Ehrlichiosis is a tick-borne 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 on the order of 2.3 cases per million people.
Five (see note below) species have been shown to cause human infection:
- Anaplasma phagocytophilum causes human granulocytic anaplasmosis. A. phagocytophilum is endemic to New England and the north-central and Pacific regions of the United States.
- Ehrlichia ewingii 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 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.
In 2008, human infection by a Panola Mountain (in Georgia, USA) Ehrlichia species was reported. On August 3, 2011, infection by a yet-unnamed bacterium in the genus Ehrlichia was reported, carried by deer ticks and causing flu-like symptoms in at least 25 people in Minnesota and Wisconsin. Until then, human ehrlichiosis was thought to be very rare or absent in both states. The new species, which is genetically very similar to an Ehrlichia species found in Eastern Europe and Japan called E. muris, was identified at a Mayo Clinic Health System hospital in Eau Claire.
Ehrlichia species are transported between cells through the host-cell filopodia during the initial stages of infection; whereas, in the final stages of infection, the pathogen ruptures the host cell membrane.
Signs and 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 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. A "toxic shock-like" syndrome is 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 support this hypothesis, as mice lacking TNF-alpha I/II receptors are resistant to liver injury caused by Ehrlichia infection.
About 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."
No human 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 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.
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, but in vitro susceptibility testing revealed resistance.
Ehrlichiosis is a nationally notifiable disease in the United States. Cases have been reported in every month of the year, but most cases are reported during April–September. These months are also the peak months for tick activity in the United States.
From 2008-2012, the average yearly incidence of ehrlichiosis was 3.2 cases per million persons. This is more than twice the estimated incidence for 2000-2007. The incidence rate increases with age, with the ages of 60–69 years being the highest age-specific years. Children less than 10 years and adults aged 70 years and older have the highest case-fatality rates. A documented higher risk of death exists among persons who are immunosuppressed.
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- Aayushi Pratap: Dog ticks may get more of a taste for human blood as the climate changes. On: ScienceNews. November 30, 2020.