Leptotrombidium Explained

Leptotrombidium is a genus of mites in the family Trombiculidae, that are able to infect humans with scrub typhus (Orientia tsutsugamushi infection) through their bite. The larval form (called chiggers) feeds on rodents, but also occasionally humans and other large mammals. They are related to the harvest mites of the North America and Europe.

Originally, rodents were thought to be the main reservoir for O. tsutsugamushi and the mites were merely vectors of infection: that is, the mites only transferred the contagion from the rodents to humans.^{[1] [2]} However, the mites are now known to only feed once in their lifetimes, which means that transmission from rodent to human via the mites is impossible (for it to have been possible, the mite would have to feed at least twice, once on the infected rodent and again on the human who would then be infected).^[3] Instead, the bacterium persists in the mites through transovarial transmission,^{[4] [5] [6]} where infected mites transmit the infection to their unborn offspring. Leptotrombidium mites are therefore both vector and reservoir for O. tsutsugamushi.^[3] The infection predominantly affects female mites,^[7] and does not appear to otherwise harm the mites.

Life history

The larvae are pale orange in colour and feed on liquified skin tissue, not blood, as their mouth parts (chelicerae) are too short to reach the blood vessels.^[8] They have three pairs of legs. The larvae most commonly target rodents, but also attach to humans.^[3] For humans, the bite is painless, but pain commonly develops only after the larvae detach from the skin, leaving red papules that may then develop into an eschar.^[9]

The larval stage lasts for 1 to 2 weeks. After feeding, the larvae drop to the ground and become nymphs. Nymphs are brick-red in colour and have four pairs of legs. The nymphal stage lasts for 1 to 3 weeks. Nymphs mature into adults which have four pairs of legs, the first pair being the largest. They are harmless to humans. In the postlarval stage, they are not parasitic and feed on plant materials.^[10] Females lay eggs singly, which hatch in about a week. Lifespan of the adult is about 6 months.

Taxonomy

• L. akamushi is endemic to Japan and is the reservoir for the Kato serotype of O. tsutsugamushi;^[11]
• L. deliense is the main vector in the south of China^[12] and in Thailand;^[13] it is also endemic to Litchfield Park, in the Northern Territory, Australia, where is carries the Litchfield serotype.^[14]
• L. pallidum is endemic to Japan and is the reservoir for Karp and Gilliam serotypes;^{[11] [15]}
• L. scutellare is endemic to Japan and is the reservoir for Kawasaki and Kuroki serotypes.^{[11] [15]}

Notes and References

1. Philip CB. Tsutsugamushi disease (scrub typhus) in World War II. J Parasitol. 1948. 34. 3. 169 - 191. 10.2307/3273264. 3273264.
2. J Immunol. 1948. 59. 109 - 114. The long persistence of Rickettsia orientalis in the blood and tissues of infected animals. Fox JP. 2. 10.4049/jimmunol.59.2.109 . 18864084 .
3. Pham XD, Suzuki H, Takaoka H . Distribution of unengorged larvae of Leptotrombidium pallidum and other species in and around the rodent nest holes. Southeast Asian J Trop Med Public Health. 32. 3. 553 - 57. 11944716. 2001.
4. Walker JS, Chan CT, Manikumaran C, Elisberg BL . 1975. Attempts to infect and demonstrate transovarial transmission of R. tsutsugamushi in three species of Leptotrombidium mites. Ann NY Acad Sci. 266. 80 - 90. 10.1111/j.1749-6632.1975.tb35090.x. 829478.
5. Takahashi M, Murata M, Nogami S, Hori E, Kawamura A, Tanaka H . 1988. Transovarial transmission of Rickettsia tsutsugamushi in Leptotrombidium pallidum successively reared in the laboratory. Jpn J Exp Med. 58. 5 . 213 - 218. 3149693 .
6. Vertical transmission of Orientia tsutsugamushi in two lines of naturally infected Leptotrombidium deliense (Acari: Trombiculidae). Frances SP, Watcharapichat P, Phulsuksombati D . J Med Entomol. 2001. 38. 1. 17 - 21. 11268685. 10.1603/0022-2585-38.1.17.
7. Sex ratios in Rickettsia tsutsugamushi-infected and noninfected colonies of Leptotrombidium (Acari: Trombiculidae). Roberts LW, Rapmund G, Gadigan FG . J Med Entomol. 14. 1. 1977. 89 - 92. 10.1093/jmedent/14.1.89 . 409845 .
8. Distribution of Rickettsia tsutsugamushi in organs of Leptotrombidium (Leptotrombidium) fletcheri (Prostigmata: Trombiculidae). J Med Entomol. 12. 3. 1975. 345–348. Roberts LW, Robinson DM, Rapmund G, etal . 10.1093/jmedent/12.3.345. 810585.
9. Kitaoka M, Asanuma K, Otsuji J . Transmission of Rickettsia orientalis to man by Leptotrombidium akamushi at a scrub typhus endemic area in Akita Prefecture, Japan. Am J Trop Med Hyg. 1974. 23. 5. 993–9. 4451238. 10.4269/ajtmh.1974.23.993.
10. Lifecycle of Leptotrombidium pallidum (Acari: Trombiculidae), one of the vector mites of scrub typhus in Japan . Takahashi M, Misumi H, Urakami H, etal . Ohara Sogo Byoin Nenpo. 45. 19–30. 2003.
11. Scrub typhus (tsutsugamushi disease) in Japan, 1996–2000. Byogen Biseibutsu Kenshutsu Joho Geppo. 22. 211–212. 2001.
12. Wang S, Jiang P, Huang J, etal . Demonstration of the natural foci of tsutsugamushi disease in Nan Peng Lie Islands in China. 32. 3. 2001. 541 - 46. 11944714. The Southeast Asian Journal of Tropical Medicine and Public Health.
13. Frances SP, Watcharapichat P, Phulsuksombati D, Tanskul P, Linthicum KJ . 1999. Seasonal occurrence of Leptotrombidium deliense (Acari: Trombiculidae) attached to sentinel rodents in an orchard near Bangkok, Thailand. J Med Entomol. 36. 869 - 874. 10593093. 6. 10.1093/jmedent/36.6.869. free.
14. 10.3201/eid0404.980416 . Odorico DM, Graves SR, Currie B, etal . New Orientia tsutsugamushi strain from scrub typhus in Australia. Emerg Infect Dis. 1998. 4. 4. 9866742. 641–4. 2640248.
15. Kawamori F, Akiyama M, Sugieda M, etal . Epidemiology of Tsutsugamushi disease in relation to the serotypes of Rickettsia tsutsugamushi isolated from patients, field mice, and unfed chiggers on the eastern slope of Mount Fuji, Shizuoka Prefecture, Japan. J Clin Microbiol. 30. 11. 2842 - 2846. 1992. 10.1128/jcm.30.11.2842-2846.1992 . 1452653. 270539.