacked clinical signs but had behavioural 'red flags' of varying severity.  With treatment the behavioural issues reduced significantly, and with combined behavioural modification the issues have resolved or seen significant improvement. 


If you do decide to test, I would really love to hear about it regardless of the result, I have put this short anonymous survey together for you to complete about your your result if you decide to test or have done previously.  I will share the results once there are enough responses.

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