Clinical, Histopathologic, and Immunohistochemical Characterization of Experimental Marburg Virus Infection in A Natural Reservoir Host, the Egyptian Rousette Bat (Rousettus aegyptiacus)

Abstract

Egyptian rousette bats (Rousettus aegyptiacus) are natural reservoir hosts of Marburg virus (MARV), and Ravn virus (RAVV; collectively called marburgviruses) and have been linked to human cases of Marburg virus disease (MVD). We investigated the clinical and pathologic effects of experimental MARV infection in Egyptian rousettes through a serial euthanasia study and found clear evidence of mild but transient disease. Three groups of nine, captive-born, juvenile male bats were inoculated subcutaneously with 10,000 TCID50 of Marburg virus strain Uganda 371Bat2007, a minimally passaged virus originally isolated from a wild Egyptian rousette. Control bats (n = 3) were mock-inoculated. Three animals per day were euthanized at 3, 5–10, 12 and 28 days post-inoculation (DPI); controls were euthanized at 28 DPI. Blood chemistry analyses showed a mild, statistically significant elevation in alanine aminotransferase (ALT) at 3, 6 and 7 DPI. Lymphocyte and monocyte counts were mildly elevated in inoculated bats after 9 DPI. Liver histology revealed small foci of inflammatory infiltrate in infected bats, similar to lesions previously described in wild, naturally-infected bats. Liver lesion severity scores peaked at 7 DPI, and were correlated with both ALT and hepatic viral RNA levels. Immunohistochemical staining detected infrequent viral antigen in liver (3–8 DPI, n = 8), spleen (3–7 DPI, n = 8), skin (inoculation site; 3–12 DPI, n = 20), lymph nodes (3–10 DPI, n = 6), and oral submucosa (8–9 DPI, n = 2). Viral antigen was present in histiocytes, hepatocytes and mesenchymal cells, and in the liver, antigen staining co-localized with inflammatory foci. These results show the first clear evidence of very mild disease caused by a filovirus in a reservoir bat host and provide support for our experimental model of this virus-reservoir host system.

Marburg and Ebola virus infections elicit a complex, muted inflammatory state in bats

“…MARV-inoculated animals showed histopathology like that observed in prior experimental infection studies 10 . Immunohistochemistry with a pan-filovirus antibody suggested that MARV was present in mammary glands and testes, despite the lack of histopathological lesions in these organs.…”

“…The most abundant group in this set comprised genes related to mitochondria (20 genes), followed by genes involved in the vascular system (19), innate immunity (16), tissue regeneration and apoptosis (15), macrophages (13), inflammation (10), metabolism and fatty-acid oxidation (8), T cells (4), complement system (2), digestion (5), and toxin processing (3).…”

“…MARV has been isolated from the Egyptian rousette bat (ERB, Rousettus aegyptiacus) [5][6][7] , and ecological and experimental studies have demonstrated that ERB are a reservoir for the virus 6,8 . Experimental infections of ERBs with MARV have consistently demonstrated that despite viral replication in multiple tissues, animals develop a mostly subclinical disease, which is characterized by mild pathology involving transient elevation of alanine aminotransferase, elevated lymphocyte and monocyte counts, and some evidence of minimal inflammatory infiltration in the liver 9,10 . Clinical signs of disease are absent [9][10][11][12][13] .…”

Bat-borne virus diversity, spillover and emergence

“…Current data suggest that the classical pathology caused by strong activation of the immune system in response to viral infection that is seen in humans and laboratory animal models does not occur in bats 37,47 . The lack of pathology observed in bats is likely due to a combination of differences in viral tissue tropism and host immune responses 48 . Viral replication and shedding in bats in combination with an apparent lack of disease may allow for the efficient maintenance and dissemination of viruses.…”

Asymptomatic Infection of Marburg Virus Reservoir Bats Is Explained by a Strategy of Immunoprotective Disease Tolerance

“…[18][19][20] Further, infecting ERB bone marrow-derived dendritic cells (BMDCs) failed to increase inflammatory gene expression, while experimental MARV infections of ERBs produced minimal, if any, gross histological signs of inflammation, even in tissues with highest viral loads. 3,5,6,21,22 The culmination of these data led us to hypothesize that ERBs have indeed developed a system of disease tolerance to MARV. However, significant testing of this hypothesis has so far been limited to immortalized cell lines, ex vivo tissue culture infections, or genomic approaches that cannot reproduce or examine the complexity and context of an immune response in a whole animal, which to our knowledge remains uncharacterized at a broad transcriptional level for any bat reservoir of a human-pathogenic virus, including ERBs infected with MARV.…”