Sennetsu Fever

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  • Human Ehrlichial Infection, Sennetsu Type
  • Human Ehrlichial Infection
  • Sennetsu Type

Disorder Subdivisions

  • None

General Discussion

Sennetsu Fever is a rare infectious disease belonging to a group of diseases known as the Human Ehrlichioses. These diseases are caused by bacteria belonging to the "Ehrlichia" family. Several forms of Human Ehrlichial infection have been identified including Sennetsu Fever, Human Monocytic Ehrlichiosis (HME), and Human Granulocytic Ehrlichiosis (HGE). Though caused by different strains of Ehrlichia bacteria, the disorders are all characterized by similar symptoms.

The symptoms of Sennetsu Fever may include a sudden high fever, headache, and muscle aches (myalgia) within a few weeks after initial infection. In some cases, affected individuals may also experience nausea, vomiting, and/or loss of appetite (anorexia). In addition, in many cases, abnormal laboratory findings may include a decrease in white blood cells (leukopenia) and/or an abnormal increase in the level of certain liver enzymes (hepatic transaminases). Sennetsu Fever is caused by the bacterium Ehrlichia sennetsu. The vector (or carrier) for this bacterium has not yet been determined; however, some researchers believe that infection may result from the ingestion of raw fish.


The symptoms of Sennetsu Fever, the first recognized form of Human Ehrlichiosis, tend to begin approximately two weeks after an individual has been infected with the bacterium Ehrlichia sennetsu. Affected individuals may then experience severe headaches, muscle aches (myalgia), sudden fever, chills, swollen lymph nodes (lymphadenopathy), nausea, vomiting, and/or loss of appetite (anorexia). In very rare cases, a rash may appear on the skin.

In the beginning of the illness, affected individuals may exhibit an abnormal decrease in the number of circulating white blood cells (leukopenia); later in the course of the infection, there may be an abnormal increase in the production of certain white blood cells (lymphocytosis) as the body's immune system works to fight the infection. Individuals with Sennetsu Fever may also exhibit an abnormally enlarged liver and spleen (hepatosplenomegaly) and a mild to moderate increase in the level of certain liver enzymes (hepatic transaminases).


The Human Ehrlichioses, including Sennetsu Fever, are caused by bacteria belonging to the "Ehrlichia" family. They are considered "gram-negative" bacteria. Bacteria may be classified as "gram negative" or "gram positive," depending upon the results of "Gram's stain," a testing method in which bacteria are stained with various solutions to help identify and classify the bacteria. Such staining may be essential in identifying a specific bacterium responsible for an infectious disorder and determining appropriate, effective treatments.

Sennetsu Fever is caused by a bacterium called Ehrlichia sennetsu (or E. sennetsu). The vector (or carrier) for this bacterium has not yet been determined; however, some researchers believe that infection may result from the ingestion of raw fish. A vector is any organism that is infected with a particular disease agent (e.g., bacterium or virus), carries it, and later transmits it to another organism, which may then become infected by the disease agent in question. The genetic makeup of the bacterium E. sennetsu is closely related to that of a bacterium called Ehrlichia risticii, which causes an Ehrlichial infection in horses (Potomac Horse Fever).

In Sennetsu Fever, the Ehrlichial bacterium (E. sennetsu) spreads through blood and lymphatic vessels. Lymph, a body fluid, carries cells that help fight infection. E. sennetsu then invades certain cells (monocytes and macrophages). These are large cells (mononuclear phagocytes) that play an essential role in the body's immune system by engulfing and digesting microorganisms (phagocytosis), such as bacteria and other foreign materials. The invading Ehrlichial bacteria grow within membrane-bound cavities (vacuoles) in monocytes and macrophages in the blood and certain body tissues (e.g., bone marrow, lymph nodes, liver, spleen, kidneys, lungs, and the fluid that surrounds the brain and spinal cord [cerebrospinal fluid]).

Affected Populations

The Ehrlichioses are a group of rare infectious diseases caused by members of the "Ehrlichia" bacteria family. Until recently, the Ehrlichioses were known primarily as veterinary disorders. Since the early 1900s, several forms of veterinary Ehrlichioses have been identified, each caused by a different strain of Ehrlichia; in most cases, the bacteria are transmitted by ticks (vectors). Veterinary Ehrlichial infections have been identified that cause disease in horses, sheep, deer, cattle, rodents, and/or dogs. For example, a form of Ehrlichial infection in dogs was first identified in Algeria in 1935 (Canine Ehrlichiosis). The disorder is now a well-recognized canine disease across the world; it is most common in subtropical and tropical areas.

In rare cases, Ehrlichiosis may affect humans. The first recognized form of Human Ehrlichial infection, Sennetsu Fever, was identified in Japan in 1954. Reported cases of Sennetsu Fever appear to be limited to Western Japan and Malaysia.

Researchers believe that the distinct geographic distributions of the Human Ehrlichioses may result from differences in the distribution of the various vectors (e.g., raw fish, Lone Star tick, American dog tick, deer tick) carrying and transmitting the different Ehrlichia bacterial strains.

Standard Therapies

Sennetsu Fever may be diagnosed based upon a thorough clinical evaluation, characteristic findings, and specialized laboratory tests. Blood tests may reveal findings often associated with the Human Ehrlichioses such as abnormally low levels of circulating blood platelets (thrombocytopenia), low levels of certain white blood cells (leukopenia), and/or elevated levels of certain liver enzymes (such as aspartate aminotransferase [AST] and alanine aminotransferase [ALT]). In some cases, laboratory tests may reveal abnormalities of the cerebrospinal fluid; chest X-rays may reveal abnormalities in the lungs (pulmonary infiltrates).

Specialized laboratory tests may include Indirect Immunofluorescence Assays (IFA) conducted on the fluid portion of an affected individual's blood (serum). Antibodies, which are proteins manufactured by certain white blood cells, help the body fight toxins and invading microorganisms. In Indirect Immunofluorescence Assays, human antibodies are marked with special fluorescent dyes and a microscope with ultraviolet light is used, enabling researchers to observe antibody response to certain microorganisms. IFA testing has been used in confirming a diagnosis of all known types of Human Ehrlichial infection. Sennetsu Fever may be diagnosed by observing the antibody response in a patient's blood serum to the bacterium Ehrlichia sennetsu.

Measurable, diagnostic rises in antibody response to the Ehrlichia sennetsu bacteria may not occur until several weeks after the onset of Sennetsu Fever. As a result, initial IFA blood serum results may be negative in some cases. Therefore, more sensitive testing techniques that can help establish early diagnosis may be used in some cases.

Information in the medical literature indicates that because it may be difficult to differentiate Human Ehrlichial infection, such as Sennetsu Fever, from other illnesses that are also characterized by high fever (febrile illnesses), Ehrlichiosis should be considered in any patient with high fever, thrombocytopenia, and leukopenia who has recently been exposed to known vectors for Ehrlichial infection. If Sennetsu Fever is suspected, treatment should not be delayed until diagnosis has been confirmed by IFA testing, since a positive antibody response may not occur until several weeks after initial infection. Therapy should begin as soon as possible after the onset of symptoms.

The treatment of Sennetsu Fever usually entails standard doses of tetracycline antibiotics. The antibiotic drugs doxycycline or minocycline have also been administered to treat this disease. In severe cases of Sennetsu Fever, hospitalization may be required. Other treatment is symptomatic and supportive.

Investigational Therapies

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