Signalment:  

Adult female grey short-tailed opossum (Monodelphis domestic).The animal was found dead.

Gross Description:  

The left ventricle was dilated with a cauliflower mass involving the aortic valves and the base of the aorta at the level of the branching of the coronary arteries from the aorta.

Histopathologic Description:

Heart: There is a mixed inflammatory infiltrate consisting predominately of neutrophils admixed with large colonies of bacterial cocci that are attached to and destroying the aortic valves.

Morphologic Diagnosis:  

Heart, aortic valve: Bacterial vegetative myocardial valvulitis.

Lab Results:  

Gram stain: Gram-positive small cocci suggestive of Streptococcus sp.

Condition:  

Streptococcal valvulitis

Contributor Comment:  

Bacterial vegetative endocarditis is a common spontaneous occurrence in the Virginia opossum (Didelphis virginiana) and this marsupial has been used as an experimental animal model for Streptococcus bacterial endocarditis. Although cardiovascular diseases are the second most common cause of death of the laboratory grey short-tailed opossum (Monodelphis domestica)(1), to the contributors knowledge, bacterial vegetative valvular endocarditis has not been previously reported in the laboratory grey short-tailed opossum. 

Bacterial endocarditis primary arises from adhesion of the microorganisms to the endocardium, leading to death of the endothelium and formation and adherence of a thrombus within which large colonies of bacteria proliferate. Such proliferative growths and thrombus are called vegetative endocarditis. Although not observed in the opossum, pieces of the vegetation may break free and circulate to other organs, causing septic infarcts or abscesses. It is not uncommon for valvular endocarditis to cause cardiac dysfunction leading to congestive heart failure.

JPC Diagnosis:  

Heart: Valvulitis, arteritis and endocarditis, fibrinosuppurative, multifocal to coalescing, subacute, severe with fibrin thrombi, myocardial degeneration and necrosis and numerous Gram-positive bacterial cocci.

Conference Comment:  

In recent years, the grey short-tailed opossum has become the most commonly utilized marsupial in biomedical research, owing to its small size, docile nature, rapid growth, high fertility and relative ease of husbandry.(3) The young are not fully developed at birth, but born at a stage somewhat comparable to 40-day-old human embryos. Thus this species is often used in reproductive research.(5) M. domestic is also routinely used in the study of UV light-induced skin and eye neoplasia, such as melanoma. Although M. domestica is a hardy species with few documented parasitic or specific infectious diseases, several spontaneous pathologic conditions are reported. Most are associated with the digestive system, including rectal prolapse, which occurs primarily in females, likely related to parturition. Dermatitis, and cardiovascular disease with secondary pulmonary lesions, are also described. Pituitary adenoma is reported as the most common neoplasm in M. domestica, followed by uterine leiomyoma and cutaneous lipoma.(1)

Spontaneous bacterial endocarditis in the Virginia opossum (Didelphis virginiana) is typically due to Streptococcus viridans or Staphylococcus aureus.(4) Although culture was not performed in this case, a tissue Gram stain reveals numerous intralesional Gram-positive cocci, supporting a similar etiology in this case. In research species, bacterial endocarditis has been associated with the use of vascular access ports and intravenous catheters. Vegetative valvular endocarditis is also seen in ruminants, swine, dogs, and rarely in cats and horses. Streptococcus sp., Staphylococcus sp. and E. coli are frequently implicated as the etiologic agents in many species. Additionally, Erysipelothrix rhusiopathiae is often isolated in pigs and (occasionally) dogs, while Bartonella sp. is more specific to the dog or the cat. Arcanobacterium pyogenes is a common pathogen in cattle and Actinobacillus equuli can occasionally cause valvular endocarditis in horses.(2) Regardless of the inciting cause, this condition can result in valve damage and the development of congestive heart failure, or detachment of the vegetations with subsequent embolic disease.(2) In this case, because the lesion is located in the left heart at the aortic valve, the kidney would be a likely anatomic location for secondary embolic lesions.

References:

1. Hubbard, GB, Mahaney MC, Gleiser CA, Taylor DE, VandeBerg JL. Spontaneous pathology of the gray short-tailed opossum. Laboratory Animal Science. 1997;47:19-26.

2. Maxie MG, Robinson WF. Cardiovascular system. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. 5th ed. Vol. 3. Philadelphia, PA: Elsevier Saunders; 2007:27-29.

3. Samollow PB. The opossum genome: insights and opportunities from an alternative mammal. Genome Res. 2008;18(8):1199-1215.

4. Sherwood BF, Rowlands MD, Vakilzadeh J, LeMay JC. Experimental bacterial endocarditis in the Opossum (Didelphis virginiana). Am J Pathol. 1971;64(3):513-520.

5. Xie Q, Mackay S, Ullmann SL, Gilmore DP, Payne AP, Gray C. Postnatal development of Leydig cells in the opossum (Monodelphis domestica): an immunohistochemical and endocrinological study. Biol Reprod. 1998;58(3):664-669.

Click the slide to view.

---

2-1. Heart

2-2. Aorta

2-3. Aorta

---