16 year old, intact female, Patas monkey (Erythrocebus pat as)End of Study. Focal area of consolidation in right caudal lung lobe noted. Submitted for histopathology.
Presented is a formalin-fixed pyramidal-shaped piece of lung with a partial cut through the middle. The 3 sides measure 2.5 cm and the base measures 1.5 cm. The tissue is firm and light gray except for one edge that is approximately 2mm x 5 mm that is purplish and somewhat aerated.
(slide variation of severity)
Multifocal to coalescing areas of varying sized infiltrates of macrophages and multinucleated giant cells engulfing and/or surrounding varying sized lipid droplets are observed primarily in alveolar airways. Many of these areas are also partially to completely surrounded by fibrous connective tissue with a mild to moderate lymphocytic and plasmacytic infiltrate. Extensive type II pneumocyte hyperplasia and multiple moderately-sized lymphoid aggregates/nodules are observed. On some slides is an abundance of necrotic mineralizing debris admixed with small amounts of fibrin within a large bronchiole that has denuded mucosal epithelium and focal areas of mild to moderate smooth muscle hypertrophy. Some slides have multifocal areas of pleural fibrosis which extends into the pulmonary parenchyma.
Lung: Pneumonia, granulomatous, multi-focal to coalescing to diffuse, severe, chronic with abundant lipid accumulation and fibrosis
Exogenous lipid pneumonia
The animal had what is known in the human literature as exogenous lipoid pneumonia. This is a condition where a lipid or fat-like substance is inhaled or aspirated into the lungs eliciting a severe inflammatory response.
There are two types of lipoid pneumonia: endogenous and exogenous. Endogenous lipoid pneumonia has been described in rats (20) dog ,(5, 19) genet,(18) cats ,(11) mongoose,(10) raccoons, (9) llama,(8) African grey parrot,(6) opossums, (3) Siberian tigers.(1) Exogenous lipoid pneumonia has been induced experimentally in mice and rats,(20) and found in two cases of horses(2,15) and a cow.(21) Lipoid pneumonia is an uncommon form of pneumonia.(7) It has been reported under different names, such as paraffinoma,(24) cholesterol pneumonia,(22) oil granulomas of the lung,(17) and lipid granulomatosis.
Exogenous lipoid pneumonia is more commonly reported in human literature(7) whereas it is the least reported in the veterinary literature.(11) It is caused by the inhalation or aspiration of lipid substances: animal fats, vegetable oils or mineral oil. Animal fat/oils elicit a very active inflammatory response. Mineral oils are fairly inert because they have no fatty acids and are rapidly emulsified and consumed by pulmonary macrophages. Vegetable oils are emulsified and not hydrolyzed by the lung lipase.(13) Aspiration may be due to age, psychiatric disorders, loss of consciousness ,(14) abnormality of deglutination (pharyngeal or esophageal) as well as gastroesophageal reflux(11,22) (oil floats on top of stomach fluids, thus oils may preferentially enter the airways).(14) Most occur when oils are used for medicinal purposes (constipation, oral health, nasal drops, etc).(12,16,17) A disproportionate population of fire breathers develop this type of pneumonia because of the liquid paraffin they use.(24)
Many lipid substances are non-irritating, can enter the tracheobronchial tree without stimulating a gag or cough reflex, and impair the mucociliary transport system. The pathophysiology is that of a chronic foreign body reaction. Once lipids are in alveoli, they are emulsified and taken up by macrophages. These cells cannot metabolize the fatty substances, so cells die, oil is released and rephagocytized. Over time, giant cell formation and fibrosis ensue. Diagnosis is made by the presence of large fat globules surrounded by macrophages and multinucleated giant cells as well as lipid-laden macrophages.(23)
Endogenous lipoid pneumonia, also called cholesterol or golden (accumulation of lipid in alveoli causing a yellow discoloration to the lungs) pneumonia, usually develop when lipids that normally reside in the lung tissue, most commonly cholesterol and its esters, escape from destroyed alveolar cell membranes (surfactant) distal to an obstructing airway lesion or damaged by an inflammatory process. It can also occur with fat emboli to the lung, pulmonary alveolar proteinosis, and lipid storage disorders (Niemann-Pick disease).(7) The pathogenesis is complex and may be related to retained epithelial secretions, cell breakdown, leakage from vessels, prolonged hypoxia, oxygen and carbon dioxide tension. It may be the result of transbronchial dissemination of breakdown products of cancer cells and secretions including mucin. Another though involves anoxic tissue injury stimulating phospholipases and mono-oxygenases, which in turn cause modification of low-density lipoprotein cholesterol. This cholesterol enhances lipid uptake by alveolar macrophages. Definitive diagnosis is demonstrating lipid-laden macrophages and cholesterol crystals.(7)
Lung: Pneumonia, interstitial, granulomatous, multifocal to coalescing, chronic, severe, with abundant intracytoplasmic lipid.
Endogenous lipid pneumonia is described as an alveolar filling disorder, encompassing several conditions in which abnormal material accumulates within alveoli. This pulmonary lesion can be an incidental finding or responsible for clinical disease. Other alveolar filling disorders include alveolar proteinosis, alveolar histiocytosis, alveolar phospholipidosis, pulmonary hyalinosis and alveolar microlithiasis. Mild forms of alveolar histiocytosis are commonly seen in the dog and alveolar phospholipidosis occurs in rodents secondary to administration of certain types of drugs as well as in conditions where there is a mutation of surfactant protein D. Conditions which result in the accumulation lipid-laden foamy macrophages must be distinguished from other conditions which result in accumulation of macrophages with foamy appearing cytoplasm such as pneumocystis and histoplasmosis. In alveolar proteinosis, acellular eosinophilic or amphophilic material composed of surfactant proteins and phospholipids accumulates, but inflammation and fibrosis are not prominent features. Pulmonary hyalinosis, an incidental finding the lungs of aged dogs, is characterized by the accumulation of amorphous, laminated or hyaline material in macrophages and giant cells. In pulmonary alveolar microlithiasis, the accumulated material is extracellular and consists of laminated concretions in alveoli and can result in clinical disease depending on distribution of deposits.(5)
Conference participants described the lung as approximately 75% affected by a predominantly granulomatous infiltrate focused on variably sized lipid vacuoles within alveolar lumina. Other prominent features include extensive type II pneumocyte hyperplasia, fibrosis, and multifocal hyaline membrane formation. Conference participants also noted the multifocal nodular lymphoid aggregates which led to a brief discussion on their histiogenesis (i.e. a chronic inflammatory response versus hyperplasia of preexisting bronchus/bronchiolar associated lymphoid tissue (BALT). The lipid expands the interstitium and prominently fills alveoli depending on location; in some areas, the precise location of the lipid, is difficult to determine due to fibrosis and inflammation. Additional histochemical stains used to highlight lipid include Oil Red O and Sudan black.
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