Diagnostic evaluation of acute diarrhea
Acute diarrhea is a common clinical problem, and one that varies in severity depending on the underlying cause. Acute diarrhea is often self-limiting and may or may not require treatment.
Often the etiology is unknown, either because diagnostic testing is not performed, the animal gets better prior to diagnostic testing, or diagnostic testing fails to reveal a definitive cause. In other instances, acute diarrhea can be severe and life threatening (as with acute parvoviral enteritis or hemorrhagic gastroenteritis). In these cases, there will be a need for a more intense diagnostic workup and treatment, with attempts to identify an underlying etiology and correct electrolyte, acid-base, and nutritional disturbances, potential sepsis, and other metabolic consequences. It is also important to keep in mind that chronic gastrointestinal disorders (such as IBD) can develop acute exacerbations.2.1.3.1 Etiology of acute diarrhea
As with acute vomiting, the causes of acute diarrhea can be divided into gastrointestinal and extra-gastrointestinal causes (Table 2.2). Again, it is beyond the scope of this chapter to discuss in detail the clinical findings and treatment of each disorder; however, many of these are discussed in other sections of this book.
2.1.3.2 Pathophysiological changes with acute diarrhea
There are several mechanisms that can result in acute diarrhea. These include osmotic diarrhea, secretory diarrhea, exudative diarrhea, and abnormal motility.4,5 Of these, osmotic diarrhea is the most common, and generally results from malassimilation of nutrients leading to an increase in osmotically active particles in the bowel lumen and therefore a net water and electrolyte loss. Examples of this include lactose intolerance, overeating, ingestion of food items that cannot be absorbed, and parvovirus enteritis.
Excessive secretion leading to diarrhea can occur from bacterial enterotoxins, malabsorbed substances (such as non-absorbed bile acids), intestinal inflammation, and certain drugs.5 Exudative diarrhea generally occurs with infectious causes, including viral enteritis and severe bacterial enteritis. Abnormal motility as a primary cause of acute diarrhea is uncommon. More commonly, motility disturbances are a consequence of the diarrhea. The most common motility derangement in the small intestine is a hypomotile gut with loss of rhythmic segmental contractions and thus decreased resistance to flow.5 In this setting, very little peristalsis is required to propel intraluminal material through the relatively flaccid tube. The pathogenesis of abnormal motility in the large intestine also involves inflammation, thus stimulating the defecation reflex resulting in more frequent defecation. In many instances of diarrhea, multiple mechanisms occur simultaneously in the same patient.Diarrhea usually results in loss of fluids that are isotonic to plasma, resulting in loss of sodium, chloride, organic anions, and potassium. This results in a body deficit of primarily sodium and water, thus leading to dehydration and potentially hypovolemic shock (as is the case with viral enteritis). Since there usually is isotonic fluid loss, serum electrolyte concentrations are often normal initially. Eventually, potassium depletion may also occur (through gastrointestinal and urinary losses). There may also be bicarbonate loss, in particular with distal small bowel disease (the site of bicarbonate secretion), resulting in metabolic acidosis. Thus, the patient with severe acute diarrhea may present with hypokalemia, and metabolic acidosis. If there is concurrent severe vomiting, this could further complicate these metabolic derangements. Additional consequences of severe mucosal disease relate to bacterial
Table 2.2: Etiology of acute diarrhea
| Gastrointestinal Causes | Extra-Gastrointestinal Causes |
| Viral | Hepatobiliary disorders |
| ■ Canine parvovirus | |
| ■ Feline panleukopenia virus | Exocrine pancreatic disease |
| ■ Canine coronavirus | ■ Acute pancreatitis |
| ■ Feline enteric corona virus | |
| ■ Canine distemper | Renal disorders |
| ■ FeLV or FIV related | |
| Endocrine disorders | |
| Bacterial | ■ Hypoadrenocorticism |
| ■ Salmonella spp. | ■ Feline hyperthyroidism |
| ■ Campylobacter spp. | |
| ■ Clostridium spp. | Miscellaneous disorders |
| ■ Yersinia spp. | ■ Drugs |
| ■ Enterotoxigenic E. coli | ■ Toxins |
| ■ Bacterial peritonitis | |
| Helminth parasites | |
| ■ Trichuris vulpis | |
| ■ Ancylostoma/Uncinaria | |
| ■ Strongyloides | |
| ■ Toxascaris / Toxocara |
Protozoan parasites
■ Giardia spp.
■ Cryptosporidium spp.
■ Othercoccidia
■ Pentatrichomonas (usually chronic)
■ Balantidium coli ■ Entamoeba
Other infectious agents
■ Neorickettsia spp. (Salmon poisoning)
■ Histoplasma (usually chronic)
■ Prototheca (usually chronic)
Miscellaneous causes
■ Hemorrhagic gastroenteritis (HGE)
■ Ingestion of garbage / foreign material
■ Diet change /overeating
■ Partial obstruction
■ Peritonitis
translocation and the subsequent development of endotoxic shock. The presence of hemorrhagic diarrhea, leukopenia, vascular collapse, and hypoglycemia should alert the clinician to this possibility. Severe gastrointestinal protein loss can occur with severe exudative processes. This can lead to decreased plasma oncotic pressure and subsequent peripheral edema. Due to the complexities of the multiple pathophysiological derangements, it is important to be thorough in the evaluation of these potential changes.
2.1.3.3 History and physical examination
The initial history should help give the clinician clues as to the cause of acute diarrhea, but also help determine the severity of the illness in order to determine the aggressiveness of the diagnostic approach and treatment plan. The same historical information needed for patients with acute vomiting is also important for patients with acute diarrhea, including previous illness, current diet and diet changes, vaccination and deworming status, onset of clinical signs, presence of systemic signs, exposure to drugs, toxins, or garbage, and also the appearance of the diarrhea.
Puppies and kittens are more likely to have parasitic diseases, viral enteritis, bacterial enteritis, intussusception, foreign body, garbage ingestion, or an adverse reaction to dietary changes. The character of the diarrhea will also help determine if there is primarily a large or small intestinal problem. This will help to allow the clinician to select appropriate diagnostic tests and medications. Small intestinal disorders typically result in a normal to increased frequency of defecation, lack of tenesmus and unproductive defecation attempts, and feces of large volume with a lack of fresh blood or mucus. Large intestinal disorders typically result in a marked increase in frequency of defecation, tenesmus, urgency, and unproductive defecation attempts. Each defecation episode is usually of small volume, with fresh blood and mucus frequently present in the feces. However, these characteristics are not absolute and patients with diffuse disease often present with mixed signs of small and large bowel disease.The physical examination should be thorough and include a rectal examination. This should also allow the clinician to obtain a fresh fecal specimen for additional diagnostic testing. The patient should also be assessed for hydration status and the need for systemic volume replacement. The presence of profuse and /or bloody diarrhea, hypothermia, hyperthermia, severe depression, weakness, dehydration, or signs suggestive of shock require a more intense diagnostic approach and treatment plan.
2.1.3.4 Laboratory and ancillary testing
Since infectious and parasitic diseases are among the most common causes of acute diarrhea, fecal testing is imperative in the initial data base regardless of the severity. A direct fecal smear is easy to perform following collection of fresh feces from the rectal examination. A small amount is placed onto a slide, mixed with one to two drops of saline, and a cover slip is placed on top. The sample should be examined under X 100 magnification for motile trophozoites of protozoan parasites, including Giardia spp., Entamoeba histolytica, Pentatrichomonas hominis, and in cats Tritrichomonas foetus.4,6,1 A fecal flotation should also be performed looking for ova, oocysts, and cysts.
A centrifugation method using zinc sulfate gives the highest yield, especially for protozoan parasites such as Giardia spp., but it is also a good choice for diagnosis of a nematode infesta- tion.8 Centrifugation improves the yield of fecal flotation.8 Though sugar centrifugation techniques are adequate for diagnosis of nematode infestation, Giardia spp. cysts are more difficult to detect.4 Some parasites are shed intermittently, such as Trichuris spp. and Giardia spp. Therefore, multiple flotations may be required to detect infestation with these parasites (a minimum of three flotations over five to seven days is considered a good screen).7 A stained rectal scraping or fecal smear is also helpful in some cases. This will allow detection of bacterial pathogens such as Campylobacter spp. and cysts of Clostridium spp. A sample is obtained by placing a moistened cotton swab in the rectum and gently rolling it a few times against the rectal mucosa. Alternatively, a gloved finger can be used to gently scrape the rectal mucosa.7 The swab is then rolled onto a slide, air dried, and stained with an in-house stain, such as Diff-Quik. The slide should be examined for white blood cells, bacterial organisms, Clostridium spp. cysts, and fungal organisms. The presence of spores of Clostridium spp. has been suggested to implicate this organism as a cause of acute or chronic diarrhea,9 but recent evidence suggests that the presence of spores is not specific for clostridial toxin-associated diarrhea.10,11 Acid-fast staining of a fecal smear may detect the presence of Cryptosporidium parvum, appearing as a 4- to 6-μm diameter organism that will stain pink to red.4 Fecal culture for specific pathogens, including Salmonella spp., Campylobacter spp., and Yersinia spp. may be warranted, especially in puppies or kittens, or when a rectal scraping reveals large numbers of neutrophils. In addition to these techniques, specific immunologic methods can be used to detect enteric pathogens. These tests include enzyme-linked immunosorbent assay (ELISA) testing for canine parvovirus, Giardia spp., Cryptosporidium par- vum, and Clostridium spp. enterotoxins. Immunofluorescent assays are also available for detecting some of these pathogens. Although Tritrichomonas foetus usually causes a more chronic clinical picture, fecal culture or PCR testing for Tritrichomonas foetus may be warranted in some cats presenting for acute gastrointestinal signs.For animals that are systemically ill, additional testing should include a CBC, serum biochemistry profile, urinalysis, and venous blood gas analysis. This will allow the clinician to look for extra-gastrointestinal causes of diarrhea, and to assess for metabolic, acid-base, and electrolyte abnormalities. Endocrine testing may be warranted, including a serum total T4 concentration in cats and an ACTH stimulation test in dogs that are suspected of having hypoadrenocorticism. Abdominal radiographs and ultrasound are low-yield test in dogs and cats with acute diarrhea. However, certain abnormalities may be detected such as peritonitis, intestinal intussusception, and partial obstruction. In these cases, vomiting usually accompanies the diarrhea.
Key Facts
■ In cases with mild clinical signs, the diagnostic evaluation may be limited, and patients may require only outpatient symptomatic care.
■ In animals with severe clinical signs, there may be severe electrolyte and acid-base disturbances, and a more thorough evaluation and in-patient care may be necessary.
■ It is critical to distinguish vomiting versus regurgitation when obtaining the initial history, because the diagnostic and treatment approaches are vastly different.
■ Acute vomiting and diarrhea can be caused by gastrointestinal and extra-gastrointestinal causes.
■ Acute diarrhea is often self-limiting and may or may not require treatment. Often the etiology remains unknown.