Experimental infection of ground squirrels (Spermophilus tridecemlineatus) with monkeypox virus

Abstract

A proposed new small-animal (rodent) model for studying the pathogenesis and treatment of severe orthopoxvirus infections is described. Thirteen-lined ground squirrels (Spermophilus tridecemlineatus) were infected intraperitoneally and intranasally with monkeypox virus (MPXV). A fulminant illness developed in all animals, and they died 6-9 days after infection. Virus was cultured from the blood and oropharynx several days before death; at necropsy, all of the organs tested contained relatively high titers of MPXV. The major pathologic findings were in the liver, which showed centrilobular necrosis, steatosis, and basophilic inclusion bodies in hepatocytes. Splenic necrosis was also observed, as well as interstitial inflammation in the lungs. The pathologic features of MPXV in ground squirrels are similar to that described with MPXV in macaques and severe variola (smallpox) virus infection in humans.

Figure 1

Representative photomicrographs of histologic changes and immunohistochemical staining of tissues from ground squirrels infected with monkeypox virus. A) Liver from a ground squirrel (intranasal infection) showing mild degenerative changes, including early steatosis, and purple-colored viral cytoplasmic inclusion bodies in the hepatocytes (40x objective). B) Spleen from a ground squirrel infected intraperitoneally, showing extensive necrosis (20x objective). C) Liver showing positive antigen staining of the intrahepatocytic inclusion bodies; antigen is present in the cytoplasm and to a lesser extent in cell membranes (40x objective). D) Spleen from a ground squirrel infected intraperitoneally, showing positive antigen staining in the interstitial cells, endothelial cells (arrowheads), and the surface mesothelial cells (arrows) (20x objective). E) Same tissue sample as D, showing the edge of the spleen with antigen-positive mesothelial layer; the adjacent fat and fibrous tissue show necrosis but are also strongly positive for antigen (20x objective). F) Lung from the same animal showing many antigen-positive interstitial cells and pneumocytes (40x objective). A and B, hematoxylin and eosin stain; C–F, immunoperoxidase stain.

Figure 2

Ultrastructural localization of monkeypox virus in hepatocytes in the liver of a ground squirrel 5 days after infection. A) Hepatocytes contain numerous groups of virions (arrows) in their cytoplasm (bar = 1 µm). B) Magnified area of A, showing typical ultrastructure of monkeypox virus virions and characteristic hepatocyte mitochondria (M) surrounded by cisterns of granular endoplasmic reticulum (er). N, fragment of hepatocyte nucleus; bar = 0.5 µm.