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Virulence of Malaysian isolates of Orientia tsutsugamushi in mice
1Department of Medical Microbiology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia. tayst@ummc.edu.my
Abstract:
The pathogenicity of Malaysian isolates of Orientia tsutsugamushi was investigated by a mouse virulence assay. The isolates could be differentiated as low (4 isolates), moderately (3 isolates) and highly virulent (2 isolates) based on the different responses in infected mice. No direct correlation between severity of human scrub typhus infections and virulence of the O. tsutsugamushi in mice was observed. Mice infected with virulent strains of O. tsutsugamushi showed splenomegaly, ascitis accumulation and enlargement of kidneys and livers whereas avirulent O. tsutsugamushi strains were asymptomatic and exhibited ruffled fur for a short period after infection. There was low antibody response in mice infected with isolates of low pathogenicity as compared with those of highly virulent isolates. Upon dissection of the infected mice, enlargement of mouse organs such as spleen, kidney and liver was noted. Presence of rickettsemia in mice was confirmed by the growth of O. tsutsugamushi in the L929 cells when inoculated with blood from infected mice. O. tsutsugamushi was also cultured from the peritoneal exudates of the infected mice. However, DNA of O. tsutsugamushi was only detected in the peritoneal exudates (by PCR) and blood (by cell culture) and not from other tissue samples.
Insights
Pathogenicity of Malaysian Orientia tsutsugamushi isolates varied in mice, showing no direct link to human scrub typhus severity. Virulent strains caused organ enlargement and detectable rickettsemia, while avirulent strains did not.
Area of Science:
- Microbiology
- Pathogen Research
- Infectious Diseases
Background:
- Orientia tsutsugamushi is the causative agent of scrub typhus.
- Understanding the pathogenicity of regional isolates is crucial for disease management.
Purpose of the Study:
- To investigate the pathogenicity of Malaysian Orientia tsutsugamushi isolates using a mouse virulence assay.
- To correlate mouse virulence with observed human scrub typhus severity.
Main Methods:
- Mouse virulence assay to assess pathogenicity.
- Clinical observation of infected mice (splenomegaly, organ enlargement).
- Detection of rickettsemia via cell culture and PCR.
Main Results:
- Malaysian Orientia tsutsugamushi isolates were categorized into low, moderate, and high virulence groups in mice.
- Virulent strains induced splenomegaly, ascites, and organ enlargement (kidneys, liver).
- No direct correlation was found between mouse virulence and human scrub typhus severity.
Conclusions:
- Mouse models can differentiate the pathogenicity of Orientia tsutsugamushi isolates.
- Virulence in mice correlates with specific clinical signs and antibody response.
- Further research is needed to understand the discrepancy between mouse virulence and human disease severity.
