Abstract

Canine parvoviral (CPV) enteritis remains a common and important cause of morbidity and mortality in young dogs, since its emergence in 1978. The continued incidence of parvoviral enteritis is partly due to the virus' capability to evolve into more virulent and resistant subspecies with significant local gastrointestinal and systemic inflammatory sequelae. This paper reviews current knowledge on the clinical manifestations, biochemical and endocrine diagnostic and prognostic biomarkers identified in CPV enteritis such as lymphopenia, thrombocytopenia, hypercoagulability, hypercortisolemia, hypothyroxinaemia, hypoalbuminemia, elevated CRP, hypocholesterolaemia and hypocitrullinaemia. The identification of several prognosticators has made management of the disease more rewarding.

Parvoviridae

Parvoviruses (Parvoviridae) are small, non-enveloped, single-stranded DNA viruses that are known to cause disease in a variety of mammalian species, although most parvoviruses are species-specific. Viral replication occurs only in certain rapidly dividing cells like intestinal crypt epithelial cells, precursor cells in the bone marrow, and myocardiocytes, resulting in cell death and loss due to failure of mitosis. The virus is better known as canine parvovirus type-2 (CPV-2). In 1967, parvovirus was first discovered as a cause of gastrointestinal and respiratory disease in dogs and was called the minute virus of canines. It was later designated CPV-1. In 1978, reports of outbreaks of an unfamiliar contagious enteric disease were reported and the causal agent was isolated as a new species of the genus Parvoviridae; subsequently named CPV-2. In the 1980s a new CPV-2 strain emerged and was designated CPV-2a. The virus quickly mutated again and a new strain, CPV-2b, emerged in 1984.¹ Today, CPV-2a and CPV-2b are still the most common parvovirus species causing disease in canines globally. Within the past decade a new highly virulent strain, CPV-2c, has emerged.²

Epidemiology

Acute CPV-2 enteritis can be seen in dogs of any breed, age, or sex, but puppies between 6 weeks and 6 months appear to be more susceptible. Immunity to CPV following infection or vaccination is long-lived, and therefore the only susceptible pool of animals is puppies born into the population. Certain breeds have been shown to be at increased risk for severe CPV enteritis, including the Rottweiler, Doberman pinscher, American pit bull terrier, Labrador retriever, and German shepherd dog.³ CPV-2 spreads rapidly among dogs via the faecal-oral route (direct transmission) or through oronasal exposure to fomites contaminated by faeces (indirect transmission). Marked plasma viraemia is observed 1 to 5 days after infection. Parvovirus infects the germinal epithelium of the intestinal crypt, causing epithelial destruction and villous collapse. As a result, normal cell turnover (usually 1 to 3 days in the small intestine) is impaired, leading to the characteristic pathologic lesion of shortened and atrophic villi. During this period of villous atrophy, the small intestine loses its absorptive capacity. The extensive lymphocytolysis in the thymic cortex, compared to other lymphoid tissues, further reflects the high mitotic rate found in this organ, and it is thus not surprising that infected puppies develop severe lymphopenia.

Clinical Manifestation

Enteritis and myocarditis were the two disease entities initially described with CPV-2 infection. CPV-2 myocarditis is very rarely seen these days. Acute enteritis is the most common manifestation of the disease. Initial clinical signs are nonspecific and include anorexia, depression, lethargy and fever. Later typical signs include vomiting and small bowel diarrhoea that can range from mucoid to haemorrhagic. Due to large fluid and protein losses through the gastrointestinal tract, dehydration and hypovolaemic shock often develop rapidly. Marked abdominal pain is a feature of CPV enteritis and can be due to either acute gastroenteritis or intestinal intussusception. Intestinal tract damage secondary to viral infection increases the risk of bacterial translocation and subsequent coliform septicemia. This may lead to the development of a systemic inflammatory response that can progress to septic shock and ultimately death. Escherichia coli has been recovered from the lungs and liver of infected puppies. Pulmonary lesions similar to those found in humans with adult respiratory distress syndrome (ARDS) have been described.^4,5 Endotoxin and tumor necrosis factor (TNF) are present in measurable quantities in the blood of infected puppies and a significant association exists between rising TNF activity and mortality.⁴ Endotoxin and proinflammatory cytokines are potent mediators of the systemic inflammatory response and activators of the coagulation cascade.

Typical ultrasonographic changes that are considered indicative of CPV enteritis included fluid-filled, atonic small and large intestines; duodenal and jejunal mucosal layer thinning with or without indistinct wall layers and irregular luminal-mucosal surfaces; extensive duodenal and/or jejunal hyperechoic mucosal speckling; and duodenal and/or jejunal corrugations. The extensive intestinal lesions correlated with the histopathological findings of villous sloughing, mucosal erosion and ulceration, and crypt necrosis, without any sonographically detectable lymphadenopathy.⁶ The severity of the sonographic changes correlated with the clinical condition of the patients.⁶ Plasma citrulline concentration is a reliable marker of global enterocyte mass in humans and is markedly decreased in diffuse small intestinal diseases. A study in parvovirus enteritis was associated with a severe decrease in plasma citrulline concentration that did not appear to have any significant prognostic value.⁷

Clinicopathologic Biomarkers

The leukocyte count during CPV enteritis is generally characterized as significantly depressed with a transient lymphopenia being the most consistent finding. The haematological changes are widely accepted to be attributable to destruction of haematopoietic progenitor cells which results in inadequate supply for the massive demand for leukocytes (specifically neutrophils) in the inflamed gastrointestinal tract.⁸ A recent study showed that a lack of cytopenia, specifically the total leukocyte and lymphocyte counts, had a positive predictive value of 100% for survival 24 hours post-admission. A rebound increase in the lymphocyte count 24 hours after admission was seen in the puppies that recovered.⁸ Studies have also shown a marked depletion of the granulocytic, erythroid and megakaryocytic cell lines in the bone marrow followed by hyperplasia of the granulocytic and erythroid elements during convalescence. These changes are nonspecific and could reflect the effect of endotoxemia. Despite the severe changes seen in the blood precursor cell lines, it appears that early pluripotent cells are spared. Increased plasma granulocyte colony-stimulating factor (G-CSF) concentration has been observed in CPV enteritis just after the onset of neutropenia which then decreases to undetectable levels once the neutropenia has resolved. Anaemia is not an uncommon haematological finding in CPV enteritis especially in the later phases of severe disease due to a combination of intestinal haemorrhage and rehydration therapy. Increased levels of lipid peroxides and an alteration in antioxidant enzyme concentrations, indicating a state of oxidative stress in these patients, may also play a role in anaemia pathogenesis. Virus-induced thrombocytopenia can occur due to decreased platelet production or as a result of direct action of viruses and/or immunologic components on platelets or endothelium. Besides haemorrhagic manifestations (which are rare), subclinical thrombocytopenia may affect vascular permeability which may potentiate extravascular dissemination of the virus. Evidence of hypercoagulability without disseminated intravascular coagulopathy has been documented in puppies with CPV enteritis and is thought to be due to an endotoxin- or cytokine-mediated procoagulant effect on endothelial cells. Loss of antithrombin (AT) through the gastrointestinal tract, as well as consumption of AT as a result of endotoxin-mediated activation of coagulation, and hyperfibrinogenemia are thought to contribute to the hypercoagulable state seen in CPV enteritis.⁹ The response of the adrenal and thyroid gland to critical illness is essential for survival. Similar to critical illness in humans, high serum cortisol and low serum thyroxine (T4) concentrations at 24 and 48 hours after admission were associated with death.^10,11 Infection-induced serum chemistry abnormalities are nonspecific. Severe hypokalemia due to anorexia, vomiting, and diarrhoea may contribute to depression and weakness. Other electrolyte abnormalities (i.e., hyponatremia and hypochloremia) may also occur secondary to vomiting and diarrhoea. Although total magnesium concentration has been found to be a prognostic indicator in critically ill humans, total as well as ionized magnesium concentrations were not associated with outcome in CPV enteritis. Hypoalbuminemia may contribute to reduced total blood calcium concentrations. Serum electrophoresis profiles have shown relative and absolute hypoalbuminemia, hypogammaglobulinemia and hyperalpha-2-globulinemia. The decrease in plasma proteins through the course of the disease are mostly due to a combination of intestinal haemorrhage followed by rehydration. The increase in alpha-2 globulins are most likely due to the hepatic synthesis of acute phase proteins (APP) stimulated by leukocyte endogenous mediators which are associated with tissue damage and inflammation. Acute phase protein generation occurs at the expense of albumin generation in critical illness. Data on serum C-reactive protein (CRP), a major APP in the dog, have shown that higher CRP levels at admission, 12 and 24 hours post admission are positively associated with an increased risk of mortality.¹² Elevated blood urea, creatinine and inorganic phosphate are associated with dehydration. Elevation in alkaline phosphatase and alanine transaminase may occur as a result of hepatic hypoxia secondary to severe hypovolemia or the absorption of toxic substances due to loss of the gut barrier. Elevated alkaline phosphatase activity can also be associated with young age. Plasma lipoproteins bind the bioactive portion of the endotoxin (LPS) molecule, preventing it from stimulating monocytes, macrophages and other LPS-responsive cells, thereby providing an important host mechanism for controlling responses to endotoxin. Several reports have shown a strong correlation between low plasma cholesterol and mortality in critically ill and infected human patients. A recent study has shown serum total cholesterol and high-density lipoprotein cholesterol levels to decrease, but serum triglyceride levels to increase in CPV enteritis. Hypocholesterolemia may be used as an index of the severity of CPV enteritis.¹³ Studies on acid-base status in CPV enteritis have shown puppies to develop either acidosis or alkalosis depending on the severity of the vomiting (i.e., loss of hydrogen and chloride ions) or the origin of the diarrhoea (i.e., small versus large intestine). The majority of cases show a decrease in venous blood pH and HCO₃^- which indicate the development of metabolic acidosis probably due to excessive loss of HCO₃^- through the intestinal tract. The metabolic acidosis seen in CPV enteritis is however readily corrected and is not exacerbated by D-lactate production by the bacterial population within the large intestine. Growing evidence supports the use of early enteral nutrition. A recent study has shown that puppies receiving early enteral nutrition via a nasoesophageal tube, compared to puppies that received nil per os until vomiting ceased, showed earlier clinical improvement, significant weight gain, as well as improved gut barrier function which could limit bacterial or endotoxin translocation.¹⁴

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