How is the disease transmission cycle maintained in nature?
Chigger mites act as the primary reservoirs for O.tsutsugamushi. Once they are infected
in nature by feeding on the body fluid of small mammals, including the rodents, they
maintain the infection throughout their life stages and, as adults, pass the infection
on to their eggs in a process called transovarial transmission. Similarly, the infection
passes from the egg to the larva or adult in a process called transtadial transmission.
In this way, chigger mite populations can autonomously maintain their infectivity
over long periods of time.
Chigger mite (Courtesy: AFRIMS
Bangkok)
Characteristic eschar in a
patient
Early workers thought that rodents were the
natural reservoir of infection, but it is now believed
that mites are both the vector and the reservoir (4).
Naturally-infected mites, reared in the laboratory,
have transmitted the infection for more than
twenty generations, while uninfected chiggers, if
fed experimentally on infected mice, take up R.
tsutsugamushi, but fail to transmit the infection
transovarially to the next generation (5).
This mite is fastidious in matters of temperature, humidity and food, and finds
everything suitable in restricted areas. Scrub typhus is generally seen in people
whose occupational or recreational activities bring them into contact with ecotypes
favourable with vector chiggers (6).