3-month-old female Yorkshire Cross pig, Sus scrofa domestics.This piglet was received at 1 month old and began receiving weekly carbon-14 injections via the ear vein with ketamine/xylazine sedation approximately one week after arrival. She was found down and extremely weak two months after arrival while other animals in same group had no clinical symptoms. The investigator elected euthanasia and a gross necropsy was performed.
On postmortem examination, connective tissues throughout the body are edematous and bright yellow.Â The liver has multifocal to coalescing pinpoint to 3 mm dark red foci, is enlarged, and flabby.
Liver: Within all lobules examined, there is massive centrilobular to diffuse hepatocellular necrosis and hemorrhage.Â Hepatocytes in these areas are often completely lost and replaced by hemorrhage.Â In other area, hepatocytes are hypereosinophilic and individualized, with pyknotic or absent nuclei and are often surrounded by cellular debris, neutrophils, and fibrin.Â Portal areas contain mildly increased numbers of lymphocytes, reactive fibroblasts, and neutrophils, with rare pyknotic cells.Â
Skeletal muscle: Diffusely throughout the tissue, there is random individual myocyte degeneration characterized by sarcoplasmic hypereosinophilia and swelling, loss of cross-striations, fragmentation, pyknosis, karyorrhexis and karyolysis.Â
1.Â Liver: Necrosis, acute, centrilobular to massive, hemorrhagic, Yorkshire cross, pig.Â
2.Â Skeletal muscle: Degeneration and necrosis, acute, multifocal, mild to moderate, Yorkshire cross, pig.Â
Clinical chemistry reveals severely icteric serum, high potassium, ALT, ALP, and amylase.Â The CBC shows a stress leukogram.Â Postmortem testing of hepatic selenium concentrations reveal levels as inadequate.
Vitamin E / selenium imbalance
Additional histopathologic findings included moderate multifocal fat necrosis of the mesentery, and minimal multifocal myocardial necrosis.Â Vitamin E/selenium deficiency was highly suspected due to the gross and histopathologic findings.Â Postmortem hepatic selenium concentrations were inadequate.Â Blood samples from the remaining three pigs in the cohort were also tested.Â A blood sample from one pig was marginal and the other two pigs were adequate.Â Vitamin E and selenium was supplemented and no clinical symptoms ensued in any of the remaining animals.Â The pigs had been fed a commercial pig diet and it is still unclear how this deficiency occurred.Â Unfortunately, there was no remaining feed from the suspect lot available for testing.Â There were no other reports of problems with other lots of this feed to the vendor.Â One possibility is that this adult diet had inadequate concentrations of vitamin E and/or selenium for such a young piglet.Â
In the pig, the most obvious effect of Vitamin E/Selenium deficiency is sudden death, typically in young, fast growing weaners.Â Two principle presentations seen at post mortem examination are mulberry heart disease (MHD) and hepatosis dietetia (HD).Â Selenium is an integral part of the membrane enzyme glutathione peroxidase, reducing toxic lipid peroxidases to hydroxyl acids.(5) Vitamin E is an antioxidant and acts synergistically with selenium to protect membranes from high concentrations of lipoperoxidases.(5)
HD is characterized by hemorrhagic centrilobular to massive hepatic necrosis.Â Animals that survive the acute disease can develop lesions of parenchymal collapse and post necrotic scarring.(1,6) The most severe injuries are often present dorsally in the diaphragmatic liver lobes.(6) The pathogenesis of HD is not completely understood, but it is thought to be caused by a deficiency in vitamin E and/or selenium.(1) Affected animals respond to vitamin E or selenium supplementation, although, interestingly, it is difficult to experimentally induce this condition by feeding diets deficient in vitamin E and selenium.(1) It is thought that oxidative injury leads to hepatocyte necrosis since vitamin E and selenium-containing enzymes are antagonists of free radical formation and are therefore important in maintaining the stability and integrity of cellular membranes.(1,6) Experimental observations have revealed the need for concurrent deficiencies of sulfur-containing amino acids, tocopherols, and trace amounts of selenium for hepatic necrosis to develop.(6) Yellow staining of the adipose tissue is a common gross finding(6) as was seen in this case.Â The gallbladder may be edematous.(5) In relapsing cases, animals may be jaundice and hemorrhagic diathesis may occur with the primary manifestation of hemorrhage into or surrounding the joints.(6)
MHD (dietary microangiopathy of pigs) is characterized by fibrinoid necrosis and thrombosis of small vessels resulting in micro hemorrhages.(8) The resulting hemorrhages lead to a mulberry-like discoloration of the epicardial surface, particularly of the right atrium.(5,8) Hydropericardium is also a common finding.Â There is often widespread fibrinoid necrosis of small arteries and arterioles with endothelial damage and fibrin thrombi, particularly in the capillaries of the myocardium.(5,8) Typically, the following microscopic lesions are observed in a heart affected by MHD: interstitial hemorrhage associated with swollen cardiac myofibers that have lost cross striations, are hypereosinophilic, and have pyknotic nuclei.(4) In this entity, selenium concentrations within the liver and heart are often within normal limits.(5) Supplementation of selenium appears to decrease the incidence of HD, but not MHD.Â Therefore, vitamin E is considered to play a central role in the development of MHD.(5) An additional lesion that can be found in animals surviving greater than 24 hours is bilaterally symmetric softening of the cerebral white matter.(5)
Aside from the disease entities in pigs, vitamin E/selenium deficiency results in a broad spectrum of diseases in a variety of animal species and humans, including but not limited to: myopathy, steatitis, liver necrosis, and encephalomalacia because of increased oxidative stress on cells.(2)
1.Â Liver, hepatocytes: Necrosis, centrilobular and midzonal, diffuse, severe, with hemorrhage and periportal hepatocellular lipidosis.Â
2.Â Skeletal muscle: Degeneration and necrosis, multifocal, marked.Â
Massive hepatic necrosis implies necrosis of the entire hepatic acinus centrilobular, midzonal, and periportal hepatocytes.Â Selenium and/or vitamin E deficiency is classically associated with massive hepatic necrosis, a disease in pigs known as hepatosis dietetica.Â Conference participants discussed the finding of intact, often regenerating periportal hepatocytes in this case and how it relates to the pathogenesis of massive necrosis.Â It was hypothesized this lesion may represent an early state of disease prior to full expression of massive necrosis.Â In addition to vitamin E and selenium deficiencies, other differentials for hepatic necrosis worthy of consideration include blue-green algae, Amanita (mushrooms), Cestrum diurnum, Xanthium sp.Â (cocklebur), iron dextran, aflatoxin, fumonisin B1, sporidesmin and pyrrolizidine alkaloids.(5) In horses, serum sickness is characterized by a near diffuse hepatic necrosis with only a narrow rim of usually vacuolated hepatocytes surrounding periportal regions.(1)
When hepatic necrosis is observed in conjunction with rhabdomyocyte necrosis, vitamin E and/or selenium deficiency should be considered most likely.Â Selenium deficiency does not only occur in managed feed situations, but also in pasture-raised livestock.Â Selenium is normally present in soil and taken up by growing plants, however, in areas such as the Pacific Northwest, the soil is naturally deficient.Â Poor quality forages are also deficient in vitamin E, thus both must be supplemented in some situations.(7) Deficiencies of either result in the bodys reduced capacity to scavenge free radicals.Â
Free radicals are molecules with unpaired electrons rendering them highly reactive as they look to unload or oxidize that electron.Â They are generated as a normal product of mitochondrial respiration, absorption of radiant energy, enzymatic metabolism of drugs or chemicals, transition metals (Fenton reaction), nitric oxide production and by activated leukocytes during inflammation.Â Free radicals are very effective at killing cells, both those of pathogens and the normal host.Â Three reactions most relevant to free radical-induced cellular injury are lipid peroxidation of membranes, oxidation of proteins and DNA damage.(3) Antioxidants serve to protect the host by counteracting this oxidation.Â The importance of antioxidants in maintaining equilibrium is exemplified by the multitude of lesions associated with vitamin E and selenium deficiencies and nicely illustrated in this case.Â
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