Robert J. Washabau, VMD, PhD, DACVIM
Professor of Medicine and Department Chair, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul, MN, USA
Pancreatic Pathophysiology--Acute Pancreatic Necrosis (APN)
The initiating event of acute pancreatitis is the premature activation of digestive zymogens within the acinar cell. Premature activation of digestive zymogen results in acinar cell necrosis and pancreatic autodigestion. In acute pancreatic necrosis, protein synthesis and intracellular transport to the Golgi complex appear to be normal, but digestive zymogens then become co-localized along with lysosomal hydrolases in large vacuoles. Cell biology studies have revealed that lysosomal and zymogen granule fractions become co-localized through a process known as crinophagy, a process used by many cells to degrade accumulated secretory products when the need for secretion is no longer present. Although this process takes place in other cells without adverse consequences, it can be lethal in pancreatic acinar cells because of the peculiarity of their secretion products (digestive zymogens). Lysosomal hydrolases, such as cathepsin B and N-acetyl glucosaminidase, activate trypsinogen to the active trypsin form, and the enhanced fragility of these large vacuoles permits release of active enzyme into the cell cytoplasm. Trypsin acts auto-catalytically to activate other trypsinogen molecules and other zymogens, each inducing a unique chemical pathology in pancreatic and extra-pancreatic cells. A variety of inflammatory mediators and cytokines (tumor necrosis factor-α, interferon-α, interferon-γ, platelet-activating factor), interleukins (IL-1, IL-2, IL-6, IL-8, IL-10), nitric oxide, and free radicals are involved in the further evolution of pancreatic acinar cell necrosis and inflammation.
New Findings in Canine Acute Pancreatitis
Canine acute pancreatitis (AP) is a common disorder which may result in death if not diagnosed in a timely fashion. This frequently encountered disease remains difficult to diagnose because the clinical signs, physical examination findings, and clinicopathologic changes are often non-specific. Therefore, knowledge of risk factors of canine AP and recognition of the clinical manifestations of this disorder are important.
The risk factors discussed in this manuscript were identified in a group of dogs in which all dogs had histopathologic confirmation of AP. The control group included dogs in which histopathologic examination excluded the possibility of AP. Clinicopathologic, radiographic, and ultrasonographic findings also pertain to dogs in which a diagnosis of AP was confirmed by histopathologic examination of the pancreas.
Risk Factors for Canine Acute Pancreatitis
Yorkshire terriers are at increased risk of developing AP, whereas miniature poodles and Labrador retrievers are at decreased risk for AP. Breed predisposition may suggest that there is a hereditary component to AP. Hereditary pancreatitis in humans can occur in association with a genetic defect of lipoprotein lipase, in individuals with hypertriglyceridemia and diabetes mellitus, or as an autosomal dominant trait, of unknown etiology, with a chronic recurrent presentation, and an early onset (usually in childhood). Further investigation is needed to determine if familial lipid metabolism disorders, or other genetic defects, predispose Yorkshire terriers to AP.
The mean age of dogs with AP is 8 years. Dogs with AP may be middle to older age dogs because several of the risk factors for AP (diabetes mellitus, hyperadrenocorticism, and hypothyroidism) develop in middle to older aged dogs. Obesity, which is another risk factor for AP may also be a problem of middle-aged dogs. Additionally, the increased age of dogs with AP could be a reflection of a degenerative pancreatic or extrapancreatic process, or a result of accumulating metabolic disorders that increase the risk of AP.
Males and neutered females are at increased risk compared to intact female dogs. This finding may indicate that sex gender specific factors are involved in the pathophysiology of AP.
Overweight Body Condition
Overweight and obese dogs are at an increased risk of developing AP. Increased body mass index (kg/m2) has been reported to be a risk factor and a poor prognostic indicator in humans. Increased retroperitoneal and peripancreatic fat deposition is thought to increase the risk of peripancreatic fat necrosis in humans.
Diabetes Mellitus, Hyperadrenocorticism, and Hypothyroidism
Diabetes mellitus, hyperadrenocorticism, and hypothyroidism are all associated with increased risk for AP. It is possible that lipid metabolism disorders are responsible for the increased risk. Hypertriglyceridemia is a risk factor in humans and is seen in dogs with diabetes mellitus, hyperadrenocorticism, and hypothyroidism. Hypertriglyceridemia has been reported in association with naturally occurring canine AP.3 Experimentally induced hypertriglyceridemia initiates pancreatic injury but does not seem to be a consequence of experimentally induced pancreatitis in the dog. These findings would support a hypothesis of hypertriglyceridemia being a risk factor, rather than a consequence of canine AP. However, many other metabolic abnormalities could be involved.
Prior Gastrointestinal Disease
Prior gastrointestinal disease (colitis, gastrointestinal parasites, hiatal hernia, or inflammatory bowel disease) is a risk factor for canine AP. Chronic inflammation of the gastrointestinal tract, e.g., proximal duodenum and transverse colon, may increase local inflammation and predispose to AP.
Epilepsy is a risk factor for canine AP. The reason for this association is not known, however, it may be due to anticonvulsant therapy (e.g., potassium bromide) or pancreatic ischemia during seizure activity.
Possible Risk Factors for Canine Acute Pancreatitis
Thromboembolism was observed more commonly in dogs with AP compared to control dogs. However, thromboembolism may develop as a result of AP and is not necessarily a risk factor for AP. It is possible that proteolytic enzymes released from the pancreas cause endothelial damage which results in infarct and thrombus formation. On the other hand, it is conceivable that an underlying coagulopathy, such as that associated with hyperadrenocorticism, causes infarct and thrombus formation, impairs pancreatic blood flow, and results in AP.
Atherosclerosis was more common in dogs with AP compared to control dogs. Hypothyroid dogs are predisposed to atherosclerosis however, atherosclerosis was also observed in a dog that had no evidence of hypothyroidism on post mortem examination. In humans, hypertriglyceridemia is a risk factor for pancreatitis, but its role in atherosclerosis remains controversial. Hypertriglyceridemia may be a risk factor for both AP and atherosclerosis in the dog.
Administration of Trimethoprim/Sulfa Antibiotics
Dogs with AP receive significantly more trimethoprim/sulfa than other dogs. This finding may reflect the severity of the disease and not necessarily a risk factor. Sulfonamides have been reported as a risk factor in humans, and in some of the human patients the association was confirmed with re-challenge. Hypersensitivity reaction or toxic effects are suspected. Although trimethoprim/sulfa administration has not been reported as a risk factor for AP in dogs, other adverse reactions have been documented, and some were suspected of being immune-mediated.
Abdominal Radiographic and Ultrasonographic Abnormalities in Canine Acute Pancreatitis
Abdominal ultrasonographic abnormalities are consistent with a diagnosis of AP more frequently than abdominal radiographic abnormalities. However, in some cases, abdominal ultrasonographic abnormalities are not apparent, while abdominal radiographs are suggestive of AP. Therefore, it is recommended that both imaging studies should be performed when faced with a suspected case of AP. Additionally, abdominal radiographs are a valuable diagnostic tool in any case of suspected AP because other causes of gastrointestinal disease must be ruled out. Abdominal radiographs are not suggestive of AP in 76% of dogs with histopathologic confirmation of AP. Therefore, in dogs suspected of having AP, abdominal ultrasonography should be performed even if abdominal radiographs are not suggestive of AP.