Antibiotic-responsive diarrhea in dogs (Proceedings)

Simplistically speaking, antibiotic-responsive diarrhea is a case of diarrhea that responds to antibiotic therapy. There are several different terms that describe similar clinical conditions: antibiotic-responsive diarrhea, tylosin-responsive diarrhea, small intestinal bacterial overgrowth (SIBO), and intestinal dysbiosis. At the current time it is unclear, whether all 4 terms describe essentially the same condition or if only one of these terms would be most appropriate.

As said above, antibiotic-responsive diarrhea is a case of diarrhea that responds to antibiotic therapy. As such, tylosin-responsive diarrhea would be similar. However, this term was coined by a Finish group after they had done several studies in dogs with chronic diarrhea. The dogs showed poor response to several different antibiotics, but all responded to tylosin. The reasons for these findings are unclear. One explanation is that tylosin has an optimal antibiotic spectrum against the intestinal bacteria that are responsible for the diarrhea. Another explanation is that tylosin has other properties in addition to its antibiotic properties. Small intestinal bacterial overgrowth refers to an expansion of unfavorable bacteria in the small intestinal tract. Finally, dysbiosis refers to a qualitative and/or quantitative derangement of the small intestinal microbiota that leads to clinical signs of small bowel diarrhea. Again, it is unclear whether these conditions can really be separated and for a lack of better understanding in the following text the term antibiotic-responsive diarrhea is used as an overarching term for all four conditions.

The microbiota

The intestinal microbiota is made of a wide variety of microorganisms, including bacteria, viruses, and fungal organisms. Most attention has been given to the intestinal bacterial ecosystem, which is made up of a complex mixture of a wide variety of bacterial species. Traditional studies describing the intestinal bacterial ecosystem have employed traditional culture techniques. Unfortunately, such studies are associated with problems in reproducibility. For example, a variety of studies reported the physiologic bacterial ecosystem in the proximal small intestine of dogs, but different studies found a preponderance of different bacterial species. It has since been recognized that a variety of factors, such as location, breed, age, collection method, culture media, culture conditions, and others all play an important role in the results of culture-based studies. However, the true diversity of the intestinal bacterial ecosystem became evident only recently with the advent of new micromolecular technologies. These newer technologies have revealed a far greater diversity of the bacterial ecosystem in the intestinal tract than previously assumed and have also shown that fungal organisms, such as Pichia spp., Cryptococcus spp., Candida spp., and Trichosporon spp. are far more frequently present in the intestinal tract of healthy dogs than previously believed. Using these new methodologies it has now been estimated that the intestinal bacterial ecosystem is made up of more than 1000 different bacterial species. These new studies are also crucial in studying the microbiota in patients gastrointestinal diseases.

Physiologic importance of the intestinal bacterial ecosystem

This ecosystem is initially established during birth and continues to develop during suckling. The impact of the intestinal microbiota and the bacterial ecosystem has been well established by studies in germ-free rodents. These rodents show a wide variety of morphological and physiological alterations that overall equate to a state of compromised intestinal function and immunity. In healthy animals the physiologic microbiota, and most prominently the bacterial ecosystem, has several important functions. Firstly, it protects the host against pathogenic bacteria, by competing for oxygen, luminal substrates, and space, but also by synthesizing and releasing substances that inhibit bacterial growth, so-called bacteriocins. Intestinal bacteria also produce short-chain fatty acids by metabolizing dietary components that are often non-digestible for the host. In turn, these short chain fatty acids serve as an important energy source for the intestinal mucosa, leading to epithelial cell proliferation and mucosal growth. Members of the intestinal bacterial ecosystem also synthesize a variety of vitamins, including riboflavin (vitamin B2), biotin (vitamin B7), folic acid (vitamin B9), cobalamin (vitamin B12), and vitamin K. It is important to note, however, that physiologically, the synthesis of some of these vitamins, for example cobalamin, is not of any significance to the host as the synthesis may occur distally to where the vitamin can be absorbed. Finally, intestinal bacteria also play a crucial role in the development of the intestinal immune system. They stimulate said intestinal immune system, which plays a crucial role in overall host defense throughout all stages of life.

Impact of the Microbiota and Intestinal Bacterial Ecosystem on Gastrointestinal Health

It has long been known that some dogs and cats with acute or chronic diarrhea respond to antibiotic therapy. While some of these patients may be infected with a primary gastrointestinal pathogen, such as Salmonella spp. or some pathogenic Campylobacter strains, a specific causative organism can't be identified in most of these patients. As detailed above, a variety of names have been associated with this condition, including small intestinal bacterial overgrowth, antibiotic-responsive diarrhea, tylosin-responsive diarrhea, and intestinal dysbiosis. Regardless of the term used for describing this condition, a response to antibiotics would suggest that these patients have an alteration of the intestinal bacterial ecosystem that leads to diarrhea and that modification of the intestinal bacterial ecosystem can lead to improvement of clinical signs. Some prebiotics and antibiotics have been used to treat these dogs, but probiotics should be considered a far more physiologic approach to treating these patients.

It is important to note that the effects of the intestinal microbiota reach further than just a direct effect of bacteria causing clinical signs. For example, in a recent study the diversity of the small intestinal microbiota was far less in dogs with inflammatory bowel disease than in healthy dogs. This would suggest that the makeup of the intestinal microbiota has an impact on the inflammatory state in the GI tract. Furthermore, adjustments in the intestinal microbiota may be able to prevent or reverse this inflammatory state and thus be able to ameliorate clinical signs in dogs with gastrointestinal disease. Simplistically, the mucosa of the gastrointestinal epithelium has only a limited number of adhesion sites to dendritic cells. Pathogenic bacteria adhere to these dendritic cells and their antigens are being presented to immune-modulating cells in the Peyer's patches. In the case of pathogenic bacteria this leads to the release of proinflammatory cytokines and an inflammatory response. In contrast, when bacteria belonging to the physiologic bacterial ecosystem adhere and their antigens are being presented to immune-modulating cells, the release of proinflammatory cytokines is being curbed.

Etiology

Antibiotic-responsive diarrhea (ARD) is caused by an abnormal proliferation of bacteria and/or the change in bacterial species present in the small intestinal lumen. However, ARD is not a primary disorder in most if not all patients with this disorder. There are several protective mechanisms that prevent ARD. Gastric acid, intestinal motility, and antibacterial activity of pancreatic juice all limit the bacterial numbers in the small intestine. Gastric acid directly destroys bacteria that are ingested with the diet and also decreases the pH of the ingesta, leading to a lower pH in the proximal small intestine. However, the lack of gastric acid secretion alone is not sufficient for ARD to develop. Propulsive movements of the small intestine are probably the most important protective factor since there is no physical barrier between the large intestine and the small intestine that would prevent retrograde cultivation of the small intestine by the large intestinal microbiota. The antibacterial properties of pancreatic juice are not well understood. Pancreatic digestive enzymes may be partly responsible for the antibacterial action of pancreatic juice.

Any disease process that affects one or more of the protective mechanisms discussed can ultimately lead to ARD.

Pathophysiology

Bacteria in the small intestinal lumen are metabolically active. This affects the host in many ways. Bacteria compete with the host for nutrients. One example is the water-soluble vitamin cobalamin. Under physiologic conditions cobalamin is absorbed in the ileum. Especially anaerobic bacteria effectively compete for dietary cobalamin leading to decreased cobalamin absorption and, over time, to a depletion of cobalamin body stores and hypocobalaminemia. Bacteria also produce a variety of substances. For example, bacteria synthesize folic acid and vitamin K, which leads to an increased serum folate concentration. This does not appear to have any pathologic effect on the host. Some bacterial species also produce bacterial proteases and glycosidases, short chain fatty acids, unconjugated bile acids, ethanol, enterotoxins, endotoxins, and peptidoglycan polysaccharide polymers. These substances can be toxic to the enterocytes or damage the brush border. Additionally, these substances can be absorbed from the small intestine and can lead to hepatic toxicity or even systemic effects, though clinically significant hepatotoxicity and systemic effects are rare in dogs with ARD.

The increased production of short chain fatty acids leads to a decreased intraluminal pH in the small intestine and to an increased osmotic load. Medium and long chain fatty acids have a more direct effect on fluid absorption and can even stimulate fluid secretion, contributing to the pathogenesis of diarrhea in dogs with ARD. Also, fatty acids can undergo hydroxylation and hydroxylated fatty acids further inhibit fluid absorption and increase fluid secretion. Bacteria in the small intestinal lumen also deconjugate bile acids. Bile acids are synthesized, conjugated, and secreted in the liver and are released into the small intestine when CCK stimulates the contraction of the gall bladder after a meal is ingested. These bile acids play an important role in fat digestion by emulsifying dietary fats. During ARD bile acids are deconjugated and reabsorbed and are no longer available for fat emulsification. This can lead to fat malabsorption and in extreme cases to steatorrhea. Fat malabsorption further increases the fatty acid load in the intestinal lumen leading to further inhibition of fluid absorption and diarrhea. Fat malabsorption also can be associated with malabsorption of fat-soluble vitamins. Finally, ARD can lead to the destruction of the brush border, leading to protein- and carbohydrate malabsorption.

While there are many functional changes that are brought about by ARD, in most cases there are no morphologic changes that can be visualized by light microscopy. In some cases villous blunting and mild lymphocytic-plasmacytic inflammation maybe observed, but ARD maybe secondary to intestinal inflammation rather than be the cause of it.

Clinical findings

ARD in dogs leads to chronic small bowel diarrhea that is often intermittent. Weight loss can be present in some cases. Other clinical signs maybe due to the primary underlying disease process such as partial obstruction, exocrine pancreatic insufficiency, or others. Complete blood count and serum chemistry profile are within normal limits in most cases, but mild elevations in hepatic enzyme activities can be observed. Abdominal radiographs and ultrasound may reveal findings that implicate an underlying disease process but otherwise will not reveal any specific findings.

Diagnosis

Part of the controversy about ARD is due to the fact that this disorder is difficult to diagnose. Traditionally, the gold standard for assessment of the small intestinal bacterial ecosystem diagnosis is the culture of duodenal juice. However, not only is the collection of duodenal juice challenging, but also the culture of duodenal juice once collected is difficult, time-consuming, and expensive and requires a laboratory that has experience in this area. Also, bacterial culture methods grossly underestimate bacterial diversity of the small intestinal bacterial ecosystem. Therefore, culture of duodenal juice is not suggested. It remains to be seen whether molecular-based methods allow a better assessment of the small intestinal microflora.

Several non-invasive diagnostic tests that have been found to be useful in human patients suspected of having ARD and have also been evaluated in dogs with suspected ARD:

Serum folate concentration - As pointed out previously folic acid is synthesized by enteric bacteria and is available for absorption. If dogs with ARD for a long period of time, serum folate concentration increases. While an increased serum folate concentration is fairly specific for ARD it is not very sensitive. In one study only 50% of all dogs with ARD had increased serum folate concentrations.

Serum cobalamin concentration - Many species of bacteria utilize cobalamin and compete with the body for dietary supplies. Unlike an increased serum folate concentration a decreased serum cobalamin concentration is not specific for ARD. Any severe small intestinal disease involving the ileum can lead to cobalamin deficiency. Also, a lack of intrinsic factor and digestive proteases in dogs with exocrine pancreatic insufficiency can cause cobalamin deficiency. A decreased serum cobalamin concentration is rather insensitive for ARD and in one study only 25% of dogs with ARD had decreased serum cobalamin concentration. A combination of a decreased serum cobalamin and an increased serum folate concentration is highly specific for ARD but rather insensitive. These two parameters are to date the most practical diagnostic tools for the diagnosis of ARD in a private practice setting.

Other non-invasive diagnostic tests, previously evaluated for the diagnosis of ARD, such as unconjugated bile acid concentration in serum, breath hydrogen concentration, and 13 C-xylose and 13 C-bile acid tests have not shown to be consistently useful for the diagnosis of this condition.

Treatment

The therapeutic goal in dogs with ARD is the identification and treatment of the inciting cause. For example, serum TLI concentration should be evaluated. Dogs with EPI and secondary ARD usually do not require specific therapy for ARD once they are treated with enzyme supplementation.

If a primary cause cannot be identified the dog should be treated with antimicrobial therapy. Oxytetracycline (10-20 mg/kg BID to TID for 4-6 weeks) used to be the therapy of choice. Unfortunately, oxytetracycline for oral use has become largely unavailable. Tylosin (25 mg/kg BID for 6 weeks) is the new antibiotic agent of choice. Other antibiotics, such as metronidazole can also be used. Some dogs respond to therapy rapidly and do not have a recurrence. However, other dogs do not respond to antibiotic therapy alone. If there is no marked improvement after 2 weeks of appropriate antibiotic therapy further work up is necessary. Some dogs may respond to therapy with a complete resolution of clinical signs but may have a recurrence of clinical signs as soon as antibiotic therapy is discontinued. These patients require further diagnostic work up. In some of these patients a specific underlying cause of the ARD can be identified and treated accordingly. However, in some dogs no specific cause can be identified and prolonged, maybe even life-long, antimicrobial therapy is required.

If serum cobalamin concentration is decreased below the lower limit of the reference range or if cobalamin is in the very low end of the reference range, cobalamin should be supplemented parenterally. Also, in a recent study the use of fructooligosaccharides (FOS) in the diet showed a lasting advantageous effect. While this has not been evaluated as of yet, other prebiotics, such as inulin or beet-pulp may also prove to be beneficial. The efficacy of probiotics for the treatment of ARD has not been evaluated. However, if a patient does not respond to therapy may provide another therapeutic modality.