Ehrlichia ewingii

Last updated

Ehrlichia ewingii is a species of  rickettsiales  bacteria. It has recently been associated with human infection, and can be detected via PCR serological testing. The name Ehrlichia ewingii was proposed in 1992. [1]

Rickettsiales Order of bacteria

The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria that are endosymbionts of eukaryotic cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales are the Wolbachia, which infect approximately two-thirds of all arthropods and nearly all filarial nematodes. Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group.


Ehrlichia ewingii
Scientific classification
E. ewingii
Binomial name
Ehrlichia ewingii

Taxonomy and characterization


The current classification is Bacteria, Proteobacteria, Alphaproteobacteria, Rickettsiales, Anaplasmataceae, Ehrlichia ewingii. [1] Classification of different members of the genus Ehrlichia has been disputed, however, it is generally agreed that close relatives of Ehrlichia ewingii are Ehrlichia chaffeensis and Ehrlichia canis . [1] It is also closely related to Wolbachia , Anaplasma , and Neorickettsia bacteria, with Rickettsia as a more distant genus. [1]

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Proteobacteria phylum of Gram-negative bacteria

Proteobacteria is a major phylum of gram-negative bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, Yersinia, Legionellales and many other notable genera. Others are free-living (non-parasitic) and include many of the bacteria responsible for nitrogen fixation.

Alphaproteobacteria class of bacteria

Alphaproteobacteria is a class of bacteria in the phylum Proteobacteria. Its members are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all Proteobacteria, its members are gram-negative and some of its intracellular parasitic members lack peptidoglycan and are consequently gram variable.


Species in the family Anaplasmataceae have unique characteristics that can help differentiate them from other families including: sensitivity to mechanical stress, changes in osmolarity, and thawing. [2]  All Ehrlichia sp. are gram-negative and spherical with a rippled outer membrane that shows no peptidoglycan layer or any lipopolysaccharides. [2]

Anaplasmataceae is a Proteobacteria family that includes genera Anaplasma, Ehrlichia, Neorickettsia and Wolbachia.

Gram-negative bacteria group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation

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 sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane.

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, forming the cell wall. The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the osmotic pressure of the cytoplasm. Peptidoglycan is also involved in binary fission during bacterial cell reproduction.



In 1971 Ewing et al. detected a new Ehrlichia strain, which was thought of as another Ehrlichia canis strain.  Ehrlichia canis is usually found in lymphocytes and monocytes, but this new strain was found in granulocytes. [3]  Due to the variable location, this strain was called Canine Granulocytic Ehrlichia (CGE). [3]

Lymphocyte Subtype of white blood cell

A lymphocyte is one of the subtypes of a white blood cell in a vertebrate's immune system. Lymphocytes include natural killer cells, T cells, and B cells. They are the main type of cell found in lymph, which prompted the name "lymphocyte".

Monocyte type of white blood cell

Monocytes are a type of leukocyte, or white blood cell. They are the largest type of leukocyte and can differentiate into macrophages and myeloid lineage dendritic cells. As a part of the vertebrate innate immune system monocytes also influence the process of adaptive immunity. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors.

Granulocyte mature white blood cells with granules in the cytoplasm

Granulocytes are a category of white blood cells characterized by the presence of granules in their cytoplasm. They are also called polymorphonuclear leukocytes or polymorphonuclear neutrophils because of the varying shapes of the nucleus, which is usually lobed into three segments. This distinguishes them from the mononuclear agranulocytes. In common parlance, the term polymorphonuclear leukocyte often refers specifically to "neutrophil granulocytes", the most abundant of the granulocytes; the other types have lower numbers. Granulocytes are produced via granulopoiesis in the bone marrow.

Method of identification

A field study was conducted by Anderson et al. on June 18, 1987 using a canine diagnosed with granulocytic ehrlichiosis: an infection of the granulocytes by a member of the Ehrlichia sp. The blood from this canine was infused in another specimen to ensure the blood transmission of the intracellular parasite. [4]  This transmission was then completed using ticks of the species Amblyomma americanum by exposing them to infected dogs and then to susceptible ones. [4]  From blood samples, 16S rRNA genes were amplified using standard polymerase chain reaction (PCR), and the genes were analyzed using gel electrophoresis and a GAP (Genetic Analysis Program) system. [4]  This data was compared to all other Ehrlichia species, and Anderson et al. found that CGE is most closely related to E. chaffeensis and E. canis. These three—E. ewingii, E. chaffeensis, and E. canis—form a group in terms of relatedness within the genus Ehrlichia. [4] Despite being related, due to the level of divergence between CGE and the other species, it has been determined that CGE deserves to have species level recognition. [4]  The name was proposed to be Ehrlichia ewingii, named after S.A. Ewing who initially identified the parasitic organism. [4]

Ehrlichiosis (; also known as canine rickettsiosis, canine hemorrhagic fever, canine typhus, tracker dog disease, and tropical canine pancytopenia is a tick-borne disease of dogs usually caused by the organism Ehrlichia canis. Ehrlichia canis is the pathogen of animals. Humans can become infected by E. canis and other species after tick exposure. German Shepherd Dogs are thought to be susceptible to a particularly severe form of the disease, other breeds generally have milder clinical signs. Cats can also be infected.

Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host. Some parasites can cause disease.

Tick order of arachnids

Ticks are small arachnids, typically 3 to 5 mm long, part of the order Parasitiformes. Along with mites, they constitute the subclass Acari. Ticks are ectoparasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks had evolved by the Cretaceous period, the most common form of fossilisation being immersed in amber. Ticks are widely distributed around the world, especially in warm, humid climates.

Metabolism and genomics


This organism mostly uses host cell machinery to replicate its genome. [4] Although the genome itself has not been sequenced, the use of the host cell for replication is known based on comparisons to other Ehrlichia sp. [5] Since Ehrlichia ewingii is unable to synthesize all the organic compounds required for growth, also known as an auxotroph, the bacterium requires a host for growth and survival. [5]

Genome entirety of an organisms hereditary information; genome of organism (encoded by the genomic DNA) is the (biological) information of heredity which is passed from one generation of organism to the next; is transcribed to produce various RNAs

In the fields of molecular biology and genetics, a genome is the genetic material of an organism. It consists of DNA. The genome includes both the genes and the noncoding DNA, as well as mitochondrial DNA and chloroplast DNA. The study of the genome is called genomics.

Organic compound chemical compound that contains carbon (except for a several compounds traditionally classified as inorganic compounds)

In chemistry, an organic compound is generally any chemical compound that contains carbon. Due to carbon's ability to catenate, millions of organic compounds are known. Study of the properties and synthesis of organic compounds is the discipline known as organic chemistry. For historical reasons, a few classes of carbon-containing compounds, along with a handful of other exceptions, are not classified as organic compounds and are considered inorganic. No consensus exists among chemists on precisely which carbon-containing compounds are excluded, making the definition of an organic compound elusive.

Autotroph Any organism that produces complex organic compounds from simple substances present in its surroundings, generally using energy from light or inorganic chemical reactions

An autotroph or primary producer, is an organism that produces complex organic compounds from simple substances present in its surroundings, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water. They do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and also create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide. Some autotrophs, such as green plants and algae, are phototrophs, meaning that they convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon.


No specific studies on E. ewingii metabolism have been conducted so far. [6] However, the organism is assumed to have a similar metabolism to members of the genus due to the high similarity between functional 16S rRNA gene sequencing and the highly related infections they cause within cells. [6]


All Ehrlichia sp. are obligate intracellular parasites that are transmitted by ticks to vertebrate animals; Ehrlichia ewingii is specifically transmitted by the North American Tick, Amblyomma americanum. [5] This bacterium then infects granulocytes in canines and humans. [3] Currently, Ehrlichia ewingii cannot be cultivated in a number of cell lines, meaning that study of this organism is dependent on extraction from arthropod and vertebrate hosts. [4]

Disease and treatment

Ehrlichia ewingii is a human pathogen which results in a serious infection if not treated in a timely manner. [7] Ehrlichiosis, the disease caused by E. ewingii and E. chaffeensis, presents with fever, headache, fatigue, and muscle aches. [8]  These symptoms are extremely unspecific which makes the infections difficult to diagnose. [8] Doxycycline is effective when administered early in the infection, but despite this, the estimated fatality rate is still 1.8%. [8] This is partially due to the difficulty diagnosing the causative agent of infection, [8] however further Ehrlichia ewingii research could allow better diagnostic tests to identify ehrlichial infections before they progress to severe cases.

Related Research Articles

Tick-borne diseases, which afflict humans and other animals, are caused by infectious agents transmitted by tick bites. Tick-borne illnesses are caused by infection with a variety of pathogens, including rickettsia and other types of bacteria, viruses, and protozoa. Because individual ticks can harbor more than one disease-causing agent, patients can be infected with more than one pathogen at the same time, compounding the difficulty in diagnosis and treatment. As of 2016, 16 tick-borne diseases of humans are known.

<i>Dermacentor variabilis</i> species of arachnid

Dermacentor variabilis, also known as the American dog tick or wood tick, is a species of tick that is known to carry bacteria responsible for several diseases in humans, including Rocky Mountain spotted fever and tularemia. It is one of the most well-known hard ticks. Diseases are spread when it sucks blood from the host, which could take several days for the host to experience some symptoms.

A canine vector-borne disease (CVBD) is one of "a group of globally distributed and rapidly spreading illnesses that are caused by a range of pathogens transmitted by arthropods including ticks, fleas, mosquitoes and phlebotomine sandflies." CVBDs are important in the fields of veterinary medicine, animal welfare, and public health. Some CVBDs are of zoonotic concern.

<i>Anaplasma phagocytophilum</i> species of bacterium

Anaplasma phagocytophilum is a Gram-negative bacterium that is unusual in its tropism to neutrophils. It causes anaplasmosis in sheep and cattle, also known as tick-borne fever and pasture fever, and also causes the zoonotic disease human granulocytic anaplasmosis.

Ehrlichia is a genus of rickettsiales bacteria that is transmitted to vertebrates by ticks. These bacteria cause the Ehrlichiosis infection, which is considered zoonotic, because the main reservoirs for the disease are animals.

<i>Amblyomma americanum</i> species of arachnid

Amblyomma americanum, also known as the lone star tick, the northeastern water tick, or the turkey tick, is a type of tick indigenous to much of the eastern United States and Mexico, that bites painlessly and commonly goes unnoticed, remaining attached to its host for as long as seven days until it is fully engorged with blood. It is a member of the phylum Arthropoda, class Arachnida. The adult lone star tick is sexually dimorphic, named for a silvery-white, star-shaped spot or "lone star" present near the center of the posterior portion of the adult female shield (scutum); adult males conversely have varied white streaks or spots around the margins of their shields.

Heartwater is a tick-borne rickettsial disease of domestic and wild ruminants. It is caused by Ehrlichia ruminantium - an intracellular gram-negative coccal bacterium. The disease is spread by bont ticks, which are members of the genus Amblyomma. Affected mammals include cattle, sheep, goats, antelope, and buffalo, but the disease has the biggest economic impact on cattle production in affected areas. The disease's name is derived from the fact that fluid can collect around the heart or in the lungs of infected animals.

Ehrlichia chaffeensis is an obligate intracellular gram-negative species of rickettsiales bacteria. It is a zoonotic pathogen transmitted to humans by the lone star tick. It is the causative agent of human monocytic ehrlichiosis.

Human granulocytic anaplasmosis human disease

Human granulocytic anaplasmosis (HGA) is a tick-borne, infectious disease caused by Anaplasma phagocytophilum, an obligate intracellular bacterium that is typically transmitted to humans by ticks of the Ixodes ricinus species complex, including Ixodes scapularis and Ixodes pacificus in North America. These ticks also transmit Lyme disease and other tick-borne diseases.

Human monocytotropic ehrlichiosis human disease

Human monocytotropic ehrlichiosis (HME) is a form of ehrlichiosis associated with Ehrlichia chaffeensis. This bacterium is an obligate intracellular pathogen affecting monocytes and macrophages.

Ehrlichiosis ewingii infection is an infectious disease caused by an intracellular bacteria, Ehrlichia ewingii. The infection is transmitted to humans by the tick, Amblyomma americanum. This tick can also transmit Ehrlichia chaffeensis, the bacteria that causes human monocytic ehrlichiosis (HME).

Neorickettsia sennetsu is a Gram-negative bacterium that causes Sennetsu ehrlichiosis.

Ehrlichia canis is an obligate, intracellular bacterium that acts as the causative agent of Ehrlichiosis, a disease most commonly affecting canine species. This pathogen is present throughout the United States, South America, Asia, and Africa. First defined in 1935, E. canis emerged in the United States in 1963 and its presence has since been found in all 48 contiguous United States. Reported primarily in dogs, E. canis has also been documented in felines and humans where it is transferred most commonly via Rhipicephalus sanguineus, the brown dog tick.

Ehrlichia Wisconsin HM543746 is an unnamed tick bacterium that spread through Minnesota and Wisconsin in 2009 and is similar to Ehrlichia muris.

Anaplasma bovis is a Gram-negative bacterium.

Anaplasma platys is a Gram-negative bacterium.

Neorickettsia risticii is a Gram-negative bacterium that causes Potomac horse fever.


  1. 1 2 3 4 Dumler, J. S.; Barbet, A. F.; Bekker, C. P.; Dasch, G. A.; Palmer, G. H.; Ray, S. C.; Rikihisa, Y.; Rurangirwa, F. R. (2001-11-01). "Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila". International Journal of Systematic and Evolutionary Microbiology. 51 (Pt 6): 2145–2165. doi:10.1099/00207713-51-6-2145. ISSN   1466-5026. PMID   11760958.
  2. 1 2 Lin, Mingqun; Rikihisa, Yasuko (2003-09-01). "Ehrlichia chaffeensis and Anaplasma phagocytophilum lack genes for lipid A biosynthesis and incorporate cholesterol for their survival". Infection and Immunity. 71 (9): 5324–5331. doi:10.1128/IAI.71.9.5324-5331.2003. ISSN   0019-9567. PMC   187327 . PMID   12933880.
  3. 1 2 3 Ewing, S. A., W. R. Roberson, R. G. Buckner, and C. S. Hayat. 1971. A new strain of Ehrlichia canis. J. Am. Vet. Med. Assoc. 159:1771-1774. 
  4. 1 2 3 4 5 6 7 8 Anderson, Burt E.; Greene, Craig E.; Jones, Dana C.; Dawson, Jacqueline E. (1992-01-01). "NOTES: Ehrlichia ewingii sp. nov., the Etiologic Agent of Canine Granulocytic Ehrlichiosis". International Journal of Systematic and Evolutionary Microbiology. 42 (2): 299–302. doi:10.1099/00207713-42-2-299. PMID   1581189.
  5. 1 2 3 Ndip, L. M.; Ndip, R. N.; Esemu, S. N.; Dickmu, V. L.; Fokam, E. B.; Walker, D. H.; McBride, J. W. (2005-11-30). "Ehrlichial infection in Cameroonian canines by Ehrlichia canis and Ehrlichia ewingii". Veterinary Microbiology. 111 (1–2): 59–66. doi:10.1016/j.vetmic.2005.08.010. PMID   16181750.
  6. 1 2 Liu, Yan; Zhang, Zhikai; Jiang, Yongquan; Zhang, Lihong; Popov, Vsevolod L.; Zhang, Jianzhi; Walker, David H.; Yu, Xue-jie (2011-03-01). "Obligate intracellular bacterium Ehrlichia inhibiting mitochondrial activity". Microbes and Infection. 13 (3): 232–238. doi:10.1016/j.micinf.2010.10.021. PMC   3031727 . PMID   21070861.
  7. Buller, Richard S.; Arens, Max; Hmiel, S. Paul; Paddock, Christopher D.; Sumner, John W.; Rikihisa, Yasuko; Unver, Ahmet; Gaudreault-Keener, Monique; Manian, Farrin A. (1999-07-15). "Ehrlichia ewingii, a Newly Recognized Agent of Human Ehrlichiosis". New England Journal of Medicine. 341 (3): 148–155. doi:10.1056/NEJM199907153410303. ISSN   0028-4793. PMID   10403852.
  8. 1 2 3 4 "Symptoms, Diagnosis, and Treatment | Ehrlichiosis | CDC". Retrieved 2017-04-23.