1. Life cycle of tsutsugamushi and infection caused by Rickettsia tsutsugamushi
     During the life cycle of the tsutsugamushi (egg, larva, nymph, adult), whereas the nymph and the adult (both have 4 pairs of legs) live freely in the soil, the larva (3 pairs of legs) (Figure 1) only lives on small mammals, such as field mice, etc., usually once (2~4 days), and it takes in their body fluids and the dissolved materials of the host tissues. There are also species of tsutsugamushi in which the unattached larva attaches to humans. If these larvae carry Orientia tsutsugamushi, which is the pathogen, humans, field mice, etc. will become infected. The rickettsia carried by these unattached larvae is assumedly transmitted through the ovaries of the parents. Kawamura et al (1917) raised in the laboratory adults of Leptotrombidium akamushi collected in endemic regions of Japan and after making incubated unattached larvae attached to a monkey, they demonstrated that the monkey contracted the disease.
     Later on Japanese researchers evidenced the transovarial transmission of rickettsia also in L. scutellare and L. pallidum. However the question is how the tsutsugamushi larva carrying these rickettsiae became carriers. As regards the transmission of rickettsia, it was thought for many years that the tsutsugamushi living on field mice carrying the rickettsia could easily catch it from field mice. However in fact the rickettsia does not easily migrate. The tsutsugamushi larva feeds by sucking the fluids between the cells without sucking the blood of the host. Moreover since the host cells around the mouth parts of the tsutsugamushi penetrating the skin of the host are destructed by digestion, the rickettsia is also possibly destructed. For this reason, though the field mouse may harbor the rickettsia, the migration of the rickettsia to the tsutsugamushi living on the mouse is apparently extremely difficult.
2. Examples of carriage of rickettsia in tsutsugamushi collected from field mice in the natural world
     Over ten species of rickettsia have been detected in tsutsugamushi living on wild animals. However the process by which these tsutsugamushi came to carry the rickettsia, namely whether they were transmitted from the parents through transovarial transmission, or they migrated from the host infected by the rickettsia is not clear. As mentioned above, three species in which the transovarial transmission has been demonstrated are found in Japan. These species are Leptotrombidium akamushi, Leptotrombidium scutellare and Leptotrombidium pallidum. Considering that cases of rickettsia infection are frequently observed in field mice captured in regions where these tsutsugamushi are absolutely not distributed, there may be tsutsugamushi transmitting the rickettsia through the ovaries, in addition to these three species.
3. Do the field mice infected by rickettsia contract the disease?
     When inoculating rickettsia to mice raised in the laboratory, ascites accumulates in their bodies, and they die about two weeks later. However wild mice injected with rickettsia at a concentration rate 300,000 times the lethal dose for mice did not contract the disease, and a multiplication was even observed. Moreover the rickettsia inoculated to the wild mice was no longer detected after several months. This suggests that the wild mice are, rather than carriers of rickettsia, considered as playing an important role as the source of supply of the nutrient necessary for the growth of tsutsugamushi.
4. Does the rickettsia migrate from mice having contracted tsutsugamushi disease to tsutsugamushi larvae?
     After experimental inoculation of rickettsia to mice at 100,000 times the lethal dose, several days before they contracted the disease and died, a migration of rickettsia at a low rate of several percent is observed in the tsutsugamushi larvae regardless of their species, after making them feed on the mice. Next are tsutsugamushi freshly infected with rickettsia transmitted through the ovaries to the next generation? Assuming there is transovarial transmission, this will prove that they have become a new source of tsutsugamushi carrying rickettsia. In fact, many researchers have attempted to demonstrate this, but they have not yet obtained any reliable evidence. However assuming that tsutsugamushi larvae could become the source of rickettsia-carrying tsutsugamushi by catching fresh rickettsia, the species of tsutsugamushi carrying rickettsia and their number will indubitably increase in the natural world. Nevertheless no supportive data have been obtained so far.
5. Transmission and preservation of rickettsia by tsutsugamushi
     As mentioned so far, most of the tsutsugamushi living in the open air do not harbor rickettsia. However the fact that patients have been afflicted with tsutsugamushi disease means that there are tsutsugamushi larvae carrying rickettsia and that humans were infected through bites and contracted the disease. Figure 2 shows the preservation and the transovarial transmission of rickettsia during the life cycle of the tsutsugamushi. Indeed the female parent infected with rickettsia transmits it through its ovaries to the next generation. The rickettsia is kept through its life cycle consisting of the following stages: engorged larva, protonymph, deutonymph, tritonymph and adult. In addition, the rickettsia proliferates as its growth progresses, and the female parent transmits again the rickettsia to the following generation through its ovaries. Incidentally the sex ratio of rickettsia infected colonies for L. pallidum is around 1:1, and both the male and female adults harbor rickettsia. However the female/male ratio of infected colonies of rickettsias, vectors of the tsutsugamushi disease, for several species inhabiting foreign countries is 1:0. This means that only the females harbor rickettsia. This is a very interesting difference. The vertical transmission, namely the transovarial transmission of rickettsia harbored by the parents to the following generation is the main route of transmission of the tsutsugamushi disease rickettsia in the natural world. Thus the rickettsia is transmitted to the following generations from the parents.
     Recent studies have progressively shed light on the detailed mechanism of transmission. Indeed it has been found that if the parents harbor rickettsia, this does not necessarily mean that all of their offspring also carry rickettsia, and that since the males harboring rickettsia do not transmit it, the transmission of rickettsia through spermatozoa is not observed. On the other hand, one cannot fully deny the fact that tsutsugamushi larvae that do not carry rickettsia may catch rickettsia secondarily and transmit it to the following generation, if they live on animals that have been infected with rickettsia,. However this probability is assumedly extremely low.
Figure 1 - Unattached larva of Leptotrombidium pallidum, one of the vectors of tsutsugamushi disease
(Dr. Hiroshi Sugikami, Niigata Pharmacology University)
Figure 2 - Transmission of tsutsugamushi disease in the tsutsugamushi
NT: no transmission of rickettsia +: carriage of rickettsia