Scrub typhus is the deadly fever caused by the bacterium Orientia tsutsugamushi identified in 1930 from the rural areas of Japan. It is also known as Bush typhus and the transmitting agent is a Mite belonging to the group Trombiculid particularly the species Leptotrobidium deliense inhabiting the areas having scrub vegetation. Bite of the mite leaves a black spot on the skin which is the useful marking for the identification of Scrub typhus infection.
Scrub typhus is an endemic fever found in the regions extending from Japan to Russia, Australia and to the Afghanistan and Pakistan. This area is named as Tsutsugamushi triangle. Now it is also appearing in India and adjacent places.
Orientia tsutsugamushi is the causative organism of Scrub typhus that enters into the human body through the bite of the mite Leptotrobidium.
It is an intracellular pathogen that infects the cells for its multiplication. It can survive and multiply, only if it is inside the cells. It is an Aerobic bacterium and its life is limited to short time when it is outside the cells. Its envelop is identical to a Gram negative bacterium but will not stain easily with Gram stain like the gram negative bacteria. There fore to identify the pathogen, Gimenez stain is commonly used. Many Serotypes of the pathogen exists and most commonly found serotypes are Karp,Kato, Gillium, Kawasaki etc. In appearance and pathogenicity, the Orientia tsutsugamushi resembles the Gram negative bacteria Rickettsia prowazekii causing endemic typhus.
The Orientia is a 0.5 micrometer wide and 1-3 micrometer long bacterium that can be cultured only in cell monolayer. Due to its very high virulence, Orientia is handled only in the laboratories having Biosafety level 3 facilities. It is found to be sensitive to antibiotics like Doxycycline, Rifampicin, Azithromycin in in-vitro studies. It is resistant to all forms of beta Lactum antibiotics like Penicillin because the bacterium lacks Peptodoglycan cell wall. Aminoglycoside antibiotics like Gentamycin is also not effective to Orientia since the pathogen is intracellur and amino glycosides do not penetrate into the cells.
Do not confuse Typhoid with Typhus. Typhus is the form of fever caused by the Gram negative bacteria Rickettsia prowazekii. The term typhus is the Greek word meaning Smoky or Hazy to describe the state of mind of the infected people. Typhus has no relation with the Typhoid fever and the diseases are entirely different. Rickettsia cannot survive outside the cells for long time and is an obligatory parasite. Typhus is transmitted through the fecal matter of Body Lice. The life cycle of the pathogen requires a vertebrate and an invertebrate host. Human Body louse is the typical invertebrate host and human is the vertebrate host.
How Scrub typhus becomes visible?
The symptoms of Scrub typhus include High fever, head ache, body pain, cough and intestinal disorders. In extreme cases, it can also cause hemorrhage and internal bleeding. The infection can be easily identified by the black spot or scar appears on the skin by the bite of the mite.
Signs and Identification
The pathogen invades the skin and multiplies in the cytoplasm of endothelial cells and macrophages .Typical signs of infection are Eschar, Spnenomegaly, Lymphadenopathy etc. Abnormal liver functions, Leucopenia etc can be considered as the result of infection. If the disease is not treated properly, it can leads to complications like Pneumonia, Encephalitis, Myocarditis etc.
The disease can be identified through serological tests. The most common test is Weil-Felix test but is not completely reliable. Immunofluorescent assay, Culturing and PCR method etc can confirm the presence of the pathogen.
Treatment and Prevention
The disease becomes fatal, without proper treatment. Treatment involves the use of antibiotics drugs. Most commonly used antibiotics are Doxycycline and Chloramphenicol. But Rifampin and Azithromycin are used as alternatives if the pathogen becomes resistant to Doxycycline or Chloramphenicol. Azithromycin is the alternative for Doxycycline in children and pregnant women when the Doxycycline resistance is suspected.
Vaccination is advised to prevent the infection of Scrub typhus strains present locally. But the vaccines used for one local strain will not be effective to prevent the infection of another strain of a different locality because of the variations in the antigenic properties.