How does the gut-brain axis influence GI disease in dogs and cats and what are the clinical implications?

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The gut-brain axis drives a bidirectional cycle in which GI disease worsens behavioral and neurological outcomes, and psychological distress in turn precipitates or amplifies GI dysfunction in dogs and cats. This communication operates through the vagus nerve, gastrointestinal peptides and hormones, cytokine and chemokine release, and immune system activation.1 In dogs with functional dyspepsia (FD), anxiety is a recognized triggering factor for dyspeptic signs, not merely a comorbidity. In approximately half of functional gastrointestinal disorder (FGID) cases, psychological distress precedes GI symptoms; in the other half, gut dysfunction develops first and psychological disorders follow.2 Dogs with FD show higher anxiety levels and higher owner-attachment scores compared to healthy dogs, and owner anxiety — transmitted to dogs through olfactory detection of human stress odors — can independently induce stress-related behaviors and increased heart rate in dogs.2 This owner-dog emotional coupling is clinically relevant: owners of FD dogs frequently report a clinical picture disproportionate to objective diagnostic findings, and toy breeds appear particularly susceptible to anxiety-driven abdominal discomfort.2 Gut dysbiosis is a mechanistic link between GI disease and behavioral or neurological comorbidities in dogs. Dysbiosis has been identified in fearful and aggressive dogs, with behavioral deviations attributed to production of neuroactive metabolites by the altered intestinal microbiome.13 In dogs with chronic intestinal enteropathy (CIE), a higher frequency of displacement behaviors and lower scores for energy and interest have been documented, supporting a direct link between chronic enteropathy and emotional disorder.2 The microbiome-gut-brain axis communicates through the vagus nerve, circulating neurotransmitters, hormones, metabolites, and immune signaling via microbial-associated molecular patterns.13 In cats and dogs with chronic kidney disease (CKD),