''Ehrlichia canis'' is an obligate intracellular

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 a ...

that acts as the

causative agent In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

of ehrlichiosis, a disease most commonly affecting

canine Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** '' Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the ...

species. This pathogen is present throughout the United States (but is most prominent in the South), South America, Asia, Africa and recently in the

Kimberley Kimberly or Kimberley may refer to: Places and historical events Australia * Kimberley (Western Australia) ** Roman Catholic Diocese of Kimberley * Kimberley Warm Springs, Tasmania * Kimberley, Tasmania a small town * County of Kimberley, a ...

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 The contiguous United States (officially the conterminous United States) consists of the 48 adjoining U.S. states and the Federal District of the United States of America. The term excludes the only two non-contiguous states, Alaska and Hawai ...

. Reported primarily in dogs, ''E. canis'' has also been documented in

felines The Felinae are a subfamily of the family Felidae. This subfamily comprises the small cats having a bony hyoid, because of which they are able to purr but not roar. Other authors have proposed an alternative definition for this subfamily: as c ...

and humans, where it is transferred most commonly via '' Rhipicephalus sanguineus'', the brown dog tick.

Transmission cycle

The brown dog tick, ''R. sanguineus'', acts as the primary vector of ''E. canis'', transferring the pathogen between hosts during blood meals. Dogs, both domestic and wild, act as reservoir hosts for this

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

and are the primary hosts of brown dog ticks. Brown dog ticks become carriers of the pathogen when they take a blood meal from a rickessemic dog. Stored in the midgut and salivary glands of an infected tick, ''E. canis'' is transferred by the saliva of ticks carrying the pathogen to hosts during blood meals. If infected while in the larval stage, the tick retains the pathogen through the next two life stages and can infect hosts during blood meals in both the

nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are ty ...

al and adult stages in transstadial transmission. Because the vector of ''E. canis'' uses canine species as a primary host, this bacterium is most commonly associated with dogs, but multiple human cases have been reported. Additionally, some cats have been found to have antibodies to ''E. canis'' suggesting that it can occasionally infect cats, as well.

Pathogenesis

''E. canis'' is a small,

coccus A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape. Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spiral ( of w ...

bacterium with a single circular chromosome. Minute in size, ''E. canis'' is smaller than the other species in the genus ''Ehrlichia''. The

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 mec ...

s of this organism lack peptidoglycans and

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer ...

s typically present in at least small amounts in most other

Gram-negative bacteria 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 wa ...

, which is believed to contribute to its ability to resist the immune response of the host. The lack of these two materials reduces the rigidity of the cell wall, allowing the cell wall’s exterior to be dynamic, in turn facilitating the ''E. canis'' cells’ evasion of antibodies in its host organism’s body. The cells of ''E. canis'' lack complex internal structures, which allow the synthesis of sugars and it, in turn, uses

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

s as its source of energy. Upon introduction, ''E. canis'' penetrates the

monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also i ...

s and macrophages of hosts. Infection may leave a host asymptomatic for months or severe clinical signs may manifest as ehrlichiosis develops.

Diagnosis and treatment

Animals infected with ''E. canis'' may experience a combination of signs, including fever, lethargy, weight loss, lymphadenopathy,

edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid in the body's tissue. Most commonly, the legs or arms are affected. Symptoms may include skin which feels tight, the area ma ...

of the appendages, discharge from nose and eyes, and seizures. A low packed cell volume in the blood is a common indicator of the presence of ''E. canis'' and a blood smear can be done to determine if morulae are present in leukocytes to definitively diagnose the infection. Because this process is often long and tedious, the presence of antibodies to ''E. canis'' in blood can be used to effectively diagnose a patient. These antibodies typically appear within seven days after infection. ''E. canis'' is commonly treated via chemoprophylaxis with doxycycline, a prescription drug used in the treatment of bacterial infections. If caught before the clinical stage, ''E. canis'' is almost completely curable, but prolonged presence of the bacteria leads to hemorrhaging, which typically results in the death of the patient. Although no

vaccine A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified. ...

is known to be on the market for ehrlichiosis caused by ''E. canis'', extensive research on the use of an attenuated strain of ''E. canis'' as an aid in the prevention of ''E. canis'' infection is being performed. Preliminary testing has proved that this method may be effective, as clinical signs and the presence of the ''E. canis'' bacteria were reduced in Beagle test subjects, but further investigations are necessary before a vaccine can be marketed.

References

{{Taxonbar, from=Q5348627 Rickettsiales Dog diseases Bacteria described in 1935