Stress in Dogs - Physical Effects on Stomach, Gut & Immune System

Stress can affect the stomach, and this is true for many dogs too. Stress hormones have a direct effect on the gastrointestinal tract, and especially with prolonged stress this can have consequences: digestive problems such as discomfort, vomiting, diarrhea, or bloating are possible signs that “the stress has reached the stomach”. 

Many dogs today no longer lead the life they were, biologically speaking, born to lead as dogs. Over the centuries, our domestic dogs have gotten used to and adapted to living in our houses, sleeping on our sofas ;), eating regular meals (which in some households are prepared far more elaborately than those of the humans), and accompanying us through thick and thin every day. Some of our dogs are used to sleeping in the office, walking through the city, waiting patiently outside shops without making a sound of complaint. 

Stimulus tolerance and nervous dogs

As much as we enjoy the constant company of our dogs, and as much as this is usually mutual, we as responsible dog owners must not forget that a dog's stimulus tolerance was not originally designed for sitting comfortably in cafés or delivering peak performance every day, as rescue or police service dogs do, for example. Even though many dogs cope well with and enjoy such a life, there are also those who have difficulty adapting, are generally nervous types, or have problems interacting with other dogs or in certain situations.

Is stress a bad thing?

Stress itself isn't necessarily a bad thing. Stress puts the body on high alert to ensure it can react as quickly as possible in an emergency. This principle is called “fight or flight” (coined by the American physiologist Walter Cannon) and describes the body's physical response to danger-induced stress. The fight-or-flight response is based on positive feedback between the adrenal medulla and the sympathetic nervous system (part of the autonomic nervous system). Impulses from the sympathetic nervous system trigger the release of adrenaline and noradrenaline, which in turn trigger physical reactions:

The eyes and ears focus sharply (tunnel vision and selective hearing), the muscles receive more blood flow and tense up, breathing rate and heart rate increase, while digestion slows down and the bladder relaxes (stress and excitement urination). The body prioritizes the functions that are important in dangerous situations. So stress ensures survival!

Stress becomes unhealthy at the point where it occurs continuously over a longer period, because the organic response persists and the animal does not distinguish between a real dangerous situation and stressors that aren't dangerous to us humans, but very much are to our dogs.

What causes stress in dogs?

Domestic dogs are almost entirely spared from real life-threatening danger, either because they never encounter it (predators) or don't even recognize it (road traffic, rail traffic, etc.). So most dogs experience human-made stress, which can have a wide variety of causes and is usually based on the fact that the dog simply isn't programmed for the stress-causing situations. (Cities, crowds, hustle and bustle, extremely intense training, sharing tight spaces with unfamiliar dogs, bringing a second dog into a previously single-dog household, moving house, New Year's Eve, etc.)

Chronic stress in dogs

When a dog is exposed to chronic stress, the level of the stress hormone cortisol rises. The adrenal gland (glandula suprarenalis) also continuously produces adrenaline and noradrenaline, which over time can have numerous negative effects on the organism, which we'll examine in more detail below.

Stress colitis is a common cause of colitis in dogs. Changes in daily routine, bad weather, a visit to the vet, or a move can trigger stress in dogs, which can lead to colitis developing.

Cortisol, adrenaline, and noradrenaline – a brief overview

• Cortisol: Cortisol is a stress hormone that activates catabolic metabolic processes, thereby providing the body with energy-rich compounds.
• Adrenaline: Adrenaline, or epinephrine, is a hormone produced in the adrenal medulla that belongs to the catecholamine group. As a stress hormone, it enhances the physical functions that improve performance so the body can react quickly to a dangerous situation.
• Noradrenaline: Noradrenaline is produced from dopamine with the help of an enzyme. In the brain, this neurotransmitter is primarily responsible for regulating alertness and attention.

The dog's stomach

As a carnivore, the dog's stomach is, relative to body weight, roughly eight times as large as that of a horse.

In the stomach, food is further processed mechanically and chemically, mixed with gastric juices, and prepared for digestion in the small intestine. Various glands produce gastric acid, which consists mainly of hydrochloric acid. The main purpose of gastric acid is to denature (break down, transform) food, but it also effectively kills pathogens or renders them largely harmless. The stomach also produces mucus, which protects the stomach wall from being attacked by the aggressive gastric acid. This mucus essentially prevents the stomach from digesting itself. In dogs, the production of gastric acid is directly tied to the trigger of “eating” and is paused during periods without food. Dogs are biologically designed for this, since even wolves only rarely had access to food twice a day. If a dog's stomach were to continuously produce gastric acid, as happens in animals classified as continuous eaters (most herbivores), it would also have to eat continuously to prevent the stomach from being damaged after only a short time.

So what happens to the stomach under stress?

As already mentioned under the fight-or-flight principle, the release of adrenaline and cortisol extremely slows down digestive activity. This happens because blood flow to the stomach decreases, as the blood is needed in the muscles. Primarily, some dogs respond with diarrhea, because the body, due to the reduced blood flow which in turn causes poorer digestion, wants to get rid of the food as quickly as possible.

Ongoing stress also leads to increased gastric secretion while simultaneously reducing mucus production, which – especially in combination – can have devastating consequences, as the stomach lining then only inadequately protects the stomach wall from the aggressive acid.

Gastritis in dogs

Gastritis generally refers to inflammation of the stomach, which can be acute or chronic. It can have various causes; in the case of stress, the inflammation is due to overproduction of gastric acid.

Although a definitive diagnosis is only possible via gastroscopy, and symptoms vary depending on the cause and degree of mucosal damage, it is possible to recognize gastritis by various symptoms. 

• Gastritis often occurs without symptoms, or only with nonspecific symptoms (loss of appetite, lethargy)

• Main symptom: vomiting

• Occasionally: excessive salivation, strongly fluctuating appetite

• Pain response when palpating the abdomen

• Diarrhea only occurs with gastroenteritis (inflammation of the stomach and intestines)

• Salty-smelling breath

• Burping

In a mild, acute case of gastritis, only the inner lining is affected - an inflammation of the stomach lining. A deeper and longer-lasting inflammation is called a stomach ulcer.

Stomach ulcers in dogs

Stomach ulcers result from long-term damage to the stomach lining and often occur as a consequence of chronic gastritis. Continuous irritation of the stomach lining leads to an ulcer, which causes pain and can bleed. Another cause is infection with Helicobacter pylori.

The symptoms of a stomach ulcer are very similar to those of gastritis, though there may be a change in the color of the stool (darkening, blackening) caused by bleeding from the ulcer into the stomach. The dog may also vomit blood.

Reducing stress in dogs - protecting the stomach

To protect the sensitive stomach from the effects of stress, it is primarily important to minimize stressors as much as possible. Since this is often only partially possible, attention should be paid to choosing a type of feeding that helps counteract excess acidity. In addition, mucus production in the stomach can be supported with various plants, such as silverweed, marshmallow root, or yarrow.

It follows, then, that chronic stress has negative consequences for a dog's stomach. In the second part of this series, we take a closer look at the effects on the gut and the immune system – which are closely linked – as well as the effects of stress on general behavior.

Part 1 of this series covered the physical effects of stress on the dog's stomach; in Part 2 we look at the gut and the immune system, which are closely linked. Stress-induced gut issues in dogs can weaken the immune system and make your dog more susceptible to pathogens.

The dog's gut

The intestine is the section of the digestive system where actual nutrient absorption takes place. The food, already chemically and mechanically pre-processed, enters the small intestine after being mixed with gastric acid and mucus.

A dog's intestine can be roughly divided into two sections, the small and large intestine. These in turn can be further subdivided and each has different functions. The duodenum, jejunum, and ileum are part of the small intestine, where the more easily digestible substances are broken down and absorbed. The cecum, colon, and rectum are part of the large intestine, whose job is to break down harder-to-digest substances and remove water from the stool before excretion. Compared to the small intestine, food passes through the large intestine more slowly. The intestinal lining has many nerves, which can cause pain in cases of inflammation and bloating. Overall, a dog's intestine is, depending on size and breed, about 2-5 meters long.

The connection between the gut and the immune system

Besides its role as a digestive organ, the gut has another vital function. Due to its immunological role, it plays a major part in a dog's overall health.

70–80% of all antibody-producing cells are located in the gut, which is directly connected to the brain via the vagus nerve (the 10th cranial nerve) – this is known as the gut-associated immune system. Together with the intestinal lining and the gut microbiota (intestinal flora), this forms an effective line of defense against pathogens, which find a large attack surface due to the gut's large surface area, known as the intestinal barrier.

The vast majority of the bacteria that make up the microbiota are non-pathogenic (not disease-causing) or at most opportunistically pathogenic (meaning the pathogens require a weakened immune status or other deficiencies in their host to become pathogenic). Gut bacteria take on important functions for health: they compete with pathogens for nutrients and also produce antibacterial substances that eliminate pathogens.

The effect of stress on the gut

In states of acute or chronic stress, blood flow to the gut is reduced due to the demand for cognitive performance (blood is needed in the brain and muscles). This in turn leads to reduced mucus production. The stimulus threshold of the so-called enteric nervous system (part of the nervous system, also known as the gut nervous system, intestinal wall nervous system, visceral nervous system, or intramural nervous system, and colloquially as the “abdominal brain” or “gut brain”) is lowered. This makes the intestinal wall more susceptible to pathogens.

Stress-induced intestinal disorders in dogs

In addition to simple digestive disorders, various intestinal diseases can develop if a dog is exposed to permanent stress. Overall, then, chronic stress is not without consequences for the gut, since ongoing diarrhea leads to fluid loss and an imbalance in the electrolyte balance, and the dog becomes more susceptible to pathogens due to a weakened immune system. The environment of the gut also changes, which makes the dog more susceptible to intestinal parasites as well. An inflamed gut is also painful and can lead to bleeding.

We can therefore see a clear correlation between the gut and the immune system, which in turn means that the negative influence of stress on the gut has a direct connection to the immune system and thus to the dog's general health.

(Of course, the immune system consists of more components than the gut, but going into that here would go beyond the scope of this article.) There are many options for supporting the gut: the dog's healthy diet is certainly the primary factor. Herbs such as silverweed (Potentilla anserina), marshmallow root, or yarrow also have a positive influence on gut health. In addition, regular checks for parasites and constant access to fresh water are essential.

Author: Hanna Nowak

Sources:

DocCheckFlexikon (www.flexicon.doccheck.com)
Anatomy for Veterinary Medicine (Salomon/Geyer/Gille)

Thieme.de (https://www.thieme.de/de/tiermedizin/akute-gastritis-beim-hund-97967%20.htm)
DocCheck Flexikon (https://flexikon.doccheck.com/de/Stresshormon)
Tierklinik.de Anatomy/Gastroenterology (https://www.tierklinik.de/medizin/erkrankungen-des-verdauungsapparates-bei-hund-und-katze/anatomie-und-funktion-des-verdauungstraktes-der-haustiere)

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