Sennetsu fever

Sennetsu fever
ICD-9	082.4
eMedicine	article/235839
MeSH	D016873

Ehrlichiosis is a tickborne

bacterial infection, caused by bacteria of the family Anaplasmataceae, genera Ehrlichia and Anaplasma. These obligate intracellular bacteria infect and kill white blood cells.

The average reported annual incidence is 0.7 cases per million population.

Species

Five (see note below) species have been shown to cause human infection:

• Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis, formerly known as human granulocytic ehrlichiosis). A. phagocytophilium is endemic to New England and the north central and Pacific regions of the United States.

• Ehrlichia ewingii (which causes human ewingii ehrlichiosis). E. ewingii primarily infects deer and dogs (see Ehrlichiosis (canine)). E. ewingii is most common in the south central and southeastern states.

• Ehrlichia chaffeensis (which causes human monocytic ehrlichiosis). E. chaffeensis is most common in the south central and southeastern states.

• Ehrlichia canis

• Neorickettsia sennetsu

The latter two infections are not well studied.

Note: In 2008, human infection by Panola Mountain (Georgia, USA) Ehrlichia species was reported. On August 3, 2011, infection by a yet-unnamed bacterium in the genus Ehrlichia carried by deer ticks that has caused flu-like symptoms in at least 25 people in Minnesota and Wisconsin was reported; human ehrlichiosis was thought to be very rare or absent in Minnesota and Wisconsin. The new species, which is very similar genetically to an Ehrlichia species found in Eastern Europe and Japan called E. muris, was identified at Mayo Clinic Health System's Eau Claire hospital.

Ehrlichia 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.

Symptoms

The most common symptoms include headache, muscle aches, and fatigue. A rash may occur, but is uncommon. Ehrlichiosis can also blunt the immune system, which may lead to opportunistic infections such as candidiasis.

Most of the symptoms of ehrlichiosis can likely be ascribed to the immune dysregulation that it causes.

Early in infection, production of TNF-alpha, a cellular product that promotes inflammation and immune response, is suppressed.

Late in infection, however, production of this substance can be upregulated by 30 fold, which is likely responsible for the "toxic shock-like" syndrome seen in some severe cases of ehrlichiosis. Some cases can present with purpura and in one such case the organisms were present in such overwhelming numbers that in 1991 Dr. Aileen Marty of the AFIP was able to demonstrate the bacteria in human tissues using standard stains, and later proved that the organisms were indeed Ehrlichia using immunoperoxidase stains.

Experiments in mouse models further supports this hypothesis, as mice lacking TNF-alpha I/II receptors are resistant to liver injury caused by ehrlichia infection.

3% of human monocytic ehrlichiosis cases result in death, however these deaths occur "most commonly in immunosuppressed individuals who develop respiratory distress syndrome, hepatitis, or opportunistic nosocomial infections."

Treatment

Doxycycline is the drug of choice. For people allergic to drugs of the tetracycline class, rifampicin is an alternative. Early clinical experience suggested that chloramphenicol may also be effective, however in vitro susceptibility testing revealed resistance.