Young adult Rhesus Macaque (Macaca mulatta)The presenting animal was inoculated with simian influenza virus, SIVmac239, and simian human immunodeficiency virus, SHIVDH12 (MD14YE) on June 1, 2005. Eighteen weeks post inoculation, only SIVmac239 was detectable in the blood. Throughout the 64-week course of the study, CD4+ T cell levels consistently declined, ranging from 26% to <3% of cells/Î¼L of blood. CD8+ T cell levels fluctuated, ranging as high as 73% of cells/Î¼L of blood, then dropped severely between weeks 62 and 64 to 26% of cells/Î¼L of blood. On August 22, 2006, week 63, the animal presented with a rapid respiration rate. Chest radiographs were obtained and results indicated diffuse pneumonia. Total white blood cell count was 13,000 cells/Î¼L of blood; lymphocyte count was 1248 cells/Î¼L, monocyte count was 1040 cells/Î¼L, and eosinophil count was 1287 cells/Î¼L. The animal was euthanized.
Upon opening the chest cavity, the lungs did not collapse.Â The lungs were grayish-pink to light brown and were mild to moderately consolidated (3-1).Â Gross appearance of the lungs was consistent with diffuse pneumonia.Â The trachea contained a small amount of frothy white fluid.Â The spleen was mildly enlarged uniformly.Â No other abnormalities were noted in the other major organs or tissues.
Lung imprints were prepared using a cut margin of lung stained with a modified Wright-Giemsa stain.Â Cytologic examination revealed red blood cells, alveolar macrophages, type II pneumocytes, neutrophils, and numerous round to oval extracellular organisms measuring approximately 4 _m in diameter.Â Nuclei were evident within the organisms and were consistent with the cystic and trophozoite forms of Pneumocystis carinii.Â
The lung exhibited diffuse alveolitis with alveoli containing foamy eosinophilic fluid with an admixture of small to moderate numbers of neutrophils, alveolar macrophages, multi-nucleated histiocytic giant cells, and numerous extracellular fungal organisms consistent with Pneumocystis carinii.Â Centrally to peripherally located within the fungal organisms was a small nucleus of the cystic and trophic forms of Pneumocystis.Â There was mild to moderate type II pneumocyte hyperplasia and within the septae, there was a mild mononuclear infiltrate comprised mainly of lymphocytes and a smaller number of plasma cells.Â Gomori methenamine silver (GMS) (3-2) stains of the lung sections and imprint smears were positive for the cystic forms of Pneumocystis carinii.
Lung: Alveolitis, diffuse, severe, acute with numerous fungal organisms consistent with Pneumocystis carinii.
Pneumocystis carinii was once mis-identified as a protozoan, but was re-classified as a fungal organism based on mitochondrial genomic gene sequences, aminoacid sequences of peptides and proteins, and comparative analysis of the 16 S ribosomal RNA.5 Transmission of the fungus can be from host to host or from the environment.15 Although Pneumocystis typically causes pneumonia in immunocompromised individuals, asymptomatic hosts may be carriers.9, 13, 14, 15 Immunocompetent hosts tend to develop transient Pneumocystis infections and can transmit infection to susceptible hosts by an airborne route.6 Pneumocystis pneumonia is the most prevalent opportunistic infection in immunocompromised patients infected with human immunodeficiency virus (HIV) and is particularly difficult to study because the organism cannot be cultured outside the host lung; therefore, animal models continue to be the best source of information on Pneumocystis.4, 14
There are 5 host-specific species of Pneumocystis: Pneumocystis carinii, Pneumocystis jiroveccii, Pneumocystis murina, Pneumocystis oryctolagi, and Pneumocystis wakefieldiae.Â Pneumocystis carinii species infects SIV positive macaques and the Pneumocystis jiroveccii species infects HIV positive humans.1, 7, 12 Wild born and laboratory bred macaques have similar susceptibilities to Pneumocystis carinii, however, wild born macaques exhibit higher levels of antibody titers to the fungus.8 The first report of infection in nonhuman primates included two aged owl monkeys and two young chimpanzees with myeloproliferative neoplasia.3
Lungs affected with severe Pneumocystis pneumonia typically exhibit alveolar type-II cell hypertrophy and intra-alveolar foamy eosinophilic material and variable neutrophilic lung inflammation that may result in diffuse alveolar damage, impaired gas exchange, and respiratory failure.6, 14 The Pneumocystis fungi attach to type-I alveolar epithelial cells and, although the organisms do not alter the metabolic, barrier, or structural functions of the alveolar cells, they are able to proliferate due to impaired cell-mediated immunity.Â Infection also leads to changes in pulmonary surfactant production initiated by the inhibition of phosphatidylcholine from type II alveolar cells.6 Pneumocystis organisms exist as cystic and haploid trophozoite forms throughout their life-cycle, but during infection, the trophozoite form is more abundant than the cystic form.14
In the transmission electron micrographs of a thin section of Pneumocystis infected lung from this macaque, cyst and trophozoite forms are evident with trophozoites predominating.Â Free ribosomes and glycogen are present in both forms.Â Cysts are typically spherical in shape, contain up to eight intracystic bodies from which trophozoites originate, and have a mean diameter of 5-8 Î¼m.Â The mean diameter of an intracystic body is 1.2 Î¼m.5 Each intracystic body contains components characteristic of eukaryotic cells including a centrally located nucleus, endoplasmic reticulum, and other organelles.Â The cyst wall is two layers thick, measuring approximately 50 nm.5 Trophozoites are generally grouped into an interdigitating cluster, are individually pleomorphic, and are closely associated with pneumocytes.Â There are very few organelles, including a nucleus with a centrally to peripherally placed nucleolus.Â Free ribosomes predominate within the infective form.Â A 20-30 nm thick, non-homogenous coat surrounds the entire trophozoite, including the interdigitations, and is a characteristic feature of the trophozoite form.Â The coat may help anchor the trophozoite to other trophozoites or alveolar epithelium or it may be involved in nutrient uptake.5
Trophozoites adhere to the alveolar epithelium to establish infection and to initiate a mitogen activated protein kinase signaling cascade for mating and proliferation of the fungus.14 CD4+ T lymphocytes are integral in the defense against Pneumocystis.6 By recruiting and activating macrophages and monocytes to attack the invading organism, CD4+ cells behave as memory cells to initiate the hosts immune response.14 Continuously declining CD4+ levels in this animal to <3% of cells/Î¼L of blood allowed the Pneumocystis organisms to extensively proliferate and exert their adverse effects.Â SIV positive monkeys infected with Pneumocystis are unable to recruit CD4+ cells to the lung and develop an inflammatory response characterized by an increased neutrophil and CD8+ cell infiltration.4
Not all of the imposed alveolar damage is due to the adherence of Pneumocystis organism to the type-I alveolar epithelial cells.Â TNF-_, an inflammatory cytokine that regulates the immune response by recruiting neutrophils, lymphocytes, and monocytes to an area of infection, is also a contributing factor in damage to the alveoli.Â While TNF-_ helps mount an immune response, it also releases oxidants, cationic proteins, and proteases responsible for subsequent damage to pulmonary tissue.14 Trimethoprim-sulfamethoxazole combined with corticosteroid therapy to suppress lung inflammation is the preferred and most effective course of treatment for Pneumocystis and is usually begun when the CD4+ cell count is less than 200 per cubic millimeter, a state of a heightened risk of infection.14 Adverse effects of the drug are common, however, and use of this prophylaxis has rapidly increased the multidrug resistance of infection bacterial pathogens found in human immunodeficiency virus-infected animals.10, 14
1.Â Lung: Pneumonia, interstitial, histiocytic and neutrophilic, chronic, diffuse, moderate, with type II pneumocyte hyperplasia, multinucleate giant cells, and myriad intraalveolar fungi, etiology consistent with Pneumocystis carinii, Rhesus macaque (Macaca mulatta), primate.
2.Â Cytological specimen, impression smear, lung: Numerous epithelial cells, macrophages with vacuolated cytoplasm, few neutrophils, and myriad 3-5um round cysts containing punctuate organisms (trophic bodies) on a blue, granular, proteinaceous background.Â
The contributor gives an excellent overview of Pneumocystis carinii.Â Pneumocystis carinii has also been described in pigs, foals, dogs, and other domestic animals with underlying immunosuppression.2 The characteristic histologic finding is an eosinophilic foamy or flocculent material within alveoli with numerous intra- and extracellular fungal bodies.Â The organism can be more readily identified with Periodic acid-Schiff (PAS) procedure, or Gomori methenamine silver (GMS) stain.2
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13.Â Stahl J, Sage MR: Radiological-pathological correlation: Alveolar pattern.Â Australasian Radiology 45:74-97, 2001
14.Â Thomas CF, Limper AH: Pneumocystis pneumonia.Â N Engl J Med 350:2487-98, 2004
15.Â Vogel P, Miller CJ, Lowenstine LL, Lackner AA: Evidence of horizontal transmission of Pneumocystis carinii pneumonia in simian immunodeficiency virus-infected Rhesus macaques.Â J Infect Dis 168:836-843, 1993