Jump to content

Ehrlichia

From Wikipedia, the free encyclopedia

Ehrlichia
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)

Ehrlichiais agenusofRickettsialesbacteria that are transmitted to vertebrates by ticks. These bacteria cause the diseaseehrlichiosis,which is consideredzoonotic,because the main reservoirs for the disease are animals.

Ehrlichiaspecies are obligatelyintracellular pathogensand are transported between cells through the host cellfilopodiaduring initial stages of infection, whereas in the final stages of infection, the pathogen ruptures the host cell membrane.[2]

History

[edit]

The genusEhrlichiais named after German microbiologistPaul Ehrlich.The first ehrlichial disease was recognized inSouth Africaduring the 19th century. Its tick-borne nature was determined in 1900. The organism itself was demonstrated in 1925 when it was recognized to be aRickettsia.It was initially namedRickettsia ruminantium,and is currently namedEhrlichia 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 andantibiotictreatment if needed. In 1985, the organism was first propagated reliably in tissue culture. A new species ofEhrlichiawas discovered inside the deer tickIxodes scapularis.This newly found organism has only been isolated from deer ticks in Wisconsin and Minnesota in the USA. The species is known asEhrlichiaWisconsin HM543746.[citation needed]

Species

[edit]

Accepted species

[edit]

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

Provisional species

[edit]

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

  • "Ehrlichia japonica"Linet al.2021
  • "Ehrlichia ovina"Moshkovski 1945
  • "Ehrlichia platys"French and Harvey 1983

Candidatusspecies

[edit]

The following species have been published ascandidatusspecies:[1]

  • "CandidatusEhrlichia corsicanum "Cicculliet al.2020
  • "CandidatusEhrlichia khabarensis "Raret al.2015
  • "CandidatusEhrlichia occidentalis "Goftonet al.2017
  • "CandidatusEhrlichia ornithorhynchi "Goftonet al.2018
  • "CandidatusEhrlichia ovata "Lynnet al.2019
  • "CandidatusEhrlichia rustica "Ehounoudet al.2016
  • "CandidatusEhrlichia senegalensis "Dahmanaet al.2020
  • "CandidatusEhrlichia shimanensis "Kawaharaet al.2006
  • "CandidatusEhrlichia urmitei "Ehounoudet al.2016
  • "CandidatusEhrlichia walkeri "corrig. Brouquiet al.2003

Evolution

[edit]

TheEhrlichiagenome contains many different variants of genes that encode outermembrane 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 resultingparalogsof 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 calledgene conversion,resulting in a new gene variant. This has a profound effect on the fitness of an organism. The survival ofEhrlichiadepends greatly on theimmune responseof its host. With a higher range of outer membrane proteins, theparasitecan evade the immune system of the host more effectively and establish persistent infection.[5]

The most pronounced evidence of evolution in the genome size ofErhlichiais the presence oftandem repeats,[3]which vary highly among individuals and species. Over time, individuals may expand or contract parts of their genes andalleles,which addsgenetic variationand may sometimes affectphenotype.[3]

Ehrlichiaand its closely related genusAnaplasmashow extreme diversity in the structure and content of their genomes.[6]This diversity is direct result of rarecloneswith extreme genomes that emerged by chance after repeatedbottleneckevents, and this diversity persists because of the lack of selective constraints on rapid growth inside the host tissue.[6]

E. ruminantium

[edit]

The evolutionary changes in the outermembrane proteinshave led to the emergence of new strains that can infect a larger variety of hosts.Heartwater,caused byE. ruminantium,is a prevalent tick-borne disease oflivestockin Africa and the Caribbean, but also threatens the American mainland. Three strains have arisen from this species due to evolutionary change in their genomes. Whensequencingtheir genomes,e many active genomic modifications have occurred, such as highsubstitutionrates, truncated genes, and the presence ofpseudogenesand tandem repeats. When analyzing substitution rates between the three strains in 888orthologouscoding 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 thatselection pressureto maintain protein function existed, and this selection acted against the nonsynonymous mutations.[3]

E. canis

[edit]

E. canisis a small, obligate-intracellular, tick-transmitted,Gram-negativeα-proteobacterium. This species is responsible for the globally distributedcanine monocytic ehrlichiosis.E. canisalso shows evolution in its complex membrane structures and immune evasion strategies. These evolutionary features arederived traitsthat 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. Uniqueglycoproteinsand 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 inE. 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 ofgenetic driftin small populations, lowrecombinationrates, and highmutationrates. 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

[edit]

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

Amblyomma americanumticks spreadE. chaffeensisandE. ewingiibacterial infection in the Eastern and Southeastern United States, whileA. phagocytophilumis spread by theIxodes scapularistick in the Upper Midwest; 1,518 cases ofE. chaffeensiswere 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 ofE. ewingiiin Oklahoma, Arkansas and Tennessee though it is observed less frequently thanE. chaffeensis.[10]

During 2008–2012, 4,613 cases ofE. chaffeensisinfections 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 ofE. ewingiiinfections 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 beE. chaffeensisorE. ewingii,but instead it was revealed as a new species, similar in genetic makeup toE. muris.[12]Ixodes scapularisticks are hypothesized to be the transmitting vector of theE. murisstrain in these states.[10]

See also

[edit]

References

[edit]
  1. ^abcdEuzéby JP, Parte AC."Ehrlichia".List of Prokaryotic names with Standing in Nomenclature(LPSN).RetrievedJune 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.PMC3004962.PMID21187937.
  3. ^abcdFrutos, 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.PMID17652027.
  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.PMID17822801.
  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.PMID15948961.
  6. ^abDale, 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.PMID16901780.
  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.PMC1482910.PMID16707693.
  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.PMC39726.PMID8610134.
  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.PMC3319926.PMID21812671.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  10. ^abHarris, 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.PMC4861533.PMID27089171.
  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.ISSN1476-1645.PMC4710445.PMID26621561.
  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.PMC3319926.PMID21812671.{{cite journal}}:CS1 maint: multiple names: authors list (link)
[edit]
Retrieved from "https://en.wikipedia.org/w/index.php?title=Ehrlichia&oldid=1203858203"