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SCRUB TYPHUS

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A relatively new scourge on the Arthropod-transmitted viruses scene since the 2nd World War. The Rickettsia Orientia tsutsugamushi is a typhus transmitted by mites. Widespread in the Asia-Pacific area, in Japan it has been called River Fever, Chigger-borne Rickettsiosis or Tsutsugamush disease. The disease ranks next to malaria in importance in the Pacific area. There are several mite species of the genus Leptotrombidium that are known or believed to be vectors, with Leptotrombium deliense and Trombicula akamushi being the most important (Service 2008). Infection occurs when being bitten by infected larval trobiculid mites, which occur in large numbers around vegetation. Rodents have been considered as reservoirs of the virus as well as the mites themselves.

LIFE CYCLE

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Trombiculid mites have a complex life cycle and different terms have been applied to the developmental stages, but the terminology used by Service (2008) is applied as follows: Adults of this group are not parasitic but rather inhabit the soil where they feed on other arthropods. During warm weather a female mite may lay up to five eggs daily on organic material located on the soil surface, in field grasses, etc. "Deutorum" larvae with six legs emerge but initially do not leave the egg shell (the "Deutovum"). Activity begins about a week later when the mites swarm all over the soil and grasses. They try to attach to mammals and birds as well as to people with which they come into contact. They gather around soft and moist areas of a host.

The larvae then penetrate into the skin, injecting saliva that destroys cells. They feed on lymphal fluid instead of blood. The continued release of saliva then results a nasty skin reaction. Some species spend a whole month on a host, but the vectors of Scrub Typhus remain on a host for only about a week. When fully fed the larvae exit the host and drop to the ground where they bury into the soil or under leaf litter, etc. There they change into a "Protonymph," which moults within week and gives rise to a "Deutonymph" with eight legs. The deutonymphs like the adults feed for a couple of weeks on arthropods in the soil. Feeding stops and the nymphs change into a "tritonymph" that moults after about two weeks giving rise to the adult stage. The total life cycle generally takes up to two months, but sometimes 8-10 months are required.

Because nymphs and adults feed on other arthropods they require habitats where there are sufficient arthropods present to sustain them. Service (2008) noted that ideal habitats are often produced when vegetation is cleared for agriculture or wood products.

A larva will remain on only one host during its lifetime, so transmission does not occur between people by them. Rather the nymphs and adult mites are vectors. Transovarial transmission among mites insures virus persistence in their population (See Service 2008 for details about transmission).

CONTROL

Repellents have been recommended for control, as it is difficult to effectively attack the mites. Once principal habitats among the vegetation are discovered these can be sprayed with insecticides or even destroyed by burning.

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Key References: <medvet.ref.htm> <Hexapoda>

Azad, A. F. 1986. Mites of public health importance and their control. WHO/VBC/86.931. Geneva, Switzerland

Frances, S. P., D. Watcharapichat, D. Phulsuksombati & P. Tanskul. 2000. Transmission of Orientia tsutsugamushi, the aetiological

agent for scrub typhus, to co-feeding mites. Parasitology 120: 601-607.

Hengbin, G., C. Min, T. Kaihua & T. Jiaqi. 2006. The foci of scrub typhus and strategies of prevention in the spring in :ingtan Island, Fujian

Province. Ann. New York Acad. Sci. 1078: 188-196.

Kawamura, A., H. Tanaka & A. Tamura (eds.). 1996. Tsutsugamushi Disease: an Overview. Tokyo Univ. Press.

Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p.

Ogawa, M., T. Hagiwara, T. Kishimoto et al. 2002. Scrub typhus in Japan: epidemiology and clinical features of cases reported in 1998.

Amer. J. Trop. Med. & Hyg. 67: 162-65.

Roberts, S. H. & J. H. Zimmerman. 1980. Chigger mites: efficacy of control with two pyrethroids. J. Econ. Ent. 73: 811-812.

Sasa, M. 1961. Biology of chiggers. Ann. Rev. Ent. 6: 221-244.

Service, M. 2008. Medical Entomology For Students. Cambridge Univ. Press. 289 p

Strickman, D. 2001. Scrub typhus. IN: The Encyclopedia of Arthropod-Transmitted Infections of Man and Domesticated Animals. CABI,

Wallingford, pp. 456-62

Takahashi, M., M. Misumi, H. Urakami et al. 2004. Mite vectors (Acari: Trombiculidae) of scrub typhus in the new endemic area in northern

Kyoto, Japan. J. Med. Ent. 41: 107-114.

Takahashi, M., M. Murata, H. Misumi, E. Hori, A. Kawamura. & H. Tanaka. 1994. Failed vertical transmission of Rickettsia tsutsugamushi

(Rickettsiales: Rickettsiaceae) acquired from rickettsemic mice by Leptotrombidium pallidum (Acari: Trombiculidae). J. Med. Ent.

31: 212-16.

Traub, R. & C. L. Wissemann. 1968. Ecological considerations in scrub typhus. 1> Emerging concepts. Bull. WHO 39: 209-218.

Traub, R. & C. L. Wissemann. 1974. The ecology of chigger-borne ricksettsiosis (scrub-typhus). J. Med. Ent. 11: 237-303.

Walter, D. E. & H. C. Proctor. 1999. Mites: Ecology, Evolution & Behavior. Univ. of New South Wales Press, Sydney.