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 people. The most common symptoms include headache, muscle aches, and fatigue. A rash may occur, but is uncommon. Ehrlichiosis can also blunt the immune system by suppressing production of TNF-alpha, which may lead to opportunistic infections such as candidiasis. Most of the signs and symptoms of ehrlichiosis can likely be ascribed to the immune dysregulation that it causes. 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. Ehrlichia/Anaplasma are tiny (0.2-2 µm) obligate, intracytoplasmic, gram-negative organisms that resemble Rickettsia; divide by binary fission; and multiply within the cytoplasm of infected white blood cells. Clusters of Ehrlichia multiply in host monocyte vacuoles (phagosomes) to form large, mulberry-shaped aggregates called morulae. Ehrlichia inclusion bodies, such as morulae, are visible in the cytoplasm of infected mononuclear phagocytic cells after 5-7 days. The type of ehrlichiosis that develops varies and depends on the infecting species and the type of leukocyte infected. Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE), is caused by Anaplasma phagocytophilum, which infect granulocytes. In contrast, human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis, which infects monocytes. HGA and HME cause the same clinical manifestations. The term ehrlichiosis is used for both types of infections. The total duration of illness for HME and HGA is unknown. No chronic cases have been reported at this time.
The distribution of ehrlichiosis in the United States mirrors the tick distribution and appropriate mammalian vectors (eg, white-footed mouse, white-tailed deer). Ehrlichiosis occurs where mammalian hosts are in contact with the appropriate tick vector. Most cases of ehrlichiosis in the United States occur in California and Texas and in the southeast and northeast regions of the country, with some cases occurring in the north-central states west of the Great Lakes. Ehrlichiosis is a seasonal disease observed mainly from April to September. In 1999, ehrlichiosis became reportable to the US Centers for Disease Control and Prevention (CDC). In 2005, 786 cases of human granulocytic anaplasmosis (HGA) were reported. The 3 states that reported the most cases were New York (221 cases), Minnesota (186 cases), and Wisconsin (155 cases). In 2006, 646 cases of HGA were reported. The 3 states that reported the most cases were New York (235 cases), Minnesota (177 cases), and Wisconsin (49 cases). A 2011 study confirmed that B burgdorferi and A phagocytophilum share the same enzootic life cycle suggesting that it is important to monitor areas endemic for Lyme disease for HGA. In this study, La Crosse, WI is centrally located in a well-documented Lyme disease focus. HGA was identified by PCR in the blood of 53 patients with clinical findings consistent with HGA confirming that this region endemic for Lyme should now also be considered part of the upper Midwestern focus of endemnicity for HGA. In 2005, 506 cases of human monocytic ehrlichiosis (HME) were reported. The 3 states that reported the most cases were New York (85 cases), Oklahoma (79 cases), and New Jersey (64 cases). In 2006, 578 cases of HME were reported. The 3 states that reported the most cases were New York (141 cases), Missouri (73 cases), and New Jersey (67 cases). A 2011 report identified a new ehrlichia species in 4 patients in the Minnesota and Wisconsin areas. All patients had the traditional clinical syndrome and responded to treatment. On testing, 17 of 697 Ixodes scapularis ticks collected in Minnesota or Wisconsin were positive for the same ehrlichia species by polymerase chain-reaction testing and genetic analyses revealed that this new ehrlichia species was closely related to E muris. International occurrence Ehrlichiosis occurs essentially worldwide, and the frequency parallels the distribution of the appropriate tick vectors for the transmission of Ehrlichia bacteria and the mammalian hosts. Ehrlichia sennetsu causes a mononucleosis-like illness in Japan and Malaysia. Doxycycline and minocycline are the medications of choice. For people allergic to antibiotics 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. The preferred drug for human monocytic ehrlichiosis (HME) and human granulocytic anaplasmosis (HGA) is doxycycline. In contrast to RMSF, chloramphenicol is not effective in ehrlichiosis. Antibiotic treatment should begin as soon as the diagnosis is ascertained. Antipyretics may be necessary. The American Academy of Pediatrics recommends doxycycline as first-line therapy for severe/life threatening suspected or proven HGA and HME. Ordinarily, tetracyclines are not administered to children younger than 8 years; however, chloramphenicol is the alternative treatment option. Oral chloramphenicol is no longer available in the United States. Thus, the American Academy of Pediatrics recommends doxycycline because the benefits outweigh the risks. Several case reports have detailed successful treatment of mild, non–life-threatening anaplasmosis (not ehrlichiosis) with rifampin in patients in whom doxycycline was contraindicated (eg, allergy, pregnancy)