Chapter 14 Intestinal Viruses
Parvoviruses, coronaviruses, and rotaviruses are established causes of viral enteritis and diarrhea in dogs and cats and are discussed in this chapter. In addition, numerous other viruses of uncertain significance and enteropathogenicity have been found in the feces or intestines of dogs and cats.
• In dogs these include herpesvirus, enteroviruses, calicivirus, parainfluenza virus, adenovirus, and picornavirus.
• In cats these include astrovirus, calicivirus, reoviruses (types 1, 2, and 3), torovirus, and togavirus.
• In addition, the intestine may be involved as part of generalized viral infections in disorders such as canine distemper in dogs (see Chapter 13), and feline leukemia virus (see Chapter 8), feline immunodeficiency virus (see Chapter 9), and feline infectious peritonitis (see Chapter 10) in cats.
CANINE PARVOVIRUS
Pathogenesis
• Intestinal epithelial damage from CPV causes breakdown of the intestinal mucosal barrier. This allows translocation of bacteria (especially Escherichia coli) and absorption of endotoxin, predisposing to bacteremia, endotoxemia, and the development of fatal systemic inflammatory response syndrome (SIRS).
Clinical Signs
• CPV causes sudden onset of anorexia, depression, fever, vomiting, diarrhea, and severe dehydration. The diarrhea can be profuse, foul smelling, and hemorrhagic. Abdominal palpation may reveal intestinal loops that are painful and distended with fluid and gas.
• Hypothermia, hypovolemic shock, icterus, hemorrhagic diathesis (disseminated intravascular coagulation), and pulmonary edema because of acute respiratory distress syndrome may develop terminally in cases complicated by bacterial septicemia, endotoxemia, and SIRS.
• Death may occur in severe cases, particularly in very young puppies and in the highly susceptible breeds, and is usually attributable to dehydration, electrolyte depletion, endotoxic shock, or overwhelming bacterial sepsis associated with severe leukopenia. Septic puppies often develop hypoglycemia.
• The severity of clinical illness may be increased by factors such as stress, overcrowded or unsanitary kennel conditions, secondary bacterial infection, and concurrent diseases such as canine distemper, coronavirus, salmonellosis, campylobacteriosis, and intestinal parasitism (e.g., nematodes or giardia).
• In susceptible mature dogs, mild or inapparent infection that results in seroconversion without obvious clinical signs is common.
• In utero or postnatal infection can cause acute neonatal myocarditis in neonatal puppies that did not receive maternal antibodies. However, this form of perinatal parvoviral infection is rare because most nursing puppies receive abundant maternal antibodies from colostrum. Signs of parvoviral myocarditis include dyspnea caused by acute heart failure, sudden death caused by arrhythmias, and sometimes delayed-onset chronic congestive heart failure caused by chronic myocardial fibrosis.
Diagnosis
Hematology
• A complete blood count is particularly useful because approximately 50% of dogs with parvoviral enteritis develop severe leukopenia caused by lymphopenia and granulocytopenia, often with a total of only 500 to 2000 leukocytes/μl and sometimes less. Depletion of circulating mature neutrophils is caused by extensive loss of neutrophils through the damaged intestinal mucosa coupled with impaired myelopoiesis caused by bone marrow disruption from the virus. The severity of the leukopenia is generally proportional to the severity of the clinical illness. A rebound neutrophilia is a positive indicator of impending recovery.
Serology
• Determination of an anti-CPV antibody titer in serum is not sufficient for diagnosis because up to 95% of dogs in the population have seroconverted from prior vaccination or natural exposure.
Virology
• Massive quantities of virus are shed in the feces during the acute illness, and shedding is over within 1 to 2 weeks after recovery. The most practical methods for detecting parvovirus in the feces are the commercially available, rapid in-office tests; including the enzyme-linked immunosorbent assays (ELISAs), such as the SNAP-Parvo Test (IDEXX Laboratories) and ASSURE-Parvo Test (Synbiotics), or the rapid immunomigration assay, such as Witness CPV (Synbiotics). These tests are sensitive and accurate.
• Consider that false-positive results are occasionally seen on these CPV immunoassays 5 to 12 days following vaccination with modified live virus (MLV) vaccines. False-negative results occur rarely when fecal antigen is bound to neutralizing antibodies or viral shedding stops early.
Treatment
Fluid Therapy
See Chapter 5 for specific guidelines and procedures for fluid and electrolyte therapy.
• Use the intravenous route for fluid and electrolyte replacement using a balanced crystalloid solution (e.g., lactated Ringer’s solution, Plasma-Lyte 148, or Normosol-R). For animals presented in hypovolemic shock, administer up to 90 ml/kg IV in the first 1 to 2 hours to restore hemodynamic stability (see Chapter 156), then switch to a maintenance rate. For most other animals, correct dehydration over the first 24 hours, then use a maintenance rate for fluids plus replacement of ongoing losses. Two to three times normal maintenance levels are often required. Continue fluid therapy until vomiting ceases and oral intake resumes.
• Add dextrose to IV fluids at a 2.5% solution to control complicating hypoglycemia of sepsis. Monitor serum glucose and increase to 5% if needed to maintain serum glucose from 120 to 160 mg/dl.
• Consider supplementing magnesium according to guidelines in Chapter 5, as magnesium is often deficient in severe cases of parvoviral enteritis.
• Infuse colloid solution (e.g., hetastarch) at 20 ml/kg, IV, if infusion of balanced crystalloid solution does not restore hemodynamic stability or if serum albumin drops below 2 g/dl.
• Avoid administration of fluids by the subcutaneous route, especially in dogs with severe leukopenia, because this has been associated with secondary infection, cellulitis, and skin necrosis at administration sites.
Antibiotics
• Administer bactericidal, broad-spectrum antibiotics to control bacterial complications. Initially, administer antibiotics parenterally.
• Use cefazolin, ampicillin, or penicillin combined with an aminoglycoside (e.g., gentamicin or amikacin) or a fluoroquinolone (e.g., enrofloxacin) in severe and leukopenic cases. Fluoroquinolones may damage joint cartilage in young, growing pups. If an aminoglycoside is used, maintain hydration to prevent nephrotoxicity and monitor the urine daily for casts and proteinuria as early indicators of nephrotoxicosis.
Dietary Restriction
• Give nothing per os (fluid needs are met by IV infusion) for the first 12 to 24 hours. Prolonged food restriction may be detrimental to intestinal recovery. One study showed improved outcome in dogs treated with early enteral nutrition (using a nasoesophageal tube feeding starting at 12 hours) compared with dogs treated by prolonged dietary restriction.
• Persistent vomiting can sometimes take 3 to 5 days to abate in severe cases, sometimes requiring partial parenteral nutrition.
Antiemetics
• For frequent or persistent vomiting associated with delayed gastric emptying that sometimes occurs in parvoviral infection, administer metoclopramide, a dopaminergic antagonist, at 0.5 mg/kg every 8 hours SC or most effectively as a constant rate infusion of 1 to 2 mg/kg every 24 hours diluted in IV fluids.
• For gastritis, control gastric acid secretion with an H2 receptor blocker, for example, ranitidine (Zantac; 2 mg/kg IV, q8–12h), or famotidine (Pepcid; 0.5 mg/kg IV, q12–24h).
• If these are unsuccessful for controlling vomiting, consider the broad-spectrum phenothiazine antiemetic, chlorpromazine (0.5 mg/kg SC or IM, q6–12h), but not until dehydration has been corrected because phenothiazines have a hypotensive effect. Avoid the use of metoclopramide and chlorpromazine together because they can produce central nervous system excitation and, rarely, seizures.
• For persistent vomiting that is refractory to other treatments, give ondansetron (Zofran; 0.1–0.2 mg/kg slow IV, q6–12h), a serotonin-antagonist antiemetic.
Infusion of Whole Blood or Plasma
• Consider whole blood transfusion (5–10 ml/kg IV) for treatment of severe blood loss anemia and hypoproteinemia.
• Consider plasma infusion (5–10 ml/kg IV) for cases that develop disseminated intravascular coagulopathy. Plasma may also help correct hypoproteinemia, but hetastarch is a preferred source of colloid for raising colloid osmotic pressure. Plasma may also provide anti-CPV antibodies, but the benefit of this is questionable.
Treatment for Septic Shock
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