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Update on Acute Hemorrhagic Diarrhea Syndrome in Dogs

      Keywords

      Key points

      • Acute hemorrhagic diarrhea syndrome (AHDS) of dogs is characterized by a sudden onset of severe bloody diarrhea associated with a dramatic loss of fluid into the intestinal lumen.
      • A normal-to-increased packed cell volume helps to differentiate AHDS from true GI bleeding, in which anemia is usually observed.
      • Whereas a positive PCR for NetF encoding Clostridium perfringens strains is suggestive for AHDS, neither a fecal culture for C perfringens nor a fecal ELISA test for C perfringens enterotoxin nor a fecal PCR for C perfringens α toxin gene are diagnostic.
      • Rapid volume replacement through a large-bore IV catheter is the mainstay of treatment. Antibiotics are usually not necessary because translocation of bacteria through the damaged intestinal mucosa can usually be compensated for by the dog.
      • Most dogs recover, but the mortality rate is high if untreated. Only 10% to 15% of dogs have repeat occurrences of AHDS, but 28% of dogs develop chronic or chronic recurrent signs of GI disease later in life.

      Introduction

      Hemorrhagic diarrhea is much more commonly observed in dogs in comparison with cats and humans, and the gut is considered as the main shock organ in this species [
      • Hackett T.B.
      Gastrointestinal complications of critical illness in small animals.
      ]. Numerous causes for acute hemorrhagic diarrhea (AHD) exist and hemorrhagic diarrhea is not a diagnosis per se, because it only reflects the severity of intestinal mucosal damage. The two most important causes of AHD are canine parvovirus infection and acute hemorrhagic diarrhea syndrome (AHDS). Dogs with canine parvovirus infection are typically young, and have a low hematocrit and/or total protein and albumin levels. Any disease causing intestinal destruction associated with significant fluid loss into the intestinal lumen can clinically resemble AHDS, but this syndrome has unique clinical features that help distinguish it from other conditions (Box 1). Clostridial overgrowth and the associated release of clostridial toxins seem to be responsible for epithelial cell injury and the rapid movement of large amounts of fluid and electrolytes into the gut lumen (Fig. 1). AHDS is not only an acute life-threatening gastrointestinal (GI) disorder, but a significant number of dogs develop long-term consequences. A recently published, retrospective, longitudinal study reported that about 30% of dogs with AHDS develop chronic diarrhea later in life, supporting the theory that severe intestinal mucosal damage and associated barrier dysfunction might trigger chronic GI diseases [
      • Skotnitzki E.
      • Suchodolski J.S.
      • Busch K.
      • et al.
      Frequency of signs of chronic gastrointestinal disease in dogs after an episode of acute hemorrhagic diarrhea.
      ].
      Unique Clinical Features of AHDS
      • Vomiting as first clinical sign in 80% of cases
      • Sudden onset of severe bloody diarrhea
      • Significant loss of fluid into the intestinal lumen
      • Blood loss is usually minor in proportion to fluid loss
      • Rapid clinical improvement with adequate fluid therapy
      • Not transmissible to other dogs or species
      Figure thumbnail gr1
      Fig. 1Severe necrohemorrhagic enteritis caused by a type A Clostridium perfringens strain in a dog with AHDS. In the intestinal lumen “raspberry jam” characteristic stool is observed.

      Pathogenesis

      A primary pathogenic role of Clostridium perfringens in dogs with AHDS was first suspected in the late 1980s, when the first cases with peracute hemorrhagic enteritis and necropsy examinations were reported. Gram-positive bacilli adherent to the necrotic epithelial surfaces of the intestinal tract were found, and large numbers of C perfringens (published at the time with the former name Clostridium welchii) were cultured from the intestinal content [
      • Prescott J.F.
      • Johnson J.A.
      • Patterson J.M.
      • et al.
      Haemorrhagic gastroenteritis in the dog associated with Clostridium welchii.
      ]. Interpretation of those findings was difficult, because C perfringens is part of the normal colonic flora and clostridial overgrowth is a possible postmortem sequela. However, a prospective study that included 10 dogs with AHD confirmed this hypothesis. Endoscopy was performed within the first 12 hours of presentation in all dogs and clostridial strains identified as C perfringens by culture and mass spectrometry using matrix-assisted laser desorption/ionization-time of flight were detected on the surface of the small intestinal mucosa by immunohistochemistry (Fig. 2) [
      • Leipig-Rudolph M.
      • Busch K.
      • Prescott J.F.
      • et al.
      Intestinal lesions in dogs with acute hemorrhagic diarrhea syndrome associated with netF-positive Clostridium perfringens type A.
      ]. C perfringens is rarely cultured from the small intestine and clostridial-type bacteria are not visible with routine histology in healthy dogs or dogs with intestinal disorders, such as chronic enteropathies and canine parvovirus infection. In 5 of 10 dogs with AHDS clostridial strains could be cultured from biopsy samples of the duodenum and each of the five genotyped isolates encoded the pore-forming toxin NetF, which is a β-pore-forming toxin that belongs to the leucocidin/hemolysin superfamily [
      • Leipig-Rudolph M.
      • Busch K.
      • Prescott J.F.
      • et al.
      Intestinal lesions in dogs with acute hemorrhagic diarrhea syndrome associated with netF-positive Clostridium perfringens type A.
      ]. In vitro experiments showed that the NetF toxin is able to form pores in susceptible cells leading to plasma membrane destruction and subsequent osmotic cell lysis. In an experimental study, a NetF insertional inactivation mutant of C perfringens was created, and this strain was no longer toxic for an equine ovarian cell line. Additionally, antiserum against NetF toxin neutralized the cytotoxicity of wild-type NetF-producing C perfringens type A strains isolated from a dog with AHD [
      • Mehdizadeh Gohari I.
      • Parreira V.R.
      • Nowell V.J.
      • et al.
      A novel pore-forming toxin in type A Clostridium perfringens is associated with both fatal canine hemorrhagic gastroenteritis and fatal foal necrotizing enterocolitis.
      ]. All of these findings suggest that NetF is likely to be the major virulence factor in C perfringens strains responsible for canine AHDS. Although NetF-encoding C perfringens strains are not present in every dog with AHDS, several studies showed a higher prevalence of C perfringens encoding the netF gene in fecal samples from dogs with AHDS compared with healthy dogs and dogs with other intestinal diseases. For example, in dogs with parvovirus infection, which show similar intestinal lesions to dogs with AHDS, no C perfringens strains encoding netF could be detected [
      • Sindern N.
      • Suchodolski J.S.
      • Leutenegger C.M.
      • et al.
      Prevalence of Clostridium perfringens netE and netF toxin genes in the feces of dogs with acute hemorrhagic diarrhea syndrome.
      ]. It can hence be concluded that the overgrowth of clostridial strains is not only a secondary consequence to a primary insult of the intestinal tract, as has been speculated previously. In addition to the pore-forming toxins, C perfringens produces numerous extracellular enzymes and minor toxins, such as collagenase (κ-toxin), neuraminidase, caseinase (λ-toxin), deoxyribonuclease (η-toxin), hyaluronidase (μ-toxin), and urease, which might be released during the bacterial proliferation phase and additionally contribute to the damage of the intestinal mucosa. C perfringens enterotoxin (CPE) is a cytotoxic (causing tissue damage) enterotoxin, and its production and release are associated with sporulation. However, CPE cannot consistently be detected in the feces of dogs with AHDS, and is therefore not considered to be the primary toxin causing mucosal lesions and hemorrhagic diarrhea. In contrast to CPE encoding strains, which are permanently present in the feces of some dogs after recovering from AHDS and in healthy dogs, netF drops lower than the detection limit of the polymerase chain reaction (PCR) assay within 7 days following presentation with AHDS in all dogs (Fig. 3) [
      • Ziese A.L.
      • Suchodolski J.S.
      • Hartmann K.
      • et al.
      Effect of probiotic treatment on the clinical course, intestinal microbiome, and toxigenic Clostridium perfringens in dogs with acute hemorrhagic diarrhea.
      ]. No specific cause inciting clostridial proliferation and toxin release, such as food poisoning, has been identified to date. Thus, it is likely that different causes are responsible for the abrupt clostridial overgrowth and clinical consequence, which is AHDS.
      Figure thumbnail gr2
      Fig. 2Histopathology (Giemsa, original magnification ×20) of a dog with AHDS. Extensive necrosis of the superficial colonic mucosa, which demonstrates a complete loss of the mucus layer. Note the plump, rod-shaped bacteria directly attached to surface of the colon. Destruction of the intestinal lining and of the mucus layer represent two main features of intestinal barrier dysfunction.
      Figure thumbnail gr3
      Fig. 3Clinical course, abundance of Clostridium perfringens encoding enterotoxin, and NetF toxin in dogs with AHDS. Characteristic clinical course of dogs with uncomplicated AHDS based on the acute hemorrhagic diarrhea index (A). Associated with the rapid clinical improvement of this self-limiting syndrome is the reduction of C perfringens strains encoding for the NetF toxin to such a degree that after 7 days these strains can usually not be detected by a sensitive PCR assay in any dog (B). Abundance of clostridial strains encoding for enterotoxin also declines, but a significant number of dogs stay positive for these toxigenic strains (C). CHDS, canine haemorrhagic diarrhoea severity index.
      A necrotizing and neutrophilic inflammation often affecting the entire gut, but with individual severity in different areas, represents the characteristic histopathologic features in dogs with AHDS. Lesions with the highest histologic scores were detectable in the colon, whereas proximal parts of the small intestines had lower scores, and the gastric mucosa was not affected [
      • Leipig-Rudolph M.
      • Busch K.
      • Prescott J.F.
      • et al.
      Intestinal lesions in dogs with acute hemorrhagic diarrhea syndrome associated with netF-positive Clostridium perfringens type A.
      ]. It can therefore be speculated that clostridial overgrowth and toxin release starts in the large intestine and subsequently affects the small intestine. In cases with a lethal outcome, the observation of C perfringens on the surface of the gastric mucosa has also been described, but this most likely represents severe, rare forms of AHDS. Based on in vivo endoscopy and histology findings from dogs with AHDS, the stomach is typically not affected, and therefore the most commonly used name “hemorrhagic gastroenteritis” seems inadequate and should be replaced by the more precise description “acute hemorrhagic diarrhea syndrome” [
      • Unterer S.
      • Busch K.
      • Leipig M.
      • et al.
      Endoscopically visualized lesions, histologic findings, and bacterial invasion in the gastrointestinal mucosa of dogs with acute hemorrhagic diarrhea syndrome.
      ].

      Signalment, clinical, and clinicopathologic abnormalities

      AHDS is often reported in small breed dogs, such as Miniature Schnauzer, Cavalier King Charles Spaniel, Dachshund, Chihuahua, Yorkshire Terrier, Maltese, Miniature Pinschers, and small mix breeds [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ,
      • Allen-Deal A.
      • Lewis D.
      Prevalence of Clostridium perfringens alpha toxin and enterotoxin in the faeces of dogs with acute haemorrhagic diarrhoea syndrome.
      ]. However, in one study Labrador Retrievers were the most prevalent breed in a dog population presenting with AHDS, so large breed dogs must also be considered predisposed [
      • Dupont N.
      • Jessen L.R.
      • Moberg F.
      • et al.
      A retrospective study of 237 dogs hospitalized with suspected acute hemorrhagic diarrhea syndrome: disease severity, treatment, and outcome.
      ]. The incidence of AHDS was higher in middle-aged dogs with a median of 5 years, compared with the general hospital population, which had a median of 10 years, as described in a study including 108 cases of dogs with AHDS [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ]. A sex predilection has not been documented.
      AHDS dogs characteristically present with an acute to peracute history of GI signs of less than 24 hours duration [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ]. Chronic enteropathies may predispose to AHDS, because a significant number of dog owners describe the presence of chronic intestinal signs before the onset of AHD (personal communication Skotnitzki). Vomiting is observed as the first clinical sign in about half of the dogs with AHDS, and 80% of dog owners describe hematemesis. It is noteworthy that some dogs with severe AHD at presentation do not have any or few bowel movements on Days 2 and 3 after presentation, because of an empty GI tract and reduced intestinal motility secondary to the acute intestinal inflammation. Soft stools are present until Day 10, because the severely damaged intestinal mucosa might need time to completely recover [
      • Unterer S.
      • Strohmeyer K.
      • Kruse B.D.
      • et al.
      Treatment of aseptic dogs with hemorrhagic gastroenteritis with amoxicillin/clavulanic acid: a prospective blinded study.
      ].
      Depending on the severity of dehydration at presentation, lethargy, weakness, tachycardia, prolonged capillary refill time, and weak pulses may be detected. The most consistent physical examination finding is tachycardia, which is a compensatory mechanism because of the hypovolemia or a stress response caused by abdominal discomfort and nausea [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ]. Although dogs with AHDS typically have a significant loss of fluid through the intestinal tract, skin turgor may appear normal in many cases because of the peracute onset and the lag time in compartmental fluid shifts. It is important to know that dogs with AHDS consistently have low to low-normal rectal temperature. Pyrexia is an unusual clinical presentation in dogs with AHDS and should raise concerns for the presence of another infectious agent (eg, Salmonella spp), translocation of bacteria from the GI tract to the systemic circulation (septicemia), or a strong inflammatory reaction involving other organs (eg, pancreatitis) [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ,
      • Dupont N.
      • Jessen L.R.
      • Moberg F.
      • et al.
      A retrospective study of 237 dogs hospitalized with suspected acute hemorrhagic diarrhea syndrome: disease severity, treatment, and outcome.
      ]. Despite the severe lesions in the intestinal tract of dogs with AHDS, significant abdominal pain is usually not present on physical examination.
      Although substantial lesions are present in the entire intestinal tract of dogs with AHDS and hematemesis and/or AHD are observed, blood loss is usually minor in proportion to fluid loss [
      • Leipig-Rudolph M.
      • Busch K.
      • Prescott J.F.
      • et al.
      Intestinal lesions in dogs with acute hemorrhagic diarrhea syndrome associated with netF-positive Clostridium perfringens type A.
      ]. As a result, dogs with AHDS characteristically have an increased packed cell volume at presentation, with a median packed cell volume of 57% and a range of 33% to 76% described in one study [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ]. A packed cell volume in the normal and greater than the normal range is helpful to differentiate this form of hemorrhagic enteritis from true GI bleeding, in which anemia is usually present. The presence of anemia at presentation in a dog with hemorrhagic diarrhea is always indicative of a true intestinal bleed (eg, hemostatic disorders, ulceration, intestinal neoplasia).
      C perfringens is an enterotoxigenic pathogen that causes significant necrosis of the intestinal mucosa but is not invasive. Translocation of bacteria through the damaged intestinal mucosa can usually be compensated for despite the significant loss of the epithelial integrity [
      • Unterer S.
      • Lechner E.
      • Mueller R.S.
      • et al.
      Prospective study of bacteraemia in acute haemorrhagic diarrhoea syndrome in dogs.
      ]. Leukogram changes, which reflect systemic bacterial infection or septicemia/sepsis, such as significant neutrophilia (>25 × 1012/L) or neutropenia and a severely increased number of band neutrophils (>2.5 × 1012/L), is not observed in dogs with an uncomplicated form of AHDS. A mild left shift is seen in about 50% of dogs, which most likely reflects the increased demand of neutrophils for the first-line defense at the surface of the destroyed intestinal mucosa [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ]. Presence of an inflammatory leukogram characterized by a significant elevation of mature neutrophils and/or banded neutrophils in contrast to a stress leukogram in any dog with AHD should always prompt a search for other causes of AHD or suspicion of another enteropathogenic, enteroinvasive bacterial infection.
      Neutropenia should also raise the suspicion of canine parvovirus infection, especially if the dog is young and/or inadequately vaccinated [
      • Schnelle A.N.
      • Barger A.M.
      Neutropenia in dogs and cats: causes and consequences.
      ]. Severe GI tract hemorrhage is also described in dogs with hypoadrenocorticism [
      • Busch K.
      • Wehner A.
      • Dorsch R.
      • et al.
      [Acute haemorrhagic diarrhoea as a presenting sign in a dog with primary hypoadrenocorticism].
      ,
      • Medinger T.L.
      • Williams D.A.
      • Bruyette D.S.
      Severe gastrointestinal tract hemorrhage in three dogs with hypoadrenocorticism.
      ]. Therefore, a basal serum cortisol concentration is generally useful in every dog with unexplained hemorrhagic diarrhea to rule out hypoadrenocorticism, but is mandatory in dogs with history of recurrent episodes of acute diarrhea, depression, and weakness and with the absence of a stress leukogram, lymphocytosis, and eosinophilia or characteristic electrolyte changes (eg, hyperkalemia, hyponatremia).
      Although few dogs have low serum albumin concentration at presentation, some dogs show a significant hypoalbuminemia after fluid resuscitation. Albumin levels should especially be monitored in dogs that have a decreased or low-normal level initially and in dogs with peripheral edema and body cavity effusions because they might require colloid osmotic support with fresh frozen plasma (small breed dogs) or albumin solutions (large breed dogs). Azotemia is only rarely observed in dogs with AHDS despite the presence of severe dehydration. Therefore, it is important to determine urine specific gravity before fluid administration to differentiate between prerenal and renal azotemia if present. Alkaline phosphatase activity is usually normal in dogs with AHDS, whereas alanine aminotransferase activity is increased in about 20% of the cases. This is most likely secondary hepatocellular damage caused by decreased hepatic perfusion during hypovolemia. Because dogs with primary acute liver failure can also present with AHD, it is important to attempt differentiating primary liver disease from reactive hepatopathy by reevaluating alanine aminotransferase, total bilirubin, and serum bile acids 3 to 5 days after rehydration. Alanine aminotransferase usually drops by about 50% and total bilirubin and serum bile acids normalize in secondary hepatopathies, such as AHDS (personal observation Unterer). Serum lactate elevation reflects the severity of hypovolemia and reduced peripheral perfusion and is hence an unspecific finding [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ].

      Recent discoveries

      In August and November 2019 an outbreak of AHD was observed in dogs living in the Oslo region of Norway. Hundreds of dogs were affected, and many dogs died across Norway. At the beginning of the outbreak a presumptive diagnosis of idiopathic AHDS had been made because no underlying cause could be identified. However, three arguments speak against an AHDS diagnosis: (1) AHDS is a syndrome affecting individual dogs and is not a transmissible disease, as suspected/documented in Oslo; (2) AHDS has a low mortality rate and this is in contrast to the high number of fatal cases during the outbreak; and (3) one major finding was the positive fecal culture results of Providencia alcalifaciens with 100% sequence identity in different affected dogs. P alcalifaciens is a gram-negative bacterium, which was not a part of the normal dog fecal microbiota in the time before the outbreak. It was concluded from this study that the increased abundance of P alcalifaciens in dogs with AHD relative to healthy dogs might explain the unexpectedly high numbers of dogs suffering from a severe form of AHDS during this outbreak. Transmission of P alcalifaciens may have triggered the concurrent outgrowths of C perfringens in a dysbiotic background microbiota [
      • Herstad K.M.V.
      • Trosvik P.
      • Haaland A.H.
      • et al.
      Changes in the fecal microbiota in dogs with acute hemorrhagic diarrhea during an outbreak in Norway.
      ]. However, a recent unpublished observational case-control study shows that P alcalifaciens can also be found in dogs with acute nonhemorrhagic diarrhea and that these dogs are not more severely affected, needing more aggressive treatment or having a worse prognosis than dogs with acute nonhemorrhagic diarrhea in which this pathogen was not isolated (personal communication Dr S. Salavati). Hence, the pathologic significance of P alcalifaciens remains unclear.
      A recently published study showed a 28% prevalence of chronic GI signs in dogs with a previous episode of AHDS, which was significantly higher compared with an age and breed matched control group with a prevalence of 14% [
      • Skotnitzki E.
      • Suchodolski J.S.
      • Busch K.
      • et al.
      Frequency of signs of chronic gastrointestinal disease in dogs after an episode of acute hemorrhagic diarrhea.
      ]. It is speculated that because of destruction of the epithelial mucosal barrier an increased number of food antigens and bacteria can pass the intestinal barrier and influence the immune system. A subset of dogs with a high AHDS-index (>9) have significantly increased intestinal permeability, as assessed by iohexol [

      Reisinger A SH, Ishii P, Suchodolski J, Unterer S, Steiner J, Lidbury JA, Hartmann K, Busch K. Iohexol as marker of intestinal barrier dysfunction in dogs with acute hemorrhagic diarrhea syndrome. presented at: ACVIM Forum; 2022; June 23-25 in Austin, Texas.

      ]. In these dogs, small proteins and allergens crossing the epithelial barrier might trigger a breakdown of oral tolerance to specific food components [
      • Wesemann D.R.
      • Nagler C.R.
      The microbiome, timing, and barrier function in the context of allergic disease.
      ].
      It is also known that the physiologic composition and diversity of the intestinal bacterial communities is essential to prevent the development of allergic and immune-mediated diseases [
      • Plunkett C.H.
      • Nagler C.R.
      The influence of the microbiome on allergic sensitization to food.
      ]. Thus, dysbiosis typically characterized by a significant clostridial overgrowth associated with reduced abundances of Blautia, Turicibacter, Faecalibacterium, and Streptococcus spp in dogs with AHDS might represent an additional factor for the increased risk for long-term consequences of acute enteritis [
      • Suchodolski J.S.
      • Markel M.E.
      • Garcia-Mazcorro J.F.
      • et al.
      The fecal microbiome in dogs with acute diarrhea and idiopathic inflammatory bowel disease.
      ].

      Diagnostic strategy and monitoring

      A diagnosis can reliably be made based on GI biopsies when C perfringens is observed on the necrotic surface, because C perfringens attached to the intestinal mucosal is not seen [
      • Leipig-Rudolph M.
      • Busch K.
      • Prescott J.F.
      • et al.
      Intestinal lesions in dogs with acute hemorrhagic diarrhea syndrome associated with netF-positive Clostridium perfringens type A.
      ,
      • Unterer S.
      • Busch K.
      • Leipig M.
      • et al.
      Endoscopically visualized lesions, histologic findings, and bacterial invasion in the gastrointestinal mucosa of dogs with acute hemorrhagic diarrhea syndrome.
      ] in either healthy dogs or in dogs with any other GI disease. However, because of the potential risk for the patient, financial expense, and lack of therapeutic consequence, a gastroduodenoscopy and colonoscopy are not recommended for the diagnosis of AHDS. Instead, noninvasive tests are preferred, but unfortunately, they all have limitations.
      Fecal culture positive for Clostridium spp is not diagnostic for AHDS, because many clostridial strains are found in feces of healthy dogs [
      • Marks S.L.
      • Rankin S.C.
      • Byrne B.A.
      • et al.
      Enteropathogenic bacteria in dogs and cats: diagnosis, epidemiology, treatment, and control.
      ]. Most fecal PCRs for clostridial toxin genes, such as CPE or α-toxin, are not helpful for the diagnosis of AHDS. Detecting CPE protein in feces based on enzyme-linked immunosorbent assay also does not confirm the diagnosis, because only up to 30% of dogs with AHDS, but also 14% of healthy dogs, can test positive according to one report [
      • Busch K.
      • Suchodolski J.S.
      • Kuhner K.A.
      • et al.
      Clostridium perfringens enterotoxin and Clostridium difficile toxin A/B do not play a role in acute haemorrhagic diarrhoea syndrome in dogs.
      ]. Other tests, such as increases in fecal inflammatory or “leakage” markers (eg, calprotectin, S100A12, α1-proteinase inhibitor), reflect intestinal damage, but these parameters are not specific for AHDS and are unhelpful to guide treatment or prognosis [
      • Heilmann R.M.
      • Guard M.M.
      • Steiner J.M.
      • et al.
      Fecal markers of inflammation, protein loss, and microbial changes in dogs with the acute hemorrhagic diarrhea syndrome (AHDS).
      ].
      A positive PCR for C perfringens type A strains, which encode for the pore-forming toxin NetF, supports the diagnosis of AHDS [
      • Sindern N.
      • Suchodolski J.S.
      • Leutenegger C.M.
      • et al.
      Prevalence of Clostridium perfringens netE and netF toxin genes in the feces of dogs with acute hemorrhagic diarrhea syndrome.
      ]. However, because fecal NetF toxin expression decreases rapidly, dogs presenting late in the course of the disease can test negative [
      • Ziese A.L.
      • Suchodolski J.S.
      • Hartmann K.
      • et al.
      Effect of probiotic treatment on the clinical course, intestinal microbiome, and toxigenic Clostridium perfringens in dogs with acute hemorrhagic diarrhea.
      ]. Hence, in many dogs, diagnosis of AHDS is still based on clinical observations and exclusion of other known causes of AHD.
      Taking a thorough history is essential to rule out the ingestion of toxins or drugs that might cause mucosal irritation, or a previous event that may have caused intestinal damage (eg, severe hypotension or heat stroke). Fecal examinations, including flotation and giardia antigen enzyme-linked immunosorbent assay, should be used to identify parasites, which although rarely solely responsible for such severe GI signs, may contribute to the clinical course. Hemoconcentration is a typical finding in AHDS, which is characterized by GI fluid loss rather than true GI bleeding. A routine serum biochemistry profile is necessary to exclude hepatic and renal disease. Baseline cortisol level should be performed as a standard screening test to rule out hypoadrenocorticism, followed by further endocrine testing if baseline cortisol concentration is less than 2 μg/dL (<55 nmol/L), even if typical electrolyte changes (hyperkalemia, hyponatremia) are not present. Diagnostic imaging, especially abdominal ultrasound, can identify other abnormalities, such as acute pancreatitis or intestinal obstruction (eg, foreign body, intussusception). A recent study showed correlation of C-reactive protein concentrations with the severity of the AHDS on the day of admission, but could not predict the duration of hospitalization or mortality [
      • Dupont N.
      • Jessen L.R.
      • Moberg F.
      • et al.
      A retrospective study of 237 dogs hospitalized with suspected acute hemorrhagic diarrhea syndrome: disease severity, treatment, and outcome.
      ]. Additional tests might need to be performed depending on the clinical course, especially if the patient does not significantly improve with fluid therapy or develops pyrexia (Fig. 4).
      Figure thumbnail gr4
      Fig. 4Summary of the approach to diagnose AHDS and to identify possible complications. ACTH, corticotropin; ALT, alanine aminotransferase; AP, alkaline phosphatase; CBC, complete blood count; CPV, canine parvovirus; ELISA, enzyme-linked immunosorbent assay; PCV, packed cell volume. aPCV reference range: 35% to 58%.
      The AHDS index (Table 1) is a useful tool to assess clinical disease activity and to monitor the clinical course (discussed later) [
      • Mortier F.
      • Strohmeyer K.
      • Hartmann K.
      • et al.
      Acute haemorrhagic diarrhoea syndrome in dogs: 108 cases.
      ,
      • Dupont N.
      • Jessen L.R.
      • Moberg F.
      • et al.
      A retrospective study of 237 dogs hospitalized with suspected acute hemorrhagic diarrhea syndrome: disease severity, treatment, and outcome.
      ].
      Table 1Criteria for Assessment of the Canine AHDS Activity Index
      Parameter0123
      Activity reductionNot reducedMildlyModeratelySeverely reduced
      Appetite reductionNot reducedMildlyModeratelySeverely reduced
      Vomiting (times/d)012–3>3
      Fecal consistencyNormalSlightly softVery softWatery diarrhea
      Defecation (n/d)12–34–5>5
      Dehydration (%)0<55–10>10
      Total AHDS score0–34–56–89 or greater
      Clinical significance of diseaseClinically insignificantMild AHDSModerate AHDSSevere AHDS
      AHD index is calculated as the sum of the scoring of all the parameters listed in the table. Each parameter is scored from 0–3 on any individual treatment day, and the sum of scores (maximum 18) yield a total cumulative score.

      Treatment and clinical course

      Severe dehydration caused by vomiting, anorexia, and diarrhea generally requires intravenous (IV) fluid replacement therapy. The peracute and severe onset of the disease, and the frequent presence of paralytic ileus, which can sequester large fluid volumes, may lead to an underestimation of actual fluid loss or requirements. Therefore, frequent reevaluation (every 2–6 hours) of clinical parameters and adjustments of fluid therapy are essential.
      Rapid volume replacement with balanced electrolyte solutions by giving a shock bolus as fast as 30 mL/kg over 10 minutes, repeated up to three times, might be needed, followed by continuous replacement of ongoing losses if there is no risk of fluid overload (eg, compromised cardiac function). Usually, patients improve quickly with aggressive fluid resuscitation, which should be continued and adapted until the dog is able to match losses with voluntary oral intake. This usually takes 2 to 3 days and electrolytes should be measured regularly (every 12–24 hours) during that time.
      Some patients suffer from severe protein loss because of gut barrier destruction, which may lead to a reduced intravascular oncotic pressure and subsequent edema and/or ascites. Therefore, serum albumin should be measured initially and after rehydration. In patients with severe hypoalbuminemia (<18 g/L) administration of synthetic colloids, albumin, or plasma should be considered.
      In addition, symptomatic treatment comprised of antiemetics (eg, maropitant, 1 mg/kg once a day IV or subcutaneously [SC]; metoclopramide, 0.2–0.5 mg/kg three times a day IV, SC, or orally) and pain medication (eg, metamizole, 30–50 mg/kg three times a day IV, SC, or orally; buprenorphine, 5–20 μg/kg IV or SC) should be administered as needed. Nonsteroidal anti-inflammatory drugs are not recommended because of their propensity to cause further GI injury. The administration of omeprazole is also not indicated in AHDS, because these dogs characteristically do not have gastric bleeding and indeed may even be contraindicated because proton pump inhibitors are suspected to further affect the integrity of the intestinal tight junction complex and might represent an additional factor responsible for intestinal dysbiosis [
      • Konig J.
      • Wells J.
      • Cani P.D.
      • et al.
      Human intestinal barrier function in health and disease.
      ].
      The general treatment recommendations in several textbooks for AHD include the administration of antimicrobials. The likely rational behind this is that either primary intestinal infection with enteropathogens is difficult to diagnose or the assumption of a compromised intestinal barrier, which is a major risk factor for translocation of commensal bacteria and subsequent septicemia. Approximately half of dogs with AHDS are classified as having systemic inflammatory response syndrome at presentation, based on the criteria defined by de Laforcade and colleagues (2003) [
      • de Laforcade A.M.
      • Freeman L.M.
      • Shaw S.P.
      • et al.
      Hemostatic changes in dogs with naturally occurring sepsis.
      ] and Hauptman and others (1997) [
      • Hauptman J.G.
      • Walshaw R.
      • Olivier N.B.
      Evaluation of the sensitivity and specificity of diagnostic criteria for sepsis in dogs.
      ]. Although most of these abnormalities might be driven by hypovolemia alone, a prospective study conducted by one of the authors showed that the incidence of detectable true bacteremia in dogs with AHDS is low (11%) and not different from healthy control dogs (14%) [
      • Unterer S.
      • Lechner E.
      • Mueller R.S.
      • et al.
      Prospective study of bacteraemia in acute haemorrhagic diarrhoea syndrome in dogs.
      ]. The likely conclusion from these observations is that even though bacterial translocation might still occur during AHDS, septicemia is often prevented by effective elimination of bacteria and their toxins from the portal circulation by the liver. We hence presume that risk of septicemia and sepsis is low in dogs with AHDS as long as portal vasculature and hepatic function are not compromised, and patients are otherwise immunocompetent. In addition to this, two independent studies (one an unpublished doctoral thesis) have evaluated the benefit of antibiotics in dogs with AHDS. When using amoxicillin/clavulanic acid in a prospective, placebo-controlled, blinded setup in dogs with AHDS without systemic inflammatory response syndrome, the clinical course (as measured by AHD activity index) and outcome (duration of hospitalization, survival) was not different between the placebo and treatment groups [
      • Unterer S.
      • Strohmeyer K.
      • Kruse B.D.
      • et al.
      Treatment of aseptic dogs with hemorrhagic gastroenteritis with amoxicillin/clavulanic acid: a prospective blinded study.
      ]. It was concluded that in dogs with aseptic AHDS, antibiotics may not be necessary as part of treatment [
      • Unterer S.
      • Strohmeyer K.
      • Kruse B.D.
      • et al.
      Treatment of aseptic dogs with hemorrhagic gastroenteritis with amoxicillin/clavulanic acid: a prospective blinded study.
      ]. Another study evaluated whether the combination of metronidazole plus amoxicillin-clavulanate is superior to the monoantibiotic therapy with amoxicillin-clavulanate, where again, there was no significant difference in hospitalization time between treatment groups, nor in daily clinical scores. Hence the addition of metronidazole does not confer a clinical benefit when treating even severe cases of hemorrhagic diarrhea in dogs [
      • Ortiz V.
      • Klein L.
      • Channell S.
      • et al.
      Evaluating the effect of metronidazole plus amoxicillin-clavulanate versus amoxicillin-clavulanate alone in canine haemorrhagic diarrhoea: a randomised controlled trial in primary care practice.
      ]. Based on the available evidence, routine administration of antibiotics is thus not recommended. Not only does routine usage of antibiotics not have a benefit, it can have detrimental effects on the intestinal microbiota, sustaining dysbiosis and promoting the development of multi-resistant bacteria [
      • Werner M.
      • Suchodolski J.S.
      • Straubinger R.K.
      • et al.
      Effect of amoxicillin-clavulanic acid on clinical scores, intestinal microbiome, and amoxicillin-resistant Escherichia coli in dogs with uncomplicated acute diarrhea.
      ,
      • Torres-Henderson C.
      • Summers S.
      • Suchodolski J.
      • et al.
      Effect of Enterococcus faecium strain SF68 on gastrointestinal signs and fecal microbiome in cats administered amoxicillin-clavulanate.
      ]. Data from human medicine suggest that dysbiosis induced by antimicrobials is associated with an increased risk for developing asthma, postinfectious irritable bowel disease, and chronic enteropathies such as Crohn disease [
      • Kummeling I.
      • Stelma F.F.
      • Dagnelie P.C.
      • et al.
      Early life exposure to antibiotics and the subsequent development of eczema, wheeze, and allergic sensitization in the first 2 years of life: the KOALA Birth Cohort Study.
      ,
      • Metsala J.
      • Lundqvist A.
      • Virta L.J.
      • et al.
      Prenatal and post-natal exposure to antibiotics and risk of asthma in childhood.
      ,
      • Klem F.
      • Wadhwa A.
      • Prokop L.J.
      • et al.
      Prevalence, risk factors, and outcomes of irritable bowel syndrome after infectious enteritis: a systematic review and meta-analysis.
      ,
      • Umamaheswari B.
      • Biswal N.
      • Adhisivam B.
      • et al.
      Persistent diarrhea: risk factors and outcome.
      ,
      • Ungaro R.
      • Bernstein C.N.
      • Gearry R.
      • et al.
      Antibiotics associated with increased risk of new-onset Crohn's disease but not ulcerative colitis: a meta-analysis.
      ]. However, because recognizing septic events and infections with enteropathogens at presentation is challenging, especially when dogs show clinical signs of hypovolemia, close monitoring and assessment of the clinical response to adequate fluid therapy is mandatory in these cases.
      Because about one-third of dogs with AHDS develop chronic GI sings, antibiotic-induced dysbiosis might represent one important factor in triggering chronicity. Thus, antibiotics should be used judiciously and be restricted to patients with clinical signs of sepsis, additional immunocompromise, or a reduced capacity to eliminate bacteria from the portal blood (eg, portosystemic shunt, significant loss of liver function) (Table 2).
      Table 2Indications for Broad-Spectrum Antibiotics in Dogs with Intestinal Disorders
      History
      • Immunocompromised patients (eg, hyperadrenocorticism, immunosuppressive treatment, portosystemic shunt)
      Physical examination finding at presentation
      • Hyperthermia (°F/°C)
      >103.1/39.5
      Physical examination findings after rehydration and pain management
      • Tachycardia (beats/min)
      >140 (small breed dogs)

      >120 (large breed dogs)
      • Tachypnea (breaths/min)
      >40
      • Hypothermia
      • Hypotension
      Hematologic and serum biochemical changes
      • Neutrophilia (cells/μL)
      >20,000
      • Neutropenia (cells/μL)
      <3000
      • Band neutrophils (cells/μL)
      >2500
      • Hypoglycemia (mmol/L)
      <3.5
      In addition to aggressive fluid and symptomatic therapy, early enteral nutrition with a highly digestible diet is important in dogs with AHD, because it promotes enterocyte proliferation and improves intestinal barrier integrity [
      • Will K.
      • Nolte I.
      • Zentek J.
      Early enteral nutrition in young dogs suffering from haemorrhagic gastroenteritis.
      ,
      • Mohr A.J.
      • Leisewitz A.L.
      • Jacobson L.S.
      • et al.
      Effect of early enteral nutrition on intestinal permeability, intestinal protein loss, and outcome in dogs with severe parvoviral enteritis.
      ]. Adding dietary fiber might be useful because it functions as an important substrate for potentially beneficial microbes in the large intestine, and acts as an energy source for colonocytes. In addition, dietary polysaccharides might have a direct impact on the colonic mucus layer, which is important for barrier function and thus prevention of bacteria translocation [
      • Desai M.S.
      • Seekatz A.M.
      • Koropatkin N.M.
      • et al.
      A dietary fiber-deprived gut microbiota degrades the colonic mucus barrier and enhances pathogen susceptibility.
      ,
      • Li N.
      • Zheng B.
      • Cai H.F.
      • et al.
      Cost-effectiveness analysis of oral probiotics for the prevention of Clostridium difficile-associated diarrhoea in children and adolescents.
      ,
      • Vernocchi P.
      • Del Chierico F.
      • Fiocchi A.G.
      • et al.
      Understanding probiotics' role in allergic children: the clue of gut microbiota profiling.
      ]. It has been suggested that restricting the diet to one protein source or even feeding a hydrolyzed diet in the acute phase of diarrhea might help to avoid sensitizing the immune system to a variety of antigens, but there is little evidence for this.
      Probiotics have so far not shown to reduce the time of clinical recovery, but a high dose of a probiotic mixture (eg, Slab51 bacterial blend) might be useful to accelerate the normalization of the intestinal microbiota and to displace toxigenic C perfringens strains. Bacteria considered as beneficial, such as Blautia spp and Faecalibacterium, had significantly higher abundance on Day 7 in dogs with AHDS treated with a probiotic compared with placebo. Probiotics might also have a positive long-term impact on the intestinal microbiota, because enterotoxin genes were significantly lower in dogs with AHDS receiving a Slab51 probiotic blend compared with a placebo group after 3 weeks of supplementation [
      • Ziese A.L.
      • Suchodolski J.S.
      • Hartmann K.
      • et al.
      Effect of probiotic treatment on the clinical course, intestinal microbiome, and toxigenic Clostridium perfringens in dogs with acute hemorrhagic diarrhea.
      ].
      Fecal microbiota transplantation (FMT) is an important treatment modality in people with Clostridium difficile infections (CDI). Similar to canine AHDS, CDI is characterized by an overgrowth of clostridial strains and bacterial toxin release causing mucosal lesions. One important difference is the comparatively milder clinical course of AHDS and rapid clinical improvement even of many severe cases with symptomatic treatment only. This is in contrast to human CDI, which can cause significant morbidity, requires aggressive antibiotic treatment, and antibiotic resistance and recurrence even after appropriate antibiotic treatment are common. Therefore, it is not unexpected that although FMT is a successful treatment of primary or recurrent CDI in people, FMT in dogs with AHDS did not have a significant effect on hospitalization times or clinical signs compared with the control treatment group [
      • Gal A.
      • Barko P.C.
      • Biggs P.J.
      • et al.
      One dog's waste is another dog's wealth: a pilot study of fecal microbiota transplantation in dogs with acute hemorrhagic diarrhea syndrome.
      ,
      • Keithlin J.
      • Sargeant J.
      • Thomas M.K.
      • et al.
      Systematic review and meta-analysis of the proportion of Campylobacter cases that develop chronic sequelae.
      ,
      • Ajene A.N.
      • Fischer Walker C.L.
      • Black R.E.
      Enteric pathogens and reactive arthritis: a systematic review of Campylobacter, Salmonella and Shigella-associated reactive arthritis.
      ]. However, the same study showed promising trends of accelerated normalization of the fecal microbiota composition and richness in dogs with AHDS after receiving a single FMT rectally. Rapid improvement of gut homeostasis may have a positive long-term impact on intestinal health in these dogs, but this still needs to be evaluated.

      Summary

      Several investigations have provided new insight into the pathogenesis and role of bacteria in AHDS. Based on current information, it is assumed that NetF-toxin is likely a major bacterial virulence factor driving this syndrome [
      • Mehdizadeh Gohari I.
      • Parreira V.R.
      • Nowell V.J.
      • et al.
      A novel pore-forming toxin in type A Clostridium perfringens is associated with both fatal canine hemorrhagic gastroenteritis and fatal foal necrotizing enterocolitis.
      ,
      • Sindern N.
      • Suchodolski J.S.
      • Leutenegger C.M.
      • et al.
      Prevalence of Clostridium perfringens netE and netF toxin genes in the feces of dogs with acute hemorrhagic diarrhea syndrome.
      ]. The trigger for transient clostridial overgrowth is not known and it is possible that AHDS is initiated by different causes (eg, motility changes, nutritional factors, food poisoning) or represents a multifactorial disorder. The peracute loss of intestinal mucosal integrity in AHDS frequently results in a dramatic fluid shift into the intestinal lumen, which requires aggressive treatment. Although response to IV fluid therapy and symptomatic treatments is usually rapid, some dogs might need prolonged fluid and colloid support because of protein loss, whereas antibiotics are rarely needed or beneficial. Development of pyrexia should raise suspicion for an additional underlying disease. Clinicians are advised to be mindful of overzealous antibiotic use in this condition, because of their detrimental effects on the intestinal microbiota, which could possibly predispose to GI problems later in life. AHDS can have long-term impacts in a significant number of dogs [
      • Skotnitzki E.
      • Suchodolski J.S.
      • Busch K.
      • et al.
      Frequency of signs of chronic gastrointestinal disease in dogs after an episode of acute hemorrhagic diarrhea.
      ], mainly an increased risk for chronic GI disorders [
      • Skotnitzki E.
      • Suchodolski J.S.
      • Busch K.
      • et al.
      Frequency of signs of chronic gastrointestinal disease in dogs after an episode of acute hemorrhagic diarrhea.
      ,
      • Kilian E.
      • Suchodolski J.S.
      • Hartmann K.
      • et al.
      Long-term effects of canine parvovirus infection in dogs.
      ,
      • Sato-Takada K.
      • Flemming A.M.
      • Voordouw M.J.
      • et al.
      Parvovirus enteritis and other risk factors associated with persistent gastrointestinal signs in dogs later in life: a retrospective cohort study.
      ,
      • Francino M.P.
      Antibiotics and the human gut microbiome: dysbioses and accumulation of resistances.
      ,
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      ,
      • Zheng D.
      • Liwinski T.
      • Elinav E.
      Interaction between microbiota and immunity in health and disease.
      ], which can develop in up to 30% of dogs following an episode of AHDS [
      • Wesemann D.R.
      • Nagler C.R.
      The microbiome, timing, and barrier function in the context of allergic disease.
      ]. Although probiotics and FMT have not been shown to have a positive effect on clinical severity, hospitalization times, or overall outcome in AHDS, they could be important adjunctive treatments that reestablish the defunct microbiota. Future research should hence focus on establishing strategies to prevent chronicity, such as modulation of the intestinal microbiota (eg, FMT, probiotics) or therapeutic agents improving intestinal integrity.

      Clinics care points

      • In any dog with acute hemorrhagic diarrhea a packed cell volume has to be performed to differentiate between true GI bleeding and hemorrhagic enteritis.
      • In young and inadequately vaccinated dogs with acute hemorrhagic diarrhea testing for canine parvovirus infection is mandatory.
      • When dogs with acute hemorrhagic diarrhea have a history of episodes with lethargy or vague and nonspecific clinical problems, check a serum basal cortisol level to rule out hypoadrenocorticism.
      • When albumin levels are already low initially at the time of presentation do not forget to reassess this parameter after fluid resuscitation to identify those dogs that may require colloids, plasma, or albumin solutions.
      • If there is evidence of hypovolemia, such as lethargy, weakness, tachycardia, prolonged capillary refill time, and weak pulse pressure, a rapid volume replacement through a large-bore IV catheter is required.
      • Do not administer antibiotics routinely, but in dogs with AHDS and signs of systemic infection, immunosuppression, and reduced liver function, and in dogs inadequately responding to symptomatic therapy, antibiotic treatment might be indicated/necessary.

      Disclosure

      The authors mentioned the study Ziese AL, Suchodolski JS, Hartmann K, et al. Effect of probiotic treatment on the clinical course, intestinal microbiome, and toxigenic Clostridium perfringens in dogs with acute hemorrhagic diarrhea. PLoS One 2018;13:e0204691. Anna Lena Ziese was working with us and in this study the probiotic (trade name Vivomixx) was provided by the company MENDES S.A. (Via Giacometti, 1, 6900 Lugano Switzerland). Anna Lena Ziese received travel support from MENDES S.A. to travel to an international conference. However, this support does not influence the data of this review article. In addition to this disclosure, the authors have nothing to disclose.

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