''Orientia tsutsugamushi'' (from

Japanese Japanese may refer to: * Something from or related to Japan, an island country in East Asia * Japanese language, spoken mainly in Japan * Japanese people, the ethnic group that identifies with Japan through ancestry or culture ** Japanese diaspor ...

''tsutsuga'' meaning "illness", and ''mushi'' meaning "insect") is a

mite Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...

-borne

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

belonging to the family

Rickettsiaceae The Rickettsiaceae are a family of bacteria. The genus ''Rickettsia'' is the most prominent genus within the family. The bacteria that eventually formed the mitochondrion (an organelle in eukaryotic cells) is believed to have originated from t ...

and is responsible for a disease called

scrub typhus Scrub typhus or bush typhus is a form of typhus caused by the intracellular parasite ''Orientia tsutsugamushi'', a Gram-negative α-proteobacterium of family Rickettsiaceae first isolated and identified in 1930 in Japan. It is a natural and an

obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism that ...

intracellular parasite Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host. Types of parasites There are two main types of intracellular parasites: Facultative and Obligate. Facultative intracellular para ...

of mites belonging to the family

Trombiculidae Trombiculidae (); commonly referred to in North America as chiggers and in Britain as harvest mites, but also known as berry bugs, bush-mites, red bugs or scrub-itch mites, are a family of mites. Chiggers are often confused with Tunga penetrans, ...

. With a genome of only 2.0–2.7 Mb, it has the most repeated DNA sequences among bacterial genomes sequenced so far. The disease, scrub typhus, occurs when infected mite larvae accidentally bite humans. Primarily indicated by undifferentiated

febrile Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using val ...

illnesses, the infection can be complicated and often fatal. ''Orientia tsutsugamushi'' infection was first reported in Japan by Hakuju Hashimoto in 1810, and to the Western world by Theobald Adrian Palm in 1878. Naosuke Hayashi first described it in 1920, giving the name ''Theileria tsutsugamushi''. Owing to its unique properties, it was renamed ''Orientia tsutsugamushi'' in 1995. Unlike other

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, it is not easily stained with

Gram stain In microbiology and bacteriology, Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish ...

, as its cell wall is devoid of

lipophosphoglycan Lipophosphoglycan (LPG) is a class of molecules found on the surface of some eukaryotes, in particular protozoa. Each is made up of two parts, lipid and polysaccharide (also called glycan). They are bonded by a phosphodiester, hence the name lip ...

and

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ...

. With highly variable membrane protein, a 56-kDa protein, the bacterium can be antigenically classified into many strains (sub-types). The classic strains are Karp (which accounts for about 50% of all infections), Gilliam (25%), Kato (less than 10%), Shimokoshi, Kuroki and Kawasaki. Within each strain, enormous variability further exists. ''Orientia tsutsugamushi'' is naturally maintained in the mite population by transmission from female to its eggs (transovarial transmission), and from the eggs to larvae and then to adults (transtadial transmission). The mite larvae, called

chiggers ''Trombicula'', known as chiggers, red bugs, scrub-itch mites, or berry bugs, are small arachnids (eight-legged arthropods) in the Trombiculidae family. In their larval stage, they attach to various animals, including humans, and feed on skin, ...

, are natural

ectoparasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

of rodents. Humans get infected upon accidental contact with infected chiggers. A scar-like scab called

eschar An eschar (; Greek: ''ἐσχάρᾱ'', ''eskhara''; Latin: ''eschara'') is a slough or piece of dead tissue that is cast off from the surface of the skin, particularly after a burn injury, but also seen in gangrene, ulcer, fungal infections, ...

is a good indicator of infection, but is not ubiquitous. The bacterium is endemic to the so-called Tsutsugamushi Triangle, a region covering the Russian Far East in the north, Japan in the east, northern Australia in the south, and Afghanistan in the west. One million infections are estimated to occur annually. Antibiotics such as

azithromycin Azithromycin, sold under the brand names Zithromax (in oral form) and Azasite (as an eye drop), is an antibiotic medication used for the treatment of a number of bacterial infections. This includes middle ear infections, strep throat, pneumon ...

and

doxycycline Doxycycline is a broad-spectrum tetracycline class antibiotic used in the treatment of infections caused by bacteria and certain parasites. It is used to treat bacterial pneumonia, acne, chlamydia infections, Lyme disease, cholera, typhus, an ...

are the main prescription drugs;

chloramphenicol Chloramphenicol is an antibiotic useful for the treatment of a number of bacterial infections. This includes use as an eye ointment to treat conjunctivitis. By mouth or by injection into a vein, it is used to treat meningitis, plague, cholera, a ...

and

tetracyclin Tetracycline, sold under various brand names, is an oral antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis. Common side effects inc ...

are also effective. Diagnosis of the infection is difficult and requires laborious techniques such as Weil–Felix test, rapid

immunochromatographic Affinity chromatography is a method of separating a biomolecule from a mixture, based on a highly specific macromolecular binding interaction between the biomolecule and another substance. The specific type of binding interaction depends on the ...

test, immunofluorescence assays, and

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...

. There is no vaccine for the infection.

History

The earliest record of ''O. tsutsugamushi'' infection was in the 3rd century (313 C.E.) in China. Japanese were also familiar with the link between the infection and mites for centuries. They gave several names such as ''shima-mushi'', ''akamushi'' (red mite) or ''kedani'' (hairy mite) disease of northern Japan, and most popularly as ''tsutsugamushi'' (from ''tsutsuga'' meaning fever or harm or illness, and ''mushi'' meaning bug or insect). Japanese physician Hakuju Hashimoto gave the first medical account from

Niigata Prefecture is a Prefectures of Japan, prefecture in the Chūbu region of Honshu of Japan. Niigata Prefecture has a population of 2,227,496 (1 July 2019) and is the List of Japanese prefectures by area, fifth-largest prefecture of Japan by geographic area ...

in 1810. He recorded the prevalence of infection along the banks of the upper tributaries of

Shinano River The , known as the in its upper reaches, is the longest and widest river in Japan and the third largest by basin area (behind the Tone River and Ishikari River). It is located in northeastern Honshu, rising in the Japanese Alps and flowing g ...

. The first report to the Western world was made by Theobald Adrian Palm, a physician of the

Edinburgh Medical Missionary Society EMMS International is a non-denominational christian Non-governmental Organization (NGO) that provides medical aid to countries around the world and operates field offices in the UK, Malawi, India, Israel, and Nepal. Founded to provide clinic ...

at Niigata in 1878. Describing his first-hand experience, Palm wrote: The aetiology of the disease was unknown until the early 20th century. In 1908, a mite theory of the transmission of tsutsugamushi disease was postulated by Taichi Kitashima and Mikinosuke Miyajima. In 1915, a British zoologist, Stanley Hirst, suggested that the larvae of mite ''Microtrombidium akamushi'' (later renamed ''Leptotrombidium akamushi'') which he found on the ears of field mice could carry and transmit the infection. In 1917, Mataro Nagayo and colleagues gave the first complete description of the developmental stages such as egg, nymph, larva, and adult of the mite. They also asserted that only the larvae bites mammals, and are thus the only carriers of the parasites. But then, the actual infectious agent was not known, and it was generally attributed to either a virus or a protozoan. The causative pathogen was first identified by Naosuke Hayashi in 1920. Confident that the organism was a protozoan, Hayashi concluded, stating, "I have reached the conclusion that the virus of the disease is the species of '' Piroplasma'' rotozoanin question... I consider the organism in Tsutsugamushi disease as a hitherto undescribed species, and at the suggestion of Dr. Henry B. Ward designate it as ''Theileria tsutsugamushi''." Discovering the similarities with the bacterium ''R. prowazekii'', Mataro Nagayo and colleagues gave a new classification with the name ''Rickettsia orientalis'' in 1930. (''R. prowazekii'' is a causative bacterium of epidemic typhus first discovered by American physicians

Howard Taylor Ricketts Howard Taylor Ricketts (February 9, 1871 – May 3, 1910) was an American pathologist after whom the family Rickettsiaceae and the order Rickettsiales are named. He was born in Findlay, Ohio. In the early part of his career, Ricketts undertook ...

and Russell M. Wilder in 1910, and described by a Brazilian physician Henrique da Rocha Lima in 1916.) The taxonomic confusion worsened. In 1931, Norio Ogata gave the name ''Rickettsia tsutsugamushi'', while Rinya Kawamüra and Yoso Imagawa independently introduced the name ''Rickettsia akamushi''. Kawamüra and Imagawa discovered that the bacteria are stored in the salivary glands of mites, and that mites feed on body (lymph) fluid, thereby establishing the fact that mites transmit the parasites during feeding. For more than 60 years there was no consensus on the choice of name – both ''R. orientalis'' and ''R. tsutsugamushi'' were equally used. Akira Tamura and colleagues reported in 1991 the structural differences of the bacterium from ''Rickettsia'' species that warranted a separate genus, and proposed the name ''Orientia tsutsugamushi''. Finally, in 1995, they made a new classification based on the morphological and biochemical properties, formally creating the new name ''O. tsutsugamushi''.

Biology

''Orientia tsutsugamushi'' is a Gram-negative bacterium and is a permanent (obligate) parasite in mites. Within a single host cell, ''O. tsutsugamushi'' rapidly divides into many individuals as shown in Figure 1. A unicellular organism, it is oval shaped and measures 0.5 to 0.8 µm wide and 1.2 to 3.0 µm long. Due to similarity, it was previously classified in the genus ''

Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "rickett ...

'' among other bacteria, but later assigned a separate genus, ''Orientia'', which it shares (as of 2010) only with ''Candidatus Orientia chuto''. It is broader but shorter than other rickettsial bacteria, which are rod shaped and on average measure 0.25 to 0.3 µm wide and 0.8 to 1 µm long. During reproduction, it divides (by

binary fission Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...

) into two daughter cells by the process of budding. While undergoing budding, it accumulates on the host cell surface, unlike other bacteria. One complete budding cycle takes 9 to 18 hours. The structure of ''O. tsutsugamushi'' (revealed by

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

) is shown in Figure 2. The bacterium is enclosed by a

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

on the outside and cell membrane on the inside. The cell covering takes up stains such as

Giemsa Giemsa stain (), named after German chemist and bacteriologist Gustav Giemsa, is a nucleic acid stain used in cytogenetics and for the histopathological diagnosis of malaria and other parasites. Uses It is specific for the phosphate groups of ...

and

Gimenez stain The Gimenez staining technique uses biological stains to detect and identify bacterial infections in tissue samples. Although largely superseded by techniques like Giemsa staining, the Gimenez technique may be valuable for detecting certain slow- ...

s. Although its cell wall has a classic bacterial double layer, its outer leaflet is much thicker than the inner one, which is just the opposite in ''Rickettsia'' species. A capsule layer that forms a spherical halo in other bacteria is missing. The cell wall is less rigid due to the absence of peptidoglycan, which is otherwise characteristic of the rigid cell walls of other bacteria. Classic bacterial lipophosphoglycans such as

muramic acid Muramic acid is an amino sugar acid. In terms of chemical composition, it is the ether of lactic acid and glucosamine. It occurs naturally as ''N''-acetylmuramic acid in peptidoglycan, whose primary function is a structural component of many typi ...

glucosamine Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is one of the most ...

, hydroxy fatty acids,

heptose A heptose is a monosaccharide with seven carbon atoms. They have either an aldehyde functional group in position 1 (aldoheptoses) or a ketone functional group in position 2, 3 or 4 (ketoheptoses). Ketoheptoses have 4 chiral centers, whereas aldoh ...

, and 2-keto-3-deoxyoctonic acid are also absent in the cell wall. Due to the absence of

, the bacterium is naturally resistant to all

β-lactam A beta-lactam (β-lactam) ring is a four-membered lactam. A ''lactam'' is a cyclic amide, and ''beta''-lactams are named so because the nitrogen atom is attached to the Β carbon, β-carbon atom relative to the carbonyl. The simplest β-lactam p ...

antibiotics (such as

penicillin Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum using ...

), to which ''Rickettsia'' species are normally sensitive to. Its genome totally lacks the genes for lipophosphoglycan synthesis, but does contain some for those of peptidoglycan. Important genes essential for peptidoglycan synthesis such as ''alr'', ''dapF'' and ''PBP1'' are missing: ''alr'' encodes an enzyme L-alanine racemase, which converts L-alanine to D-alanine in the first step of peptidoglycan synthesis pathway; ''dapF'' encodes diaminopimelate epimerase, which convert LL-2,6-diaminoheptanedioate (L,L-DAP) to meso-diaminoheptanedioate (meso-DAP); and ''PBP1'' encodes penicillin-binding protein-1 (PBP1), which converts periplasmic lipid II to peptidoglycan. Thus, the bacterium cannot synthesise a typical peptidoglycan cell wall, and instead makes a peptidoglycan-like structure on its surface. The cell membrane is also chemically different in its protein composition, and this difference gives rise to strain variations within the species itself. The cytoplasm is clear and shows distinct DNA and ribosomes. The bacterium is highly virulent, such that its isolation and cell culture are done only in a laboratory facility with biosafety level 3. Unlike other bacteria which can easily grow on different culture media, rickettsiales can be cultured only in living cells. ''O. tsutsugamushi'' specifically can be grown only in the yolk sacs of developing chicken embryos and in cultured cell lines such as

HeLa HeLa (; also Hela or hela) is an immortalized cell line used in scientific research. It is the oldest and most commonly used human cell line. The line is derived from cervical cancer cells taken on February 8, 1951, named after Henrietta La ...

, BHK, Vero, and L929. In contrast to ''Rickettsia'' species which reside in the nucleus of the host cell, ''O. tsutsugamushi'' mostly grows within the cytoplasm of the host cell. Genetically, it differs from other ''Rickettsia'' by only 9%. Even though adaptation to obligate intracellular parasitism among bacteria generally results in a reduced genome, it has a genome size of about 2.0–2.7 Mb depending on the strains (Figure 3), which is comparatively larger than those of other rickettsiales – two times larger than that of ''

Rickettsia prowazekii ''Rickettsia prowazekii'' is a species of gram-negative, alphaproteobacteria, obligate intracellular parasitic, aerobic bacillus bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North America, the ...

'', the most well-known member. The entire genome is distributed in a single circular chromosome. Whole genome sequences are available only for Ikeda and Boryong strains, both from the Republic of Korea. The genome of the Ikeda strain is 2,008,987 base pairs (bp) long, and contains 1,967 protein-coding genes. The Boryong strain is larger with 2,127,051 bp and 2,179 protein-coding genes. Genome comparison shows only 657 core genes among the different strains. With about 42-47% of repetitive sequences, ''O. tsutsugamushi'' has the most highly repeated bacterial genome sequenced as of 2013. The repeated DNA sequence includes short repetitive sequences,

transposable elements A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transpo ...

(including insertion sequence elements, miniature inverted-repeat transposable elements, a

Group II intron Group II introns are a large class of self-catalytic ribozymes and mobile genetic elements found within the genes of all three domains of life. Ribozyme activity (e.g., self- splicing) can occur under high-salt conditions ''in vitro''. However, ...

), and a greatly amplified integrative and conjugative element (ICE) called the rickettsial amplified genetic element (RAGE). RAGE is also found in other rickettsial bacteria. In ''O. tsutsugamushi'', however, RAGE contains a number of genes including ''tra'' genes typical of type IV secretion systems and gene for

ankyrin repeat The ankyrin repeat is a 33-residue motif in proteins consisting of two alpha helices separated by loops, first discovered in signaling proteins in yeast Cdc10 and ''Drosophila'' Notch. Domains consisting of ankyrin tandem repeats mediate prote ...

–containing protein. Ankyrin repeat–containing proteins are secreted through a type I secretion system into the host cell. The precise role of type IV secretion system in ''O. tsutsugamushi'' is not known. It may be involved in

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

between the different strains.

Life cycle and transmission

''Orientia tsutsugamushi'' is naturally transmitted in the mite population belonging to the genus ''

Leptotrombidium ''Leptotrombidium'' () is a genus of mites in the family Trombiculidae, that are able to infect humans with scrub typhus (''Orientia tsutsugamushi'' infection) through their bite. The larval form (called chiggers) feeds on rodents, but also occa ...

''. It can be transmitted by a female to its eggs through the process called transovarial transmission, and from the eggs to larvae and adults through the process of transstadial transmission. Thus, the bacterial life cycle is maintained entirely in mites. Infection to rodents and humans is an accidental transmission from the bite of mite larvae, and not required for reproduction or survival of the bacterium. In fact, in humans the transmission is stopped, and the bacterium meets a dead end. However, uninfected mites can acquire the infection from infected rodents. In rodent and human infections, ''

Leptotrombidium deliense ''Leptotrombidium deliense'' is a species of mite. It is a vector and reservoir for scrub typhus Scrub typhus or bush typhus is a form of typhus caused by the intracellular parasite '' Orientia tsutsugamushi'', a Gram-negative α-proteobacter ...

'' is the most common vector of ''O. tsutsugamushi''. ''L. pallidum'', ''L. fletcheri'' and ''L. scutellare'' are also carriers in many countries. In addition, ''L. akamushi'' is an endemic carrier in Japan, ''L. chiangraiensis'' and ''L. imphalum'' in Thailand, ''L. gaohuensis'' in China, and ''L. arenicola'' in Malaysia and Indonesia. In parts of India, a different mite species, ''Schoengastiella ligula'' is also a major vector. The life cycle of mites consists of egg, prelarva, larva, protonymph, deutonymph, tritonymph, and adult. The larvae, commonly referred to as chiggers (Figure 4), are the only ectoparasitic stage feeding on the body fluids of rodents and other opportunistic mammals. Thus, they are the only stage in the life of mites that transmit the infection. Wild rats of the genus ''

Rattus ''Rattus'' is a genus of muroid rodents, all typically called rats. However, the term rat can also be applied to rodent species outside of this genus. Species and description The best-known ''Rattus'' species are the black rat (''R. rattus'') ...

'' are the principal natural hosts of the chiggers. Chiggers feed only once on a mammalian host. The feeding usually takes 2 to 4 days. In contrast to most parasites, they do not feed on blood, but instead on the body fluid through the hair follicles or skin pores. They possess a special feeding apparatus called a Stylostome, which is a tube formed by solidified saliva. Their saliva can dissolve the host tissue around the feeding site, so that they ingest the liquefied tissue. ''O. tsutsugamushi'' is present in the

salivary glands The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands (parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary gla ...

of mites and is released into the host tissue during this feeding.

Cellular invasion

''Orientia tsutsugamushi'' initially attacks the

myelocyte A myelocyte is a young cell of the granulocytic series, occurring normally in bone marrow (can be found in circulating blood when caused by certain diseases). Structure When stained with the usual dyes, the cytoplasm is distinctly basophilic ...

s (young white blood cells) in the area of inoculation, and then the

endothelial cells The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...

lining the

vasculature The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

. The process of cellular invasion is shown in Figure 5. In the blood circulation, it targets professional phagocytes ("cell eaters", white blood cells) such as

dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. ...

s and

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

in all organs as the secondary targets. The parasite first attaches itself to the target cells using surface proteoglycans present on the host cell and bacterial surface proteins such as type specific protein 56 (or type specific antigen, TSA56) and surface cell antigens (ScaA and ScaC, which are membrane transporter proteins). These proteins interact with the host

fibronectin Fibronectin is a high- molecular weight (~500-~600 kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. Fibronectin also binds to other extracellular matrix proteins such as collage ...

to induce

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

(the process of ingesting the bacterium). The ability to actually enter the host cell depends on

integrin Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, ...

-mediated signaling and reorganisation of the

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...

cytoskeleton. ''Orientia tsutsugamushi'' has a special adaptation for surviving in the host cell by evading the host immune reaction. Once it interacts with the host cells, it causes the host cell membrane to form a transportation bubble called a clathrin-coated vesicle by which it gets transported into the cytoplasm. Inside the cytoplasm, it makes an exit from the vesicle (now known as an

endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can ...

) before the endosome is destroyed (in the process of cell-eating called

autophagy Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...

) by the

lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane prot ...

s. It then moves towards the nucleus, specifically at the perinuclear region, where it starts to grow and multiply. Unlike other closely related bacteria which use actin-mediated processes for movement in the cytoplasm (called intracellular trafficking or transport), ''O. tsutsugamushi'' is unusual in using

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...

-mediated processes similar to those employed by viruses such as

adenoviruses Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from thei ...

and

herpes simplex virus Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also known by their taxonomical names ''Human alphaherpesvirus 1'' and '' Human alphaherpesvirus 2'', are two members of the human ''Herpesviridae'' family, a set of viruses that produce viral inf ...

es. Further, the escape (

exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use o ...

) from an infected host cell is also unusual. It forms another vesicle using the host cell membrane, gives rise to a small bud, and releases itself from the host cell surface while still enclosed in the vesicle. The membrane-bound bacterium is formed by interaction between cholesterol-rich lipid rafts as well as HtrA, a 47-kDa protein on the bacterial surface. However, the process of budding and importance of the membrane-bound bacterium are not yet understood.

Strains

''Orientia tsutsugamushi'' is a diverse species of bacteria. Ida A. Bengtson of the

United States Public Health Service The United States Public Health Service (USPHS or PHS) is a collection of agencies of the Department of Health and Human Services concerned with public health, containing nine out of the department's twelve operating divisions. The Assistant S ...

was the first to note the existence of different strains using antigen-antibody interaction (

complement fixation test The complement fixation test is an immunological medical test that can be used to detect the presence of either specific antibody or specific antigen in a patient's serum, based on whether complement fixation occurs. It was widely used to diagnos ...

) in 1944. She observed that different strains had varying degree of virulence, and that the antibodies in the blood sera of patients cross-react to different strains. By 1946, she established that there were three principal strains (serotypes), namely Karp (from New Guinea), Gilliam (from India) and Seerangay (from British Malaya). Akira Shishido described the Kato strain, in addition to Gilliam and Karp, in Japan in 1958. Since then, six basic antigenic strains are recognised, namely Gilliam, Karp, Kato, Shimokoshi, Kawasaki, and Kuroki. Karp is the most abundant strain, accounting for about 50% of all infections. In Korea, the major strain is Boryong. As of 2009, more than 20 different strains have been established in humans based on antigenic variation using serological tests such as complement fixation and immunofluorescence assay. The number is much higher if the strains in rodents and mites are taken into account. For example, a study in Japan in 1994 reported 32 strains, 14 from human patients, 12 from wild rodents, and 6 from trombiculid mites. The different strains exert different levels of virulence, and the most virulent is KN-3, which is predominant among wild rodents. Another study in 1996 reported 40 strains. Genetic methods have revealed even greater complexity than had been previously described (for example, Gilliam is further divided into Gilliam and JG types). Due to immunological differences of the serotypes, simultaneous and repeated infection with different strains is possible.

Antigenic variation

''Orientia tsutsugamushi'' has four major surface-membrane proteins (

antigens In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

) having molecular weights 22 kDa, 47 kDa, 56 kDa and 110 kDa. A 56-kDa type specific antigen (TSA56) is the most important because it is not produced by any other bacteria, and is responsible for making the genetic diversity in different strains. It accounts for about 10–15% of the total cell proteins. The 22-kDa, 47-kDa or 110-kDa antigens are not strain specific so that TSA56 is the main target in sophisticated diagnostic tests such as immunoblotting, ELISA and DNA analysis. The protein assists the adhesion and entry of the bacterium into host cells, as well as evasion of the host's immune reaction. It varies in size from 516 to 540 amino acid residues between different strains, and its gene is approximately 1,550 base pairs long. Its gene contains four hypervariable regions, indicating that it synthesises many antigenically different proteins. There are also 11-kDa and 60-kDa proteins inside the bacterium which are very similar to

GroES Heat shock 10 kDa protein 1 (Hsp10), also known as chaperonin 10 (cpn10) or early-pregnancy factor (EPF), is a protein that in humans is encoded by the ''HSPE1'' gene. The homolog in ''E. coli'' is GroES that is a chaperonin which usually works ...

and

GroEL GroEL is a protein which belongs to the chaperonin family of molecular chaperones, and is found in many bacteria. It is required for the proper folding of many proteins. To function properly, GroEL requires the lid-like cochaperonin protein comp ...

of the bacterium ''

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

'', but not that of ''Rickettsia'' species. GroES and GroEL are

heat shock protein Heat shock proteins (HSP) are a family of proteins produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses including expo ...

s belonging to the family of

molecular chaperones In molecular biology, molecular chaperones are proteins that assist the conformational folding or unfolding of large proteins or macromolecular protein complexes. There are a number of classes of molecular chaperones, all of which function to assi ...

in bacteria. DNA analyses have shown that the ''GroES'' and ''GroEL'' genes are indeed present in ''O. tsutsugamushi'' with slight variation in different strains, and they produce the 11-kDa and 60-kDa proteins.

Disease

''Orientia tsutsugamushi'' causes a complex and potentially life-threatening disease known as scrub typhus. Infection starts when chiggers bite on the skin during their feeding. The bacteria are deposited at the site of feeding (inoculation), where they multiply. They cause progressive tissue damage (

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

), which leads to formation of an

on the skin. Necrosis progresses to inflammation of the blood vessels, called

vasculitis Vasculitis is a group of disorders that destroy blood vessels by inflammation. Both arteries and veins are affected. Lymphangitis (inflammation of lymphatic vessels) is sometimes considered a type of vasculitis. Vasculitis is primarily caused b ...

. This in turn causes inflammation of the lymph nodes, called

lymphadenopathy Lymphadenopathy or adenopathy is a disease of the lymph nodes, in which they are abnormal in size or consistency. Lymphadenopathy of an inflammatory type (the most common type) is lymphadenitis, producing swollen or enlarged lymph nodes. In cli ...

. Within a few days, vasculitis extends to various organs including the liver, brain, kidney,

meninges In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...

and lungs. The disease is responsible for nearly a quarter of all the febrile (high fever) illness in endemic areas. Mortality in severe cases or due to improper treatment or misdiagnosis may be as high as 30–70%. About 6% of infected people die untreated, and 1.4% of the patients die even with medical treatment. Moreover, the death rate can be as high as 14% with neurological problems and 24% with multi-organ dysfunction among treated patients. In cases of misdiagnosis and failure of treatment, systemic complications rapidly develop including

acute respiratory distress syndrome Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin colo ...

, acute kidney failure,

encephalitis Encephalitis is inflammation of the brain. The severity can be variable with symptoms including reduction or alteration in consciousness, headache, fever, confusion, a stiff neck, and vomiting. Complications may include seizures, hallucinations, ...

, gastrointestinal bleeding,

hepatitis Hepatitis is inflammation of the liver tissue. Some people or animals with hepatitis have no symptoms, whereas others develop yellow discoloration of the skin and whites of the eyes (jaundice), poor appetite, vomiting, tiredness, abdominal pa ...

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...

myocarditis Myocarditis, also known as inflammatory cardiomyopathy, is an acquired cardiomyopathy due to inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. The ...

pancreatitis Pancreatitis is a condition characterized by inflammation of the pancreas. The pancreas is a large organ behind the stomach that produces digestive enzymes and a number of hormones. There are two main types: acute pancreatitis, and chronic pancr ...

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...

septic shock Septic shock is a potentially fatal medical condition that occurs when sepsis, which is organ injury or damage in response to infection, leads to dangerously low blood pressure and abnormalities in cellular metabolism. The Third International Con ...

subacute thyroiditis Subacute thyroiditis is a form of thyroiditis that can be a cause of both thyrotoxicosis and hypothyroidism. It is uncommon and can affect individuals of both sexes, occurring three times as often in women than in men. and people of all ages. The m ...

, and

multiple organ dysfunction syndrome Multiple organ dysfunction syndrome (MODS) is altered organ (anatomy), organ function in an acutely ill patient requiring medicine, medical intervention to achieve homeostasis. Although Irwin and Rippe cautioned in 2005 that the use of "multiple ...

. Harmful effects involving multiple organ failure and neurological impairment are difficult to treat, and can cause lifelong debilitation or be directly fatal. The central nervous system is often affected and results in various complications including cerebellitis,

cranial nerve palsies Cranial nerve disease is an impaired functioning of one of the twelve cranial nerves. Although it could theoretically be considered a mononeuropathy, it is not considered as such under MeSH. It is possible for a disorder of more than one crania ...

meningoencephalitis Meningoencephalitis (; from ; ; and the medical suffix ''-itis'', "inflammation"), also known as herpes meningoencephalitis, is a medical condition that simultaneously resembles both meningitis, which is an infection or inflammation of the menin ...

plexopathy Plexopathy is a disorder of the wikt:network, network of nerves in the brachial plexus, brachial or lumbosacral plexus. Symptoms include pain, muscle weakness, and sensory deficits (numbness). Types There are two main types of plexopathy, based on ...

transverse myelitis Transverse myelitis (TM) is a rare neurological condition wherein the spinal cord is inflamed. The adjective ''transverse'' implies that the spinal inflammation (myelitis) extends horizontally throughout the cross section of the spinal cord; the ...

, and Guillan-Barré syndrome. Death rates due to complications can be up to 14% in brain infections, and 24% with multiple organ failure. In India, scrub typhus has become the major cause of acute encephalitis syndrome, which was earlier caused mainly by a viral infection,

Japanese encephalitis Japanese encephalitis (JE) is an infection of the brain caused by the Japanese encephalitis virus (JEV). While most infections result in little or no symptoms, occasional inflammation of the brain occurs. In these cases, symptoms may include he ...

Epidemiology

The

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...

in 1999 stated that: Scrub typhus is historically endemic to the Asia-Pacific region, covering the Russian Far East and Korea in the north, to northern Australia in the south, and Afghanistan in the west, including islands of the western Pacific Oceans such as Japan, Taiwan, Philippines, Papua New Guinea, Indonesia, Sri Lanka, and the Indian Subcontinent. This geographic region is popularly called the Tsutsugamushi Triangle as shown in Figure 6. However, it has spread to Africa, Europe and South America. One billion people are estimated to be at risk of infection at any moment and an average of one million cases occur every year in the Tsutsugamushi Triangle. The burden of scrub typhus in rural areas of Asia is huge, accounting for up to 20% of febrile sickness in hospital, and seroprevalence (positive infection on blood test) over 50% of the population. More than one-fifth of the population carry the bacterial antibodies, i.e. they had been infected, in endemic areas. South Korea has the highest level incidence (with its highest of 59.7 infections out of 100,000 people in 2013), followed by Japan, Thailand, and China at top of the list. The age group of 60–69 years is at highest risk of infection. Higher infection (57.3%) is seen in females compared to males (42.7%). Farmers are most vulnerable, accounting for 70% of the cases in China. The disease is more prevalent in rural areas, but there is a rapid increase in urban areas. For example, in Korea, the annual incidence increased 21-fold between 2003 and 2013 in metropolitan areas.

Diagnosis

Symptom

The main symptom of ''O. tsutsugamushi'' infection is high (febrile) fever; however, the symptom is similar to other vector-borne tropical diseases such as

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

leptospirosis Leptospirosis is a blood infection caused by the bacteria ''Leptospira''. Signs and symptoms can range from none to mild (headaches, muscle pains, and fevers) to severe ( bleeding in the lungs or meningitis). Weil's disease, the acute, severe ...

typhoid Typhoid fever, also known as typhoid, is a disease caused by '' Salmonella'' serotype Typhi bacteria. Symptoms vary from mild to severe, and usually begin six to 30 days after exposure. Often there is a gradual onset of a high fever over several ...

murine typhus Murine typhus, also known as endemic typhus or flea-borne typhus, is a form of typhus transmitted by fleas (''Xenopsylla cheopis''), usually on rats, in contrast to epidemic typhus which is usually transmitted by lice. Murine typhus is an under- ...

chikungunya Chikungunya is an infection caused by the ''Chikungunya virus'' (CHIKV). Symptoms include fever and joint pains. These typically occur two to twelve days after exposure. Other symptoms may include headache, muscle pain, joint swelling, and a ra ...

, and

dengue fever Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. These may include a high fever, headache, vomiting, muscle and joint pains, and a characterist ...

. This makes precise clinical diagnosis difficult, which often leads to misdiagnosis. The initial indications are fever with chills, associated with headache, muscle pain (

myalgia Myalgia (also called muscle pain and muscle ache in layman's terms) is the medical term for muscle pain. Myalgia is a symptom of many diseases. The most common cause of acute myalgia is the overuse of a muscle or group of muscles; another likel ...

), sweating and vomiting. The appearance of symptoms ( the incubation period) takes between 6 and 21 days. A simple visual diagnosis is the presence of an inflamed scar-like scab called eschar, which is regarded as "the most useful diagnostic clue in patients with acute febrile illness". Eschar is formed on the skin where an infected mite bit, usually seen in the armpit, groin or any abdominal area (Figure 7). In rare cases, it can be seen on the cheek, ear lobe and dorsum of the feet. But, the problem is that eschar is not always present; at the highest record, only 55% of scrub typhus patients had eschar during an outbreak in south India. Also, eschar is not specific to scrub typhus, occurring in other rickettsial diseases such as

Rocky Mountain spotted fever Rocky Mountain spotted fever (RMSF) is a bacterial disease spread by ticks. It typically begins with a fever and headache, which is followed a few days later with the development of a rash. The rash is generally made up of small spots of bleedin ...

, Brazilian spotted fever, and Indian tick typhus. Using DNA analysis by advanced polymerase chain reaction, different rickettsial infections can be identified from eschars.

Blood test

Suspected infections are confirmed with serological tests. ''O. tsutsugamushi'' is most often detected from blood serum using the Weil–Felix test. Weil–Felix is the simplest and most rapid test, but it is not sensitive or specific, as it detects any kind of rickettsial infection. More sensitive tests such as rapid immunochromatographic test (RICT), immunofluorescence assays (IFA), ELISA, and DNA analysis using polymerase chain reaction (PCR) are used. IFA is regarded as the gold standard test, as it gives a reliable result; however, it is expensive and not specific for different rickettsial bacteria. ELISA and PCR can detect ''O. tsutsugamushi''-specific proteins such as the TSA56 and GroEL, so that they are highly specific and sensitive. On the other hand, they are highly sophisticated and expensive techniques.

Treatment

''Orientia tsutsugamushi'' infection can be treated with antibiotics such as

rifampicin Rifampicin, also known as rifampin, is an ansamycin antibiotic used to treat several types of bacterial infections, including tuberculosis (TB), mycobacterium avium complex, ''Mycobacterium avium'' complex, leprosy, and Legionnaires’ disease. ...

roxithromycin Roxithromycin is a semi-synthetic macrolide antibiotic. It is used to treat respiratory tract, urinary and soft tissue infections. Roxithromycin is derived from erythromycin, containing the same 14-membered lactone ring. However, an N-oxime side ...

, and

. Doxycycline is the most commonly used and is considered as the drug of choice because of its high efficacy and quick action. But, in pregnant women and babies, it is

contraindicated In medicine, a contraindication is a condition that serves as a reason not to take a certain medical treatment due to the harm that it would cause the patient. Contraindication is the opposite of indication, which is a reason to use a certain tre ...

, and azithromycin is the drug of choice. In Southeast Asia, where doxycycline and chloramphenicol resistance have been experienced, azithromycin is recommended for all patients. A randomized controlled trial and systematic review showed that azithromycin is the safest medication.

Vaccine

No licensed ''O. tsutsugamushi'' vaccines are currently available. The first vaccines were developed in the late 1940s, but failed in clinical trials. Considered an ideal target, the unique TSA56 itself is highly variable in its chemical composition in different strains. An effective vaccine for one strain is not useful for another. An ideal vaccine should give protection to all the strains present locally. This complexity makes it difficult to produce a usable vaccine. A vaccine targeting the 47-kDa outer membrane protein (OMP) is a promising candidate with experimental success in mice against the Boryong strain. Combined targeting of TSA56 and ScaA is also a good candidate for mixed-strain infection.

Immunity

There is no lasting immunity to ''O. tsutsugamushi'' infection. Antigenic variation prevents the development of cross immunity to the various strains of ''O. tsutsugamushi''. An infected individual may develop a short-term immunity but that disappears after a few months, and immunity to one strain does not confer immunity to another. An immunisation experiment was done in 1950 in which 16 volunteers still developed the infection after 11–25 months of primary infection. It is now known that the longevity of immunity depends on the strains of the bacterium. When reinfection occurs with the same strain as the previous infection, there can be immunity for 5–6 years in monkeys. But in humans, immunity declines after one year, and disappears within two years.

References

{{Taxonbar, from=Q3018329 Rickettsiaceae Bacteria described in 1995 Rickettsioses