Ehrlichia

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Ehrlichia
EhrlichiaEwingiiUS CDCMorulaInCytoplasmOfNeutrophil.jpg
Ehrlichia ewingii
Scientific classification
Domain:
Bacteria
Phylum:
Pseudomonadota
Class:
Alphaproteobacteria
Order:
Rickettsiales
Family:
Ehrlichiaceae [1]
Genus:
Ehrlichia
Species:
See text.
Synonyms
  • CowdriaMoshkovski 1947 (Approved Lists 1980)

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

Contents

Ehrlichia species are obligately intracellular pathogens and are transported between cells through the host cell filopodia during initial stages of infection, whereas in the final stages of infection, the pathogen ruptures the host cell membrane. [2]

History

The genus Ehrlichia is named after German microbiologist Paul Ehrlich. The first ehrlichial disease was recognized in South Africa during the 19th century. Its tick-borne nature was determined in 1900. The organism itself was demonstrated in 1925 when it was recognized to be a Rickettsia . It was initially named Rickettsia ruminantium, and is currently named Ehrlichia ruminantium . In 1945, an "infection and treatment" method for livestock was developed. This is still the only commercially available "vaccine" against the disease, which is not a true vaccine, but intentional exposure to the disease with monitoring and antibiotic treatment if needed. In 1985, the organism was first propagated reliably in tissue culture. A new species of Ehrlichia was discovered inside the deer tick Ixodes scapularis . This newly found organism has only been isolated from deer ticks in Wisconsin and Minnesota in the USA. The species is known as Ehrlichia Wisconsin HM543746.[ citation needed ]

Species

Accepted species

The following species have been effectively and validly published: [1]

Provisional species

The following species have been published, but are not valid according to the Bacteriological Code: [1]

Candidatus species

The following species have been published as candidatus species: [1]

Evolution

The Ehrlichia genome contains many different variants of genes that encode outer membrane proteins, [3] which have gone through intense modification over long periods of time. [4] The great diversity in outer membrane protein genes is thought to originate from gene duplication events, followed by the fusion and fission of resulting paralogs of the gene. These duplication, fusion, and fission events form multiple gene copies and fragments, which are able to accumulate mutations. These copies and fragments of membrane proteins can then recombine, through a process called gene conversion, resulting in a new gene variant. This has a profound effect on the fitness of an organism. The survival of Ehrlichia depends greatly on the immune response of its host. With a higher range of outer membrane proteins, the parasite can evade the immune system of the host more effectively and establish persistent infection. [5]

The most pronounced evidence of evolution in the genome size of Erhlichia is the presence of tandem repeats, [3] which vary highly among individuals and species. Over time, individuals may expand or contract parts of their genes and alleles, which adds genetic variation and may sometimes affect phenotype. [3]

Ehrlichia and its closely related genus Anaplasma show extreme diversity in the structure and content of their genomes. [6] This diversity is direct result of rare clones with extreme genomes that emerged by chance after repeated bottleneck events, and this diversity persists because of the lack of selective constraints on rapid growth inside the host tissue. [6]

E. ruminantium

The evolutionary changes in the outer membrane proteins have led to the emergence of new strains that can infect a larger variety of hosts. Heartwater, caused by E. ruminantium, is a prevalent tick-borne disease of livestock in Africa and the Caribbean, but also threatens the American mainland. Three strains have arisen from this species due to evolutionary change in their genomes. When sequencing their genomes,e many active genomic modifications have occurred, such as high substitution rates, truncated genes, and the presence of pseudogenes and tandem repeats. When analyzing substitution rates between the three strains in 888 orthologous coding DNA sequences, three coding DNA sequences were biased towards nonsynonymous substitutions that affect phenotype. In contrast, 181 coding DNA sequences were biased towards synonymous substitutions, which do not affect phenotype. This indicates that selection pressure to maintain protein function existed, and this selection acted against the nonsynonymous mutations. [3]

E. canis

E. canis is a small, obligate-intracellular, tick-transmitted, Gram-negative α-proteobacterium. This species is responsible for the globally distributed canine monocytic ehrlichiosis. E. canis also shows evolution in its complex membrane structures and immune evasion strategies. These evolutionary features are derived traits that do not show up in the previous lineages, which may indicate that these features may have contributed to a fitness advantage that kept this lineage going. Unique glycoproteins and major outer membrane proteins can be expressed variously using 25 different genes. The glycoproteins are important targets of the host immune response, attachment to the host cell, and other features in the immune response. The more outer-membrane protein genes that can be expressed, the higher the chance the organism can avoid being recognized by the host's immune system. [7]

Also, reductive evolution is present in E. canis. The genome has had a severe loss of metabolic pathway enzymes compared to its ancestors. Reductive evolution in obligate intracellular pathogens is usually the direct result of genetic drift in small populations, low recombination rates, and high mutation rates. The host metabolic pathway enzymes take control of the functions lost due to reductive evolution, and this contributes to its need for a host. Natural selection may not be the reason for small genomes. [8]

Epidemiology

Despite there being multiple strains of ehrlichiosis, only two species, E. chaffeensis and E. ewingii, are currently known to cause the disease in humans. [9]

Amblyomma americanum ticks spread E. chaffeensis and E. ewingii bacterial infection in the Eastern and Southeastern United States, while A. phagocytophilum is spread by the Ixodes scapularis tick in the Upper Midwest; 1,518 cases of E. chaffeensis were recorded in southeastern, south-central and mid-Atlantic areas of the country in 2013. Despite the first cases of "E. ewingii" appearing in the Missouri in the year 1999, this strain was not reportable to health officials until 2008. Since 2008, there have been reported human cases of E. ewingii in Oklahoma, Arkansas and Tennessee though it is observed less frequently than E. chaffeensis. [10]

During 2008–2012, 4,613 cases of E. chaffeensis infections were reported through the National Notifiable Diseases Surveillance System (NNDSS). The incidence rate (IR) was 3.2 cases per million person-years (PYs). The hospitalization rate (HR) was 57% and the case fatality rate (CFR) was 1%. During that same time, 55 cases of E. ewingii infections were reported through NNDSS. The national IR was 0.04 cases per million PY. The HR was 77% and the case fatality rate was 0%. [11]

In Minnesota and Wisconsin, four people reported symptoms that are associated with ehrlichiosis, and upon further research, neither of these cases was found to be E. chaffeensis or E. ewingii, but instead it was revealed as a new species, similar in genetic makeup to E. muris. [12] Ixodes scapularis ticks are hypothesized to be the transmitting vector of the E. muris strain in these states. [10]

See also

Related Research Articles

<i>Treponema pallidum</i> Species of bacterium

Treponema pallidum, formerly known as Spirochaeta pallida, is a microaerophilic spirochaete bacterium with subspecies that cause the diseases syphilis, bejel, and yaws. It is transmitted only among humans. It is a helically coiled microorganism usually 6–15 μm long and 0.1–0.2 μm wide. T. pallidum's lack of either a tricarboxylic acid cycle or oxidative phosphorylation results in minimal metabolic activity. The treponemes have a cytoplasmic and an outer membrane. Using light microscopy, treponemes are visible only by using dark-field illumination. T. pallidum consists of three subspecies, T. p. pallidum, T. p. endemicum, and T. p. pertenue, each of which has a distinct associated disease.

<i>Rickettsia</i> Genus of bacteria

Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci, bacilli, or threads. The genus was named after Howard Taylor Ricketts in honor of his pioneering work on tick-borne spotted fever.

Ehrlichiosis is a tick-borne disease of dogs usually caused by the rickettsial agent 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.

Mycoplasma pneumoniae is a very small bacterium in the class Mollicutes. It is a human pathogen that causes the disease mycoplasma pneumonia, a form of atypical bacterial pneumonia related to cold agglutinin disease. M. pneumoniae is characterized by the absence of a peptidoglycan cell wall and resulting resistance to many antibacterial agents. The persistence of M. pneumoniae infections even after treatment is associated with its ability to mimic host cell surface composition.

<i>Wolbachia</i> Genus of bacteria in the Alphaproteobacteria class

Wolbachia is a genus of gram-negative bacteria that can either infect many species of arthropod as an intracellular parasite, or act as a mutualistic microbe in filarial nematodes. It is one of the most common parasitic microbes of arthropods, and is possibly the most common reproductive parasite in the biosphere. Its interactions with its hosts are often complex. Some host species cannot reproduce, or even survive, without Wolbachia colonisation. One study concluded that more than 16% of neotropical insect species carry bacteria of this genus, and as many as 25 to 70% of all insect species are estimated to be potential hosts.

<i>Rickettsia rickettsii</i> Species of bacterium

Rickettsia rickettsii is a Gram-negative, intracellular, coccobacillus bacterium that was first discovered in 1902. R. rickettsii is the causative agent of Rocky Mountain Spotted Fever and is transferred to its host via a tick bite. It is one of the most pathogenic Rickettsia species and affects a large majority of the Western Hemisphere, most commonly the Americas.

<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.

<span class="mw-page-title-main">Anaplasmosis</span> Medical condition

Anaplasmosis is a tick-borne disease affecting ruminants, dogs, and horses, and is caused by Anaplasma bacteria. Anaplasmosis is an infectious but not contagious disease. Anaplasmosis can be transmitted through mechanical and biological vector processes. Anaplasmosis can also be referred to as "yellow bag" or "yellow fever" because the infected animal can develop a jaundiced look. Other signs of infection include weight loss, diarrhea, paleness of the skin, aggressive behavior, and high fever.

<span class="mw-page-title-main">Ehrlichiosis</span> Medical condition

Ehrlichiosis is a tick-borne bacterial infection, caused by bacteria of the family Anaplasmataceae, genera Ehrlichia and Anaplasma. These obligate intracellular bacteria infect and kill white blood cells.

<span class="mw-page-title-main">Lyme disease microbiology</span>

Lyme disease, or borreliosis, is caused by spirochetal bacteria from the genus Borrelia, which has 52 known species. Three main species are the main causative agents of the disease in humans, while a number of others have been implicated as possibly pathogenic. Borrelia species in the species complex known to cause Lyme disease are collectively called Borrelia burgdorferisensu lato (s.l.) not to be confused with the single species in that complex Borrelia burgdorferi sensu stricto which is responsible for nearly all cases of Lyme disease in North America.

<i>Orientia tsutsugamushi</i> Species of bacterium

Orientia tsutsugamushi is a mite-borne bacterium belonging to the family Rickettsiaceae and is responsible for a disease called scrub typhus in humans. It is a natural and an obligate intracellular parasite of mites belonging to the family Trombiculidae. 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 illnesses, the infection can be complicated and often fatal.

<i>Ehrlichia ruminantium</i> Ruminant disease

Heartwater is a tick-borne rickettsial disease. The name is derived from the fact that fluid can collect around the heart or in the lungs of infected animals. It is caused by Ehrlichia ruminantium —an intracellular Gram-negative coccal bacterium. The disease is spread by various Amblyomma ticks, and has a large economic impact on cattle production in affected areas. There are four documented manifestations of the disease, these are acute, peracute, subacute, and a mild form known as heartwater fever. There are reports of zoonotic infections of humans by E. ruminantium, similar to other Ehrlichia species, such as those that cause human ehrlichiosis.

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.

<i>Ehrlichia ewingii</i> Species of bacterium

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.

<span class="mw-page-title-main">Human granulocytic anaplasmosis</span> Medical condition

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.

<span class="mw-page-title-main">Human monocytotropic ehrlichiosis</span> Medical condition

Human monocytotropic ehrlichiosis 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).

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, Africa and recently in the Kimberley region of Australia. 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.

<i>Theileria parva</i> Species of single-celled organism

Theileria parva is a species of parasites, named in honour of Arnold Theiler, that causes East Coast fever (theileriosis) in cattle, a costly disease in Africa. The main vector for T. parva is the tick Rhipicephalus appendiculatus. Theiler found that East Coast fever was not the same as redwater, but caused by a different protozoan.

Ehrlichia muris is a species of pathogenic bacteria first isolated from mice, with type strain AS145T. Its genome has been sequenced.

References

  1. 1 2 3 4 Euzéby JP, Parte AC. "Ehrlichia". List of Prokaryotic names with Standing in Nomenclature (LPSN). Retrieved June 1, 2021.
  2. ↑ Thomas, S; Popov, VL; Walker, DH (2010). "Exit Mechanisms of the Intracellular Bacterium Ehrlichia". PLOS ONE. 5 (12): e15775. Bibcode:2010PLoSO...515775T. doi: 10.1371/journal.pone.0015775 . PMC   3004962 . PMID   21187937.
  3. 1 2 3 4 Frutos, Roger; Viari, Alain; Vachiery, Nathalie; Boyer, Frédéric; Martinez, Dominique (September 2007). "Ehrlichia ruminantium: genomic and evolutionary features". Trends in Parasitology. 23 (9): 414–419. doi:10.1016/j.pt.2007.07.007. PMID   17652027.
  4. ↑ Darby, Alistair C.; Cho, Nam-Huyk; Fuxelius, Hans-Henrik; Westberg, Joakim; Andersson, Siv G.E. (October 2007). "Intracellular pathogens go extreme: genome evolution in the Rickettsiales". Trends in Genetics. 23 (10): 511–520. doi:10.1016/j.tig.2007.08.002. PMID   17822801.
  5. ↑ Futse, James E.; Brayton, Kelly A.; Knowles, Donald P.; Palmer, Guy H. (July 2005). "Structural basis for segmental gene conversion in generation of Anaplasma marginale outer membrane protein variants". Molecular Microbiology. 57 (1): 212–221. doi: 10.1111/j.1365-2958.2005.04670.x . PMID   15948961.
  6. 1 2 Dale, C.; Moran, N. (2006). "Molecular interactions between bacterial symbionts and their hosts". Cell. 126 (3): 453–465. doi: 10.1016/j.cell.2006.07.014 . PMID   16901780.
  7. ↑ Mavromatis, K.; Doyle, C. K.; Lykidis, A.; Ivanova, N.; Francino, M. P.; Chain, P.; Shin, M.; Malfatti, S.; Larimer, F.; Copeland, A.; Detter, J. C.; Land, M.; Richardson, P. M.; Yu, X. J.; Walker, D. H.; McBride, J. W.; Kyrpides, N. C. (17 May 2006). "The Genome of the Obligately Intracellular Bacterium Ehrlichia canis Reveals Themes of Complex Membrane Structure and Immune Evasion Strategies". Journal of Bacteriology. 188 (11): 4015–4023. doi:10.1128/JB.01837-05. PMC   1482910 . PMID   16707693.
  8. ↑ Moran, N. A. (1996). "Accelerated evolution and Muller's rachet in endosymbiotic bacteria". Proc. Natl. Acad. Sci. USA. 93 (7): 2873–2878. Bibcode:1996PNAS...93.2873M. doi: 10.1073/pnas.93.7.2873 . PMC   39726 . PMID   8610134.
  9. ↑ Pritt, Sloan, Johnson, Munderloh, Paskewtiz, McElroy, McFadden, Binnicker, Neitzel, Liu, Nicholson, Nelson, Franson, Martin, Cunningham, Steward, Bogumill, Bjorgaard, Davis, McQuiston, Warshauer, Wilhelm, Patel, Trivedi, Eremeeva, Bobbi, Lynne, Diep, Ulrike, Susan, Kristina, Jevon, Matthew, David, Gongping, William, Curtis, Joni, Scott, Scott, Christopher, Kay, Mary, Jeffrey, Jennifer, david, Mark, Robin, Vipul, Marina (August 4, 2011). "Emergence of New Pathogenic Ehrlichia Species, Wisconsin and Minnesota, 2009". The New England Journal of Medicine. 365 (5): 422–429. doi:10.1056/NEJMoa1010493. PMC   3319926 . PMID   21812671.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. 1 2 Harris, Rebecca M.; Couturier, Brianne A.; Sample, Stephan C.; Coulter, Katrina S.; Casey, Kathleen K.; Schlaberg, Robert (2016). "Expanded Geographic Distribution and Clinical Characteristics of Ehrlichia ewingii Infections, United States". Emerging Infectious Diseases. 22 (5): 862–865. doi:10.3201/eid2205.152009. PMC   4861533 . PMID   27089171.
  11. ↑ Nichols Heitman, Kristen; Dahlgren, F. Scott; Drexler, Naomi A.; Massung, Robert F.; Behravesh, Casey Barton (2016-01-01). "Increasing Incidence of Ehrlichiosis in the United States: A Summary of National Surveillance of Ehrlichia chaffeensis and Ehrlichia ewingii Infections in the United States, 2008-2012". The American Journal of Tropical Medicine and Hygiene. 94 (1): 52–60. doi:10.4269/ajtmh.15-0540. ISSN   1476-1645. PMC   4710445 . PMID   26621561.
  12. ↑ Pritt, Sloane, Hoang-Johnson, Munderloh, Paskewitz, McElroy, McFadden, Binnicker, Neitzel, Liu, Nicholson, Nelson, Franson, Martin, Cunningham, Steward, Bogumill, Bjorgaard, Davis, McQuiston, Warshauer, Wilhelm, Patel, Trivedi, Eremeeva, Bobbi, Lynne, Diep, Ulrike, Susan, Kristina, Jevon, Matthew, David, Gongping, William, Curtis, Joni, Scott, Scott, Christopher, Kay, Mary, Jeffrey, Jennifer, David, Mark, Robin, Vipul, Marina (2011). "Emergence of a New Ehrlichia Species, Wisconsin and Minnesota, 2009". The New England Journal of Medicine. 365 (5): 422–429. doi:10.1056/NEJMoa1010493. PMC   3319926 . PMID   21812671.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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