UNDER CONSTRUCTION
VECTORS:
Scrub typhus is transmitted to humans and rodents by some species of trombiculid mites, including a number of different species throughout the geographic range of O. tsutsugamushi. Details of the relationship of mites and scrub typhus can be obtained in the review of Santibáñez et al. (2015). The mites are very small (0.2 – 0.4mm), often virtually invisible by eye and only easily visible when seen through a microscope or magnifying glass.
Trombiculid mites have a complex life history, including egg, larva, three nymph stages and adult. The only life stage that can transmit the bacteria to mammals is the larval or “chigger” phase. Humans acquire the disease when bitten by an infected chigger. Because only the chigger stage bites to take a blood meal from mammals, the scrub typhus bacterium cannot be transmitted directly though the mite from one mammal (for instance a rodent) to another (human). Transmission can occur because the bacteria are capable of being transmitted from one generation of mite to the next via transovarial transmission from mother through the egg.
The vectors of scrub typhus are almost all members of the mite genus Leptotrombidium. A SEM of the larval stage of a representative member of Leptotrombidium appears below (figure from Living with Insects Blog/ Jonathan Neal).
In any geographical location only a small proportion (often less than 10%) of mites will be carrying O. tsutsugamushi. [Need reference]
SPECIES IN THE GENUS Leptotrombidium THAT HAVE BEEN IMPLICATED AS VECTORS OF HUMAN SCRUB TYPHUS, and their geographic foci (from Kelly et al., 2009):
L. deliense – Prevalent in Australia, China, India, Malaysia, New Guinea, Pakistan, Philippines, and Thailand; present in Sumatra (Indonesia), Myanmar, , Kazakhstan, Uzbekistan, Afghanistan, Nepal, Bangladesh, Vietnam, Cambodia, Sri Lanka, Singapore, Brunei, and Taiwan and Pescadores islands (Taiwan)
L. akamushi – Prevalent in Japan and Solomon Islands
L. arenicola – Present in Indonesia and Malaysia
L. chiangraiensis – Present in Thailand
L. fletcheri – Present in Indonesia, Malaysia, New Guinea, and Philippines
L. gaohuensis – Present in Zhejiang Province, China
L. imphalum – Present in Thailand
L. pallidum – Prevalent in Japan and Korea; present in Primorski Krai (Russia)
L. palpale – Present in Japan, Korea, and Primorski Krai, Russia
L. pavlovskyi – Prevalent in Primorski Krai (Russia)
L. scutellare – Prevalent in Japan; present in China, Korea, Malaysia, and Thailand
Other potential mite vectors that have been observed as carrying O. tsutsugamushi, but have not been demonstrated to transmit the bacteria to mammals include:
L. fuji – Present in Japan
L. orientale – Present in Japan, Korea, and Primorski Krai, Russia
L. tosa – Present in Japan
Eushoengastia koreaensis – Present in Korea
OTHER POSSIBLE VECTORS
In addition to the known mite vectors for scrub typhus, there is at least anecdotal evidence that other vectors may be involved in the transmission of the disease. Among the most likely alternate vectors are leeches. The evidence is substantial in the case of scrub typhus in Chile that the vector is likely to be a leech, since no evidence of mite or tick bites have been associated with cases of Chilean scrub typhus on the island of Chiloe (Balcells et al., 2011). There is also additional anecdotal reference in Asia to scrub typhus cases in which leech bites are known to be present in a patient’s history, but in which no history of a mite bite, or general absence or low abundance of mites in a geographic area is noted [Stone JH (ed.) . Crisis Fleeting: Original reports on military medicine in India, and Burma in the Second World War. Office of the Surgeon General, Department of the Army, Washington, D.C. 1969, page 124-125)].
Further study of leeches as a potential vector of scrub typhus may be profitable, especially in areas with endemic scrub typhus, but where ecological conditions would not favor mites.
ADDITIONAL CONSIDERATIONS
Mite islands
Reference: Audy, J.R., and J.L. Harrison. 1951. A review of investigations on mite typhus in Burma and Mayaya, 1945-1950. Trans. R. Soc. Trop. Med. Hyg. 44: 371-395.