Tree shrew is a novel animal model used in biology and virology. Although wild captured tree shrews have been spared from HEV infection [
16], Asian musk shrew (
suncus murinus), another species of shrew, is susceptible to rat HEV [
17]. In the present study, the HEV-infected tree shrews manifested symptoms of HEV infection similar to HEV-infected humans. These symptoms include HEV shedding in feces (Table
1), detectable HEV antigens in the liver and other extra-hepatic tissues (Fig.
2), induced humoral response (Fig.
3), elevated liver-specific enzyme activities, and aggravated liver lesions (Fig.
4). These manifestations strongly suggest that tree shrews are susceptible to HEV. HEV RNA was first detected in the feces at 3–4 dpi; this result is consistent with that involving HEV-infected Balb/C nude mice (4 dpi) [
5], swine (3 dpi) [
18], or cynomolgus macaques (4–6 dpi) [
19]. HEV RNA also occurred earlier in shrews than in rats inoculated with rat HEV infectious clone (14 dpi) [
20]. Similarly, a negative HEV strand was detectable from 3–4 dpi to 28 dpi (i.e., end of the experiment, Table
1), which indicates that HEV RNA was shed in the feces during at least 4 weeks. HEV usually causes self-limited diseases with viral shedding in feces for 2–4 weeks [
3]. HEV infection animal models showed similar syndromes. Swine HEV-infected nude mice shed virus for 3 weeks [
5], and the infection of rhesus macaque lasts for 4 weeks [
21]. In the present study, a large extent of HEV replication in the liver and serum was detected even at 28 dpi, suggesting that HEV replicates last for more than 28 dpi and that the HEV infection of tree shrews is successful.
HEV is replicated in multiple tissues, such as the spleen and kidneys, as confirmed by the results for Balb/C nude mice [
5] and swine [
22]. Similarly, HEV RNA and capsid protein are simultaneously detected in the liver, spleen, and kidneys of the HEV-infected tree shrews. This result strongly indicates that the spleen and kidney are additional extra-hepatic replication sites of HEV, and a recent study has confirmed that the virus is detectable in the urine of HEV-infected patients [
21]. Although HEV antigens have been identified in the colon of HEV-infected Balb/C nude mice by indirect immunofluorescence assay [
5], HEV RNA is detected in the PBS washed colon of only one tree shrew (1/3). Capsid protein is not found in any of the HEV-infected tree shrews based on the results of the Western blot. Thus, the possibility of the colon being a replication site of HEV should be further studied. HEV RNA is detected in the bile of the HEV-infected tree shrews; this result is similar to that for HEV-infected nonhuman primates or swine. Moreover, the level of ALT and AST is obviously elevated in the HEV-infected tree shrews. More interestingly, the presence of HEV antibodies (both IgM and IgG antibodies) in the tree shrews indicates that these animals can be successfully used as models for acute HEV infection studies.
Tree shrews have considerable genetic homology with both humans and primates and are therefore useful models for viral infections, including herpes simplex virus [
23], rotavirus [
24], and hepatitis viruses, such as HBV [
9] and HCV [
25]. Tree shrews are especially suitable for hepatitis studies. For example, liver cirrhosis and hepatocellular carcinoma have been observed in HCV-infected tree shrews [
8]. Chronic HBV infection has also been simulated in HBV-infected tree shrews, and the results show histopathological changes similar to the clinical symptoms in HBV-infected humans [
9,
26]. Furthermore, trees shrews are advantageous in anti-viral drug development and preclinical studies [
27]. For instance, tree shrews have been successfully used to establish a fatty liver model [
28]. Thus, tree shrews may be highly suitable animal models for HEV studies.