|Classification and external resources|
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.
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.
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."
Doxycycline is the drug of choice. For people allergic to drugs of the tetracycline class, rifampin is an alternative. Early clinical experience suggested that chloramphenicol may also be effective, however in vitro susceptibility testing revealed resistance.
At the present time no commercial vaccine is available for ehrlichiosis. Tick control is the main preventive measure against the disease. However, in late 2012 a breakthrough in the prevention of CME (canine monocytic ehrlichiosis) was announced when a vaccine was accidentally discovered by Prof. Shimon Harrus, Dean of the Hebrew University of Jerusalem's Koret School of Veterinary Medicine in Rehovot, Israel. The vaccine has proven effective in an experimental study in dogs, which was published in the Vaccine journal.
See also 
- CDC "Questions and Answers" page for tickborne rickettsial diseases
- Dawson, Jacqueline E., Marty, Aileen M. (1997). "Ehrlichiosis". In Horsburgh CR, Nelson AM. Pathology of emerging Infections 1. American Society for Microbiology.
- Goddard J (September 1, 2008). "What Is New With Ehrlichiosis?". Infections in Medicine.
- Dumler JS, Madigan JE, Pusterla N, Bakken JS (July 2007). "Ehrlichioses in humans: epidemiology, clinical presentation, diagnosis, and treatment". Clin. Infect. Dis. 45 (Suppl 1): S45–51. doi:10.1086/518146. PMID 17582569.
- Reeves WK, Loftis AD, Nicholson WL, Czarkowski AG (2008). "The first report of human illness associated with the Panola Mountain Ehrlichia species: a case report". Journal of medical case reports 2: 139. doi:10.1186/1752-1947-2-139. PMC 2396651. PMID 18447934.
- Julie Steenhuysen. 2011. New tick-borne bacterium found in upper Midwest. Reuters, 8/3/2011, http://www.trust.org/alertnet/news/new-tick-borne-bacterium-found-in-upper-midwest/, accessed August 4, 2011.
- Thomas S, Popov VL, Walker DH (2010). "Exit Mechanisms of the Intracellular Bacterium Ehrlichia". In Kaushal, Deepak. PLoS ONE 5 (12): e15775. doi:10.1371/journal.pone.0015775. PMC 3004962. PMID 21187937.
- Marty AM, Dumler JS, Imes G, Brusman HP, Smrkovski LL, Frisman DM (August 1995). "Ehrlichiosis mimicking thrombotic thrombocytopenic purpura. Case report and pathological correlation". Hum. Pathol. 26 (8): 920–5. doi:10.1016/0046-8177(95)90017-9. PMID 7635455.
- McBride, Jere W.; Walker, David H. (31 January 2011). "Molecular and cellular pathobiology of Ehrlichia infection: targets for new therapeutics and immunomodulation strategies". Expert Reviews in Molecular Medicine 13. doi:10.1017/S1462399410001730.
- Thomas, Rachael J; Dumler, J Stephen, Carlyon, Jason A (1 August 2009). "Current management of human granulocytic anaplasmosis, human monocytic ehrlichiosis and ehrlichiosis". Expert Review of Anti-infective Therapy 7 (6): 709–722. doi:10.1586/eri.09.44. PMC 2739015. PMID 19681699.
- (RO) Ehrlichiosis in dogs. Real cases Ehrlichia cases in dogs. Case studies