Sennetsu fever

Sennetsu fever

Description, Causes and Risk Factors:

A disease of humans in western Japan caused by the rickettsia Ehrlichia sennetsu and characterized by fever, malaise, anorexia, backache, and lymphadenopathy.

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.


Sennetsu fever interferes with the body's immune system by attacking white blood cells, a vital part of the body's defense against invading bacteria, viruses, and other microbes or harmful substances. Untreated, the disease can leave people vulnerable to other infections.

Sennetsu fever does not spread from person-to-person. The Ehrlichia bacteria that cause the disease are spread through tick bites. The species that most commonly spread the disease are the lone star tick, the blacklegged tick, and the western black-legged tick.

For many years, Sennetsu fever was the only form of ehrlichiosis known to afflict humans. Comparative studies of emerging Human Ehrlichiosis Agents, Anaplasma (formerly Ehrlichia) phagocytophilum, Ehrlichia chaffeensis, and Neorickettsia (formerly Ehrlichia) sennetsu will enhance the opportunities for investigation of virulence factors, pathogenesis, immune modulation, and novel targets for antimicrobial therapy and vaccines.

Between 1986 and 1997, 1,223 cases of Sennetsu fever were reported in the United States. Most occurred along the Atlantic Coast and in southern and central states, although cases were reported in almost every region of the country. The number of cases peaks during tick season, which runs from April to October.

Incubation period is 14 -18 days.


Sennetsu fever is a relatively mild infection that resembles infectious mononucleosis. It is characterized by fever,malaise, anorexia, lymphadenopathy and hepatosplenomegaly. Fatigue, chills, headache, backache and myalgias havealso been reported. Increased circulating mononuclear cellsand atypical lymphocytes are often found. A rash is rare, butgeneralized erythematous reactions or localized petechiaehave been described.


The initial diagnosis is usually based on the history,clinical signs, hematologic abnormalities and changes inserum chemistry. Serology is used for confirmation. Humanmonocytic or granulocytic ehrlichiosis is usually diagnosedwith an indirect immunofluorescence assay (IFA). ELISAsare also in development. The serologic tests for Sennetsufever include an IFA and complement fixation. Cross-reactions can occur.Morulae can occasionally be found in neutrophils ormononuclear cells. Only a small percentage of cells areinfected. Polymerase chain reaction (PCR) testing can also detectEhrlichia. Immunohistochemistry and in situ hybridizationhave been described on spleen and lymph node samples.Culture of the organism is difficult and time-consuming, and may not be practical in clinical cases. Anaplasmaphagocytophilum and E. chaffeensis have been isolated fromthe blood of acutely ill patients, using various cell lines suchas canine DH82 and human HL-60 cells. E. chaffeensis isusually found after 7 to 36 days, and A. phagocytophilum in7 to 12 days. Species can be identified by the sequencing andanalysis of 16S rRNA.



Sennetsu fever in humans is usually treated with tetracylines; doxycycline is currently the drug of choice. Other antibiotics may be used in some circumstances. Uncomplicated cases usually respond promptly, but prolonged treatment may be necessary for severe or complicated disease.




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