The digestive tract is the cause of many health issues in our dogs. Here we get to grips with exactly what each part does, the signs that something is wrong, and the possible diseases that affect it. By understanding more, we can detect issues early and identify concerning symptoms.
The digestive tract
Vets consider the digestive system to start at the mouth, with the teeth and tongue. After chewing, food moves through the pharynx (the part of the throat that ‘swallows’ food) and down the oesophagus into the stomach. The stomach is filled with acidic digestive juices and made up of muscle to allow good mixing of the digestive juices with the food.
Periodically, the stomach empties into the small intestine, where the food is mixed with new digestive juices and begins moving through the length of the gut. Food moves with the rhythmic squeezing of the muscles in the gut wall in a process called peristalsis. The small intestine is entirely devoted to digestion. Each part is perfectly designed to break down food, absorb the essential parts and deliver each molecule to exactly where it needs to be.
The small intestine eventually moves into the large intestine, which is considerably shorter in length. In the large intestine, water is absorbed, and what was once food forms faeces. The large intestine is also the home of the natural ‘friendly bacteria’ that occupy the gut. These bacteria help digestion and protect against ‘bad’ bacteria.
The final stop in the digestive tract is the rectum, which holds the stool before it is passed.
Several other organs in the abdomen are also key for digestion, although food does not pass through them directly. One of the most important is the pancreas. This small organ lies next to the small intestine, near the stomach. It produces enzymes that break up pretty much everything we eat, including fats and proteins. It also produces bicarbonate, which it flushes into the small intestine to neutralise the stomach acid.
Another key organ is the liver. The liver makes bile acids, which are essential for digesting fats and absorbing fat-soluble vitamins. The liver is also ‘next in line’ for the blood from the intestines. This means it has a huge role in collecting, storing and utilising the valuable food molecules from the blood. It also helps to filter the toxins to prevent them from continuing around the body in the bloodstream.
The cells involved
The lining of the digestive tract, from mouth to rectum, consists of broadly the same layers and cells, albeit in slightly different proportions.
The innermost layer – the mucosa – is the most important for the digestive process. It isn’t smooth but instead covered in finger-like projections called ‘villi’, which increase the wall’s surface area for absorption of food molecules. It also contains glands which produce the digestive juices. The mucosa itself is made up of hundreds of cells arranged in a single layer. This is to make absorption easier as there is only one layer for the nutrients to cross to enter the bloodstream. The cells in this layer are joined tightly, however, so that whilst it is easy for food to pass, bacteria are prevented from getting into the bloodstream. Good bacteria on the inside of the gut also help by blocking any holes that might exist so that bad bacteria remain trapped in the gut. The mucosa cells only live for a couple of days before they are shed and replaced with new ones. This also helps to reduce the risk of bacterial damage to the cells, affecting their permeability. Interestingly, the cells themselves are also not smooth and are covered in microvilli – microscopic finger-like projections that again increase the surface area for food absorption.
Beneath the mucosa is the submucosa. This layer contains the blood vessels that carry nutrients away from the gut. It also contains nerve cells to send messages about hunger and pain. Beneath this is the muscularis externa, an external muscle layer. This is the layer that contracts to mix the food with digestive juices and keep it moving through the gut in the process called peristalsis.
What can go wrong in the digestive system?
Despite the system being so complicated, there are only a few ways that the body can show something isn’t right with the digestive system.
Vomiting
Special cells in the brain form the ‘vomiting centre’, which coordinates and controls vomiting. Nerves from the brain and other areas like the stomach end in this region. These nerves are triggered when the stomach is over-full or irritated or when the body experiences travel sickness. Signals are also received from the chemoreceptor trigger zone, which detects hormones and blood-borne toxins.
Once the vomiting centre has decided to cause vomiting, it sends signals to the muscles of the ribs and the diaphragm, causing a huge intake of breath to fill the chest and push the diaphragm down and to the abdominal muscles, which contract. The subsequent increase in pressure in the abdomen squishes the stomach contents high enough that the food reaches the mouth. The feeling of food at the back of the throat causes a further push from the vomiting centre, expelling the stomach contents out of the mouth.
Diarrhoea
Diarrhoea occurs when the body secretes too much fluid or fails to absorb as much fluid as it should. The fluid moves through the digestive tract and causes expansion of the intestine, which in turn causes faster movement of the gut contents. Faster movement means that there is even less chance for the body to absorb the water, and so begins the vicious cycle. Diarrhoea can be caused by bacteria or viruses that damage the intestines and stop the absorption of water, by allergic reactions, or even by the body in order to remove toxins as quickly as possible. More chronic diarrhoea can be caused when the wrong type of bacteria live in the gut, perhaps because antibiotics wiped out the good bacteria or sometimes because the good bacteria never establish themselves properly.
Diarrhoea caused by anxiety is also a possibility in dogs. This occurs when the nerves in the gut react to stress hormones, which increase the rate of peristalsis.
Malabsorption
Malabsorption occurs when, for whatever reason, the body cannot absorb the nutrients from the food. This could be because the digestive system is not working properly (perhaps because there are not enough digestive juices being formed) or because the absorption is compromised, for example, by a virus that destroys the microvilli. The body struggles without the necessary nutrients and will lose weight and condition. Malabsorption often accompanies diarrhoea because the nutrients that were not properly absorbed cause liquid to be drawn into the intestines.
Which diseases affect the digestive system?
Dietary indiscretion and allergies
Dietary indiscretion, or eating the wrong things, is common in dogs. Unfortunately, so are allergies. Either way, symptoms are usually loose stools and sometimes vomiting. Dogs, in these instances, are rarely very ill and usually improve within a couple of days. It’s possible, though, for these dogs to become more ill if the food they’ve eaten is mouldy or filled with bacteria, in which case they may get bloody diarrhoea. Hospitalisation may be necessary.
Parasites
There are several types of internal parasites that cause problems in dogs. These include roundworms, heartworms, tapeworms, hookworms, and whipworms.
Intestinal worms can be a serious problem in young puppies. Hookworms can cause anaemia and roundworms can lead to poor growth and development.
In adult dogs, however, intestinal parasites are only occasionally life-threatening. Debilitated animals or those with a weakened immune system are more likely to experience severe intestinal parasitism and show clinical signs. Having said this, heartworm is a life-threatening parasite spread by mosquitoes and causes significant damage to the heart and lungs even in healthy dogs. The disease caused by heartworm is one of the most serious conditions seen in small animal practice.
Blockages
Objects that have been removed from dog intestines are too numerous to list here but include everything from children’s toys to chunks of bone to soft toy stuffing to clothing. Dogs with a blockage will vomit and cannot keep food or water down for long. They’ll sometimes have diarrhoea, but more often than not, will have no faeces at all after a time. As these dogs can look very much like dietary indiscretion dogs, an X-ray is generally the best way to diagnose them. Even if that object can’t be seen on X-ray, the vet often sees gas build-up behind the blockage. Surgical removal is the only option, and the surgery can have complications, so it’s best to do what you can to avoid a blockage.
Intussusception
Intussusception is predominantly an issue in puppies and young dogs. It occurs when part of the gut telescopes inside another piece of the gut. It’s hugely painful and causes vomiting and other signs similar to a blockage. Surgery is needed to correct it, and, often, part of the gut has to be removed as it is too damaged to survive.
Parvovirus (and other viral diseases)
Dogs can pick up many viral diseases that cause vomiting and diarrhoea. The vast majority pass within a day or two, but parvovirus is extremely serious. The virus replicates in the cells of the small intestine, causing severe haemorrhagic diarrhoea and vomiting. It is highly contagious and often fatal. If the dog survives, it is likely they will have digestive issues for a long time, as the virus destroys the lining of the small intestine, making it less efficient at absorbing nutrients.
Pancreatitis
Pancreatitis is inflammation of the pancreas. It is intensely painful and can be fatal. Dogs with pancreatitis will refuse to eat and will often vomit. They may or may not have normal faeces, but they have a tender abdomen on examination. This cycle often goes on for several days, so these dogs normally need hospitalisation for fluid, pain relief and sometimes syringe or tube feeding. Luckily, blood tests have been developed that quickly diagnose pancreatitis, so treatment can start promptly.
Pancreatitis can become recurrent in some dogs, and they may need to go onto a low-fat food for life to avoid flare-ups.
Liver disease
Liver disease can be acute, such as infectious hepatitis, or chronic, such as storage diseases of the Border Terrier. Either way, liver diseases can cause vomiting as the liver fails to do its job and toxins enter the bloodstream. Thankfully, the liver can regenerate, given time, so treatment is usually supportive until the liver has the chance to do so. Blood tests and ultrasounds are important for diagnosing liver disease.
Cancers
Cancers in the intestine can be diffuse (meaning spread throughout) or localised as a mass. Masses can block the intestines and cause symptoms similar to a foreign body or go unnoticed until they eventually get in the way. Sometimes, they can be removed, depending on their location. Diffuse cancers, like lymphoma, are impossible to remove, and chemotherapy is the only treatment option.
Take home message
The digestive system is complex and needs to be treated with care. If you have any concerns about the digestive health of your canine companion, contact your vet today.
