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Researchers from the Department of Entomology at the South Korean National Institute of Health decided to test three essential oils and their major constituents against a form of typhus for which there is no effective vaccine.
“Scrub typhus” is a form of typhus that is caused by Orientia tsutsugamushi, a Gram-negative α-proteobacterium. A Wikipedia article states that this typhus is found in what is called the “tsutsugamushi triangle” which extends from northern Japan and far-eastern Russia in the north, to the territories around the Solomon Sea into northern Australia in the south, and to Pakistan and Afghanistan in the west.”1
The typhus is transmitted to humans by the genus of mites called Leptotrombidium. In the larval form (called chiggers) the mites feed on rodents or humans who walk through brushy areas. “Leptotrombidium mites are therefore both vector and reservoir for O. tsutsugamushi.”2
While other forms of typhus belong to the genus Rickettsia, scrub typhus is categorized as Orientia. The Wikipedia article states that “Without treatment, the disease is often fatal. Since the use of antibiotics, case fatalities have decreased from 4–40% to less than 2%.”3 However, strains of the scrub typhus are becoming resistant to doxycycline and chloramphenicol. Because of great antigenic variations in the strains, there is no vaccine available for this chigger- and tick-borne bacterium.
Here is where the South Korean study4 brings hope. The researchers studied the essential oils of cassia bark, eucalyptus, and star anise along with their major constitutents: (E)-cinnamaldehyde; 1,8-cineole; and (E)-anethole.
The study showed that the efficacy of the essential oils deserves even more research. The researchers stated: “Results were compared with those of conventional repellents DEET . . ., IR3535 . . . , and permethrin. Based on the median repellent concentration (RC50) values, (E)-cinnamaldehyde, (E)-anethole, cassia bark oil, and star anise oil . . . exhibited significantly more potent repellency than DEET. . . . (E)-cinnamaldehyde, (E)-anethole, cassia bar oil, and star anise oil were approximately 43, 16,11, 8, and 4 times more effective than IR3535 (CC5, 6.51%). . . . The median residual duration time of repellency (RT50) was significantly more pronounced in DEET (RT50, 323 min) than in all essential oils and constituents (108-167 min). In the light of global efforts to reduce the level of highly toxic synthetic repellents, the three essential oils and their major constituents described merit further study as potential biorepellents for the control of L. pallidum populations.”5
- Scrub typhus, op cited.
- Shin EH, et al., “Repellency of cassia bark, eucalyptus, and star anise oils and their major constituents to Leptotrombidium pallidum (Acari: Trombiculidae),” J Med Entomol. 2013 May;50(3):579-84.
Copyright © 2013 AIRASE. Posted with permission from AIRASE