Dog behaviour and the gut microbiome

For many years now owners and vets have often speculated about whether there is a link between gut health and behaviour, as lots of dogs seemed to behave worse (more anxious or aggressive) when they were having a flare up of their gastrointestinal disease. Was it just pain or discomfort? Or could it be something more? Could it be the microbiome? We now know for sure that the microbiome does influence behaviour. 

There is actually such a close link between the gut and the brain that some have termed the gut the ‘second brain’ because so many of the neurotransmitters used by the brain and nervous system are produced in the gut. Others argue that the gut should be called the ‘primary brain’ because gut health and the microbiome have such an influence on mental conditions and behaviours, such as anxiety and aggression. The connection between the bacteria in the gut microbiome producing chemicals, and the brain which is receiving these messages through hormones and nerves, is known as the ‘gut-brain axis’. This connection is so key that studies have shown germ-free mice (those without a microbiome) show marked defects in their nerve structure and function, leading to impaired central nervous system (CNS) immune responses.

The gut microbiome can influence the brain and CNS in many ways: Excessive stimulation of the innate immune system in dysbiosis can lead to systemic or CNS inflammation; bacterial enzymes can produce neurotoxic metabolites, such as ammonia; gut microbes produce hormones and neurotransmitters that are identical to those produced by the body; bacterial proteins may cross-react with antigens to stimulate dysfunction within the innate immune system; and gut bacteria can send messages directly to the brain via the vagus nerve. In this way they can have a huge impact on memory, mood, sleep, stress and cognition. They can also be closely linked with specific disorders of the nervous system and brain.^[1]

Three of the most important chemicals produced by the gut that influence behaviour are Serotonin, Dopamine and GABA (Gamma-Aminobutyric Acid). 

• Serotonin is known as the ‘happy hormone’ because it impacts the mood and levels of anxiety, contributes to emotional wellbeing, and low levels have been linked with depression. It is thought that around 70% of serotonin is made in the gut. 
• Dopamine is the ‘reward’ neurotransmitter, which helps you or your pet feel good when anticipating a reward. Unfortunately for you and your dog this means that just thinking about eating that cookie or treat releases the dopamine, and not the actual eating of it. It is also involved in mood, decision making, motor function and the control of other hormones. It is estimated that around 50% of dopamine is produced in the gut. 
• GABA regulates anxiety, sleep patterns and stress. It is known to be modulated by the gut microbiome. 

With the recent introduction of more faecal microbiome testing for dogs it is becoming clearer that dogs with behavioural issues often have evidence of dysbiosis (an imbalance in the gut microbiome). Which bacteria are out of balance will influence how the behaviour changes, as some bacteria release chemicals that have calming effects, whilst others may promote anxiety or depression. A study in 2019 looking at the microbiome of rescue dogs in a shelter, found a clear link between the gut microbiome and aggressive behaviour.^[2] What we don’t yet know for sure is whether aggression causes a change in the microbiome or vice versa. Another study looking for links between behavioural issues and the gut microbiome found that phobic or fearful dogs had a microbiome enriched with Lactobacillus, a bacterial species known to have probiotic and psychobiotic properties. The same study found that aggressive dogs tended to have high biodiversity and be enriched with less dominant bacterial species, such as Megamonas.^[3] What this opens up is the possibility that manipulating the microbiome could help to correct or improve behavioural issues. An early study on mice found that supplementing with Lactobacillus rhamnosus reduced stress, anxiety and depression related behaviours through influences on GABA^[4]. It also presents the possibility of being able to predict the appearance of unwanted behaviours by finding certain changes within the gut microbiome. This could allow us to compensate for, or correct these changes in advance and before we have an actual behavioural issue. In the current climate, where we are seeing increasing numbers of behavioural issues, this is very exciting research.

Age is also thought to be a factor in the gut-brain axis. A study in 2020 looking at older dogs found that those with a lower proportion of Actinobacteria performed better in memory tests, whilst older dogs generally showed fewer Fusobacterium^[5]. In humans they have found specific links between cognitive disorders, such as Alzheimers, and dysbiosis, with recent research looking at how probiotics might help in the treatment of this disease[6]. A study in elderly patients even found that altering the cognitive capacity of the participants, through various brain stimulating activities, actually led to corresponding changes in the gut microbiota[7]. This all opens up the possibility of helping to manage or prevent canine cognitive dysfunction syndrome (‘doggy dementia’) through manipulation of the microbiome, and even in improving the microbiome through manipulation of behaviour. 

Looking at all of this information it becomes impossible to argue with the fact that there is a very close link between the gut microbiome and the CNS. As research progresses there is so much potential for how we can help to improve brain and nerve health, as well as gut microbiome health, through microbiome testing and manipulation. But, when it comes down to it, behavioural issues in dogs are always likely to need a multifaceted approach, with training being the number 1 focus. 

References

^[1] Galland L. The gut microbiome and the brain. J Med Food. 2014 Dec;17(12):1261-72. doi: 10.1089/jmf.2014.7000. PMID: 25402818; PMCID: PMC4259177.

^[2] Kirchoff NS, Udell MAR, Sharpton TJ. The gut microbiome correlates with conspecific aggression in a small population of rescued dogs (Canis familiaris). PeerJ. 2019 Jan 9;7:e6103. doi: 10.7717/peerj.6103. PMID: 30643689; PMCID: PMC6330041.

^[3] Mondo E, Barone M, Soverini M, D'Amico F, Cocchi M, Petrulli C, Mattioli M, Marliani G, Candela M, Accorsi PA. Gut microbiome structure and adrenocortical activity in dogs with aggressive and phobic behavioral disorders. Heliyon. 2020 Jan 29;6(1):e03311. doi: 10.1016/j.heliyon.2020.e03311. PMID: 32021942; PMCID: PMC6994854.

^[4] Bravo JA, Forsythe P, Chew MV, Escaravage E, Savignac HM, Dinan TG, Bienenstock J, Cryan JF. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proc Natl Acad Sci U S A. 2011 Sep 20;108(38):16050-5. doi: 10.1073/pnas.1102999108. Epub 2011 Aug 29. PMID: 21876150; PMCID: PMC3179073.

^[5] Kubinyi E, Bel Rhali S, Sándor S, Szabó A, Felföldi T. Gut Microbiome Composition is Associated with Age and Memory Performance in Pet Dogs. Animals (Basel). 2020 Aug 24;10(9):1488. doi: 10.3390/ani10091488. PMID: 32846928; PMCID: PMC7552338.

^[6] Kesika P, Suganthy N, Sivamaruthi BS, Chaiyasut C. Role of gut-brain axis, gut microbial composition, and probiotic intervention in Alzheimer's disease. Life Sci. 2021 Jan 1;264:118627. doi: 10.1016/j.lfs.2020.118627. Epub 2020 Oct 22. PMID: 33169684.

^[7] Khine WWT, Voong ML, Ng TKS, Feng L, Rane GA, Kumar AP, Kua EH, Mahendran R, Mahendran R, Lee YK. Mental awareness improved mild cognitive impairment and modulated gut microbiome. Aging (Albany NY). 2020 Dec 9;12(23):24371-24393. doi: 10.18632/aging.202277. Epub 2020 Dec 9. PMID: 33318317; PMCID: PMC7762482.