Female 4 year old cynomolgus monkey (Macaca fascicularis).This monkey was a control animal in a 4 week oral gavage toxicity study with an experimental compound. It exhibited no clinical signs while on the study and was euthanized at the scheduled terminal sacrifice. The lesion was found incidentally during routine histopathologic evaluation of tissues.

Gross Description:  

Necropsy findings included a discolored duodenum, liver cyst, small pancreas, and an abnormally shaped spleen.

Histopathologic Description:

Heart: There are moderate infiltrates of lymphocytes, plasma cells, macrophages, and rare neutrophils predominantly within the myocardium, and to a lesser extent affecting the endocardium and minimally affecting the epicardium. Inflammatory cell infiltrates and mild edema expand the interstitium and separate cardiac myofibers. Scattered myofibers exhibit degeneration characterized by variation in fiber size, hypereosinophilia, loss of cross striations, fragmentation, and nuclear hypertrophy. Rare myofibers contain protozoal pseudocysts. Pseudocysts are irregularly shaped, approximately 30 microns in length, and contain myriad amastigotes. Amastigotes are spherical, basophilic, approximately 2 microns in diameter, and have a darkly basophilic kinetoplast.

Morphologic Diagnosis:  

Heart: Moderate, chronic, multifocal to coalescing lymphoplasmacytic and histiocytic myocarditis and endocarditis with myofiber degeneration and protozoal pseudocysts containing amastigotes (Trypanosoma cruzi)

Lab Results:  

Three days prior to euthanasia, electrocardiography revealed multiple premature ventricular complexes, including periods of ventricular bigeminy and paroxysmal ventricular tachycardia.


Trypanosoma cruzi

Contributor Comment:  

Trypanosoma cruzi is the causative organism of Chagas disease. Trypanosoma, along with Leishmania, belongs to the subphylum Sarcomastigophora. T. cruzi is a hemoflagellate protozoan that is endemic in South America, Central America, Mexico, and in the southern United States. Natural reservoirs include opossums, armadillos, rodents, dogs, cats, pigs, raccoons, and monkeys.(1,4)

Insects serve as the vector for most Trypanosoma species. Trypanosoma cruzi resides in the hindgut of reduviid bugs.(1,3) At night these bloodsucking insects emerge to feed upon sleeping hosts. They usually target the face, and because of this are also known as kissing bugs. Once fed the insects will often defecate, depositing infective organisms onto the skin. Trypanosomes gain entry through a wound in the skin or by crossing mucous membranes.(3) While circulating in the body, they are in a form known as trypomastigotes. Most species of Trypanosoma exist solely in this form, which is also the reproductive stage. T. cruzi is unique among trypanosomes in that it also forms a tissue pseudocyst containing amastigotes(3), and this is the form that undergoes multiplication in this species.(1) Amastigotes can then differentiate into trypomastigotes, which rupture out from their cysts and can either invade another cell or circulate within the blood to infect another intermediate host. Amastigotes are most commonly found in cardiac and skeletal muscle, but can also occur in reticuloendothelial, neural, and glial cells.(1,4)

T. cruzi infection in primates often results in nonspecific clinical signs including edema, anemia, hepatosplenomegaly, and lymphadenitis. The most severe sequela of infection is myocarditis. Myocarditis can result in dilated cardiomyopathy, arrhythmias, and eventually death.(7) Myocarditis is likely the result of a variety of factors, including a reaction to degenerating parasites as well as an autoimmune component caused by a release of proteins from degenerating myofibers.(6,8)

This case is unusual in that it occurred in a monkey originally from Asia, where T. cruzi is not found. This particular monkey originated from China and was shipped to a holding facility in Texas, where it was held indoors in quarantine for approximately 2 -+ months. Following quarantine, the monkey was placed in a single-sex corn crib style outdoor housing unit for 11 months. This is most likely where the animal became infected. The clinical history of this monkey while at the holding facility was unremarkable. A few cases of T. cruzi in animals housed at this facility have occurred, and human cases are not uncommon in the surrounding area.

JPC Diagnosis:  

Heart: Pancarditis, lymphoplasmacytic, and histiocytic, multifocal, mild to moderate, with myocardiocyte degeneration and necrosis and rare intracytoplasmic protozoal amastigotes.

Conference Comment:  

In most cases of cardiac T. cruzi infection, granulomatous myocarditis with myocyte degeneration and necrosis is most severe in the right atrium and ventricle, while chronic disease, such as in this case, presents as lymphoplasmacytic myocarditis concentrated in the apex of the heart with fewer organisms. In addition the organs listed by the contributor, T. cruzi amastigotes have been found within germinal cells in the testicle of dogs surrounded by an intense lymphoplasmacytic interstitial orchitis(4).

Two proteins on the surface of T. cruzi are involved in its entry into macrophages and other host cells. Transsialidase removes host cell sialic residues and transfers them to a parasite surface protein (Ssp-3), which binds to host cells. Penetrin binds extracellular matrix proteins, heparin, heparin sulfate, and collagen and mediates parasite invasion into host cells. Intramacrophage survival is due to rapid movement from lysosomes to the cytosol, which is mediated by neuraminidase which removes sialic acids from host proteins and destabilizes the lysosomes, and hemolysins, in which lysosomal acid pH stimulates release and the formation of pores in lysosomal membranes(2).

Protozoal pseudocysts were difficult to find, and were often located in myofibers unaffected by the inflammatory response. Due to the low numbers of observable parasites and atypical host, several viral etiologies were also considered in the differential diagnosis including picornaviridae, such as encephalomyocarditis virus or Coxsackie B virus, and morbillivirus (measles), adenovirus, and betaherpesvirus (cytomegalovirus).(5)


1.Bowman, DD. Georgis Parasitology for Veterinarians. 7th edition. Philadelphia, PA: W.B. Saunders Company; 1999
2. de Souza W, de Carvalho TMU, Barrias ES. Review on Trypanosoma cruzi: Host Cell Interaction. Int J Cell Biol. 2010; 2010: 295394.
3. Gardiner CH, Fayer R, Dubey JP. An Atlas of Protozoan Parasites in Animal Tissues. 2nd edition. Armed Forces Institute of Pathology; 1998.
4. Jones TC, Hunt RD, King NW, Veterinary Pathology. 6th edition. Baltimore, MD: Wiliams & Wilkins; 1997.
5. Masek-Hammerman K, et al. Epizootic Myocarditis Associated with Encephalomyocarditis Virus in a Group of Rhesus Macaques (Macaca mulatta). Vet Pathol. 2012 Mar;49(2):386-92.
6. McAdam AJ, Sharpe AH. Infectious Diseases. In: Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th ed. Philadelphia, PA: Elsevier Saunders; 2005:405-406.
7. Toft JD, Eberhard ML. Parasitic diseases. In: Bennett TB, Abee CR, Henrickson R. Nonhuman Primates in Biomedical Research Diseases. San Diego, CA: Academic Press; 1998: 115-116.
8. Valli VEO. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. 5th ed., vol. 3. Philadelphia, PA: Elsevier Ltd; 2007:254.

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