Primary Investigation Of Experimental Infection Of Relapsing Fever Borrelia In Tree Shrew

Objective(s):Relapsing fever(RF)is an acute infectious disease caused by RF Borrelia and its clinical characteristics are periodic high fever and frequent recurrences,accompanied by muscle aches,liver and spleen enlargement and other symptoms.Severe patients have a tendency to jaundice and bleeding,and involve neurological symptoms.This condition is similar to a cold,so it is easy to be confused with viral influenza at the time of diagnosis,which can lead to delayed treatment and serious consequences.Therefore,the establishment of an RF animal infection model allows us to have a better understanding of the occurrence and development of the disease and the pathogenic mechanism.Currently,there are animal models of RF in Mus musculus,and as a new experimental animal,tree shrew(Tupaia Belangeris,tree shrew)have been studied in depth by researchers in evolution,classification and genetics in recent years.It has been found that tree shrews have similar biological characteristics as humans and other primates,and have been widely used in biomedical research,so we tried to use relapsing fever(RF)Borrelia to infect tree shrews to explore the replication of its pathogens in the body,and the characteristics of immune response,clinical symptoms and pathological changes caused by relapsing fever(RF)Borrelia infection in tree shrews.Methods:Intraperitoneal injection of pathogens is a faster route of absorption and easy to operate.We selected 1 to 2 year old F1 generation healthy adult tree shrews and injected Borrelia hermsii,a species of relapsing fever(RF)Borrelia,into the tree shrew via intraperitoneal injection.Before injection of pathogens,we need to take tree shrew anticoagulant as a control.After the injection,tail blood anticoagulation were collected daily and the number of RF Borrelia in peripheral blood was counted by dark field microscope.At the same times,we recorded the temperature changes and observed the symptoms of tree shrews.Later in the experiment,tree shrew were anesthetized and dissected,and collect its organs for histopathological analysis by HE staining.Meanwhile,we locate the Borrelia hermsii in the tree shrews by immunohistochemistry(IHC)experiments,and the liver function and changes in splenic lymphocytes of the tree shrews also be detected.Results:After experimental infection of tree shrews with Borrelia hermsii,the results show that:1)we monitored the bacteremia,it was found that Borrelia hermsii can effective multiply and periodically in tree shrews.After RF Borrelia infected tree shrews,the first peak of bacteremia appeared around the 7th dpi,then it would start to decline and the next peak appeared form around the 12th to 14th dpi,and finally formed a "recurunt" shaped proliferation curve;2)There was no significant change in the body temperature of tree shrews during the infection period,which was the same as that of non-infected tree shrews.Generally,the body temperature of the tree shrew is maintained between 40℃ and 42℃,however,the body temperature usually droped suddenly by 2℃ to 10℃ before death,and death occured within 24 hours afterwards;3)Observing the symptoms of tree shrews,we found that neurological symptoms such as weakness of the hind limbs and tail weakness appeared after RF Borrelia infected tree shrews;4)After dissecting the tree shrews,we clearly observed that their livers and spleens were obviously swollen,especially the spleens which about twice as large of the normal tree shrews;5)Pathological analysis of the tissues in tree shrews showed that,RF Borrelia infection caused severe inflammatory infiltration in the liver and spleen,and there are a large number of inflammatory cells infiltrated the tissues;6)The test result of liver function indicators ALT and AST suggest that a large amount of necrosis may have occurred in the liver cells of tree shrews;7)Flow cytometry revealed that the number of lymphocytes were decreased in the soleen,especially B lymphocytes.Conclusion(s):RF Borrelia can effectively infect tree shrews and form a "recurunt"shape of bacteremia,caused neurological symptoms.This infection also caused severe inflammatory infiltration in the liver and spleen of the tree shrew.At the same time,liver cells are necrotic and lymphocytes in the spleen are depleted.Therefore,this study suggests that tree shrews can be used as animal models for studying RF Borrelia infection and pathogenic mechanisms.