By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
What is a Protein-Losing Enteropathy?
Normal small intestine under the microscope. The finger-like projections are the microvilli that are responsible for absorbing nutrients. We want nutrients to be absorbed in, not leaked out and lost. Photo by Nephron via Wikimedia Commons
If the intestine is leaking nutrients out instead of absorbing them in, clearly the result is a nutritional disaster. In protein-losing enteropathy, that is exactly what is happening: the intestine and/or stomach is actually leaking out protein. This is obviously a serious problem as without protein coming in, the body's only choice to make the proteins it needs is to break down other proteins and reassemble the amino acids that make them up into other proteins. Muscle is broken down to make way for higher priority blood proteins. The immune system's ability to make antibodies may be compromised. Debilitation results from protein break-down and the intestinal disease that started it all facilitates this by creating diarrhea. Lymphangiectasia is one of several protein-losing enteropathies.
Albumin Loss in Protein-Losing Enteropathy
The main protein that one cannot afford to lose is called albumin. This protein normally is produced by the liver and it circulates in the bloodstream acting as a carrier for biochemicals that require transport but cannot actually dissolve in blood by themselves. Albumin can be considered sort of a mass transit system in the bloodstream, a bus or subway, if you will, carrying important biochemicals from one place to another.
Albumin, by being the most prevalent blood protein, also is responsible for keeping water in the bloodstream. When water cannot be held within the vasculature, it leaks out, causing fluid accumulation in tissue (i.e. edema) or in the chest or abdomen (i.e. effusion).
Of course, in a protein-losing enteropathy other proteins are lost, too. Antibodies, proteins of blood clotting, enzymes, etc. all leak out the intestine and are forever lost in the feces that exits the body.
The body tries hard to maintain its albumin level by extracting protein from other sources (like muscle), and having the liver make albumin from the components of these other proteins. This may help maintain a workable amount of albumin in the bloodstream but it comes at the expense of muscle tissue and other working proteins.
There are several described causes of protein-losing enteropathy:
- Inflammatory bowel disease
- Histoplasmosis (a fungal infection)
- Diseases associated with intestinal bleeding (parvovirus, intestinal parasites, cancers such as lymphoma)
- Breed associated PLE (as in soft-coated wheaten terrier, shar pei, Maltese, and basenji)
- Crypt disease (where the intestinal crypts dilate and plug with mucus)
- Intestinal lymphangiectasia
We will limit our discussion to lymphangiectasia.
What is Intestinal Lymphangiectasia?
Lymph is a fluid that circulates through the body similar to the way blood does; however, the heart actively pumps blood through the body while lymph is pumped passively via the normal muscle activity of the body. Lymph consists of cells called lymphocytes. They are cells of the immune system as well as the fluid that collects from the tissues and shunts into lymphatic vessels, which are similar to veins.
The word lymphangiectasia means dilated lymph vessels. In the intestinal tract, lymphangiectasia is usually caused by some kind of inflammation that puts pressure back on the lymph vessels, leading them to dilate. Lymph flow may be blocked by the inflammatory events in the intestine or local structures.
Lacteals are a specific type of lymph vessels in the intestinal tract designed to absorb nutritional fats. The intestine is able to absorb our nutrients through small finger-like structures called microvilli. Lacteals run though the center of the microvilli happily imbibing fats from the food we have eaten. When there is high pressure within the lymph vessels, the tender lacteals burst and instead of absorbing fats, the lymph inside, its cells, fats, and precious proteins are lost. The intestine may be able to reabsorb some of these valuable substances at other sites but if the inflammatory intestinal disease that started the problem in the first place is widespread, the balance may have shifted to nutritional loss rather than gain.
Small terrier breeds, particularly the Yorkshire terrier, appear predisposed to the development of lymphangiectasia. Lymphangiectasia is extremely rare in cats.
What Does One See at Home?
Weight loss is the most consistent sign along with chronic diarrhea, vomiting, and, in more advanced cases, fluid accumulation in the abdomen creating a bloated appearance.
How Do We Make a Diagnosis?
In most cases, an obviously sick, skinny dog is brought to the veterinarian. Sometimes the above classical signs are seen but sometimes there is no specific hint of this condition until blood test results are in.
Low Lymphocyte Count
Animals with lymphangiectasia have lymphocytes rolling out their lacteals and into their intestine to be lost forever. The low blood lymphocyte count is so consistent with lymphangiectasia that it is difficult to make this diagnosis if this finding is not present.
Cholesterol is part of the lymph fluid that gets lost.
Low Albumin Level
Low blood albumin level is the most consistent finding in lymphangiectasia, though it is possible to have lymphangiectasia in only a small portion of the intestine and still maintain a normal albumin level. There are a limited number of conditions that can lead to a low albumin level: a protein-losing enteropathy; a protein-losing nephropathy where the albumin is lost via leaking kidney membranes; reduced albumin production by a diseased liver; hypoadrenocorticism, a deficiency in cortisone; or serum leakage through extreme skin damage such as third-degree burns.
These conditions can be easily ruled out one by one. It is obvious if there are third-degree burns. If there are none, a routine urinalysis will indicate if there is significant protein loss in the urine and if glomerular disease should be pursued. A liver function test such as a bile acids test will indicate whether or not there is latent liver failure. Blood testing can be easily done to rule out hypoadrenocorticism (also called Addison's disease). If there aren't any of these conditions, then there must be a protein-losing enteropathy by exclusion. Biopsy will be needed to determine the nature of the intestinal disease causing the protein-losing enteropathy.
Sample from a human with lymphangiectasia. The villi are short and squat compared to the normal finger-like villi shown at the top of this page. Note the large white areas within the intestinal microvillus shown. These are dilated lacteals as described. Photo by Patho via Wikimedia Commons
Tissue sampling is necessary to distinguish the intestinal diseases that cause protein-losing enteropathy.
With most PLE patients being deficient in albumin, this means they are not in a good position to heal from a surgical biopsy so endoscopy is generally recommended as it is less invasive. With endoscopy, a small camera on a long flexible scope travels down the mouth, through the stomach and into the intestine (or up the anus past the large intestine and into the lower small intestine) where biopsy samples can be pinched off with a grabber. The procedure requires general anesthesia but is otherwise low risk and most patients can go home the same day. Often the areas of intestinal disease are patchy and surrounded by large areas of normal intestine so it is important to biopsy the right area. Ultrasound of the abdomen prior to endoscopy is helpful in determining where to sample, plus often a few spoons of corn oil or other fat are often fed a few hours prior to biopsy so that dilated lacteals with plump and leaking fat visibly in the best areas for biopsy. Because PLE patients often lose important vitamins and blood clotting factors, blood clotting tests are generally done just prior to endoscopy.
The image here is actually from a human with lymphangiectasia but the concept is the same. Note the large pale areas within the intestinal microvillus shown. These are dilated lacteals. (The small white areas in the layer of cells lining the villi are "goblet cells" that produce mucus. The dilated lacteals are the large expanses of white or lavender underneath the villus lining).
The first step in treatment is to address the underlying cause. In many cases of lymphangiectasia, there is an association with inflammatory bowel disease (IBD). Whether or not the IBD is actually causing the lymphangiectasia remains a matter of speculation but anti-inflammatories are often used aggressively in lymphangiectasia patients. Prednisolone has been the traditional immune suppressive medication in this situation but as it causes an increase in water consumption that can contribute to edema, other medications - particularly cyclosporine - have become more popular as it does not have steroid side effects.
Intestinal loss of a body protein called "anti-thrombin III" leads to a tendency to form abnormal blood clots which can lodge in inappropriate places (embolization). As prevention, anti-clotting medications are often used. Aspirin has been traditional but clopidogrel is gaining popularity.
Nutritional management of PLE is a bit tricky. The diet must be especially digestible and high in protein (20-25% protein on a dry matter basis) so as to replace all the protein being lost through the leaky GI tract. Further, the diet should be no more than 15% fat on a dry matter basis (lower for more severely affected patients). Reading a diet label shows percentages on an "as fed" basis, so to compare diets, it is necessary to convert values to a dry matter basis through knowing the moisture content. To see how this is done click here. There are therapeutic diets available for diseases like this one where fat restriction is crucial. Non-prescription diets are unlikely to meet the above criteria but if you know how to read the food label properly you may be able to find one. Injectable vitamin supplements are likely to be needed.
Severely affected dogs will find even the fat content of the prescription diets too high and may need to begin with what is called an "elemental diet." These are typically made for humans and are powders that can be mixed with water. They are not nutritionally balanced for dogs and can only be used for a few weeks. Alternatively, home cooked diets can be used but a professional veterinary nutritionist should be consulted to get a proper recipe. Again, because of intestinal absorption issues injectable vitamin supplements are likely to be needed. The goal is to eventually return to a standard commercial dog food.
If the underlying condition is treatable then prognosis for lymphangiectasia is good. It should be understood that lymphangiectasia is unlikely to be cured and at best can be managed.
How to Convert Food Values from "As Fed" to "Dry Matter"
First, find the "Guaranteed Analysis" on the label of the food. It should look something like this:
Crude Protein (min) 7.5%
Crude Fat (min) 4%
Crude Fiber (max) 2%
Moisture (max) 76%
In this case the moisture content is 76%. Subtracting from 100%, that means that there is 24% dry matter (the actual food). In other words, this can of puppy food is 76% water and 24% actual nutrients.
To convert the guaranteed analysis numbers to dry matter basis, simply divide them by the percentage of the diet that is dry matter:
Crude protein: 7.5 divided by 24 = 0.31 or 31%
Crude fat: 4 divided by 24 = 0.166 or 16.6%
Crude fiber: 2 divided by 24 =0.08 or 8%
Foods can only truly be compared by converting to dry matter basis.