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Abstract
Scrub typhus threatens one billion people in the Asia-Pacific area and cases have emerged outside this region. It is caused by infection with any of the multitude of strains of the bacterium Orientia tsutsugamushi. A vaccine that affords heterologous protection and a commercially-available molecular diagnostic assay are lacking. Herein, we determined that the nucleotide and translated amino acid sequences of outer membrane protein A (OmpA) are highly conserved among 51 O. tsutsugamushi isolates. Molecular modeling revealed the predicted tertiary structure of O. tsutsugamushi OmpA to be very similar to that of the phylogenetically-related pathogen, Anaplasma phagocytophilum, including the location of a helix that contains residues functionally essential for A. phagocytophilum infection. PCR primers were developed that amplified ompA DNA from all O. tsutsugamushi strains, but not from negative control bacteria. Using these primers in quantitative PCR enabled sensitive detection and quantitation of O. tsutsugamushi ompA DNA from organs and blood of mice that had been experimentally infected with the Karp or Gilliam strains. The high degree of OmpA conservation among O. tsutsugamushi strains evidences its potential to serve as a molecular diagnostic target and justifies its consideration as a candidate for developing a broadly-protective scrub typhus vaccine.
Highlights
Scrub typhus is an acute, febrile, and potentially deadly disease caused by infection with the larval Leptotrombidium mite-vectored bacterium, Orientia tsutsugamushi
DNA samples recovered from 51 geographically-diverse O. tsutsugamushi isolates (Table 1) were subjected to polymerase chain reaction (PCR) using primer set OTT_RS06375-1F/615R (Table 2), which targets the full-length ompA gene (OTT_RS06375) of annotated O. tsutsugamushi strain Ikeda
Aligning all outer membrane protein A (OmpA) amino acid sequences revealed that several segments thereof were 100% conserved among the isolates (Figure 3)
Summary
Scrub typhus is an acute, febrile, and potentially deadly disease caused by infection with the larval Leptotrombidium mite-vectored bacterium, Orientia tsutsugamushi. Long known to be endemic to the Asia-Pacific, a densely-populated region where more than one million cases are estimated to occur annually, scrub typhus affects all organs and the central nervous system. The disease can account for up to 20% of all acute undifferentiated febrile episodes and up to 27% of blood culture-negative fever patients in endemic areas [3,4,5]. Recent outbreaks in the Asia-Pacific [10,11,12,13,14,15,16,17,18,19,20,21,22,23] as well as evidence for scrub typhus or scrub typhus-like infections in Cameroon, Kenya, Congo, Djibouti, Tanzania, Chile, and Peru signify these illnesses as both emerging and reemerging diseases of global importance [24,25,26,27,28,29,30]
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